Patent Publication Number: US-11387796-B2

Title: Power amplifier circuit

Description:
This application claims priority from Japanese Patent Application No. 2018-233484 filed on Dec. 13, 2018. The content of this application is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to a power amplifier circuit. Mobile communication devices such as mobile phones include a power amplifier circuit that amplifies the power of a signal using transistors. For example, Japanese Unexamined Patent Application Publication No. 2018-85689 discloses a power amplifier circuit including two vertically connected transistors. In the disclosed power amplifier circuit, the emitter of the upper-stage transistor is connected to the collector of the lower-stage transistor via a capacitor and is also connected to ground via an inductor, thereby rendering the upper- and lower-stage transistors conductive for alternating current (AC) and cut-off for direct current (DC). Accordingly, a signal having a voltage amplitude that is approximately twice as high as a power supply voltage is output from the collector of the upper-stage transistor, and consequently the power amplifier circuit provides larger maximum output power than a power amplifier circuit that amplifies power using a single transistor. 
     However, in the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2018-85689, due to the effect of the parasitic capacitance of the upper-stage transistor, the capacitor connected between the upper- and lower-stage transistors, or the like, the upper-stage transistor may be less stable and might oscillate in certain condition. 
     BRIEF SUMMARY 
     The present disclosure provides a power amplifier circuit that provides large maximum output power while providing improved stability. 
     According to embodiments of the present disclosure, a power amplifier circuit includes a lower-stage transistor having a first terminal, a second terminal, and a third terminal, wherein a first power supply voltage is supplied to the first terminal, the second terminal is connected to ground, and a first signal is supplied to the third terminal; a first capacitor; an upper-stage transistor having a first terminal, a second terminal, and a third terminal, wherein a second power supply voltage is supplied to the first terminal, a second signal obtained by amplifying the first signal is output from the first terminal, the second terminal is connected to the first terminal of the lower-stage transistor via the first capacitor, and the third terminal is connected to ground via a ground path; an inductor that connects the second terminal of the upper-stage transistor to ground; and an adjustment circuit that adjusts impedance seen from the third terminal of the upper-stage transistor. The adjustment circuit includes a second capacitor and at least one resistance element, which are connected in series with the ground path between the third terminal of the upper-stage transistor and ground. 
     According to embodiments of the present disclosure, a power amplifier circuit includes a lower-stage transistor having a first terminal, a second terminal, and a third terminal, wherein a first power supply voltage is supplied to the first terminal, the second terminal is connected to ground, and a first signal is supplied to the third terminal; a first capacitor; an upper-stage transistor having a first terminal, a second terminal, and a third terminal, wherein a second power supply voltage is supplied to the first terminal, a second signal obtained by amplifying the first signal is output from the first terminal, the second terminal is connected to the first terminal of the lower-stage transistor via the first capacitor, and a bias current or bias voltage is supplied to the third terminal from a bias circuit via a bias supply path; an inductor that connects the second terminal of the upper-stage transistor to ground; and an adjustment circuit that adjusts impedance seen from the third terminal of the upper-stage transistor. The adjustment circuit includes a second capacitor and at least one resistance element, which are connected in series with the bias supply path between the third terminal of the upper-stage transistor and an output end of the bias circuit. 
     According to embodiments of the present disclosure, it may be possible to provide a power amplifier circuit that provides large maximum output power while providing improved stability. 
     Other features, elements, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of embodiments of the present disclosure with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  illustrates an example configuration of a power amplifier circuit according to an embodiment of the present disclosure; 
         FIG. 2  illustrates an example configuration of the power amplifier circuit, in which an adjustment circuit illustrated in  FIG. 1  is specifically illustrated; 
         FIG. 3A  illustrates in a simplified manner a power-stage amplifier illustrated in  FIG. 2 ; 
         FIG. 3B  illustrates an equivalent circuit of a transistor illustrated in  FIG. 3A ; 
         FIG. 4  illustrates an example configuration of a power amplifier circuit including a first modification of the adjustment circuit illustrated in  FIG. 2 ; 
         FIG. 5  illustrates an example configuration of a power amplifier circuit including a second modification of the adjustment circuit illustrated in  FIG. 2 ; 
         FIG. 6  illustrates an example configuration of a power amplifier circuit including a third modification of the adjustment circuit illustrated in  FIG. 2 ; 
         FIG. 7  illustrates an example configuration of a power amplifier circuit in which each of upper- and lower-stage transistors illustrated in  FIG. 2  includes a plurality of unit transistors; 
         FIG. 8  illustrates an example configuration of a power amplifier circuit, in which bias circuits illustrated in  FIG. 2  are specifically illustrated; 
         FIG. 9  illustrates an example configuration of a power amplifier circuit including a first modification of the bias circuit illustrated in  FIG. 8 ; and 
         FIG. 10  illustrates an example configuration of a power amplifier circuit including a second modification of the bias circuit illustrated in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure in detail with reference to the drawings. The same or substantially the same elements are denoted by the same numeral, and will not be repeatedly described. 
