Patent Publication Number: US-2023155556-A1

Title: Matching circuit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2021-184605 filed on Nov. 12, 2021. The content of this application is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure relates to matching circuits. 
     2. Description of the Related Art 
     There is a radio frequency (RF) power amplifier including a transmission line transformer (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-88770).  FIG.  13    is an equivalent circuit diagram of the last amplifier stage of a RF power amplifier described as the related art in Japanese Unexamined Patent Application Publication No. 2009-88770. In this RF power amplifier, a transmission line transformer (TLT) is connected as an impedance matching circuit between a heterojunction bipolar transistor Q and an output terminal Pout of a RF power module. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     In the transmission line transformer described in Japanese Unexamined Patent Application Publication No. 2009-88770, there is a possibility that its band is narrowed. Here, a method of widening the band by connecting a plurality of transmission line transformers can be thought. However, this is not preferable because the circuit size is increased. 
     The present disclosure was made in consideration of these circumstances, and is to provide a matching circuit capable of favorably matching impedance between a circuit at a preceding stage and a circuit at a subsequent stage in a wide frequency band, while an increase in circuit size is suppressed. 
     A matching circuit according to an aspect of the present disclosure includes: a first wire having one end connected to a first terminal and another end; a second wire having one end connected to the other end of the first wire and another end connected to a first reference potential and electromagnetically coupled to the first wire; and a third wire having one end connected to the one end of the second wire and another end connected to a second terminal and electromagnetically coupled to at least one of the first wire and the second wire. 
     According to the present disclosure, it is possible to provide a matching circuit capable of favorably matching impedance between a circuit at a preceding stage and a circuit at a subsequent stage in a wide frequency band, while an increase in circuit size is suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG.  1    is a circuit diagram of a power amplifier circuit  111 ; 
         FIG.  2    is a circuit diagram of a power amplifier circuit  91  as a reference example; 
         FIG.  3    is a graph depicting one example of a result of simulation of return loss in the power amplifier circuit  111 ; 
         FIG.  4    is a graph depicting one example of a result of simulation of return loss in the power amplifier circuit  91  as a reference example; 
         FIG.  5    is a graph depicting one example of a result of simulation of pass band loss in the power amplifier circuit  111 ; 
         FIG.  6    is a graph depicting one example of a result of simulation of pass band loss in the power amplifier circuit  91  as a reference example; 
         FIG.  7    is a diagram schematically depicting a layout of a transformer  301 ; 
         FIG.  8    is a circuit diagram of a power amplifier circuit  112 ; 
         FIG.  9    is a circuit diagram of a power amplifier circuit  113 ; 
         FIG.  10    is a circuit diagram of a power amplifier circuit  114 ; 
         FIG.  11    is a circuit diagram of a power amplifier circuit  115 ; 
         FIG.  12    is a circuit diagram of a power amplifier circuit  116 ; and 
         FIG.  13    is an equivalent circuit diagram of the last amplifier stage of a RF power amplifier described as the related art in Japanese Unexamined Patent Application Publication No. 2009-88770. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Embodiments of the present disclosure are described in detail below with reference to the drawings. Note that the same components are provided with the same reference characters and redundant description is omitted as much as possible. 
     Embodiment 1 
     A matching circuit  171  and a power amplifier circuit  111  according to Embodiment 1 are described.  FIG.  1    is a circuit diagram of the power amplifier circuit  111 . As depicted in  FIG.  1   , a semiconductor device  1  includes the power amplifier circuit  111 . The semiconductor device  1  is, for example, a semiconductor chip having the power amplifier circuit  111  formed thereon. The power amplifier circuit  111  is a two-stage amplifier circuit which amplifies a signal RF 1  and outputs an amplified signal RF 3 . 
     The power amplifier circuit  111  includes an amplifier  151  (first amplifier), an amplifier  152  (second amplifier), and a matching circuit  171 . The matching circuit  171  includes a transformer  301 , a capacitor  331  (first capacitor), and a capacitor  332  (second capacitor). The transformer  301  includes a coil  311  (first wire), a coil  312  (second wire), and a coil  313  (third wire). 
     In the present embodiment, the amplifier such as the amplifiers  151  and  152  is configured of a bipolar transistor such as, for example, a heterojunction bipolar transistor (HBT). Note that the amplifier such as the amplifiers  151  and  152  may be configured of another transistor such as a metal-oxide-semiconductor field-effect transistor (MOSFET). In this case, the base, the collector, and the emitter are read as the gate, the drain, and the source, respectively. 
     The amplifiers  151  and  152  are amplifiers at an initial stage (driver stage) and a subsequent stage (power stage), respectively. The matching circuit  171  is an interstage matching circuit provided between the amplifier  151  and the amplifier  152  in cascading connection to the amplifier  151  to match impedance between the amplifier  151  and the amplifier  152 . 
     The amplifier  151  has an input terminal  151   a  and an output terminal  151   b . The input terminal  151   a  is supplied with the signal RF 1 . The output terminal  151   b  is connected to an input terminal  31  (first terminal) in the matching circuit  171  and outputs an amplified signal RF 2  obtained by amplifying the signal RF 1 . 
     The capacitor  332  in the matching circuit  171  has one end connected to the input terminal  31  and another end. Note that the capacitor  332  does not have to be provided. 
     The coil  311  in the transformer  301  has one end connected to the other end of the capacitor  332  and another end. The coil  312  has one end connected to the other end of the coil  311  and another end connected to the ground (first reference potential) to be electromagnetically coupled to the coil  311 . Here, the polarity of the coil  311  is different from the polarity of the coil  312 . 
