Abstract:
A cascode amplifier includes: first transistors; second transistors cascode-connected with respective first transistors; a first line connected at spaced points to control terminals of the first transistors; a second line connected at spaced points to control terminals of the second transistors; and a capacitance connected between one end of the second line and ground. The second line includes at least two lines connected in parallel with each other.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cascode amplifier primarily for use in mobile communication devices such as cellular phones. 
     2. Background Art 
     Increasing effort is currently being spent in developing cascode amplifiers formed by a CMOS process which are used as a means for reducing the cost of power amplifiers for CDMA cellular phones and other types of cellular phones (see, e.g., Japanese Laid-Open Patent Publication No. H05-259765). 
       FIG. 16  is a circuit diagram showing the basic configuration of a cascode amplifier. The cascode amplifier is shown within a dashed line rectangle, and the other circuit components shown are required for forming a power amplifier. Transistors Tr 1  and Tr 2  are n-channel MOS transistors and are connected together in a cascode configuration. An amplifier using cascode-connected transistors is referred to as a “cascode amplifier.” 
     The gate of the transistor Tr 1  is connected to an RF input signal terminal IN through an input matching circuit and also connected to a gate bias terminal Vg 1 . The source of the transistor Tr 1  is grounded. That is, the transistor Tr 1  is a source-grounded transistor. 
     The gate of the transistor Tr 2  is connected to a gate bias terminal Vg 2 , and grounded through a capacitance C 1 . That is, the transistor Tr 2  is a gate-grounded transistor. The source of the transistor Tr 2  is connected to the drain of the transistor Tr 1 . The drain of the transistor Tr 2  is connected through a transmission line L 1  to a drain power terminal Vd of this cascode amplifier, and also connected through an output matching circuit to an RF output signal terminal OUT. The transmission line L 1  has a specific electrical length and acts as an inductor. 
     SUMMARY OF THE INVENTION 
     Since the transistors of a cascode amplifier constituting a power amplifier have a large gate width, each transistor is divided into smaller transistors. More specifically, the source-grounded transistor of the cascode amplifier is divided into smaller source-grounded transistors, and the gate-grounded transistor is divided into smaller gate-grounded transistors. Each smaller source-grounded transistor and the smaller gate-grounded transistor connected thereto form a cell. The gates of the smaller gate-grounded transistors are connected together by a gate line, and a capacitance is connected between one end of this gate line and ground. It has been found, however, that since the resistance of the gate line increases with the total gate width of the cascode amplifier, an increase in the total gate width may not result in an increase in the output power of the power amplifier, or cascode amplifier. 
     In view of the above-described problems, an object of the present invention is to provide a cascode amplifier which can minimize reduction of its output power due to wiring resistance. 
     According to the present invention, a cascode amplifier includes: a plurality of first transistors; a plurality of second transistors cascode-connected with the plurality of first transistors respectively; a first line connected at spaced points along its length to control terminals of the plurality of first transistors; a second line connected at spaced points along its length to control terminals of the plurality of second transistors; and a capacitance connected between one end of the second line and ground. The second line includes at least two lines connected in parallel with each other. 
     The present invention makes it possible to minimize reduction of its output power due to wiring resistance. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a cascode amplifier in accordance with a first embodiment of the present invention. 
         FIG. 2  is a circuit diagram of the cascode amplifier of the first embodiment. 
         FIG. 3  is a top view of the cascode amplifier of Comparative Example 1. 
         FIG. 4  is a circuit diagram of the cascode amplifier of Comparative Example 1. 
         FIG. 5  is a diagram showing the relationship between the resistance of wiring in the cascode amplifier of Comparative Example 1 and the power gain. 
         FIG. 6  is a diagram showing the relationship between the resistance of wiring in the cascode amplifier of Comparative Example 1 and the output power when the cascode amplifier is provided with 6 transistor cells and when it is provided with 3 transistor cells. 
         FIG. 7  is a circuit diagram of the cascode amplifier of Comparative Example 2. 
