Patent Publication Number: US-2022239273-A1

Title: Combining balun and differential amplification device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2021-008674 filed on Jan. 22, 2021. The content of this application is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention relates to a combining balun and a differential amplification device. 
     There is a power amplification device that combines output power of a plurality of amplifier pairs with use of a transformer (see, for example, Japanese Unexamined Patent Application Publication No. 2011-66599). Further, there is a technique in which each of peaking amplifiers and carrier amplifiers is composed of a pair of differential amplifiers and output power of the amplifier pair of the peaking amplifier side and output power of the amplifier pair of the carrier amplifier side are combined with each other by a transformer, in a Doherty amplifier (see, for example, Chenxi Zhao and three others, “Analysis and Design of CMOS Doherty Power Amplifier Based on Voltage Combining Method”, IEEE Access, the U.S., IEEE, Mar. 6, 2017, Vol. 5, pp. 5001-5012, and Chenxi Zhao and three others, “Analysis and Design of CMOS Doherty Power Amplifier Using Voltage Combining Method”, (online), 14-18 Apr. 2013, 2013 IEEE International Wireless Symposium (IWS), (searched on Nov. 19, 2020, Internet ieeexplore.ieee.org/abstract/document/6616725). 
     BRIEF SUMMARY 
     In the power amplification device described in Japanese Unexamined Patent Application Publication No. 2011-66599 and the Doherty amplifiers described in “Analysis and Design of CMOS Doherty Power Amplifier Based on Voltage Combining Method” and “Analysis and Design of CMOS Doherty Power Amplifier Using Voltage Combining Method”, two inductors are provided on the input side of the transformer and two signals having opposite phases and the same amplitudes are supplied to both ends of these respective inductors. 
     In these two inductors, for example, the direction of a magnetic field generated in one inductor is sometimes opposite to the direction of a magnetic field generated in the other inductor. In this case, the magnetic fields sometimes cancel each other out and efficiency in conversion from a differential signal into a single phase signal is sometimes degraded. 
     The present invention provides a combining balun and a differential amplification device that suppress efficiency degradation in converting from a differential signal to a single phase signal. 
     A combining balun according to one aspect of the present invention includes: a first input side conductive member that is wound around a first axis on a first surface, which intersects with the first axis, and has a first portion which is positioned between a second axis, which is substantially parallel to the first axis, and the first axis and through which a first input current flows; a second input side conductive member that is wound around the second axis on the first surface and has a second portion which is positioned between the second axis and the first portion and through which a second input current flows in a same direction as a direction of the first input current; a first output side conductive member that is wound around the first axis on a second surface, which faces the first surface, and has a third portion which faces the first portion; a second output side conductive member that is wound around the second axis on the second surface and has a fourth portion which faces the second portion; and a first output terminal that outputs a current or a voltage, which is generated in the first output side conductive member and the second output side conductive member, based on the first input current and the second input current. 
     According to the present invention, a combining balun and a differential amplification device that suppress efficiency degradation in converting from a differential signal to a single phase signal can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a differential amplification circuit  11 ; 
         FIG. 2  is a plan view of a current combining balun  101 ; 
         FIG. 3  illustrates an example of a layout of an input unit  40  in the current combining balun  101 ; 
         FIG. 4  illustrates an example of a layout of an upper inductor set  50   a  in the current combining balun  101 ; 
         FIG. 5  illustrates an example of a layout of a lower inductor set  50   b  in the current combining balun  101 ; 
         FIG. 6  is a circuit diagram used in a simulation of the current combining balun  101 ; 
         FIG. 7  illustrates an example of frequency change of an inductor relative to each signal source; 
         FIG. 8  illustrates an example of frequency change of a coupling coefficient of each transformer in the current combining balun  101 ; 
         FIG. 9  is a circuit diagram of a differential amplification circuit  12 ; 
         FIG. 10  illustrates an example of a layout of the input unit  40  and an input unit  140 ; 
         FIG. 11  illustrates an example of a layout of the upper inductor set  50   a  and an upper inductor set  150   a;    
         FIG. 12  illustrates an example of a layout of the lower inductor set  50   b  and a lower inductor set  150   b;    
         FIG. 13  illustrates an example of a layout of an input unit  240  in the current combining balun  101 ; 
         FIG. 14  illustrates an example of a layout of a differential amplification device according to a reference example; 
         FIG. 15  illustrates an example of a layout of an upper inductor set  90   a  in a current combining balun according to the reference example; 
         FIG. 16  illustrates an example of frequency change of an inductor according to the reference example relative to each signal source; 
         FIG. 17  illustrates an example of frequency change of a coupling coefficient of each transformer in the current combining balun according to the reference example; 
         FIG. 18  is a circuit diagram of a differential amplification circuit  13 ; 
         FIG. 19  illustrates an example of a layout of an upper inductor set  350   a  in a voltage combining balun  301 ; 
         FIG. 20  illustrates an example of a layout of a lower inductor set  350   b  in the voltage combining balun  301 ; and 
         FIG. 21  is a circuit diagram of a differential amplification circuit  14 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments according to the present invention will be described in detail below with reference to the accompanying drawings. Here, the mutually-same components will be provided with the same reference characters and duplicate description thereof will be omitted as much as possible. 
     First Embodiment 
     A differential amplification device according to a first embodiment will be described. 
       FIG. 1  is a circuit diagram of a differential amplification circuit  11 . As illustrated in  FIG. 1 , the differential amplification circuit  11  is a circuit provided to the differential amplification device and includes a first amplifier  31   a,  a second amplifier  31   b,  a third amplifier  31   c,  a fourth amplifier  31   d,  an inductor  47 , and a current combining balun  101 . The current combining balun  101  includes an input unit  40  and an output unit  50 . The input unit  40  includes a first input side inductor  41  (first input side conductive member) and a second input side inductor  42  (second input side conductive member). The output unit  50  includes an output side inductor  81  and an output side inductor  82 . 
     The differential amplification circuit  11  amplifies a first differential signal and a second differential signal and converts the first differential signal and the second differential signal, which are subjected to the amplification, into two respective single end signals (single phase signals). Then, the differential amplification circuit  11  outputs an output signal RFout that is obtained by combining these single end signals from an output terminal  22 . Here, each of the differential signals is, for example, a radio frequency signal. 
     More specifically, the first differential signal is composed of a first signal RF 1  and a second signal RF 2  that has a different phase from that of the first signal RF 1 . Specifically, the phase of the first signal RF 1  is different from the phase of the second signal RF 2  by approximately 180°, for example. The first signal RF 1  and the second signal RF 2  are generated by, for example, a balun that is provided on the previous stage of the differential amplification circuit  11 . 
     The second differential signal is composed of a third signal RF 3  that has substantially the same phase as that of the first signal RF 1  and a fourth signal RF 4  that has substantially the same phase as that of the second signal RF 2 . As the phase of the first signal RF 1  and the phase of the second signal RF 2  are different from each other by approximately 180° as described above, the phase of the third signal RF 3  and the phase of the fourth signal RF 4  are different from each other by, for example, approximately 180°. The third signal RF 3  and the fourth signal RF 4  are generated by, for example, a balun that is provided on the previous stage of the differential amplification circuit  11 . 
     The first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  and the fourth amplifier  31   d  are composed, for example, of a bipolar transistor such as a heterojunction bipolar transistor (HBT). However, these amplifiers may be composed of a field effect transistor (FET). 
     The first amplifier  31   a  has an input terminal, which is connected with an input terminal  21   a,  and an output terminal. The first amplifier  31   a  amplifies the first signal RF 1  supplied to the input terminal thereof through the input terminal  21   a  and outputs a first amplified signal ARF 1  from the output terminal thereof. 
     The second amplifier  31   b  has an input terminal, which is connected with an input terminal  21   b,  and an output terminal. The second amplifier  31   b  amplifies the second signal RF 2  supplied to the input terminal thereof through the input terminal  21   b  and outputs a second amplified signal ARF 2  from the output terminal thereof. 
     The third amplifier  31   c  has an input terminal, which is connected with an input terminal  21   c,  and an output terminal. The third amplifier  31   c  amplifies the third signal RF 3  supplied to the input terminal thereof through the input terminal  21   c  and outputs a third amplified signal ARF 3  from the output terminal thereof. 
     The fourth amplifier  31   d  has an input terminal, which is connected with an input terminal  21   d,  and an output terminal. The fourth amplifier  31   d  amplifies the fourth signal RF 4  supplied to the input terminal thereof through the input terminal  21   d  and outputs a fourth amplified signal ARF 4  from the output terminal thereof. 
     In the current combining balun  101 , the first input side inductor  41  in the input unit  40  has a first end  41   a,  a second end  41   b,  and an intermediate tap  41   c.  The first end  41   a  is connected with the output terminal of the first amplifier  31   a.  The second end  41   b  is connected with the output terminal of the second amplifier  31   b.  The intermediate tap  41   c  is connected with a power source voltage supply node N 1  via the inductor  47 . The first amplifier  31   a  and the second amplifier  31   b  perform an amplification operation by using a voltage received from the power source voltage supply node N 1  via the inductor  47  and the intermediate tap  41   c.    
     The output side inductor  81  in the output unit  50  is electromagnetically coupled mainly with the first input side inductor  41  and generates an output current based on an electromagnetic field that is generated by the first amplified signal ARF 1  and the second amplified signal ARF 2  which are supplied to the first input side inductor  41 . The output current will be described in detail later. In the present embodiment, the output side inductor  81  has a first end  81   a,  which is connected with the output terminal  22 , and a second end  81   b  which is grounded. Described in detail later, the output side inductor  81  is configured by connecting two inductors in series. 
     The second input side inductor  42  in the input unit  40  is a similar inductor to the first input side inductor  41 . Specifically, the second input side inductor  42  has a first end  42   a,  a second end  42   b,  and an intermediate tap  42   c.  The first end  42   a  is connected with the output terminal of the third amplifier  31   c.  The second end  42   b  is connected with the output terminal of the fourth amplifier  31   d.  The intermediate tap  42   c  is connected with the power source voltage supply node N 1  via the inductor  47 . The third amplifier  31   c  and the fourth amplifier  31   d  perform an amplification operation by using a voltage received from the power source voltage supply node N 1  via the inductor  47  and the intermediate tap  42   c.    
     The output side inductor  82  in the output unit  50  is electromagnetically coupled mainly with the second input side inductor  42  and generates an output current based on an electromagnetic field that is generated by the third amplified signal ARF 3  and the fourth amplified signal ARF 4  which are supplied to the second input side inductor  42 . The output current will be described in detail later. In the present embodiment, the output side inductor  82  is a similar inductor to the output side inductor  81  and has a first end  82   a,  which is connected with the output terminal  22 , and a second end  82   b  which is grounded. Described in detail later, the output side inductor  82  is configured by connecting two inductors in series. 