     The coil  313  has one end connected to the one end of the coil  312  and another end to be electromagnetically coupled to the coil  312 . Here, the polarity of the coil  313  is the same as the polarity of the coil  312 . The inductance of the coil  313  is smaller than the inductance of the coil  311  and the inductance of the coil  312 . Note that the coils  311 ,  312 , and  313  function as inductors in a lumped constant circuit when the signal RF 1  is at low frequency. On the other hand, the coils  311 ,  312 , and  313  function as transmission lines (wires for transmitting high-frequency signals) handled as distributed constant circuits when the signal RF 1  is at high frequency. 
     The capacitor  331  has one end connected to the other end of the coil  313  and another end connected to an output terminal  32  (second terminal). Note that the capacitor  331  does not have to be provided. 
     The amplifier  152  has an input terminal  152   a  and an output terminal  152   b . The input terminal  152   a  is connected to the output terminal  32  and is supplied with the amplified signal RF 2  passing though the matching circuit  171 . The output terminal  152   b  outputs the amplified signal RF 3  obtained by amplifying the amplified signal RF 2 . 
     Reference Example 
     A power amplifier circuit  91  as a reference example is described.  FIG.  2    is a circuit diagram of the power amplifier circuit  91  as a reference example. In the power amplifier circuit  91 , compared with the power amplifier circuit  111  (refer to  FIG.  1   ), a transformer  391  is provided in place of the transformer  301 . 
     The transformer  391  has a structure similar to that of the transmission line transformer (TLT) described in Japanese Unexamined Patent Application Publication No. 2009-88770 and includes coils  391   a  and  391   b . The coil  391   a  has one end connected to the other end of the capacitor  332  and another end connected to the one end of the capacitor  331 . The coil  391   b  has one end connected to the other end of the coil  391   a  and another end connected to the ground. 
     [Frequency Change of Return Loss] 
     The frequency change of return loss at the output terminal  151   b  of the amplifier  151  at the driver stage is described.  FIG.  3    is a graph depicting one example of a result of simulation of return loss in the power amplifier circuit  111 . Note in  FIG.  3    that the horizontal axis represents frequency in units of “GHz” and the vertical axis represents return loss in units of “dB”. 
     A curve L 1  indicates the frequency change of return loss in the power amplifier circuit  111 . Here, the return loss is 20×log(|Gin1|). |Gin1| is an absolute value of a reflection coefficient based on an impedance ZL_Q 1  when the amplifier  152  is viewed from the output terminal  151   b  in the power amplifier circuit  111  (refer to  FIG.  1   ). As the value of the return loss is smaller, impedance matching by the matching circuit  171  is more favorable. 
     The inventors conducted a simulation of frequency change of return loss by taking a circuit constant of each circuit element in the power amplifier circuit  111  and the power amplifier circuit  91  as a parameter. The inventors optimized the parameters so that a frequency range with small return loss is large in a frequency range, for example, approximately from 1.2 GHz to 4.0 GHz. 
     As depicted in  FIG.  3   , for example, if the impedance can be favorably matched when the return loss is lower than or equal to −10, in the power amplifier circuit  111 , the impedance can be favorably matched by the matching circuit  171  in a wide frequency range of 1 GHz from 2.1 GHz to 3.1 GHz. 
       FIG.  4    is a graph depicting one example of a result of simulation of return loss in the power amplifier circuit  91  as a reference example. Note that  FIG.  4    is read in a manner similar to that in  FIG.  3   . 
     A curve L 2  indicates return loss in the power amplifier circuit  91 , that is, the frequency change of 20×log(|Gin2|). Here, |Gin2| is an absolute value of a reflection coefficient based on an impedance ZL_Q 2  when the amplifier  152  is viewed from the output terminal  151   b  in the power amplifier circuit  91  (refer to  FIG.  2   ). 
     For example, when the impedance is favorably matched where the return loss is lower than or equal to −10, the frequency range in which the impedance can be favorably matched in the power amplifier circuit  91  is a narrow range of 0.5 GHz from 1.5 GHz to 2.0 GHz. Therefore, in the power amplifier circuit  111 , compared with the power amplifier circuit  91 , the impedance can be favorably matched in a wider frequency range. 
     [Frequency Change of Pass Band Loss] 
     The frequency change of pass band loss from the output terminal  151   b  of the amplifier  151  at the driver stage to the input terminal  152   a  of the amplifier  152  at the power stage is described.  FIG.  5    is a graph depicting one example of a result of simulation of pass band loss in the power amplifier circuit  111  (refer to  FIG.  1   ). Note in  FIG.  5    that the horizontal axis represents frequency in units of “GHz” and the vertical axis represents pass band loss in units of “dB”. As the value of the pass band loss is larger, the output power of the amplifier  151  is favorably transferred to the amplifier  152 . 
     A curve L 3  indicates the frequency change of pass band loss in the power amplifier circuit  111 . Here, the pass band loss is 10×log(1−|Gin1|×|Gin1|). 
     In the power amplifier circuit  111 , the pass band loss is −2.5 dB to −2.4 dB in 2.1 GHz to 3.1 GHz where the return loss is lower than or equal to −10. 
       FIG.  6    is a graph depicting one example of a result of simulation of pass band loss in the power amplifier circuit  91  as a reference example. Note that  FIG.  6    is read in a manner similar to that in  FIG.  5   . 
     A curve L 4  indicates pass band loss in the power amplifier circuit  91 , that is, the frequency change of 10×log(1−|Gin2|×|Gin2|). 
     In the power amplifier circuit  91 , the pass band loss is −4.0 dB to −2.8 dB in 1.5 GHz to 2.0 GHz where the return loss is lower than or equal to −10. Therefore, in the power amplifier circuit  111 , compared with the power amplifier circuit  91 , the output power of the amplifier  151  can be favorably transferred to the amplifier  152  in a wider frequency range. 