         FIG. 8  is a diagram showing the relationship between the input power and the output power of the cascode amplifier of Comparative Example 2 when the resistance of the wiring in the cascode amplifier is assumed to be zero, which is ideal. 
         FIG. 9  is a diagram showing the relationship between the input power and the output power of the cascode amplifier of Comparative Example 2 when the wiring in the cascode amplifier has some resistance. 
         FIG. 10  is a top view of a cascode amplifier in accordance with a second embodiment of the present invention. 
         FIG. 11  is a circuit diagram of the cascode amplifier of the second embodiment. 
         FIG. 12  is a top view of a cascode amplifier in accordance with a third embodiment of the present invention. 
         FIG. 13  is a circuit diagram of the cascode amplifier of the third embodiment. 
         FIG. 14  is a circuit diagram of a cascode amplifier in accordance with a fourth embodiment of the present invention. 
         FIG. 15  is a circuit diagram of the cascode amplifier of Comparative Example 3. 
         FIG. 16  is a circuit diagram showing the basic configuration of a cascode amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A cascode amplifier according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted. 
     First Embodiment 
       FIG. 1  is a top view of a cascode amplifier in accordance with a first embodiment of the present invention.  FIG. 2  is a circuit diagram of the cascode amplifier of the first embodiment. Transistors Tr 1   a  to Tr 1   f  are cascode-connected with transistors Tr 2   a  to Tr 2   f , respectively. These transistors are n-channel MOS transistors. 
     A line Wg 1  is connected at spaced points along its length to the gates of the transistors Tr 1   a  to Tr 1   f . A line Wg 2  and a line Wg 3  are connected at spaced points along their length to the gates of the transistors Tr 2   a  to Tr 2   f . The lines Wg 2  and Wg 3  are connected in parallel with each other. One end of a capacitance C 1  is connected to one end of the line Wg 2  and one end of the line Wg 3 , and the other end of the capacitance C 1  is connected to GND. 
     The sources of Tr 1   a  to Tr 1   f  are connected through a line Ws to GND. The drains of Tr 1   a  to Tr 1   f  are connected through lines Wsd to the sources of Tr 2   a to Tr 2   f,  respectively. The drains of Tr 2   a  to Tr 2   f  are connected through a line Wd to an RF output signal terminal OUT. 
     The line Wg 2  is connected to a Vg 2  terminal through vias VIA 1  and VIA 3  and a line Wg 4 , and the line Wg 3  is connected to the Vg 2  terminal through vias VIA 2  and VIA 3  and the line Wg 4 . It should be noted that the vias VIA 1  to VIA 3  are used to interconnect upper layer wiring and lower layer wiring in the multilayer wiring process of the LSI process. 
     In  FIG. 2 , the symbols Rg 1   b  to Rg 1   f  represent the resistances of the line Wg 1  as measured between the gates of neighboring transistors Tr 1   a  and Tr 1   b , Tr 1   b  and Tr 1   c,  Tr 1   c  and Tr 1   d , Tr 1   d  and Tr 1   e , and Tr 1   e  and Tr 1   f , respectively. The symbols Rg 2   b  to Rg 2   f  represent the resistances of the line Wg 2  as measured between the gates of neighboring transistors Tr 2   a  and Tr 2   b , Tr 2   b  and Tr 2   c , Tr 2   c  and Tr 2   d , Tr 2   d  and Tr 2   e,  and Tr 2   e  and Tr 2   f , respectively. Further, the symbols Rg 3   b  to Rg 3   f  represent the resistances of the line Wg 3  as measured between the gates of neighboring transistors Tr 2   a  and Tr 2   b , Tr 2   b  and Tr 2   c , Tr 2   c  and Tr 2   d , Tr 2   d  and Tr 2   e , and Tr 2   e  and Tr 2   f,  respectively. A resistance Rc 1  represents the sum of the wiring resistance between the Vg 2  terminal and the capacitance C 1  and the parasitic resistance of the capacitance C 1 . A resistance Rg 1   a  represents the wiring resistance between a Vg 1  terminal and the gate of Tr 1   a . A resistance Rg 2   a  represents the sum of the wiring resistance between the Vg 2  terminal and the gate of Tr 2   a  and the contact resistances of the vias VIA 1  and VIA 3 . A resistance Rg 3   a  represents the sum of the wiring resistance between the Vg 2  terminal and the gate of Tr 2   a  and the contact resistances of the vias VIA 2  and VIA 3 . 