       FIG. 1  illustrates an example configuration of a power amplifier circuit  100  according to an embodiment of the present disclosure. The power amplifier circuit  100  is mounted in, for example, a mobile communication device such as a mobile phone and is used to amplify the power of a radio-frequency (RF) signal to be transmitted to a base station. For example, the power amplifier circuit  100  amplifies transmission signals conforming to communication standards such as the second-generation mobile communication system (2G), the third-generation mobile communication system (3G), the fourth-generation mobile communication system (4G), the fifth-generation mobile communication system (5G), Long Term Evolution Frequency Division Duplex (LTE-FDD), LTE Time Division Duplex (LTE-TDD), LTE-Advanced, and LTE-Advanced Pro. The RF signal has a frequency of about several hundreds of megahertz (MHz) to about several tens of gigahertz (GHz), for example. The power amplifier circuit  100  may amplify signals having other frequencies and conforming to other communication standards. 
     As illustrated in  FIG. 1 , the power amplifier circuit  100  includes transistors  110  to  112 , bias circuits  120  to  122 , an adjustment circuit  130 , capacitors  140  to  142 , inductors  150  to  153 , resistance elements  160  and  161 , and matching networks (MNs)  170  to  172 . 
     The power amplifier circuit  100  includes two-stage amplifiers. The first-stage (drive-stage) amplifier includes the transistor  110 , and the second-stage (power-stage) amplifier includes the transistor  111  and the transistor  112 . The drive-stage amplifier amplifies an RF signal RF 1  and outputs an RF signal RF 2 . The power-stage amplifier amplifies the RF signal RF 2  and outputs an RF signal RF 3 . In this way, the power amplifier circuit  100  amplifies the power of a transmission signal in two stages. 
     In this embodiment, each of the transistors  110  to  112  is constituted by a bipolar transistor such as a heterojunction bipolar transistor (HBT). Each of these transistors may be constituted by a field-effect transistor such as a metal-oxide-semiconductor field-effect transistor (MOSFET) instead of by an HBT. In this case, the terms “collector”, “base”, and “emitter” in the following description are changed to the terms “drain”, “gate”, and “source”, respectively. 
     In the drive-stage amplifier, the collector of the transistor  110  is supplied with a power supply voltage Vcc 1  via the inductor  150 , the emitter of the transistor  110  is connected to ground, and the base of the transistor  110  is supplied with the RF signal RF 1  via the matching network  170  and the capacitor  140 . The base of the transistor  110  is further supplied with a bias current or bias voltage from the bias circuit  120  via the resistance element  160 . As a result, the RF signal RF 2 , which is obtained by amplifying the RF signal RF 1 , is output from the collector of the transistor  110 . 
     In the power-stage amplifier, the transistor  111  and the transistor  112  synchronously perform similar amplification operations. The collector (first terminal) of the lower-stage transistor  111  (lower-stage transistor) is supplied with a power supply voltage Vcc 2  (first power supply voltage) via the inductor  151 , the emitter (second terminal) of the lower-stage transistor  111  is connected to ground, and the base (third terminal) of the lower-stage transistor  111  is supplied with the RF signal RF 2  (first signal) via the capacitor  141 . The base of the transistor  111  is further supplied with a bias current or bias voltage (second bias current or bias voltage) from the bias circuit  121  via the resistance element  161 . 