     Note that while description has been made to the structure of the matching circuit  171  including the transformer  301  formed of the coils  311 ,  312 , and  313  functioning as inductors or transmission lines, this is not meant to be restrictive. The matching circuit  171  may have a structure including a coupled line formed of three parallel wires in place of the transformer  301 . Here, the three parallel wires may be transmission lines such as microstrip lines. Also, each of the three parallel wires may have a linear shape or a coiled shape. 
     Also, while the structure of the transformer  301  has been described in which the coil  311  and the coil  312  are electromagnetically coupled together and the coil  312  and the coil  313  are electromagnetically coupled together, this is not meant to be restrictive. In the transformer  301 , the structure may be such that the coil  311  and the coil  313  are further electromagnetically coupled together. Also, the structure may be such that the coil  311  and the coil  312  are electromagnetically coupled together and the coil  311  and the coil  313  are electromagnetically coupled together, or the structure may be such that the coil  311  and the coil  313  are electromagnetically coupled together and the coil  312  and the coil  313  are electromagnetically coupled together. Also, by these matching circuits including the transformers as described above, the impedance can be favorably matched in a wide frequency range. 
     [Layout of Transformer  301 ] 
     The layout of the transformer  301  is described. Note that the layout of the coupled line can be achieved by a layout similar to the layout of the transformer  301 . 
     In each drawing, an x axis, a y axis, and a z axis may be depicted. The x axis, the y axis, and the z axis form three-dimensional orthogonal coordinates in a right-handed system. In the following, an arrow direction on the x axis may be referred to as an x-axis + side, a direction opposite to the arrow direction may be referred to as an x-axis − side. The same goes for the other axes. Note that the z-axis + side and the z-axis − side may be referred to an “upper side” and a “lower side”, respectively. Here, a direction rotating in a clockwise direction when viewed from the upper side to the lower side is defined as a clockwise direction cw. Also, a direction rotating in a counterclockwise direction when viewed from the upper side to the lower side is defined as a counterclockwise direction ccw. 
       FIG.  7    is a diagram schematically depicting the layout of the transformer  301 . As depicted in  FIG.  7   , the semiconductor device  1  includes two layers, that is, wiring layers  211  and  212 , for example. The wiring layers  211  and  212  are provided in this order from the lower side toward the upper side. Note that the semiconductor device  1  may be configured to include three or more wiring layers. 
     The wiring layers  211  and  212  have a surface  211   a  (first surface) and a surface  212   a  (second surface), respectively. Each of the surface  211   a  and the surface  212   a  intersects with an axis  201  which is parallel to the z axis. In the present embodiment, each of the surface  211   a  and the surface  212   a  is set to be orthogonal to the axis  201 . Note that the structure may be such that each of the surface  211   a  and the surface  212   a  of the respective wiring layers  211  and  212  is not parallel to an xy plane due to, for example, variations in manufacturing or the like, and these surfaces may be substantially parallel to the xy plane, that is, substantially orthogonal to the axis  201 . 
     The coil  311  is formed of a metal wire  701  (first conductive member) wound around the axis  201  on the surface  211   a  of the wiring layer  211 , and the metal wire  701  has one end and another end respectively corresponding to the one end and the other end of the coil  311 . 
     In the present embodiment, when the surface  211   a  is viewed in plan view from the upper side along a direction perpendicular to the surface  211   a  (which may be hereinafter simply referred to as “when the surface  211   a  is viewed in plan view” and the same goes for the other surfaces), the metal wire  701  is wound around the axis  201  in the clockwise direction cw by two turns or more and less than two and half turns from the one end toward the other end as approaching the axis  201 . 
     The coil  312  is formed of a metal wire  702  (second conductive member) wound around the axis  201  on the surface  212   a  of the wiring layer  212 , and the metal wire  702  has one end and another end respectively corresponding to the one end and the other end of the coil  312 . In the present embodiment, when the surface  212   a  is viewed in plan view, the metal wire  702  is formed in a C shape with its x-axis + side open. 
     When the surface  212   a  is viewed in plan view, the direction in which the metal wire  702  is wound from the one end toward the other end of the metal wire  702  is identical to the direction in which the metal wire  701  is wound from the one end toward the other end of the metal wire  701 . 
     Specifically, the metal wire  702  is wound around the axis  201  from the one end toward the other end in the clockwise direction cw by ¾ turn or more and less than one turn. The one end of the metal wire  702  is connected to the other end of the metal wire  701  through an interlayer via  721 . 
     The coil  313  is formed of a metal wire  703  (third conductive member) wound around the axis  201  on the surface  211   a  of the wiring layer  211 , and the metal wire  703  has one end and another end respectively corresponding to the one end and the other end of the coil  313 . 
     When the surface  211   a  is viewed in plan view, the direction in which the metal wire  703  is wound from the one end toward the other end of the metal wire  703  is identical to the direction in which the metal wire  701  is wound from the one end toward the other end of the metal wire  701 . 
     Specifically, on the y-axis − side of the axis  201 , the metal wire  703  extends from the one end toward the other end to the x-axis − side substantially in parallel to the x axis. This corresponds to the metal wire  703  wound around the axis  201  from the one end toward the other end in the clockwise direction cw by ¼ turn or more and less than ½ turn. The one end of the metal wire  703  is connected to the one end of the metal wire  702  through an interlayer via  722 . 
     When the surface  211   a  is viewed in plan view, at least part of the metal wire  701  and at least part of the metal wire  703  overlap the metal wire  702 . Specifically, when the surface  211   a  is viewed in plan view, an area of a portion where the metal wire  701  and the metal wire  702  overlap (which may be hereinafter referred to as a first overlapping area) is more than or equal to 50% of the area of the metal wire  701 . Preferably, the first overlapping area is more than or equal to 60% of the area of the metal wire  701 . In the present embodiment, the first overlapping area is more than or equal to 75% of the area of the metal wire  701 . 