     Advantages of the present embodiment will now be described in comparison with two conventional cascode amplifiers designated as Comparative Examples 1 and 2, respectively.  FIG. 3  is a top view of the cascode amplifier of Comparative Example 1.  FIG. 4  is a circuit diagram of the cascode amplifier of Comparative Example 1. The cascode amplifier of Comparative Example 1 does not have the line Wg 3  described in connection with the first embodiment; that is, only the line Wg 2  is connected at spaced points to the gates of the transistors Tr 2   a  to Tr 2   f.    
       FIG. 5  is a diagram showing the relationship between the resistance of wiring in the cascode amplifier of Comparative Example 1 and the power gain. As can be seen from  FIG. 5 , the power gain and the maximum value of the output power of the cascode amplifier decrease with increasing resistance of the line Wg 2 . 
       FIG. 6  is a diagram showing the relationship between the resistance of wiring in the cascode amplifier of Comparative Example 1 and the output power when the cascode amplifier is provided with 6 transistor cells and when it is provided with 3 transistor cells. As can be seen from  FIG. 6 , the output power of the cascode amplifier is lower when the cascode amplifier has 6 transistor cells than when it has 3 transistor cells. That is, in the case of the cascode amplifier of Comparative Example 1, an increase in the total gate width of the cascode amplifier results in a decrease in the output power, rather than an increase. 
       FIG. 7  is a circuit diagram of the cascode amplifier of Comparative Example 2. This circuit differs from that of Comparative Example 1 shown in  FIG. 4  in that it is provided with a power meter PM 0  for monitoring the output power of the entire cascode amplifier and also provided with power meters PM 1  to PM 6  for monitoring the output power of each cell of the cascode amplifier. 
       FIG. 8  is a diagram showing the relationship between the input power and the output power of the cascode amplifier of Comparative Example 2 when the resistance of the wiring in the cascode amplifier is assumed to be zero, which is ideal. The output power levels of the cells, monitored by the power meters PM 1  to PM 6 , are equal. 
       FIG. 9  is a diagram showing the relationship between the input power and the output power of the cascode amplifier of Comparative Example 2 when the wiring in the cascode amplifier has some resistance. The output power levels of the cells of the cascode amplifier, monitored by the power meters PM 1  to PM 6 , are different from one another, indicating that these cells are operating in different conditions. 
     Thus, the cascode amplifiers of Comparative Examples 1 and 2 are disadvantageous in that an increase in the total gate width results in a decrease in the output power due to the resistance of the line Wg 2 . (It should be noted that an increase in the total gate width would result in an increase in the output power, not a decrease, if the resistance of the wiring in the cascode amplifiers were zero.) For example, the wiring resistance between the gate of Tr 2   f  and the capacitance C 1  is higher than that between the gate of Tr 2   a  and the capacitance C 1  by an amount equal to the sum of the resistances Rg 2   b , Rg 2   c , Rg 2   d , Rg 2   e , and Rg 2   f . This results in a decrease in the power gain of the cascode amplifier formed by Tr 1   f  and Tr 2   f  and a decrease in the maximum output power that can be drawn from the cascode amplifier. 
     On the other hand, in the cascode amplifier of the present embodiment, two lines, namely the lines Wg 2  and Wg 3 , are connected in parallel to the gates of Tr 2   a  to Tr 2   f , making it possible to reduce the wiring resistance between the capacitance C 1  and the gates of Tr 2   a  to Tr 2   f  by half, as compared with the cascode amplifiers of Comparative Examples 1 and 2. This configuration may also be applied to a cascode amplifier having a larger gate width so as to minimize reduction of its output power due to wiring resistance. Further, this configuration makes it possible to reduce the differences between the wiring resistances between the gates of the transistors Tr 2   a  to Tr 2   f  and thereby minimize the differences in operating conditions between the cells of the cascode amplifier. 