     [Layout] 
     A layout of the differential amplification circuit  11  will be described. Some drawings show some of an x axis, a y axis, and a z axis. The x axis, the y axis, and the z axis form a three-dimensional orthogonal coordinate of right-hand system. Hereinafter, an arrow direction of the z axis is sometimes referred to as a z-axis positive side and an opposite direction to the arrow is sometimes referred to as a z-axis negative side. The same is applied to the rest of the axes. Here, the z-axis positive side and the z-axis negative side are sometimes referred to as “upper” and “lower” respectively. 
       FIG. 2  is a plan view illustrating the current combining balun  101  viewed from the y-axis positive side.  FIG. 3  illustrates an example of a layout of the input unit  40  in the current combining balun  101 .  FIG. 4  illustrates an example of a layout of an upper inductor set  50   a  in the current combining balun  101 .  FIG. 5  illustrates an example of a layout of a lower inductor set  50   b  in the current combining balun  101 . 
       FIGS. 3 to 5  are plan views obtained by viewing the input unit  40 , the upper inductor set  50   a,  and the lower inductor set  50   b  from the z-axis positive side respectively. Here,  FIG. 3  illustrates the first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  the fourth amplifier  31   d,  and the inductor  47  in a schematic manner. 
     The output unit  50  includes the upper inductor set  50   a  and the lower inductor set  50   b  as illustrated in  FIGS. 2 to 5 . Each of a first axis  61  and a second axis  62  is substantially parallel to the z axis. The second axis  62  is positioned on the x-axis positive side of the first axis  61  with a certain interval from the first axis  61 . The direction from the first axis  61  toward the second axis  62  (sometimes referred to as a first direction hereinafter) is substantially the same as the x-axis direction. Here, the first axis  61  and the second axis  62  are virtual axes for facilitating deeper understanding with respect to the invention and they are accordingly not provided to actual products. 
     Each of a first surface  66 , a second surface  67 , and a third surface  68 , which are illustrated in  FIG. 2 , intersects with the first axis  61  and the second axis  62  substantially orthogonally. The second surface  67  faces the first surface  66  and is positioned above the first surface  66 . The third surface  68  is opposed to the second surface  67  with the first surface  66  interposed therebetween and is positioned below the first surface  66 . An interval between the first surface  66  and the second surface  67  is substantially the same as an interval between the first surface  66  and the third surface  68 , for example. 
     The input unit  40  illustrated in  FIGS. 2 and 3  is provided on a second layer along the first surface  66 . The upper inductor set  50   a  illustrated in  FIGS. 2 and 4  is provided on a first layer along the second surface  67 . The lower inductor set  50   b  illustrated in  FIGS. 2 and 5  is provided on a third layer along the third surface  68 . 
     In the present embodiment, the first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  and the fourth amplifier  31   d  are provided, for example, on the second layer in an order of the second amplifier  31   b,  the first amplifier  31   a,  the third amplifier  31   c,  and the fourth amplifier  31   d  toward the x-axis positive side (see  FIG. 3 ). 
     The first input side inductor  41  in the input unit  40  illustrated in  FIG. 3  is wound around the first axis  61 . The first input side inductor  41  is wound around the first axis  61  by substantially a half circumference, in the present embodiment. Specifically, the first input side inductor  41  is provided on the y-axis negative side of the second amplifier  31   b  and the first amplifier  31   a  and includes a positive side extending portion  41   d,  a negative side extending portion  41   e,  a coupling portion  41   f,  and a protruding portion  41   g.    
     More specifically, the positive side extending portion  41   d  is positioned on the y-axis negative side of the first amplifier  31   a  and between the first axis  61  and the second axis  62 . The positive side extending portion  41   d  has a shape that extends from the first end  41   a,  connected with the output terminal of the first amplifier  31   a,  toward the y-axis negative side. 
     The negative side extending portion  41   e  is positioned on the y-axis negative side of the second amplifier  31   b  and on an opposite side of the positive side extending portion  41   d  about the first axis  61  used as a reference. The negative side extending portion  41   e  has a shape that extends from the second end  41   b,  connected with the output terminal of the second amplifier  31   b,  toward the y-axis negative side. 
     The coupling portion  41   f  is positioned on the y-axis negative side of the first axis  61  and has a shape that extends in substantially parallel to the x-axis direction. The coupling portion  41   f  couples an end portion on the y-axis negative side of the positive side extending portion  41   d  and an end portion on the y-axis negative side of the negative side extending portion  41   e  to each other. 
     The protruding portion  41   g  has a shape that protrudes from an intermediate point between both end portions of the coupling portion  41   f  toward the y-axis negative side and functions as the intermediate tap  41   c.  The first input side inductor  41  has a U shape that is opened in a direction directed from the y-axis negative side toward the y-axis positive side (sometimes referred to as a second direction hereinafter) as a whole. 
     The second input side inductor  42  illustrated in  FIG. 3  is wound around the second axis  62 . The second input side inductor  42  is wound around the second axis  62  by substantially a half circumference, in the present embodiment. Specifically, the second input side inductor  42  is provided on the y-axis negative side of the third amplifier  31   c  and the fourth amplifier  31   d  and includes a positive side extending portion  42   d,  a negative side extending portion  42   e,  a coupling portion  42   f,  and a protruding portion  42   g.    
     The second input side inductor  42  has a shape that is substantially symmetrical to the first input side inductor  41  across a symmetry plane  63 . Here, the symmetry plane  63  is positioned between the first axis  61  and the second axis  62  and is a plane that is substantially parallel to the yz plane. That is, the second input side inductor  42  is positioned on the x-axis positive side of the first input side inductor  41  and has a U shape that is opened in the second direction as a whole. The symmetry plane  63  is a virtual plane for facilitating better understanding with respect to the invention and the symmetry plane  63  is accordingly not provided to actual products. 
     More specifically, the positive side extending portion  42   d  is positioned on the y-axis negative side of the third amplifier  31   c  and between the positive side extending portion  41   d  of the first input side inductor  41  and the second axis  62 . The positive side extending portion  42   d  has a shape that extends from the first end  42   a,  connected with the output terminal of the third amplifier  31   c,  toward the y-axis negative side. 
     The negative side extending portion  42   e  is positioned on the y-axis negative side of the fourth amplifier  31   d  and on an opposite side of the positive side extending portion  42   d  about the second axis  62  used as a reference. The negative side extending portion  42   e  has a shape that extends from the second end  42   b,  connected with the output terminal of the fourth amplifier  31   d,  toward the y-axis negative side. 
     The coupling portion  42   f  is positioned on the y-axis negative side of the second axis  62  and has a shape that extends in substantially parallel to the x-axis direction. The coupling portion  42   f  couples an end portion on the y-axis negative side of the positive side extending portion  42   d  and an end portion on the y-axis negative side of the negative side extending portion  42   e  to each other. 
     The protruding portion  42   g  has a shape that protrudes from an intermediate point between both end portions of the coupling portion  42   f  toward the y-axis negative side and functions as the intermediate tap  42   c.    
     In the positive side extending portion  41   d  of the first input side inductor  41 , more specifically, in a first portion  41   h  positioned between the first axis  61  and the second axis  62  in the positive side extending portion  41   d,  a first input current i 1  flows based on the first amplified signal ARF 1  and the second amplified signal ARF 2  supplied from the first amplifier  31   a  and the second amplifier  31   b  respectively. 
     In the positive side extending portion  42   d  of the second input side inductor  42 , more specifically, in a second portion  42   h  positioned between the first portion  41   h  and the second axis  62  in the positive side extending portion  42   d,  a second input current i 2  flows based on the third amplified signal ARF 3  and the fourth amplified signal ARF 4  supplied from the third amplifier  31   c  and the fourth amplifier  31   d  respectively. 
     The difference between the phase of the first amplified signal ARF 1  and the phase of the second amplified signal ARF 2  is approximately 180° and similarly, the difference between the phase of the third amplified signal ARF 3  and the phase of the fourth amplified signal ARF 4  is approximately 180°. Further, the phase of the first amplified signal ARF 1  and the phase of the third amplified signal ARF 3  are substantially the same as each other. Thus, the current of the first amplified signal ARF 1  and the current of the third amplified signal ARF 3  flow side by side and the phases of these signals are the same as each other, whereby the direction of the second input current i 2  and the direction of the first input current i 1  are the same as each other. 
     Accordingly, the direction of the magnetic field that is generated by the first input current i 1  in an inner side portion  41   i  of the first input side inductor  41  is opposite to the direction of the magnetic field that is generated by the second input current i 2  in an inner side portion  42   i  of the second input side inductor  42 . Here, the inner side portion  41   i  of the first input side inductor  41  is a region surrounded by the positive side extending portion  41   d,  the negative side extending portion  41   e,  and the coupling portion  41   f,  for example. In a similar manner, the inner side portion  42   i  of the second input side inductor  42  is a region surrounded by the positive side extending portion  42   d,  the negative side extending portion  42   e,  and the coupling portion  42   f,  for example. 
     Here, the direction of the magnetic field in the inner side portion  41   i  of the first input side inductor  41  is opposite to the direction of the magnetic field in an outer side portion of the first input side inductor  41 . In a similar manner, the direction of the magnetic field in the inner side portion  42   i  of the second input side inductor  42  is opposite to the direction of the magnetic field in an outer side portion of the second input side inductor  42 . 
     An outer side portion  41   j  of the first input side inductor  41  is, for example, an outside region of an outline that follows the outer circumference of the first input side inductor  41  and shapes a rectangle overall. That is, the inner side portion  42   i  of the second input side inductor  42  is included in the outer side portion  41   j  of the first input side inductor  41 . 
     An outer side portion  42   j  of the second input side inductor  42  is, for example, an outside region of an outline that follows the outer circumference of the second input side inductor  42  and shapes a rectangle overall. That is, the inner side portion  41   i  of the first input side inductor  41  is included in the outer side portion  42   j  of the second input side inductor  42 . 
     Accordingly, for example, when the first input current i 1  generates a magnetic field directed to the z-axis positive side in the inner side portion  41   i  of the first input side inductor  41 , a magnetic field directed to the z-axis negative side is generated in the outer side portion  41   j  of the first input side inductor  41 . At this time, the second input current i 2  generates a magnetic field directed to the z-axis negative side in the inner side portion  42   i  of the second input side inductor  42  and generates a magnetic field directed to the z-axis positive side in the outer side portion  42   j  of the second input side inductor  42 . 
     On the other hand, for example, when the first input current i 1  generates a magnetic field directed to the z-axis negative side in the inner side portion  41   i  of the first input side inductor  41 , a magnetic field directed to the z-axis positive side is generated in the outer side portion  41   j  of the first input side inductor  41 . At this time, the second input current i 2  generates a magnetic field directed to the z-axis positive side in the inner side portion  42   i  of the second input side inductor  42  and generates a magnetic field directed to the z-axis negative side in the outer side portion  42   j  of the second input side inductor  42 . 