     Further, when the surface  211   a  is viewed in plan view, an area of a portion where the metal wire  703  and the metal wire  702  overlap (which may be hereinafter referred to as a second overlapping area) is more than or equal to 50% of the area of the metal wire  703 . Preferably, the second overlapping area is more than or equal to 60% of the area of the metal wire  703 . In the present embodiment, the second overlapping area is more than or equal to 75% of the area of the metal wire  703 . 
     With the above-described layout of the metal wires  701 ,  702 , and  703  and the interlayer vias  721  and  722 , the area required for the arrangement of the coils  311 ,  312 , and  313  when viewed from the upper side, that is, the area of the transformer  301 , can be reduced. This allows the matching circuit  171  with a wide frequency band to be formed in a compact manner. 
     Note that while the structure has been described in which the wiring layers  211  and  212  are provided in this order from the lower side toward the upper side, this is not meant to be restrictive. The order in which the wiring layers  211  and  212  are provided is not limited to this order and may be a reversed order. 
     Also, while the structure has been described in which the metal wires  701  and  703  are formed on the surface  211   a , this is not meant to be restrictive. The structure may be such that the metal wires  701  and  703  are formed on different surfaces. 
     Embodiment 2 
     A power amplifier circuit  112  according to Embodiment 2 is described. In Embodiment 2 and later embodiments, description of matters common to Embodiment 1 is omitted, and only a different point is described. In particular, similar operation and effect by a similar structure are not mentioned one by one for each embodiment. 
       FIG.  8    is a circuit diagram of the power amplifier circuit  112 . As depicted in  FIG.  8   , the power amplifier circuit  112  according to Embodiment 2 is different from the power amplifier circuit  111  according to Embodiment 1 in that the coils  311  and  312  are compound transformers. 
     Compared with the power amplifier circuit  111  depicted in  FIG.  1   , the power amplifier circuit  112  includes a matching circuit  172  in place of the matching circuit  171 . Compared with the matching circuit  171  depicted in  FIG.  1   , the matching circuit  172  includes a transformer  302  in place of the transformer  301 . The transformer  302  includes the coil  311  (first wire), the coil  312  (second wire), and the coil  313  (third wire). 
     The coil  311  in the transformer  302  has one end connected to the other end of the capacitor  332  and another end connected to the ground (first reference potential). The coil  312  has one end and another end connected to the ground (second reference potential), and is electromagnetically coupled to the coil  311 . Here, the polarity of the coil  311  is the same as the polarity of the coil  312 . 
     The coil  313  has one end connected to the one end of the coil  312  and another end connected to the one end of the capacitor  331 , and is electromagnetically coupled to the coil  312 . Here, the polarity of the coil  312  is the same as the polarity of the coil  313 . 
     Note that while the structure of the transformer  302  has been described in which the coil  311  and the coil  312  are electromagnetically coupled together and the coil  312  and the coil  313  are electromagnetically coupled together, this is not meant to be restrictive. In the transformer  302 , the structure may be such that the coil  311  and the coil  313  are further electromagnetically coupled together. Also, the structure may be such that the coil  311  and the coil  312  are electromagnetically coupled together and the coil  311  and the coil  313  are electromagnetically coupled together, or the structure may be such that the coil  311  and the coil  313  are electromagnetically coupled together and the coil  312  and the coil  313  are electromagnetically coupled together. 
     Also in the power amplifier circuit  112  according to Embodiment 2, the matching circuit  172  is configured of the coils  311 ,  312 , and  313  electromagnetically coupled to one another. Therefore, as with the power amplifier circuit  111  according to Embodiment 1, also in the present embodiment, in a frequency range wider than ever, it is possible to favorably match impedance and favorably transfer output power of the amplifier  151  to the amplifier  152 . 
     Embodiment 3 
     A power amplifier circuit  113  according to Embodiment 3 is described.  FIG.  9    is a circuit diagram of the power amplifier circuit  113 . As depicted in  FIG.  9   , the power amplifier circuit  113  according to Embodiment 3 is different from the power amplifier circuit  111  according to Embodiment 1 in that a circuit combining two transformers  301  is used as an interstage matching circuit of a fully-differential two-stage amplifier. 
     The power amplifier circuit  113  is a circuit which amplifies a balanced signal including signals RFp 1  and RFm 1  to output a balanced signal including amplified signals RFp 3  and RFm 3 . The phase of the signal RFp 1  is different from the phase of the signal RFm 1  by approximately 180°. Also, the phase of the amplified signal RFp 3  is different from the phase of the amplified signal RFm 3  by approximately 180°. Note that the phase difference may be greatly different from 180° depending on the imbalance of the wire length of the circuit. 
     The power amplifier circuit  113  includes a differential pair  151   d  (first differential pair), a differential pair  152   d  (second differential pair), and a matching circuit  173 . The differential pair  151   d  includes an amplifier  151   p  and an amplifier  151   m . The differential pair  152   d  includes an amplifier  152   p  and an amplifier  152   m . The amplifier  151   m  has input/output characteristics substantially identical to input/output characteristics of the amplifier  151   p . The amplifier  152   m  has input/output characteristics substantially identical to input/output characteristics of the amplifier  152   p.    
     The matching circuit  173  includes transformers  301   p  and  301   m , capacitors  331   p  and  331   m , and a capacitor  333 . The transformer  301   p  includes the coil  311  (first wire), the coil  312  (second wire), and the coil  313  (third wire). The transformer  301   m  includes a coil  314  (fourth wire), a coil  315  (fifth wire), and a coil  316  (sixth wire). 
     The matching circuit  173  is an interstage matching circuit provided between the differential pair  151   d  at a driver stage and the differential pair  152   d  at a power stage to match impedance between the differential pair  151   d  and the differential pair  152   d.    