     It should be noted that although in the present embodiment two lines, namely the lines Wg 2  and Wg 3  are connected in parallel to the gates of Tr 2   a  to Tr 2   f , it is to be understood that three or more lines may be connected in parallel to the gates of Tr 2   a  to Tr 2   f  to provide the same effect. 
     Further, the resistance of the line Wg 1  connected to the gates of Tr 1   a  to Tr 1   f  may also serve to vary the levels of power input to these gates, thereby varying the operating conditions of the cells of the cascode amplifier. Therefore, two or more lines may be connected in parallel to the gates of Tr 1   a  to Tr 1   f  to reduce the differences in input power between these gates and thereby minimize the differences in operating conditions between the cells of the cascode amplifier. 
     Second Embodiment 
       FIG. 10  is a top view of a cascode amplifier in accordance with a second embodiment of the present invention.  FIG. 11  is a circuit diagram of the cascode amplifier of the second embodiment. In the cascode amplifier of the second embodiment, unlike that of the first embodiment, a first capacitance C 1   a  is connected between one end of the line Wg 2  and ground and a second capacitance C 1   b  is connected between the other end of the line Wg 2  and ground. A resistance Rc 1   a  represents the sum of the parasitic resistances of the line and vias connected in series between the Vg 2  terminal and the first capacitance C 1   a . A resistance Rc 1   b  represents the sum of the parasitic resistances of the line and vias connected in series between the gate of Tr 2   f  and the second capacitance C 1   b.    
     The cascode amplifier of the present embodiment is provided with two RF grounding capacitances, namely the first capacitance C 1   a  and the second capacitance C 1   b,  instead of only one RF grounding capacitance (as in the first embodiment). This configuration allows the parasitic resistance of each capacitance to be lower than the parasitic resistance of the grounding capacitance C 1  of the cascode amplifier of the first embodiment. Further, since the capacitances C 1   a  and C 1   b  are connected to the opposite ends of the line Wg 2 , the wiring resistance between, e.g., the transistor Tr 2   c  (which is located near the center of the length of the cascode amplifier) and the capacitance C 1   a  or C 1   b  is substantially half the wiring resistance between the transistor Tr 2   f  and the capacitance C 1  of the cascode amplifier of Comparative Example 1, making it possible to minimize reduction of the output power due to wiring resistance. Further, this configuration makes it possible to reduce the differences between the wiring resistances between the gates of the transistors Tr 2   a  to Tr 2   f  and thereby minimize the differences in operating conditions between the cells of the cascode amplifier. 
     Third Embodiment 
       FIG. 12  is a top view of a cascode amplifier in accordance with a third embodiment of the present invention.  FIG. 13  is a circuit diagram of the cascode amplifier of the third embodiment. In the cascode amplifier of the third embodiment, unlike that of the first embodiment, capacitances C 1   a  to C 1   f  are connected at one end to the gates of the transistors Tr 2   a  to Tr 2   f  by lines Wg 5   a  to Wg 5   f,  respectively, and connected at the other end to GND. Resistances Rc 1   a  to Rc 1   f  (see  FIG. 13 ) represent the sum of the parasitic resistances of the line and vias connected in series between the capacitances C 1   a  to C 1   f  and the gates of the transistors Tr 2   a  to Tr 2   f,  respectively; specifically, the resistance Rc 1   a  represents the sum of the parasitic resistances of the line Wg 5   a  and vias by which the capacitance C 1   a  is connected to the gate of the transistor Tr 2   a,  the resistance Rc 1   b  represents the parasitic resistances of the line Wg 5   b  and vias by which the capacitance C 1   b  is connected to the gate of the transistor Tr 2   b,  and so on. 