     That is, the direction of the magnetic field generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  and the direction of the magnetic field generated by the second input current i 2  in the outer side portion  42   j  of the second input side inductor  42  are the same as each other. Also, the direction of the magnetic field generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42  and the direction of the magnetic field generated by the first input current i 1  in the outer side portion  41   j  of the first input side inductor  41  are the same as each other. 
     That is, the magnetic field generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  is strengthened by a magnetic field existing in the inner side portion  41   i  of the first input side inductor  41  out of the magnetic field generated by the second input current i 2  in the outer side portion  42   j  of the second input side inductor  42 . Also, the magnetic field generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42  is strengthened by a magnetic field existing in the inner side portion  42   i  of the second input side inductor  42  out of the magnetic field generated by the first input current i 1  in the outer side portion  41   j  of the first input side inductor  41 . Accordingly, a coupling coefficient in the current combining balun  101  can be increased, being able to suppress efficiency degradation in converting from a differential signal to a single phase signal. 
     The upper inductor set  50   a  (see  FIG. 4 ) and the lower inductor set  50   b  (see  FIG. 5 ) will be described. 
     The upper inductor set  50   a  includes a first output side inductor  51  (first output side conductive member), a second output side inductor  52  (second output side conductive member), a coupling portion  50   aa,  and a protruding portion  50   ab.  The lower inductor set  50   b  includes a third output side inductor  53  (third output side conductive member) and a fourth output side inductor  54  (fourth output side conductive member). 
     The first output side inductor  51  in the upper inductor set  50   a  is positioned on the z-axis positive side of the first input side inductor  41  and is wound around the first axis  61 . More specifically, the first output side inductor  51  has a first end  51   a  and a second end  51   b  that is provided on a position overlapped with the first axis  61 , in plan view of the upper inductor set  50   a  viewed from the z-axis positive side. The first output side inductor  51  winds clockwise by approximately 360° from the second end  51   b  to the first end  51   a  in a manner to separate from the first axis  61 , in the plan view. This configuration can shorten the entire length of the first output side inductor  51 . 
     The second output side inductor  52  is positioned on the z-axis positive side of the second input side inductor  42 . The second output side inductor  52  has a shape that is substantially symmetrical to the first output side inductor  51  across the symmetry plane  63 . That is, the second output side inductor  52  is positioned on the x-axis positive side of the first output side inductor  51  and is wound around the second axis  62  in an opposite direction to the winding direction of the first output side inductor  51 . 
     More specifically, the second output side inductor  52  has a first end  52   a  and a second end  52   b  that is provided on a position overlapped with the second axis  62 , in plan view of the upper inductor set  50   a  viewed from the z-axis positive side. The second output side inductor  52  winds counterclockwise by approximately 360° from the second end  52   b  to the first end  52   a  in a manner to separate from the second axis  62 , in the plan view. This configuration can shorten the entire length of the second output side inductor  52 . 
     The coupling portion  50   aa  has a shape that extends in substantially parallel to the x-axis direction and couples the first end  51   a  of the first output side inductor  51  and the first end  52   a  of the second output side inductor  52  to each other. The protruding portion  50   ab  has a shape that protrudes from an intermediate point between both end portions of the coupling portion  50   aa  toward the y-axis negative side. 
     The first output side inductor  51  has a third portion  51   d  that faces the first portion  41   h  of the first input side inductor  41 . The third portion  51   d  is positioned between the first axis  61  and the second axis  62 . 
     The second output side inductor  52  has a fourth portion  52   e  that faces the second portion  42   h  of the second input side inductor  42 . The fourth portion  52   e  is positioned between the third portion  51   d  and the second axis  62 . 
     A first output current i 3  based on the first input current i 1  flows through the third portion  51   d.  More specifically, in response to change of a magnetic field that is generated mainly by the first input current i 1  flowing through the first input side inductor  41 , an electric field is generated along a direction in which the first output side inductor  51  is wound, in the first output side inductor  51 . The first output current i 3  flows through the third portion  51   d  in accordance with the electric field thus generated. 
     A second output current i 4  based on the second input current i 2  flows through the fourth portion  52   e  in the same direction as the first output current i 3 . More specifically, in response to change of a magnetic field that is generated mainly by the second input current i 2  flowing through the second input side inductor  42 , an electric field is generated along a direction in which the second output side inductor  52  is wound, in the second output side inductor  52 . The direction of the magnetic field generated by the first input current i 1  is opposite to the direction of the magnetic field generated by the second input current i 2  as described above and therefore, the second output current i 4  having the same direction as the first output current i 3  flows through the fourth portion  52   e  in accordance with the electric field in the second output side inductor  52 . 
     The third output side inductor  53  and the fourth output side inductor  54  in the lower inductor set  50   b  are respectively wound in the opposite direction to the winding direction of the first output side inductor  51  and the winding direction of the second output side inductor  52  in the upper inductor set  50   a.    
     More specifically, the third output side inductor  53  in the lower inductor set  50   b  is positioned on the z-axis negative side of the first input side inductor  41  and is wound around the first axis  61  in the opposite direction to the winding direction of the first output side inductor  51 . More specifically, the third output side inductor  53  has a first end  53   a  that is grounded and a second end  53   b  that is provided on a position overlapped with the first axis  61  in plan view of the lower inductor set  50   b  viewed from the z-axis positive side. The third output side inductor  53  winds counterclockwise by approximately 360° from the second end  53   b  to the first end  53   a  in a manner to separate from the first axis  61 , in the plan view. The second end  53   b  is connected with the second end  51   b  of the first output side inductor  51  through a via  56   a  (see  FIG. 2 ). The via  56   a  is formed along the first axis  61  and penetrates through the inner side portion  41   i  of the first input side inductor  41 . 
     The fourth output side inductor  54  is positioned on the z-axis negative side of the second input side inductor  42 . The fourth output side inductor  54  has a shape that is substantially symmetrical to the third output side inductor  53  across the symmetry plane  63 . That is, the fourth output side inductor  54  is positioned on the x-axis positive side of the third output side inductor  53  and is wound around the second axis  62  in an opposite direction to the winding direction of the third output side inductor  53 . In other words, the fourth output side inductor  54  is wound around the second axis  62  in the opposite direction to the winding direction of the second output side inductor  52 . 
     More specifically, the fourth output side inductor  54  has a first end  54   a  that is grounded and a second end  54   b  that is provided on a position overlapped with the second axis  62  in plan view of the lower inductor set  50   b  viewed from the z-axis positive side. The fourth output side inductor  54  winds clockwise by approximately 360° from the second end  54   b  to the first end  54   a  in a manner to separate from the second axis  62 , in the plan view. The second end  54   b  is connected with the second end  52   b  of the second output side inductor  52  through a via  56   b  (see  FIG. 2 ). The via  56   b  is formed along the second axis  62  and penetrates through the inner side portion  42   i  of the second input side inductor  42 . 
     The third output side inductor  53  has a fifth portion  53   d  that is opposed to the third portion  51   d  of the first output side inductor  51  with the first portion  41   h  of the first input side inductor  41  interposed therebetween. The fifth portion  53   d  is positioned between the first axis  61  and the second axis  62 . 
     The fourth output side inductor  54  has a sixth portion  54   e  that is opposed to the fourth portion  52   e  of the second output side inductor  52  with the second portion  42   h  of the second input side inductor  42  interposed therebetween. The sixth portion  54   e  is positioned between the fifth portion  53   d  and the second axis  62 . 
     A third output current i 5  based on the first input current i 1  flows through the fifth portion  53   d,  similarly to the first output current i 3  flowing through the third portion  51   d  in the first output side inductor  51 . The direction of the third output current i 5  and the direction of the first output current i 3  are the same as each other. A fourth output current i 6  based on the second input current i 2  flows through the sixth portion  54   e  in the same direction as the third output current i 5 , similarly to the second output current i 4  flowing through the fourth portion  52   e  in the second output side inductor  52 . 
     An end portion on the y-axis negative side of the protruding portion  50   ab  in the upper inductor set  50   a  is a first output terminal  55   a  that outputs a combined output current i 7 . The combined output current i 7  is obtained by combining a current induced by the first output side inductor  51  and the third output side inductor  53  and a current induced by the second output side inductor  52  and the fourth output side inductor  54  with each other. The first output terminal  55   a  is connected with the output terminal  22  via a connection conductive member  57   a.    
     Thus, the first output side inductor  51  and the third output side inductor  53  are connected in series and therefore, the first output side inductor  51  and the third output side inductor  53  function as the output side inductor (see  FIG. 1 ). 
     In a similar manner, the second output side inductor  52  and the fourth output side inductor  54  are connected in series and therefore, the second output side inductor  52  and the fourth output side inductor  54  function as the output side inductor  82  (see  FIG. 1 ). Further, the output signal RFout having the combined output current i 7  is outputted from the output terminal  22 . 
     [Simulation] 
     A simulation of the current combining balun  101  will be described. 
       FIG. 6  is a circuit diagram used in the simulation of the current combining balun  101 . As illustrated in  FIG. 6 , a signal source Src 1  supplies a signal to the other end of the first input side inductor  41  whose one end is grounded. The signal supplied to the other end of the first input side inductor  41  is obtained by combining the first amplified signal ARF 1  and the second amplified signal ARF 2  that are outputted from the first amplifier  31   a  and the second amplifier  31   b  respectively. 
     A signal source Src 2  supplies a signal to the other end of the second input side inductor  42  whose one end is grounded. The signal supplied to the other end of the second input side inductor  42  is obtained by combining the third amplified signal ARF 3  and the fourth amplified signal ARF 4  that are outputted from the third amplifier  31   c  and the fourth amplifier  31   d  respectively. 
     A signal source Src 3  is capable of supplying a signal to the other end of the output side inductor  81  whose one end is grounded and to the other end of the output side inductor  82  whose one end is grounded. 
       FIG. 7  illustrates an example of frequency change of an inductor relative to each signal source. In  FIG. 7 , the horizontal axis represents a frequency whose unit is “GHz” and the vertical axis represents inductance whose unit is “nH”. 
     As illustrated in  FIG. 7 , a curve L 1  represents, for example, frequency change of inductance from the signal source Src 3  to the ground. Curves L 2  and L 4  represent, for example, frequency change of inductance from the signal source Src 1  and the signal source Src 2  to the ground respectively. 
     For example, at 5 GHz, the curves L 1 , L 2 , and L 4  respectively show 1.30 nH, 0.48 nH, and 0.50 nH. Further, the curves L 1 , L 2 , and L 4  show inductance at which frequency change is small but larger than zero in a frequency region of 6 GHz or lower, for example. That is, the first input side inductor  41 , the second input side inductor  42 , the output side inductor  81 , and the output side inductor  82  function as inductors in the frequency region of 6 GHz or lower, being able to make the balun favorably function in this frequency region. 