     The amplifier  151   p  in the differential pair  151   d  has an input terminal  151   pa  and an output terminal  151   pb . The input terminal  151   pa  is supplied with the signal RFp 1 . The output terminal  151   pb  is connected to an input terminal  31   p  (first terminal) in the matching circuit  173  and outputs an amplified signal RFp 2  obtained by amplifying the signal RFp 1 . 
     The amplifier  151   m  has an input terminal  151   ma  and an output terminal  151   mb . The input terminal  151   ma  is supplied with the signal RFm 1 . The output terminal  151   mb  is connected to an input terminal  31   m  (third terminal) in the matching circuit  173  and outputs an amplified signal RFm 2  obtained by amplifying the signal RFm 1 . 
     The capacitor  333  in the matching circuit  173  is provided between the input terminal  31   p  and the input terminal  31   m . Note that the capacitor  333  may be omitted. 
     The coil  311  in the transformer  301   p  has one end connected to the input terminal  31   p  and another end. The coil  312  has one end connected to the other end of the coil  311  and another end connected to a node N 1  (first reference potential), and is electromagnetically coupled to the coil  311 . The node N 1  is supplied with power for operating the amplifiers  151   p  and  151   m , for example, power supply voltage VCC. 
     The coil  313  has one end connected to the one end of the coil  312  and another end, and is electromagnetically coupled to the coil  312 . The capacitor  331   p  has one end connected to the other end of the coil  313  and another end connected to an output terminal  32   p  (second terminal). Note that the capacitor  331   p  may be omitted. 
     The coil  314  in the transformer  301   m  has one end connected to the input terminal  31   m  and another end. The coil  315  has one end connected to the other end of the coil  314  and another end connected to the node N 1 , and is electromagnetically coupled to the coil  314 . 
     The coil  316  has one end connected to the one end of the coil  315  and another end, and is electromagnetically coupled to the coil  315 . The inductance of the coil  316  is smaller than the inductance of the coil  314  and the inductance of the coil  315 . 
     The capacitor  331   m  has one end connected to the other end of the coil  316  and another end connected to an output terminal  32   m  (fourth terminal). Note that the capacitor  331   m  may be omitted. 
     The amplifier  152   p  in the differential pair  152   d  has an input terminal  152   pa  and an output terminal  152   pb . The input terminal  152   pa  is connected to the output terminal  32   p , and is supplied with the amplified signals RFp 2  and RFm 2  passing through the matching circuit  173 . The output terminal  152   pb  outputs the amplified signal RFp 3  obtained by amplifying the amplified signals RFp 2  and RFm 2 . 
     The amplifier  152   m  has an input terminal  152   ma  and an output terminal  152   mb . The input terminal  152   ma  is connected to the output terminal  32   m , and is supplied with the amplified signals RFp 2  and RFm 2  passing through the matching circuit  173 . The output terminal  152   mb  outputs the amplified signal RFm 3  obtained by amplifying the amplified signals RFp 2  and RFm 2 . 
     Note that, with the matching circuit  173  provided at a preceding stage or a subsequent stage of the differential pair, the matching circuit  173  can be also used as an input matching circuit or an output matching circuit of that differential pair. 
     Embodiment 4 
     A power amplifier circuit  114  according to Embodiment 4 is described.  FIG.  10    is a circuit diagram of the power amplifier circuit  114 . As depicted in  FIG.  10   , the power amplifier circuit  114  according to Embodiment 4 is different from the power amplifier circuit  113  according to Embodiment 3 in that the coils  311   p  and  312   p  and the coils  311   m  and  312   m  are compound transformers. 
     Compared with the power amplifier circuit  113  depicted in  FIG.  9   , the power amplifier circuit  114  includes a matching circuit  174  in place of the matching circuit  173 . Compared with the matching circuit  173  depicted in  FIG.  9   , the matching circuit  174  includes a transformer  303  in place of the transformers  301   p  and  301   m . The transformer  303  includes a coil  311   p  (first wire), a coil  312   p  (second wire), a coil  313   p  (third wire), a coil  311   m  (fourth wire), a coil  312   m  (fifth wire), and a coil  313   m  (sixth wire). 
     The coil  311   p  in the transformer  303  has one end connected to the input terminal  31   p  and another end connected to a center tap  311   c  (first reference potential). The center tap  311   c  is supplied with power supply voltage VCC. 
     The coil  312   p  has one end and another end connected to a center tap  312   c  (second reference potential), and is electromagnetically coupled to the coil  311   p . The center tap  312   c  is in an imaginary short. 
     The coil  313   p  has one end connected to the one end of the coil  312   p  and another end connected to one end of the capacitor  331   p , and is electromagnetically coupled to the coil  312   p . The inductance of the coil  313   p  is smaller than the inductance of the coil  311   p  and the inductance of the coil  312   p.    
     The coil  311   m  has one end connected to the input terminal  31   m  and another end connected to the center tap  311   c . The coil  312   m  has one end and another end connected to the center tap  312   c , and is electromagnetically coupled to the coil  311   m.    
     The coil  313   m  has one end connected to the one end of the coil  312   m  and another end connected to one end of the capacitor  331   m , and is electromagnetically coupled to the coil  312   m . The inductance of the coil  313   m  is smaller than the inductance of the coil  311   m  and the inductance of the coil  312   m.    
     Note that, with the matching circuit  174  provided at a preceding stage or a subsequent stage of the differential pair, the matching circuit  174  can be also used as an input matching circuit or an output matching circuit of that differential pair. 
     Embodiment 5 
     A power amplifier circuit  115  according to Embodiment 5 is described.  FIG.  11    is a circuit diagram of the power amplifier circuit  115 . As depicted in  FIG.  11   , the power amplifier circuit  115  according to Embodiment 5 is different from the power amplifier circuit  111  according to Embodiment 1 in that the transformer  301  is used as an output matching circuit. 