     Thus, in the cascode amplifier of the present embodiment, each of the six cells has a grounding capacitance connected thereto. This means that these grounding capacitances can be smaller in area than the grounding capacitance C 1  of the first embodiment, making it possible to reduce wiring resistance. Further, whereas in the cascode amplifier of the first embodiment the gates of the transistors Tr 2   a  to Tr 2   f  are connected to the grounding capacitance C 1  through a single common line, in the cascode amplifier of the present embodiment the gates of Tr 2  to Tr 2   f  are connected to the capacitances C 1   a  to C 1   f  through different lines. This configuration allows the wiring resistance between the gates of the transistors Tr 2  to Tr 2   f  and the capacitances C 1   a  to C 1   f,  respectively, to be lower than the wiring resistance between the gates of the transistors Tr 2  to Tr 2   f  and the capacitance C 1  in the cascode amplifier of the first embodiment, making it possible to minimize reduction of the output power due to wiring resistance. Further, since each of the cells of the cascode amplifier of the present embodiment has a grounding capacitance connected thereto, the resistances Rg 2   b  to Rg 2   f  of the line Wg 2  as measured between the gates of neighboring transistors Tr 2   a  and Tr 2   b,  Tr 2   b  and Tr 2   c,  Tr 2   c  and Tr 2   d,  Tr 2   d  and Tr 2   e,  and Tr 2   e  and Tr 2   f,  respectively, can be ignored, making it possible to minimize the differences in operating conditions between the cells of the cascode amplifier. 
     Fourth Embodiment 
       FIG. 14  is a circuit diagram of a cascode amplifier in accordance with a fourth embodiment of the present invention. This cascode amplifier is a differential amplifier. In this cascode amplifier, a cascode amplifier made up of transistors Tr 1   a,  Tr 1   b,  Tr 2   a,  and Tr 2   b  and a cascode amplifier made up of transistors Tr 3   a,  Tr 3   b,  Tr 4   a,  and Tr 4   b  form a differential pair. Capacitances C 2   a,  C 2   b,  C 4   a,  and C 4   b  are connected at one end to the gates of Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b,  respectively, and at the other end to GND. 
     Advantages of the present embodiment will now be described in comparison with a conventional cascode amplifier designated as Comparative Example 3.  FIG. 15  is a circuit diagram of the cascode amplifier of Comparative Example 3. In the cascode amplifier of Comparative Example 3, one grounding capacitance, namely capacitance C 1 , is connected at one end to the junction n 1  between the gates of Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b,  and at the other end to ground. This configuration, however, is disadvantageous in that in order to virtual grounding in the cascode amplifier, it is necessary to increase the grounding capacitance if the total gate width of the cascode amplifier is increased, since an increase in the total gate width results in increased impact of the wiring resistances Rc 1 , Rg 2   a,  Rg 2   b,  Rg 4   a,  and Rg 4   b  shown in  FIG. 15 . 
     In the cascode amplifier of the present embodiment, on the other hand, each of the four cells has a grounding capacitance connected thereto. This means that these grounding capacitances can be smaller in area than the grounding capacitance C 1  of the cascode amplifier of Comparative Example 3, making it possible to reduce wiring resistance. Further, whereas in the cascode amplifier of Comparative Example 3 the gates of the transistors Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b  are connected to the grounding capacitance C 1  through a single common line, in the cascode amplifier of the present embodiment the gates of Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b  are connected to the grounding capacitances C 2   a,  C 2   b,  C 4   a,  and C 4   b  through different lines. This configuration allows the wiring resistance between the gates of Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b  and the grounding capacitances C 2   a,  C 2   b,  C 4   a,  and C 4   b,  respectively, to be lower than the wiring resistance between the gates of the transistors Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b  and the grounding capacitance C 1  in the cascode amplifier of Comparative Example 3, making it possible to achieve virtual grounding at the junction n 1  between the gates of Tr 2   a,  Tr 2   b,  Tr 4   a,  and Tr 4   b  in the differential amplifier (or cascode amplifier of the present embodiment) while minimizing the values of the grounding capacitances C 2   a , C 2   b,  C 4   a , and C 4   b.    
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of Japanese Patent Application No. 2012-247144, filed on Nov. 9, 2012, including specification, claims, drawings, and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.