     The collector (first terminal) of the upper-stage transistor  112  (upper-stage transistor) is supplied with a power supply voltage Vcc 3  (second power supply voltage) via the inductor  152 , the emitter (second terminal) of the upper-stage transistor  112  is connected to ground via the inductor  153 , and the base (third terminal) of the upper-stage transistor  112  is supplied with a bias current or bias voltage (first bias current or bias voltage) from the bias circuit  122  via the adjustment circuit  130 . The emitter of the transistor  112  is connected to the collector of the lower-stage transistor  111  via the capacitor  142 . Accordingly, the upper-stage transistor  112  outputs the RF signal RF 3  (second signal), which is obtained by amplifying the RF signal RF 2 , from the collector of the upper-stage transistor  112 . The operation of the power-stage amplifier will be described in detail below. 
     The capacitor  142  (first capacitor) has an end connected to the emitter of the upper-stage transistor  112  and another end connected to the collector of the lower-stage transistor  111 . The capacitor  142  has a function of isolating the upper-stage transistor  112  and the lower-stage transistor  111  from each other for DC and connecting the upper-stage transistor  112  and the lower-stage transistor  111  to each other for AC. 
     The inductor  153  has an end connected to the emitter of the upper-stage transistor  112  and another end connected to ground. The inductor  153  has a function of connecting the emitter of the upper-stage transistor  112  to ground for DC. 
     The bias circuits  120 ,  121  (second bias circuit), and  122  (first bias circuit) generate the respective bias currents or bias voltages and supply the bias currents or bias voltages to the bases of the transistors  110 ,  111 , and  112 , respectively. The bias circuits  120  to  122  respectively adjust the bias currents or bias voltages to control bias conditions for the transistors  110  to  112 . 
     The adjustment circuit  130  is provided between the bias circuit  122  and the base of the upper-stage transistor  112  and adjusts the impedance seen from the base of the transistor  112 . A specific configuration of the adjustment circuit  130  will be described below. 
     The capacitors  140  and  141  are provided on the input side of the transistors  110  and  111 , respectively. Each of the capacitors  140  and  141  blocks the DC component included in the RF signal and allows the AC component included in the RF signal to pass therethrough. 
     The inductor  150  has an end to which the power supply voltage Vcc 1  is supplied, and another end connected to the collector of the transistor  110 . The inductor  151  has an end to which the power supply voltage Vcc 2  is supplied, and another end connected to the collector of the transistor  111 . The inductor  152  has an end to which the power supply voltage Vcc 3  is supplied, and another end connected to the collector of the transistor  112 . The inductors  150  to  152  prevent the RF signal from leaking toward a power supply circuit (not illustrated). 
     The resistance element  160  is provided between an output end of the bias circuit  120  and the base of the transistor  110 . The resistance element  161  is provided between an output end of the bias circuit  121  and the base of the transistor  111 . The resistance elements  160  and  161  are provided to adjust the bias currents or bias voltages supplied from the bias circuits  120  and  121 , respectively, or to prevent an increase in the temperature of the transistors  110  and  111 , which is caused by an excessive amount of current flow. While a similar resistance element may be provided between an output end of the bias circuit  122  and the base of the transistor  112 , such a resistance element is included in the adjustment circuit  130  and will be described below. 
     The matching networks  170  to  172  match the impedances of the preceding components and the subsequent components. The matching network  170  matches the impedances of the preceding circuit (not illustrated) of the matching network  170  and the transistor  110 . The matching network  171  matches the impedances of the transistor  110  and the transistor  111 . The matching network  172  matches the impedances of the transistor  112  and the circuit (not illustrated) subsequent to the matching network  172 . Each of the matching networks  170  to  172  is constituted by, for example, a capacitor and an inductor. The power amplifier circuit  100  may not include some or any of the matching networks  170  to  172  if any other component has some or all of the functions of the matching networks  170  to  172 . 
     Next, the operation of the power-stage amplifier will be described in detail. In the following description, both the power supply voltage Vcc 2  and the power supply voltage Vcc 3  are assumed to be 3 V. 