     The power amplifier circuit  115  includes the amplifier  152  at a power stage and a matching circuit  175 . The matching circuit  175  includes the transformer  301 . 
     The other end of the coil  313  in the transformer  301  is connected to the output terminal  152   b  of the amplifier  152  through the input terminal  31 . The one end of the coil  313  is connected to the one end of the coil  312 . The coil  313  is electromagnetically coupled to the coil  311 . 
     The other end of the coil  312  is connected to a voltage source for supplying power for operating the amplifier  152 , for example, power supply voltage VCC (first reference potential). The coil  312  is electromagnetically coupled to the coil  311 . The other end of the coil  311  is connected to the one end of the coil  312 . The one end of the coil  311  is connected to the output terminal  32 . 
     Embodiment 6 
     A power amplifier circuit  116  according to Embodiment 6 is described.  FIG.  12    is a circuit diagram of the power amplifier circuit  116 . As depicted in  FIG.  12   , the power amplifier circuit  116  according to Embodiment 6 is different from the power amplifier circuit  115  according to Embodiment 5 in that a capacitor is connected in parallel to a coil. 
     Compared with the power amplifier circuit  115  depicted in  FIG.  11   , the power amplifier circuit  116  includes a matching circuit  176  in place of the matching circuit  175 . Compared with the matching circuit  175  depicted in  FIG.  11   , the matching circuit  176  further includes a capacitor  311   a  (third capacitor). 
     The capacitor  311   a  has one end connected to the one end of the coil  311  and another end connected to the other end of the coil  311 . 
     As described above, with the structure in which the capacitor  311   a  is provided in parallel to the coil  311 , an LC parallel circuit can be provided to a wire to which the amplified signal RF 3  is transmitted. Also, for example, by setting the resonant frequency of the LC parallel circuit at harmonic frequency of the amplified signal RF 3 , the transmission of the harmonic waves to the output terminal  32  can be suppressed. 
     Note that while the structure has been described in which the capacitor  311   a  is provided in parallel to the coil  311 , this is not meant to be restrictive. The structure may be such that the capacitor  311   a  (fourth capacitor) is provided in parallel to the coil  312 . 
     As described above, with the structure in which the capacitor  311   a  is provided in parallel to the coil  312 , an LC parallel circuit can be provided to a wire branched from a wire to which the amplified signal RF 3  is transmitted. Also, for example, by setting the resonant frequency of the LC parallel circuit at the basic wave frequency of the amplified signal RF 3 , a short circuit of the basic wave at a voltage source as a low-impedance node for an alternating current signal can be suppressed. Also, a short circuit of the harmonic wave of the amplified signal RF 3  can be made at that voltage source. That is, while the basic wave of the amplified signal RF 3  can be favorably transmitted to the output terminal  32 , the transmission of the harmonic wave of the amplified signal RF 3  to the output terminal  32  can be suppressed. 
     Also, the structure may be such that the capacitor  311   a  is connected in parallel to the coil  313 . 
     Also, in the power amplifier circuits  111  to  116 , while the structure has been described in which a signal is transmitted from the input terminal  31  to the output terminal  32  (from the input terminals  31   p  and  31   m  to the output terminals  32   p  and  32   m ), this is not meant to be restrictive. The structure may be such that a signal is transmitted from the output terminal  32  to the input terminal  31  (from the output terminals  32   p  and  32   m  to the input terminals  31   p  and  31   m ). 
     In the foregoing, the exemplary embodiments of the present disclosure have been described. The band of a transmission line transformer may be narrowed due to element characteristics or a parasitic element. For example, one end Lin(B) of a sub-line Lin in a RF power amplifier depicted in  FIG.  13    is ideally connected to the ground for a RF signal. 
     However, there is a possibility that the one end Lin(B) is not ideally connected to the ground due to characteristics of a bypass capacitance Cpass provided between the one end Lin(B) and the ground and a wire from the one end Lin(B) to the bypass capacitance Cpass. Therefore, the band of the transmission line transformer is narrowed. Here, a method of widening the band by connecting a plurality of transmission line transformers can be thought. However, this is not preferable because the circuit size is increased. 
     By contrast, in the matching circuits  171 ,  175 , and  176 , the first wire has one end connected to the input terminal  31  and another end. The second wire has one end connected to the other end of the first wire and another end connected to the first reference potential and is electromagnetically coupled to the first wire. The third wire has one end connected to the one end of the second wire and another end connected to the output terminal  32  and is electromagnetically coupled to at least one of the first wire and the second wire. 
     As described above, with the structure in which the first wire and the second wire are electromagnetically coupled together and the third wire and at least one of the first wire and the second wire are electromagnetically coupled together, a matching circuit having characteristics similar to those of a matching circuit using two or more stages of transformers or coupled lines formed of two wires can be achieved by a one-stage transformer including three wires. That is, by having characteristics similar to those of a matching circuit using two or more stages of transformers or coupled lines, it is possible to suppress an increase in return loss and a decrease in pass band loss for the basic frequency of a radio frequency signal in a wide frequency band. Also, while four or more wires are required for a conventional matching circuit using two or more stages of transformers or coupled lines, the number of wires can be reduced by a one-stage transformer including three wires. Thus, an increase in circuit size can be suppressed. Therefore, it is possible to provide a matching circuit capable of favorably matching impedance between a circuit at a preceding stage and a circuit at a subsequent stage in a wide frequency band, while an increase in circuit size is suppressed. 
     Also, in the matching circuit  172 , the first wire has one end connected to the input terminal  31  and another end connected to the first reference potential. The second wire has one end and another end connected to the second reference potential and is electromagnetically coupled to the first wire. The third wire has one end connected to the one end of the second wire and another end connected to the output terminal  32  and is electromagnetically coupled to at least one of the first wire and the second wire. 