       FIG. 8  illustrates an example of frequency change of a coupling coefficient of each transformer in the current combining balun  101 . In  FIG. 8 , the horizontal axis represents a frequency whose unit is “GHz” and the vertical axis represents a coupling coefficient. 
     As illustrated in  FIG. 8 , a curve k 1  represents frequency change of a coupling coefficient between the first input side inductor  41  and the first and third output side inductors  51  and  53 , for example. A curve k 2  represents frequency change of a coupling coefficient between the second input side inductor  42  and the second and fourth output side inductors  52  and  54 , for example. 
     A coupling coefficient is a coefficient whose maximum value is 1. As the value of the coupling coefficient increases, conversion efficiency rises. In the present embodiment, the curves k 1  and k 2  respectively show 0.53 and 0.51 at 5 GHz, for example, and thus, the balun is capable of converting a differential signal into a single end signal at a favorable conversion efficiency. 
     Second Embodiment 
     A differential amplification device according to a second embodiment will be described. The second and following embodiments will omit the description of matters common to those of the first embodiment and describe only different points. In particular, the same advantageous effects obtained from the same configuration will not be sequentially mentioned in each embodiment. 
       FIG. 9  is a circuit diagram of a differential amplification circuit  12 . As illustrated in  FIG. 9 , the differential amplification circuit  12  according to the second embodiment is different from the differential amplification circuit  11  according to the first embodiment in that four differential signals are inputted into the differential amplification circuit  12 . 
     The differential amplification circuit  12  is a circuit provided to a differential amplification device and further includes a fifth amplifier  31   e,  a sixth amplifier  31   f,  a seventh amplifier  31   g,  an eighth amplifier  31   h,  and a current combining balun  1101 , compared to the differential amplification circuit  11  illustrated in  FIG. 1 . The current combining balun  1101  includes an input unit  140  and an output unit  150 . The input unit  140  includes the first input side inductor  41  and the second input side inductor  42 . The output unit  150  includes the output side inductor  81  and the output side inductor  82 . 
     The differential amplification circuit  12  amplifies each of first to fourth differential signals and converts the first to fourth differential signals, which are subjected to the amplification, into four respective single end signals. Then, the differential amplification circuit  12  outputs an output signal RFout that is obtained by combining these single end signals from the output terminal  22 . 
     More specifically, the third differential signal is composed of a fifth signal RF 5  that has substantially the same phase as that of the first signal RF 1  and a sixth signal RF 6  that has substantially the same phase as that of the second signal RF 2 . In a similar manner, the fourth differential signal is composed of a seventh signal RF 7  that has substantially the same phase as that of the first signal RF 1  and an eighth signal RF 8  that has substantially the same phase as that of the second signal RF 2 . 
     As the phase of the first signal RF 1  and the phase of the second signal RF 2  are different from each other by approximately 180° as described above, the difference between the phase of the fifth signal RF 5  and the phase of the sixth signal RF 6  is approximately 180° and the difference between the phase of the seventh signal RF 7  and the phase of the eighth signal RF 8  is approximately 180°. The third differential signal and the fourth differential signal are generated by, for example, a balun that is provided on the previous stage of the differential amplification circuit  12 . 
     The fifth amplifier  31   e,  the sixth amplifier  31   f,  the seventh amplifier  31   g,  and the eighth amplifier  31   h  are similar amplifiers to the first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  and the fourth amplifier  31   d.    
     The fifth amplifier  31   e  amplifies the fifth signal RF 5  supplied via an input terminal  21   e  and outputs a fifth amplified signal ARF 5  from an output terminal thereof. The sixth amplifier  31   f  amplifies the sixth signal RF 6  supplied via an input terminal  21   f  and outputs a sixth amplified signal ARF 6  from an output terminal thereof. The seventh amplifier  31   g  amplifies the seventh signal RF 7  supplied via an input terminal  21   g  and outputs a seventh amplified signal ARF 7  from an output terminal thereof. The eighth amplifier  31   h  amplifies the eighth signal RF 8  supplied via an input terminal  21   h  and outputs an eighth amplified signal ARF 8  from an output terminal thereof. 
     The current combining balun  1101  is a similar balun to the current combining balun  101  and converts the third differential signal and the fourth differential signal into two respective single end signals. In the current combining balun  1101 , the first input side inductor  41  in the input unit  140  has the first end  41   a,  the second end  41   b,  and the intermediate tap  41   c.  The first end  41   a  is connected with the output terminal of the fifth amplifier  31   e.  The second end  41   b  is connected with the output terminal of the sixth amplifier  31   f.  The intermediate tap  41   c  is connected with the power source voltage supply node N 1  via the inductor  47 . The fifth amplifier  31   e  and the sixth amplifier  31   f  perform an amplification operation by using a voltage received from the power source voltage supply node N 1  via the inductor  47  and the intermediate tap  41   c.    
     The output side inductor  81  in the output unit  150  is electromagnetically coupled mainly with the first input side inductor  41  in the input unit  140  and generates an output current based on an electromagnetic field that is generated by the fifth amplified signal ARF 5  and the sixth amplified signal ARF 6  which are supplied to the first input side inductor  41 . The output current will be described in detail later. The output side inductor  81  has the first end  81   a,  which is connected with the output terminal  22 , and the second end  81   b  which is grounded. 
     The second input side inductor  42  in the input unit  140  has the first end  42   a,  the second end  42   b,  and the intermediate tap  42   c.  The first end  42   a  is connected with the output terminal of the seventh amplifier  31   g.  The second end  42   b  is connected with the output terminal of the eighth amplifier  31   h.  The intermediate tap  42   c  is connected with the power source voltage supply node N 1  via the inductor  47 . The seventh amplifier  31   g  and the eighth amplifier  31   h  perform an amplification operation by using a voltage received from the power source voltage supply node N 1  via the inductor  47  and the intermediate tap  42   c.    
     The output side inductor  82  in the output unit  150  is electromagnetically coupled mainly with the second input side inductor  42  in the input unit  140  and generates an output current based on an electromagnetic field that is generated by the seventh amplified signal ARF 7  and the eighth amplified signal ARF 8  which are supplied to the second input side inductor  42 . The output current will be described in detail later. The output side inductor  82  has the first end  82   a,  which is connected with the output terminal  22 , and the second end  82   b  which is grounded. 
     [Layout] 
     A layout of the differential amplification circuit  12  will be described. 
       FIG. 10  illustrates an example of a layout of the input unit  40  and the input unit  140 .  FIG. 11  illustrates an example of a layout of the upper inductor set  50   a  and an upper inductor set  150   a.    FIG. 12  illustrates an example of a layout of the lower inductor set  50   b  and a lower inductor set  150   b.  The way of reading  FIGS. 10 to 12  is the same as that of  FIGS. 3 to 5 . 
     As illustrated in  FIGS. 10 and 12 , the input unit  140  is provided on the x-axis negative side of the input unit  40  (see  FIG. 10 ). The first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  the fourth amplifier  31   d,  the fifth amplifier  31   e,  the sixth amplifier  31   f,  the seventh amplifier  31   g,  and the eighth amplifier  31   h  are provided on, for example, the second layer in an order of the sixth amplifier  31   f,  the fifth amplifier  31   e,  the seventh amplifier  31   g,  the eighth amplifier  31   h,  the second amplifier  31   b,  the first amplifier  31   a,  the third amplifier  31   c,  and the fourth amplifier  31   d  toward the x-axis positive side. 
     In the positive side extending portion  41   d  of the first input side inductor  41  in the input unit  140 , more specifically, in the first portion  41   h,  a third input current i 9  flows based on the fifth amplified signal ARFS and the sixth amplified signal ARF 6  supplied from the fifth amplifier  31   e  and the sixth amplifier  31   f  respectively. 
     In the positive side extending portion  42   d  of the second input side inductor  42  in the input unit  140 , more specifically, in the second portion  42   h,  a fourth input current i 10  flows based on the seventh amplified signal ARF 7  and the eighth amplified signal ARF 8  supplied from the seventh amplifier  31   g  and the eighth amplifier  31   h  respectively. 
     The phase of the first amplified signal ARF 1  and the phase of the second amplified signal ARF 2  are different from each other by approximately 180°. Further, the fifth amplified signal ARFS and the sixth amplified signal ARF 6  have substantially the same phases as those of the first amplified signal ARF 1  and the second amplified signal ARF 2  respectively. Therefore, the direction of the third input current i 9  and the direction of the first input current i 1  are the same as each other. In a similar manner, the seventh amplified signal ARF 7  and the eighth amplified signal ARF 8  have substantially the same phases as those of the first amplified signal ARF 1  and the second amplified signal ARF 2  respectively. Therefore, the direction of the fourth input current i 10  and the direction of the first input current i 1  are the same as each other. 
     The output unit  150  includes the upper inductor set  150   a  similar to the upper inductor set  50   a  and the lower inductor set  150   b  similar to the lower inductor set  50   b  and is provided on the x-axis negative side of the output unit  50  (see  FIGS. 11 and 12 ). More specifically, the upper inductor set  150   a  in the output unit  150  is provided on the x-axis negative side of the upper inductor set  50   a  and above the input unit  140  (see  FIGS. 10 and 11 ). The lower inductor set  150   b  is provided on the x-axis negative side of the lower inductor set  50   b  and below the input unit  140  (see  FIGS. 10 and 12 ). 
     A fifth output current i 11  based on the third input current i 9  flows through the third portion  51   d  (see  FIG. 11 ) in the upper inductor set  150   a.  The direction of the fifth output current i 11  and the direction of the first output current i 3  are the same as each other (see  FIG. 11 ). A sixth output current i 12  based on the fourth input current i 10  flows through the fourth portion  52   e  in the same direction as the fifth output current i 11 , that is, in the same direction as the first output current i 3 . 
     A seventh output current i 13  based on the third input current i 9  flows through the fifth portion  53   d  (see  FIG. 12 ) in the lower inductor set  150   b.  The direction of the seventh output current i 13  and the direction of the first output current i 3  are the same as each other. An eighth output current i 14  based on the fourth input current i 10  flows through the sixth portion  54   e  in the same direction as the seventh output current i 13 , that is, in the same direction as the first output current i 3 . 
     The first output terminal  55   a  in the upper inductor set  150   a  is connected with the output terminal  22  via a connection conductive member  57   d,  a node N 2 , and the connection conductive member  57   a.  The first output terminal  55   a  in the upper inductor set  150   a  outputs a combined output current i 15 . The combined output current i 15  is obtained by combining a current induced by the first output side inductor  51  in the upper inductor set  150   a  and the third output side inductor  53  in the lower inductor set  150   b  and a current induced by the second output side inductor  52  in the upper inductor set  150   a  and the fourth output side inductor  54  in the lower inductor set  150   b  with each other. 