     As described above, with the structure in which the first wire and the second wire are electromagnetically coupled together and the third wire and at least one of the first wire and the second wire are electromagnetically coupled together, a matching circuit having characteristics similar to those of a matching circuit using two stages of transformers or coupled lines can be achieved by three wires. That is, by having characteristics similar to those of a matching circuit using two stages of transformers or coupled lines, it is possible to suppress an increase in return loss and a decrease in pass band loss for the basic frequency of a radio frequency signal in a wide frequency band. Also, by reducing the number of wires to three, an increase in circuit size can be suppressed. Furthermore, the electrical isolation between the first wire and the second wire can be enhanced. Therefore, it is possible to provide a matching circuit capable of favorably matching impedance between a circuit at a preceding stage and a circuit at a subsequent stage in a wide frequency band, while an increase in circuit size is suppressed. 
     Furthermore, in the matching circuit  171 , the first wire is formed of the metal wire  701  on the surface  211   a . The second wire is formed of the metal wire  702  on the surface  212   a . The third wire is formed of the metal wire  703  on the surface  211   a . When the surface  211   a  is viewed in plan view along a direction perpendicular to the surface  211   a , at least part of the metal wire  701  and at least part of the metal wire  703  overlap the metal wire  702 . When the surface  211   a  is viewed in plan view along the direction, a direction in which the metal wire  701  is wound from the one end toward the other end of the first wire, a direction in which the metal wire  702  is wound from the one end toward the other end of the second wire, and a direction in which the metal wire  703  is wound from the one end toward the other end of the third wire are identical. 
     As described above, with the structure in which the metal wires  701 ,  702 , and  703  are respectively formed on the surfaces  211   a ,  212   a , and  211   a , the first wire, the second wire, and the third wire can be formed by two wiring layers. Thus, the thickness of a substrate where the matching circuit  171  is provided can be decreased. Also, with the structure in which, when the surface  211   a  is viewed in plan view, at least part of the metal wire  701  and at least part of the metal wire  703  overlap the metal wire  702 , electromagnetic coupling between wires can be sufficiently ensured, while the area occupied by the matching circuit  171  is suppressed. 
     Still further, in the matching circuits  171  and  172 , the capacitor  331  is provided between the other end of the third wire and the output terminal  32 . 
     As described above, with the structure in which the capacitor  331  is provided between the other end of the third wire and the output terminal  32 , the frequency band in which impedance can be favorably matched can be further widened. Also, since conduction of a direct current can be prevented, for example, when the input terminal  152   a  of the amplifier  152  is connected to the output terminal  32 , a short circuit of bias of the amplifier  152  to the ground can be prevented. 
     Still further, in the matching circuits  171  and  172 , the capacitor  332  is provided between the input terminal  31  and the one end of the first wire. 
     As described above, with the structure in which the capacitor  332  is provided between the input terminal  31  and the one end of the first wire, the frequency band in which impedance can be favorably matched can be further widened. Also, since the size of the capacitor  332  may be small, an increase in circuit size can be suppressed. Furthermore, since conduction of a direct current can be prevented, for example, when the input terminal of the amplifier is connected to the input terminal  31 , a short circuit of bias of the amplifier to the ground can be prevented. 
     Still further, in the matching circuit, the input terminal  31  is connected to the amplifier  151 . 
     With this structure, for example, when the input terminal  31  is connected to the output terminal  151   b  of the amplifier  151 , even if the impedance matching ratio is large, favorable matching can be achieved by the matching circuit. Also, for example, when the input terminal  31  is connected to the input terminal  151   a  of the amplifier  151 , the matching circuit can be used as an input matching circuit. 
     Still further, in the matching circuit, the output terminal  32  is connected to the amplifier  152 . 
     With this structure, for example, when the output terminal  32  is connected to the input terminal  152   a  of the amplifier  152 , the matching circuit can be used as an input matching circuit. Also, for example, when the output terminal  32  is connected to the output terminal  152   b  of the amplifier  152 , even if the impedance matching ratio is large, favorable matching can be achieved by the matching circuit. 
     Still further, each of the matching circuits  171  and  172  is provided between the amplifier  151  and the amplifier  152  in cascading connection to the amplifier  151 . 
     With this structure, even if the impedance matching ratio is large, favorable matching can be achieved by the matching circuits  171  and  172 . 
     Still further, in the matching circuit  176 , the capacitor  311   a  is connected in parallel to the first wire or the third wire. 
     As described above, with the structure in which the capacitor  311   a  is provided in parallel to the first wire or the third wire, an LC parallel circuit can be provided to a wire to which the amplified signal RF 3  is transmitted. Also, for example, by setting the resonant frequency of the LC parallel circuit at the harmonic frequency of the amplified signal RF 3 , the transmission of the harmonic waves to the output terminal  32  can be suppressed. 
     Still further, in the matching circuit  176 , the capacitor  311   a  is connected in parallel to the second wire. 
     As described above, with the structure in which the capacitor  311   a  is provided in parallel to the second wire, an LC parallel circuit can be provided to a wire branched from a wire to which the amplified signal RF 3  is transmitted. Also, for example, by setting the resonant frequency of the LC parallel circuit at the basic wave frequency of the amplified signal RF 3 , a short circuit of the basic wave at a voltage source as a low-impedance node for an alternating current signal can be suppressed. That is, the basic wave of the amplified signal RF 3  can be favorably transmitted to the output terminal  32 . 
     Still further, in the matching circuits  171 ,  172 ,  175 , and  176 , inductance of the third wire is smaller than inductance of the first wire and inductance of the second wire. 