     The first output terminal  55   a  in the upper inductor set  50   a  is connected with the output terminal  22  via a connection conductive member  57   c,  the node N 2 , and the connection conductive member  57   a  and outputs a combined output current i 7 . 
     Then, the output signal RFout having a combined output current, which is obtained by combining the combined output current i 7  and the combined output current i 15  with each other, is outputted from the output terminal  22 . 
     Third Embodiment 
     A differential amplification device according to a third embodiment will be described. 
       FIG. 13  illustrates an example of a layout of an input unit  240  in the current combining balun  101 . The way of reading  FIG. 13  is the same as that of  FIG. 3 . As illustrated in  FIG. 13 , the input unit  240  according to the third embodiment is different from the input unit  40  according to the first embodiment in that two U-shaped inductors are opened in mutually-opposite directions. 
     The current combining balun  101  according to the present embodiment includes the input unit  240  instead of the input unit  40 , compared to the current combining balun  101  illustrated in  FIG. 3 . 
     In the present embodiment, the first input side inductor  41  has the U shape that is opened in the second direction. The second input side inductor  42  is positioned on the x-axis positive side of the first input side inductor  41  and has the U shape that is opened in a third direction which is opposite to the second direction. 
     The first amplifier  31   a  and the second amplifier  31   b  are provided on the y-axis positive side of the first input side inductor  41  in an order of the second amplifier  31   b  and the first amplifier  31   a  toward the x-axis positive side (see  FIG. 13 ). The third amplifier  31   c  and the fourth amplifier  31   d  are provided on the y-axis negative side of the second input side inductor  42  in an order of the fourth amplifier  31   d  and the third amplifier  31   c  toward the x-axis positive side. 
     The negative side extending portion  42   e  of the second input side inductor  42  is positioned on the y-axis positive side of the fourth amplifier  31   d  and between the positive side extending portion  41   d  of the first input side inductor  41  and the second axis  62 . The negative side extending portion  42   e  has a shape that extends from the second end  42   b,  connected with the output terminal of the fourth amplifier  31   d,  toward the y-axis positive side. 
     The positive side extending portion  42   d  is positioned on the y-axis positive side of the third amplifier  31   c  and on an opposite side of the negative side extending portion  42   e  about the second axis  62  used as a reference. The positive side extending portion  42   d  has a shape that extends from the first end  42   a,  connected with the output terminal of the third amplifier  31   c,  toward the y-axis positive side. 
     The second input side inductor  42  has the U shape that is opened in the third direction which is directed from the y-axis positive side toward the y-axis negative side, as a whole. In the negative side extending portion  42   e  of the second input side inductor  42 , more specifically, in the second portion  42   h,  which is positioned between the first portion  41   h  and the second axis  62 , in the negative side extending portion  42   e,  the second input current i 2  flows based on the third amplified signal ARF 3  and the fourth amplified signal ARF 4  supplied from the third amplifier  31   c  and the fourth amplifier  31   d  respectively. 
     The phase of the first amplified signal ARF 1  and the phase of the second amplified signal ARF 2  are different from each other by approximately 180°. Further, the third amplified signal ARF 3  and the fourth amplified signal ARF 4  have substantially the same phases as those of the first amplified signal ARF 1  and the second amplified signal ARF 2  respectively. Therefore, the direction of the second input current i 2  and the direction of the first input current i 1  are the same as each other. 
     REFERENCE EXAMPLE 
     A differential amplification device according to a reference example will be described. 
       FIG. 14  illustrates an example of a layout of the input unit  40  in a current combining balun according to the reference example.  FIG. 15  illustrates an example of a layout of an upper inductor set  90   a  in the current combining balun according to the reference example. The way of reading  FIGS. 14 and 15  is the same as that of  FIGS. 3 and 4 . 
     As illustrated in  FIGS. 14 and 15 , in the differential amplification device according to the reference example, the first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  and the fourth amplifier  31   d  are provided, for example, on the second layer in an order of the first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  and the fourth amplifier  31   d  toward the x-axis positive side (see  FIG. 14 ). 
     The first end  41   a  and the second end  41   b  of the first input side inductor  41  are connected with the output terminal of the second amplifier  31   b  and the output terminal of the first amplifier  31   a  respectively. The first end  42   a  and the second end  42   b  of the second input side inductor  42  are connected with the output terminal of the third amplifier  31   c  and the output terminal of the fourth amplifier  31   d  respectively. 
     Accordingly, the direction of an input current i 91  that flows through the first portion  41   h  of the first input side inductor  41  is opposite to the direction of an input current i 92  that flows through the second portion  42   h  of the second input side inductor  42 . 
     Accordingly, the direction of a magnetic field that is generated by the input current i 91  in an inner side portion of the first input side inductor  41  is the same as the direction of a magnetic field that is generated by the input current i 92  in an inner side portion of the second input side inductor  42 . 
     That is, the direction of the magnetic field generated by the input current i 91  in the inner side portion of the first input side inductor  41  and the direction of a magnetic field generated by the input current i 92  in an outer side portion of the second input side inductor  42  are opposite to each other. Also, the direction of the magnetic field generated by the input current i 92  in the inner side portion of the second input side inductor  42  and the direction of a magnetic field generated by the input current i 91  in an outer side portion of the first input side inductor  41  are opposite to each other. 
     That is, the magnetic field generated by the input current i 91  in the inner side portion of the first input side inductor  41  is weakened by the magnetic field generated by the input current i 92  in the outer side portion of the second input side inductor  42 . Also, the magnetic field generated by the input current i 92  in the inner side portion of the second input side inductor  42  is weakened by the magnetic field generated by the input current i 91  in the outer side portion of the first input side inductor  41 . 
     The upper inductor set  90   a  will now be described. The upper inductor set  90   a  is provided on the first layer. A first output side inductor  91  and a second output side inductor  92  in the upper inductor set  90   a  are positioned above the first input side inductor  41  and the second input side inductor  42  respectively. The first output side inductor  91  and the second output side inductor  92  are wound in the same direction as each other. 
     More specifically, the first output side inductor  91  has a first end  91   a,  which is connected with a first output terminal  95   a,  and a second end  91   b,  which is provided on a position overlapped with the first axis  61  and is grounded. The second output side inductor  92  has a first end  92   a,  which is connected with the first output terminal  95   a,  and a second end  92   b,  which is provided on a position overlapped with the second axis  62  and is grounded. 
     The first output side inductor  91  and the second output side inductor  92  wind counterclockwise in plan view of the upper inductor set  90   a  viewed from the z-axis positive side. 
     In the first output side inductor  91 , an output current i 93  based on the input current i 91  flows through a third portion  91   d  facing the first portion  41   h  of the first input side inductor  41 . 
     In the second output side inductor  92 , an output current i 94  based on the input current i 92  flows through a fourth portion  92   e  facing the second portion  42   h  of the second input side inductor  42 , in the opposite direction to the output current i 93 . The first output terminal  95   a  outputs a combined output current i 95 , which is obtained by combining the output current i 93  and the output current i 94 , to the output terminal  22 . 
     Not illustrated, a lower inductor set similar to the upper inductor set  90   a  is provided on the third layer and a combined output current similar to the combined output current i 95  is outputted to the output terminal  22 . The output signal RFout is outputted from the output terminal  22 . The output signal RFout has a current that is obtained by combining the combined output current i 95  and the combined output current from the lower inductor set with each other. 
     [Simulation] 
     A simulation of the current combining balun according to the reference example will be described. This simulation was performed by using the circuit illustrated in  FIG. 6 . 
       FIG. 16  illustrates an example of frequency change of the inductor according to the reference example relative to each signal source. The way of reading  FIG. 16  is the same as that of  FIG. 7 . As illustrated in  FIGS. 6 and 16 , a curve L 91  represents, for example, frequency change of inductance from the signal source Src 3  to the ground. Curves L 92  and L 94  represent, for example, frequency change of inductance from the signal source Src 1  and the signal source Src 2  to the ground respectively. 
     For example, at 5 GHz, the curves L 91 , L 92 , and L 94  respectively show 1.02 nH, 0.46 nH, and 0.44 nH. As the curves L 1 , L 2 , and L 4  respectively show 1.30 nH, 0.48 nH, and 0.50 nH at 5 GHz (see  FIG. 7 ) as described above, inductance of each inductor constituting the current combining balun  101  can be raised in the current combining balun  101  compared to the current combining balun according to the reference example. 
       FIG. 17  illustrates an example of frequency change of a coupling coefficient of each transformer in the current combining balun according to the reference example. The way of reading  FIG. 17  is the same as that of  FIG. 8 . 
     As illustrated in  FIG. 17 , a curve k 91  represents frequency change of a coupling coefficient between the first input side inductor  41  and the first output side inductors  91  on the first and third layers, for example. A curve k 92  represents frequency change of a coupling coefficient between the second input side inductor  42  and the second output side inductors  92  on the first and third layers, for example. 
     For example, at 5 GHz, the curves k 91  and k 92  respectively show 0.50 and 0.47. As described above, the curves kl and k 4  respectively show 0.53 and 0.51 at 5 GHz (see  FIG. 8 ) and thus, the current combining balun  101  is capable of converting a differential signal into a single end signal at more favorable conversion efficiency, compared to the current combining balun according to the reference example. 
     In the above description, the current combining balun  101  has the configuration in which the upper inductor set  50   a  and the lower inductor set  50   b  are provided on the first layer and the third layer respectively. However, the configuration is not limited to this. The current combining balun  101  may be configured to include either one of the upper inductor set  50   a  on the first layer and the lower inductor set  50   b  on the third layer. 
     The description has been provided on the configuration in which each of the first surface  66 , the second surface  67 , and the third surface  68  intersects with the first axis  61  and the second axis  62  substantially orthogonally. However, the configuration is not limited to this. In the configuration, the first surface  66 , the second surface  67 , or the third surface  68  does not have to intersect with the first axis  61  and the second axis  62  substantially orthogonally as long as the first surface  66 , the second surface  67 , or the third surface  68  intersects with the first axis  61  and the second axis  62 . 
     The description has been provided on the configuration in which the first input side inductor  41  is wound around the first axis  61  by a half circumference in the current combining balun  101 . However, the configuration is not limited to this. The first input side inductor  41  may be configured to be wound one or more times around the first axis  61  or wound less than once around the first axis  61 . 
     The description has been provided on the configuration in which the second input side inductor  42  is wound around the second axis  62  by a half circumference in the current combining balun  101 . However, the configuration is not limited to this. The second input side inductor  42  may be configured to be wound one or more times around the second axis  62  or wound less than once around the second axis  62 . 