     With this structure, impedance between a circuit at a preceding stage and a circuit at a subsequent stage can be matched by a simple structure. 
     Still further, in the matching circuits  171 ,  172 ,  175 , and  176 , each of the first wire, the second wire, and the third wire is a transmission line. 
     With this structure, while electromagnetic coupling is sufficiently ensured in the frequency band of a radio frequency signal, a coupled line can be formed with ease. 
     Still further, in the matching circuits  171 ,  172 ,  175 , and  176 , each of the first wire, the second wire, and the third wire is an inductor. 
     With this structure, while electromagnetic coupling is sufficiently ensured in the frequency band of a radio frequency signal and attenuation effects are obtained for signals with frequencies other than a desired frequency (operating frequency), a coupled line can be formed with ease. 
     Still further, in the matching circuits  171 ,  172 ,  175 , and  176 , each of the first wire, the second wire, and the third wire is a coil. 
     As described above, with the structure in which each of the first wire, the second wire, and the third wire is configured of a coil in which large inductance can be easily ensured, electromagnetic coupling between wires can be sufficiently ensured. 
     Still further, in the matching circuit  173 , the first wire has one end connected to the input terminal  31   p  and another end. The second wire has one end connected to the other end of the first wire and another end connected to the first reference potential and is electromagnetically coupled to the first wire. The third wire has one end connected to the one end of the second wire and another end connected to the output terminal  32   p  and is electromagnetically coupled to at least one of the first wire and the second wire. The fourth wire has one end connected to the input terminal  31   m  and another end. The fifth wire has one end connected to the other end of the fourth wire and another end connected to the first reference potential and is electromagnetically coupled to the fourth wire. The sixth wire has one end connected to the one end of the fifth wire and another end connected to the output terminal  32   m  and is electromagnetically coupled to at least one of the fourth wire and the fifth wire. 
     With this structure, it is possible to provide a matching circuit capable of favorably matching impedance between a circuit at a preceding stage and a circuit at a subsequent stage in a wide frequency band for a balanced signal, while an increase in circuit size is suppressed. 
     Still further, in the matching circuit  174 , the first wire has one end connected to the input terminal  31   p  and another end connected to the first reference potential. The second wire has one end and another end connected to the second reference potential and is electromagnetically coupled to the first wire. The third wire has one end connected to the one end of the second wire and another end connected to the output terminal  32   p  and is electromagnetically coupled to at least one of the first wire and the second wire. The fourth wire has one end connected to the input terminal  31   m  and another end connected to the first reference potential. The fifth wire has one end and another end connected to the second reference potential and is electromagnetically coupled to the fourth wire. The sixth wire has one end connected to the one end of the fifth wire and another end connected to the output terminal  32   m  and is electromagnetically coupled to at least one of the fourth wire and the fifth wire. 
     With this structure, it is possible to provide a matching circuit capable of favorably matching impedance between a circuit at a preceding stage and a circuit at a subsequent stage in a wide frequency band for a balanced signal, while an increase in circuit size is suppressed. Also, since the first wire and the fourth wire and the second wire and the fifth wire are insulated, the electrical isolation between the first wire and the fourth wire and the second wire and the fifth wire can be enhanced. 
     Still further, in the matching circuit, the input terminals  31   p  and  31   m  are connected to the differential pair  151   d.    
     With this structure, for example, when the input terminals  31   p  and  31   m  are respectively connected to the output terminal  151   pb  of the amplifier  151   p  and the output terminal  151   mb  of the amplifier  151   m , even if the impedance matching ratio is large, favorable matching can be achieved by the matching circuit. Also, for example, when the input terminals  31   p  and  31   m  are respectively connected to the input terminal  151   pa  of the amplifier  151   p  and the input terminal  151   ma  of the amplifier  151   m , the matching circuit can be used as an input matching circuit. 
     Still further, in the matching circuits  173  and  174 , the output terminals  32   p  and  32   m  are connected to the differential pair  152   d.    
     With this structure, for example, when the output terminals  32   p  and  32   m  are respectively connected to the input terminal  152   pa  of the amplifier  152   p  and the input terminal  152   ma  of the amplifier  152   m , each matching circuit can be used as an input matching circuit. Also, for example, when the output terminals  32   p  and  32   m  are respectively connected to the output terminal  152   pb  of the amplifier  152   p  and the output terminal  152   mb  of the amplifier  152   m , even if the impedance matching ratio is large, favorable matching can be achieved by each matching circuit. 
     Still further, each of the matching circuits  173  and  174  is provided between the differential pair  151   d  and the differential pair  152   d  in cascading connection to the differential pair  151   d.    
     With this structure, even if the impedance matching ratio is large, favorable matching can be achieved by the matching circuits  173  and  174 . 
     Also, in the matching circuits  173  and  174 , the input terminals  31   p  and  31   m  are connected to the differential pair  151   d . The first reference potential is supplied with power for operating the differential pair  151   d.    
     With this structure, power for operating the differential pair  151   d  can be supplied without separate provision of a power supply line. Thus, an increase in circuit size of the matching circuits  173  and  174  can be suppressed. 
     Note that each of the above-described embodiments is for ease of understanding the present disclosure and is not for restrictively interpreting the present disclosure. The present disclosure can be modified/improved without deviating from the gist of the present disclosure and also includes its equivalents. That is, those having the design of each embodiment modified by a person skilled in art as appropriate are also included in the scope of the present disclosure as long as they include the features of the present disclosure. For example, each of the components included in each embodiment and their arrangements, materials, conditions, shapes, sizes, and so forth are not limited to those exemplarily described and can be changed as appropriate. Also, each embodiment is merely an example, and it goes without saying that the structures described in different embodiments can be partially replaced or combined and these are also included in the scope of the present disclosure as long as they include the features of the present disclosure.