     The description has been provided on the configuration in which the first output side inductor  51  and the third output side inductor  53  are wound approximately once around the first axis  61  in the current combining balun  101 . However, the configuration is not limited to this. The first output side inductor  51  or the third output side inductor  53  may be configured to be wound more than once around the first axis  61  or wound less than once around the first axis  61 . 
     The description has been provided on the configuration in which the second output side inductor  52  and the fourth output side inductor  54  are wound approximately once around the second axis  62  in the current combining balun  101 . However, the configuration is not limited to this. The second output side inductor  52  or the fourth output side inductor  54  may be configured to be wound more than once around the second axis  62  or wound less than once around the second axis  62 . 
     The description has been provided on the configuration in which the first output side inductor  51  and the fourth output side inductor  54  wind clockwise and the second output side inductor  52  and the third output side inductor  53  wind counterclockwise in the current combining balun  101  in plan view of the current combining balun viewed from the z-axis positive side. However, the configuration is not limited to this. A configuration may be employed in which the first output side inductor  51  and the fourth output side inductor  54  wind counterclockwise and the second output side inductor  52  and the third output side inductor  53  wind clockwise. 
     The description has been provided on the configuration in which the second direction intersects with the first direction substantially orthogonally in the current combining balun  101 . However, the configuration is not limited to this. In the configuration, the second direction does not have to intersect with the first direction substantially orthogonally as long as the second direction intersects with the first direction. 
     The description has been provided on the configuration in which two current combining baluns  101  are aligned along the x axis in the differential amplification circuit  12 . However, the configuration is not limited to this. A configuration may be employed in which three or more current combining baluns  101  are aligned along the x axis. 
     The description has been provided on the configuration of the differential amplification device that is provided with the current combining balun  101  that combines currents which are respectively induced by the first output side inductor  51 , the second output side inductor  52 , the third output side inductor  53 , and the fourth output side inductor  54 . However, the configuration is not limited to this. The differential amplification device may be configured to include a current combining balun that combines voltages which are respectively generated in the first output side inductor  51 , the second output side inductor  52 , the third output side inductor  53 , and the fourth output side inductor  54 . 
     Fourth Embodiment 
     A differential amplification device according to a fourth embodiment will be described. 
       FIG. 18  is a circuit diagram of a differential amplification circuit  13 . As illustrated in  FIG. 18 , the differential amplification circuit  13  according to the fourth embodiment is different from the differential amplification circuit  11  according to the first embodiment in that voltages generated in output side inductors are combined with each other. 
     The differential amplification circuit  13  is a circuit provided to a differential amplification device and includes a voltage combining balun  301  instead of the current combining balun  101 , compared to the differential amplification circuit  11  illustrated in  FIG. 1 . The voltage combining balun  301  includes an output unit  350  instead of the output unit  50 , compared to the current combining balun  101  illustrated in  FIG. 1 . The output unit  350  includes an output side inductor  381  and an output side inductor  382 . 
     The output side inductor  381  in the output unit  350  is electromagnetically coupled mainly with the first input side inductor  41  and generates an output voltage based on an electromagnetic field that is generated by the first amplified signal ARF 1  and the second amplified signal ARF 2  which are supplied to the first input side inductor  41 . The output voltage will be described in detail later. In the present embodiment, the output side inductor  381  has a first end  381   a  and a second end  381   b  that is connected with the output terminal  22 . Described in detail later, the output side inductor  381  is configured by connecting two inductors in series. 
     The output side inductor  382  in the output unit  350  is electromagnetically coupled mainly with the second input side inductor  42  and generates an output voltage based on an electromagnetic field that is generated by the third amplified signal ARF 3  and the fourth amplified signal ARF 4  which are supplied to the second input side inductor  42 . The output voltage will be described in detail later. In the present embodiment, the output side inductor  382  has a first end  382   a,  which is connected with the first end of the output side inductor  381 , and a second end  382   b  which is grounded. Described in detail later, the output side inductor  382  is configured by connecting two inductors in series. 
     [Layout] 
       FIG. 19  illustrates an example of a layout of an upper inductor set  350   a  in the voltage combining balun  301 .  FIG. 20  illustrates an example of a layout of a lower inductor set  350   b  in the voltage combining balun  301 . The way of reading  FIGS. 19 and 20  is the same as that of  FIGS. 4 and 5 . 
     The output unit  350  includes the upper inductor set  350   a  and the lower inductor set  350   b  as illustrated in  FIG. 3  and  FIGS. 18 to 20 . The input unit  40  (see  FIG. 3 ) is provided on the second layer along the first surface  66  (see  FIG. 2 ). The upper inductor set  350   a  illustrated in  FIG. 19  is provided on the first layer along the second surface  67  (see  FIG. 2 ). The lower inductor set  350   b  illustrated in  FIG. 20  is provided on the third layer along the third surface  68  (see  FIG. 2 ) . 
     The upper inductor set  350   a  illustrated in  FIG. 19  includes a first output side inductor  351  (first output side conductive member) and a second output side inductor  352  (second output side conductive member). The lower inductor set  350   b  includes a third output side inductor  353  (third output side conductive member) and a fourth output side inductor  354  (fourth output side conductive member). 
     The first output side inductor  351  in the upper inductor set  350   a  is positioned on the z-axis positive side of the first input side inductor  41  and is wound around the first axis  61 . More specifically, the first output side inductor  351  has a first end  351   a,  which is grounded, and a second end  351   b  , in plan view of the upper inductor set  350   a  viewed from the z-axis positive side. The first output side inductor  351  winds clockwise by less than 360° from the first end  351   a  to the second end  351   b  in the plan view. 
     The second output side inductor  352  is positioned on the z-axis positive side of the second input side inductor  42 . The second output side inductor  352  is positioned on the x-axis positive side of the first output side inductor  351  and is wound around the second axis  62  in the winding direction of the first output side inductor  351 . 
     More specifically, the second output side inductor  352  has a first end  352   a  and a second end  352   b.  The first end  352   a  is connected with the second end  351   b  of the first output side inductor  351 , and the second end  352   b  is provided on a position overlapped with the second axis  62  in plan view of the upper inductor set  350   a  viewed from the z-axis positive side. The second output side inductor  352  winds clockwise by approximately 360° from the second end  352   b  to the first end  352   a  in a manner to separate from the second axis  62 , in the plan view. This configuration can shorten the entire length of the second output side inductor  352 . 
     The first output side inductor  351  has a third portion  351   d  that faces the first portion  41   h  of the first input side inductor  41 . The third portion  351   d  is positioned between the first axis  61  and the second axis  62 . 
     The second output side inductor  352  has a fourth portion  352   e  that faces the second portion  42   h  of the second input side inductor  42 . The fourth portion  352   e  is positioned between the third portion  351   d  and the second axis  62 . 
     The first output current i 3  based on the first input current i 1  flows through the third portion  351   d.  More specifically, in response to change of a magnetic field that is generated mainly by the first input current i 1  flowing through the first input side inductor  41 , an electric field is generated along a direction in which the first output side inductor  351  is wound, in the first output side inductor  351 . In accordance with the electric field thus generated, a voltage is generated between the first end  351   a  and the second end  351   b  of the first output side inductor  351 . Here, a potential of the second end  351   b  with respect to the first end  351   a,  that is, with respect to a ground is denoted as V 1 . The first output current i 3  flows through the third portion  351   d  based on the voltage between the first end  351   a  and the second end  351   b.    
     The second output current i 4  based on the second input current i 2  flows through the fourth portion  352   e  in the same direction as the first output current i 3 . More specifically, in response to change of a magnetic field that is generated mainly by the second input current i 2  flowing through the second input side inductor  42 , an electric field is generated along a direction in which the second output side inductor  352  is wound, in the second output side inductor  352 . In accordance with the electric field thus generated, a voltage is generated between the first end  352   a  and the second end  352   b  of the second output side inductor  352 . Here, a potential of the second end  352   b  with respect to the first end  352   a  is denoted as V 2 . The direction of the magnetic field generated by the first input current i 1  is opposite to the direction of the magnetic field generated by the second input current i 2  as described above and therefore, the second output current i 4  having the same direction as the first output current i 3  flows through the fourth portion  352   e  in accordance with the electric field in the second output side inductor  352 . 
     The third output side inductor  353  and the fourth output side inductor  354  in the lower inductor set  350   b  illustrated in  FIG. 20  are wound in the opposite direction to the winding direction of the first output side inductor  351  or the second output side inductor  352  in the upper inductor set  350   a.    
     More specifically, the fourth output side inductor  354  in the lower inductor set  350   b  is positioned on the z-axis negative side of the second input side inductor  42  and is wound around the second axis  62  in the opposite direction to the winding direction of the first output side inductor  351  or the second output side inductor  352 . More specifically, the fourth output side inductor  354  has a first end  354   a  that is provided on a position overlapped with the second axis  62  in plan view of the lower inductor set  350   b  viewed from the z-axis positive side and a second end  354   b.  The fourth output side inductor  354  winds counterclockwise by approximately 360° from the first end  354   a  to the second end  354   b  in a manner to separate from the second axis  62 , in the plan view. The first end  354   a  is connected with the second end  352   b  of the second output side inductor  352  through the via  56   b  (see  FIG. 2  and  FIG. 19 ). 
     The third output side inductor  353  is positioned on the z-axis negative side of the first input side inductor  41 . The third output side inductor  353  is positioned on the x-axis negative side of the fourth output side inductor  354  and is wound around the first axis  61  in the opposite direction to the winding direction of the first output side inductor  351  or the second output side inductor  352 . In other words, the third output side inductor  353  is wound around the first axis  61  in the winding direction of the fourth output side inductor  354 . 
     More specifically, the third output side inductor  353  has a first end  353   a  and a second end  353   b.  The first end  353   a  is connected with the second end  354   b  of the fourth output side inductor  354 , and the second end  353   b  is provided on a position overlapped with the first axis  61  in plan view of the lower inductor set  350   b  viewed from the z-axis positive side. The third output side inductor  353  winds counterclockwise by approximately 360° from the second end  353   b  to the first end  353   a  in a manner to separate from the first axis  61 , in the plan view. 
     The third output side inductor  353  has a fifth portion  353   d  that is opposed to the third portion  351   d  of the first output side inductor  351  with the first portion  41   h  of the first input side inductor  41  interposed therebetween. The fifth portion  353   d  is positioned between the first axis  61  and the second axis  62 . 
     The fourth output side inductor  354  has a sixth portion  354   e  that is opposed to the fourth portion  352   e  of the second output side inductor  352  with the second portion  42   h  of the second input side inductor  42  interposed therebetween. The sixth portion  354   e  is positioned between the fifth portion  353   d  and the second axis  62 . 
     The fourth output current i 6  based on the second input current i 2  flows through the sixth portion  354   e.  More specifically, in response to change of a magnetic field that is generated mainly by the second input current i 2  flowing through the second input side inductor  42 , an electric field is generated along a direction in which the fourth output side inductor  354  is wound, in the fourth output side inductor  354 . In accordance with the electric field thus generated, a voltage is generated between the first end  354   a  and the second end  354   b  of the fourth output side inductor  354 . Here, a potential of the second end  354   b  with respect to the first end  354   a,  that is, with respect to the second end  352   b  of the second output side inductor  352  is denoted as V 3 . The fourth output current i 6  flows through the fourth output side inductor  354  based on the voltage between the first end  354   a  and the second end  354   b.    
     The third output current i 5  based on the first input current i 1  flows through the fifth portion  353   d  in the same direction as the fourth output current i 6 . More specifically, in response to change of a magnetic field that is generated mainly by the first input current i 1  flowing through the first input side inductor  41 , an electric field is generated along a direction in which the third output side inductor  353  is wound, in the third output side inductor  353 . In accordance with the electric field thus generated, a voltage is generated between the first end  353   a  and the second end  353   b  of the third output side inductor  353 . Here, a potential of the second end  353   b  with respect to the first end  353   a  is denoted as V 4 . The direction of the magnetic field generated by the first input current i 1  is opposite to the direction of the magnetic field generated by the second input current i 2  as described above and therefore, the third output current i 5  having the same direction as the fourth output current i 6  flows through the fifth portion  353   d  in accordance with the electric field in the third output side inductor  353 . 
     A connection conductive member  357   a  illustrated in  FIG. 19  is, for example, a straight member that extends along the y axis in plan view of the lower inductor set  350   b  viewed from the z axis positive side, and has a first end and a second end. The first end is connected with the second end  353   b  of the third output side inductor  353  through the via  56   a.  The second end is the output terminal  22 . 
     The output terminal  22  outputs a combined output voltage that is obtained by combining voltages generated in the first output side inductor  351  (see  FIG. 19 ), the second output side inductor  352  (see  FIG. 19 ), the third output side inductor  353  (see  FIG. 20 ), and the fourth output side inductor  354  (see  FIG. 20 ) respectively. Specifically, the first output side inductor  351 , the second output side inductor  352 , the third output side inductor  353 , and the fourth output side inductor  354  are connected in series and therefore, the output terminal  22  has a potential of (V 1 +V 2 +V 3 +V 4 ) with respect to the ground. 
     Further, the first output side inductor  351  and the second output side inductor  352  function as the output side inductor  381  (see  FIG. 18 ). The second output side inductor  352  and the fourth output side inductor  354  function as the output side inductor  382  (see  FIG. 18 ). 
     In the above description, the voltage combining balun  301  has the configuration in which the upper inductor set  350   a  and the lower inductor set  350   b  are provided on the first layer and the third layer respectively. However, the configuration is not limited to this. The voltage combining balun  301  may be configured to include either one of the upper inductor set  350   a  on the first layer and the lower inductor set  350   b  on the third layer. 
     The description has been provided on the configuration in which one voltage combining balun  301  is provided in the differential amplification circuit  13 . However, the configuration is not limited to this. A configuration may be employed in which two or more voltage combining baluns  301  are aligned along the x axis. 
     The description has been provided on the configuration in which the voltage combining balun  301  includes the input unit  40  in the differential amplification circuit  13 . However, the configuration is not limited to this. The voltage combining balun  301  may be configured to include the input unit  240  (see  FIG. 13 ) instead of the input unit  40 . 
     Fifth Embodiment 
     A differential amplification device according to a fifth embodiment will be described. 
       FIG. 21  is a circuit diagram of a differential amplification circuit  14 . As illustrated in  FIG. 21 , the differential amplification circuit  14  according to the fifth embodiment is different from the differential amplification circuit  13  according to the fourth embodiment in that two differential pairs of a driver stage are provided on the previous stage of the first amplifier  31   a,  second amplifier  31   b,  third amplifier  31   c,  and fourth amplifier  31   d.    
     The differential amplification circuit  14  is a circuit provided to a differential amplification device and further includes a balun  401 , a first amplifier  131   a,  a second amplifier  131   b,  a third amplifier  131   c,  and a fourth amplifier  131   d,  compared to the differential amplification circuit  13  illustrated in  FIG. 18 . The balun  401  has a similar configuration to the configuration of the current combining balun  101  illustrated in  FIG. 1 . 
     The first amplifier  131   a  and the second amplifier  131   b  constitute the first driver-stage differential pair. The third amplifier  131   c  and the fourth amplifier  131   d  constitute the second driver-stage differential pair. 
     The first amplifier  31   a  and the second amplifier  31   b  constitute the first power-stage differential pair. The third amplifier  31   c  and the fourth amplifier  31   d  constitute the second power-stage differential pair. 
     The balun  401  is provided between the two driver-stage differential pairs and the two power-stage differential pairs and functions as an inter-stage matching circuit. 
     Specifically, the first amplifier  131   a  has an input terminal, which is connected with the input terminal  21   a,  and an output terminal, which is connected with the first end  41   a  of the first input side inductor  41  in the balun  401 . The first amplifier  131   a  amplifies the first signal RF 1  supplied to the input terminal thereof through the input terminal  21   a  and outputs an amplified signal ARF 9  from the output terminal thereof. 
     The second amplifier  131   b  has an input terminal, which is connected with the input terminal  21   b,  and an output terminal, which is connected with the second end  41   b  of the first input side inductor  41  in the balun  401 . The second amplifier  131   b  amplifies the second signal RF 2  supplied to the input terminal thereof through the input terminal  21   b  and outputs an amplified signal ARF 10  from the output terminal thereof. 
     The third amplifier  131   c  has an input terminal, which is connected with the input terminal  21   c,  and an output terminal, which is connected with the first end  42   a  of the second input side inductor  42  in the balun  401 . The third amplifier  131   c  amplifies the third signal RF 3  supplied to the input terminal thereof through the input terminal  21   c  and outputs an amplified signal ARF 11  from the output terminal thereof. 
     The fourth amplifier  131   d  has an input terminal, which is connected with the input terminal  21   d,  and an output terminal, which is connected with the second end  42   b  of the second input side inductor  42  in the balun  401 . The fourth amplifier  131   d  amplifies the fourth signal RF 4  supplied to the input terminal thereof through the input terminal  21   d  and outputs an amplified signal ARF 12  from the output terminal thereof. 
     The first end  81   a  and the second end  81   b  of the output side inductor  81  in the balun  401  are connected with the input terminal of the first amplifier  31   a  and the input terminal of the second amplifier  31   b  respectively. The first end  82   a  and the second end  82   b  of the output side inductor  82  in the balun  401  are connected with the input terminal of the third amplifier  31   c  and the input terminal of the fourth amplifier  31   d  respectively. 
     Thus, the balun  401  that has a high coupling coefficient among inductors is provided as an inter-stage matching circuit. This configuration can increase an impedance conversion ratio. Accordingly, even when an input impedance of two power-stage differential pairs is reduced to increase an output, for example, impedances between the two driver-stage differential pairs and the two power-stage differential pairs can be favorably matched. 
     The description has been provided on the configuration of the differential amplification circuit  14  that is provided with the voltage combining balun  301  on the subsequent stage of the two power-stage differential pairs. However, the configuration is not limited to this. The differential amplification circuit  14  may be configured to be provided with the current combining balun  101  on the subsequent stage of the two power-stage differential pairs. 
     The exemplary embodiments of the present invention have been described above. A combining balun includes: the first input side inductor  41  that is wound around the first axis  61  on the first surface  66 , which intersects with the first axis  61 , and has the first portion  41   h  which is positioned between the second axis  62 , which is substantially parallel to the first axis  61 , and the first axis  61  and through which the first input current i 1  flows; the second input side inductor  42  that is wound around the second axis  62  on the first surface  66  and has the second portion  42   h  which is positioned between the second axis  62  and the first portion  41   h  and through which the second input current i 2  flows in the same direction as the direction of the first input current i 1 ; the first output side inductor  51  that is wound around the first axis  61  on the second surface  67 , which faces the first surface  66 , and has the third portion  51   d  which faces the first portion  41   h;  the second output side inductor  52  that is wound around the second axis  62  on the second surface  67  and has the fourth portion  52   e  which faces the second portion  42   h;  and the first output terminal  55   a  that outputs a current or a voltage, which is generated in the first output side inductor  51  and the second output side inductor  52 , based on the first input current i 1  and the second input current i 2 . 
     This configuration can make the direction of the magnetic field that is generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  be opposite to the direction of the magnetic field that is generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42 . That is, the direction of the magnetic field generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  and the direction of the magnetic field generated by the second input current i 2  in the outer side portion  42   j  of the second input side inductor  42  are the same as each other and further, the direction of the magnetic field generated by the first input current i 1  in the outer side portion  41   j  of the first input side inductor  41  and the direction of the magnetic field generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42  are the same as each other. Accordingly, the magnetic field generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  is strengthened by the magnetic field generated by the second input current i 2  in the outer side portion  42   j  of the second input side inductor  42  and also, the magnetic field generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42  is strengthened by the magnetic field generated by the first input current i 1  in the outer side portion  41   j  of the first input side inductor  41 . Thus, a coupling coefficient among inductors in the combining balun can be increased, that is, the combining balun can be brought closer to an ideal transformer. This can reduce leakage inductance, improve an impedance conversion ratio of the combining balun, and suppress efficiency degradation in converting from a differential signal to a single phase signal. Further, the combining balun can be used as an inter-stage matching circuit that matches impedances between two driver-stage differential pairs and two power-stage differential pairs. Here, even when the impedance of the two driver-stage differential pairs is largely different from the impedance of the two power-stage differential pairs, the impedance of the two driver-stage differential pairs and the impedance of the two power-stage differential pairs can be favorably matched. 
     In the input unit  40  in the current combining balun  101 , the first input side inductor  41  and the second input side inductor  42  have a U shape that is opened in a second direction, which intersects with a first direction that is directed from the first axis  61  toward the second axis  62 , on the first surface  66 . 
     With the configuration in which the first input side inductor  41  and the second input side inductor  42  have the U shapes that are opened in the mutually-same direction, the first amplifier  31   a,  the second amplifier  31   b,  the third amplifier  31   c,  and the fourth amplifier  31   d  can be aligned on the first surface  66 . This can realize simple wiring around these amplifiers. Specifically, for example, wiring between the first, second amplifiers  31   a,    31   b  and the first input side inductor  41 , wiring between the third, fourth amplifiers  31   c,    31   d  and the second input side inductor  42 , and wiring between a circuit on the previous stage and each amplifier can be simplified. Further, forming each amplifier can be simplified. 
     In the input unit  240  in the differential amplification circuit  11 , the first input side inductor  41  has a U shape that is opened in a second direction, which intersects with the first direction, on the first surface  66 . Further, the second input side inductor  42  has a U shape that is opened in a third direction, which is an opposite direction to the second direction. 
     With the configuration in which the first input side inductor  41  and the second input side inductor  42  have the U shapes that are opened in the opposite directions to each other, the first amplifier  31   a  and the second amplifier  31   b  can be arranged on the y-axis positive side of the first input side inductor  41  and the third amplifier  31   c  and the fourth amplifier  31   d  can be arranged on the y-axis negative side of the second input side inductor  42 , for example. 
     Further, in the differential amplification circuit  11 , the second input side inductor  42  has a shape that is substantially symmetrical to the first input side inductor  41  across the symmetry plane  63  that is positioned between the first axis  61  and the second axis  62 . 
     This configuration can align the magnetic field generated by the first input side inductor  41  with the magnetic field generated by the second input side inductor  42 , being able to make it easier to align characteristics in converting from a first differential signal to a single end signal with characteristics in converting from a second differential signal to a single end signal. Further, a compact arrangement can be realized as a whole and each inductor can be designed to have a shape with which favorable characteristics are exerted. 
     In the current combining balun  101  in the differential amplification circuit  11 , the second output side inductor  52  is wound around the second axis  62  in the opposite direction to the winding direction of the first output side inductor  51 . The first output current i 3  based on the first input current i 1  flows through the third portion  51   d.  The second output current i 4  based on the second input current i 2  flows through the fourth portion  52   e  in the same direction as the first output current i 3 . The first output terminal  55   a  outputs the combined output current i 7  that is obtained by combining the first output current i 3  and the second output current i 4  with each other. 
     This configuration can provide a function as the current combining balun that inputs the first input current i 1  and the second input current i 2  constituting a differential signal into the first input side inductor  41  and the second input side inductor  42  respectively and outputs the combined output current i 7  as a single phase signal. 
     The current combining balun  101  further includes: the third output side inductor  53  that is wound around the first axis  61  in the opposite direction on the third surface  68 , which is opposed to the second surface  67  with the first surface  66  interposed therebetween, has the fifth portion  53   d  which is opposed to the third portion  51   d  with the first portion  41   h  interposed therebetween, and is connected with the first output side inductor  51  in series; and the fourth output side inductor  54  that is wound around the second axis  62  in the winding direction on the third surface  68 , has the sixth portion  54   e  which is opposed to the fourth portion  52   e  with the second portion  42   h  interposed therebetween, and is connected with the second output side inductor  52  in series. 
     The third output current i 5  based on the first input current i 1  flows through the fifth portion  53   d.  The fourth output current i 6  based on the second input current i 2  flows through the sixth portion  54   e  in the same direction as the third output current i 5 . 
     This configuration can raise current supply capability of the current combining balun  101 , increase a coupling coefficient of the current combining balun  101 , and effectively suppress efficiency degradation in converting from a differential signal to a single phase signal. 
     Further, in the differential amplification circuit  11 , the second output side inductor  52  has a shape that is substantially symmetrical to the first output side inductor  51  across the symmetry plane  63  that is positioned between the first axis  61  and the second axis  62 . 
     This configuration can make it easier to align characteristics in converting from a first differential signal to a single end signal with characteristics in converting from a second differential signal to a single end signal, being able to output the first output current i 3  and the second output current i 4  in a well-balanced manner. Further, a compact arrangement can be realized as a whole and each inductor can be designed in a shape with which favorable characteristics are exerted. 
     In the voltage combining balun  301  in the differential amplification circuit  13 , the second output side inductor  352  is wound in the winding direction of the first output side inductor  351  and is connected with the first output side inductor  351  in series. The first output current i 3  based on the first input current i 1  flows through the third portion  351   d.  The second output current i 4  based on the second input current i 2  flows through the fourth portion  352   e  in the same direction as the first output current i 3 . The output terminal  22  outputs a combined output voltage that is obtained by combining voltages, which are generated in the first output side inductor  351  and the second output side inductor  352  respectively, with each other. 
     This configuration can provide a function as the voltage combining balun that inputs the first input current i 1  and the second input current i 2  constituting a differential signal into the first input side inductor  41  and the second input side inductor  42  respectively and outputs a combined output voltage as a single phase signal. 
     The voltage combining balun  301  further includes: the third output side inductor  353  that is wound around the first axis  61  in the opposite direction on the third surface  68 , which is opposed to the second surface  67  with the first surface  66  interposed therebetween, has the fifth portion  353   d  which is opposed to the third portion  351   d  with the first portion  41   h  interposed therebetween, and is connected with the first output side inductor  351  in series; and the fourth output side inductor  354  that is wound around the second axis  62  in the opposite direction on the third surface  68 , has the sixth portion  354   e  which is opposed to the fourth portion  352   e  with the second portion  42   h  interposed therebetween, and is connected with the first output side inductor  351  in series. The output terminal  22  outputs a combined output voltage that is obtained by combining voltages, which are generated in the first output side inductor  351 , the second output side inductor  352 , the third output side inductor  353 , and the fourth output side inductor  354  respectively, with each other. 
     This configuration can raise a voltage outputted by the voltage combining balun  301 , increase a coupling coefficient of the voltage combining balun  301 , and effectively suppress efficiency degradation in converting from a differential signal to a single phase signal. 
     Further, in the differential amplification circuit  12 , two sets, each of which includes the first input side inductor  41 , the second input side inductor  42 , the first output side inductor  51 , the second output side inductor  52 , and the first output terminal  55   a,  are provided along the first direction. 
     This configuration can convert first to fourth differential signals into four respective single end signals and combine these four single end signals to output the combined signal. Accordingly, a load per amplifier can be reduced and therefore, the output of the differential amplification circuit  12  can be raised. Further, for example, when output is performed at the same power as the differential amplification circuit  11  including four amplifiers, a load per amplifier is reduced and therefore, output impedance relative to the amplifier can be increased. This can raise an output voltage of each amplifier. That is, compared to each amplifier included in the differential amplification circuit  11 , a gain of each amplifier included in the differential amplification circuit  12  can be increased. 
     A differential amplification device includes: the first amplifier  31   a  that amplifies the first signal RF 1  and outputs the first amplified signal ARF 1 ; the second amplifier  31   b  that amplifies the second signal RF 2 , which has a phase that is different from a phase of the first signal RF 1 , and outputs the second amplified signal ARF 2 ; the third amplifier  31   c  that amplifies the third signal RF 3 , which has a phase that is substantially the same phase as the phase of the first signal RF 1 , and outputs the third amplified signal ARF 3 ; the fourth amplifier  31   d  that amplifies the fourth signal RF 4 , which has a phase that is substantially the same phase as the phase of the second signal RF 2 , and outputs the fourth amplified signal ARF 4 ; the first input side inductor  41  that is wound around the first axis  61  on the first surface  66 , which intersects with the first axis  61 , and has the first end  41   a  which is connected with the first amplifier  31   a,  the first portion  41   h  which is positioned between the second axis  62 , which is substantially parallel to the first axis  61 , and the first axis  61  and through which the first input current i 1  flows, and the second end  41   b  which is connected with the second amplifier  31   b;  the second input side inductor  42  that is wound around the second axis  62  on the first surface  66 , and has the first end  42   a  which is connected with the third amplifier  31   c,  the second portion  42   h  which is positioned between the second axis  62  and the first portion  41   h  and through which the second input current i 2  flows in the same direction as the direction of the first input current i 1 , and the second end  42   b  which is connected with the fourth amplifier  31   d;  the first output side inductor  51  that is wound around the first axis  61  on the second surface  67 , which faces the first surface  66 , and has the third portion  51   d  which faces the first portion  41   h;  the second output side inductor  52  that is wound around the second axis  62  on the second surface  67  and has the fourth portion  52   e  which faces the second portion  42   h;  and the first output terminal  55   a  that outputs a current or a voltage, which is generated in the first output side inductor  51  and the second output side inductor  52 , based on the first input current i 1  and the second input current i 2 . 
     This configuration can make the direction of the magnetic field that is generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  be opposite to the direction of the magnetic field that is generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42 . That is, the direction of the magnetic field generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  and the direction of the magnetic field generated by the second input current i 2  in the outer side portion  42   j  of the second input side inductor  42  are the same as each other and further, the direction of the magnetic field generated by the first input current i 1  in the outer side portion  41   j  of the first input side inductor  41  and the direction of the magnetic field generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42  are the same as each other. Accordingly, the magnetic field generated by the first input current i 1  in the inner side portion  41   i  of the first input side inductor  41  is strengthened by the magnetic field generated by the second input current i 2  in the outer side portion  42   j  of the second input side inductor  42  and also, the magnetic field generated by the second input current i 2  in the inner side portion  42   i  of the second input side inductor  42  is strengthened by the magnetic field generated by the first input current i 1  in the outer side portion  41   j  of the first input side inductor  41 . Thus, a coupling coefficient among inductors in the combining balun can be increased, that is, the combining balun can be brought closer to an ideal transformer. This can reduce leakage inductance, improve an impedance conversion ratio of the combining balun, and suppress efficiency degradation in converting from a differential signal to a single phase signal. Further, the combining balun can be used as an inter-stage matching circuit that matches impedances between two driver-stage differential pairs and two power-stage differential pairs. Here, even when the impedance of the two driver-stage differential pairs is largely different from the impedance of the two power-stage differential pairs, the impedance of the two driver-stage differential pairs and the impedance of the two power-stage differential pairs can be favorably matched. 
     Further, first and second differential signals can be converted into two respective single end signals and these two single end signals can be combined to be outputted. Accordingly, a load per amplifier can be reduced and therefore, the output of the differential amplification device can be raised. Further, for example, when output is performed at the same power as the differential amplification circuit including two amplifiers, a load per amplifier is reduced and therefore, output impedance relative to the amplifier can be increased. This can raise an output voltage of each amplifier. That is, compared to each amplifier included in the differential amplification circuit, a gain of each amplifier included in the differential amplification device can be increased. 
     It should be noted that each of the embodiments is described above for facilitating the understanding of the present invention, and is not described for limiting the interpretation of the present invention. The present invention can be modified/improved without necessarily departing from the spirit thereof, and the present invention also includes an equivalent thereof. That is, each embodiment whose design is appropriately changed by those skilled in the art is also included in the scope of the present invention as long as the embodiment has the features of the present invention. For example, elements included in each embodiment and those arrangement, materials, conditions, shapes, sizes, and the like are not limited to those exemplified, and can be appropriately changed. Further, each of the embodiments is exemplary and it goes without necessarily saying that partial substitution or combination of the configurations described in different embodiments can be performed, and this is also included in the scope of the present invention as long as the features of the present invention are included.