     The power supply voltage Vcc 2  (DC 3 V) is supplied to the collector of the lower-stage transistor  111  for DC, and thus the collector voltage of the lower-stage transistor  111  changes in a range of DC 3 V±AC 3 V. The emitter voltage of the upper-stage transistor  112  changes in a range of DC 0 V±AC 3 V since the emitter of the upper-stage transistor  112  is connected to ground for DC and is connected to the collector of the lower-stage transistor  111  for AC. The collector voltage of the upper-stage transistor  112  changes in a range of DC 3 V±AC 6 V since the power supply voltage Vcc 3  (DC 3 V) is supplied to the collector of the upper-stage transistor  112  for DC and the signal amplitudes at the collector and the emitter of the transistor  112  are added together for AC. Accordingly, the signal amplitude across the collector and emitter of the upper-stage transistor  112  is equal to the signal amplitude across the collector and emitter of the lower-stage transistor  111 , whereas the signal amplitude at the collector of the upper-stage transistor  112  is approximately twice as high as the signal amplitude across the collector and emitter of the upper-stage transistor  112 . 
     Given that the output power of a signal is denoted by P, the collector voltage by V, and the load impedance of an amplifier by R, a relationship given by P=V 2 /R holds. That is, to double the voltage amplitude and double the output power, the load impedance is doubled. In the power amplifier circuit  100 , accordingly, the load impedance is doubled without necessarily increasing the power supply voltages Vcc 2  and Vcc 3 . As a result, the maximum output power of the signal is increased. 
     Next, the configuration and function of the adjustment circuit  130  will be described in detail. 
       FIG. 2  illustrates an example configuration of a power amplifier circuit  100 A, in which the adjustment circuit illustrated in  FIG. 1  is specifically illustrated. In  FIGS. 2 and 4 to 10  described below, the components related to the drive stage are not illustrated, and the components related to the power stage are illustrated, for convenience of illustration. 
     As illustrated in  FIG. 2 , in the power amplifier circuit  100 A, the base of the upper-stage transistor  112  is connected to ground via a ground path X 1 . The bias circuit  122  supplies a bias current or bias voltage to a supply point (node) Y 1  on the ground path X 1  via a bias supply path X 2 . 
     An adjustment circuit  130 A includes a capacitor  200 , an inductor  210 , and resistance elements  220  and  221 . 
     The resistance element  220  (first resistance element) and the inductor  210  are connected in series with the bias supply path X 2  between the output end of the bias circuit  122  and the supply point Y 1 . Like the resistance elements  160  and  161  illustrated in  FIG. 1 , the resistance element  220  is also provided to adjust the bias current or bias voltage output from the bias circuit  122  or to prevent an increase in the temperature of the transistor  112 . The inductor  210  is provided to match the impedance seen from the base of the transistor  112 . 
     The capacitor  200  (second capacitor) and the resistance element  221  are connected in series with the ground path X 1  between the supply point Y 1  and ground. The capacitor  200  is provided to change the base voltage of the transistor  112  with the amplitude of the signal amplified by the transistor  112  and to match the impedance seen from the base of the transistor  112 . The capacitance value of the capacitor  200  can be smaller than the capacitance value of the capacitor  142 , for example. This is because if the capacitor  200  has an excessively large capacitance value, a change in the base voltage of the transistor  112  is suppressed. The resistance element  221  has predetermined impedance and is provided to intentionally consume the energy of the signal. 
     With the configuration described above, the adjustment circuit  130 A has the following two functions. The first function of the adjustment circuit  130 A is to prevent, by using the capacitor  200 , the amplitude operation of the base voltage of the transistor  112  from being restricted by the output end of the bias circuit  122 . That is, to turn on the upper-stage transistor  112 , the base-emitter voltage of the transistor  112  needs to be greater than or equal a predetermined voltage. In other words, the base voltage of the transistor  112  needs to change as the emitter voltage of the transistor  112  changes. The adjustment circuit  130 A, which includes, for example, the capacitor  200 , has a function of changing the base voltage of the transistor  112  with the signal amplitude. 
     The second function of the adjustment circuit  130 A is to adjust the impedance seen from the base of the transistor  112  to improve the stability of the transistor  112  and to suppress oscillation. The stability of a transistor will be described with reference to  FIGS. 3A and 3B . 
       FIG. 3A  illustrates in a simplified manner the power-stage amplifier illustrated in  FIG. 2 . A parasitic capacitor  300  represents the parasitic capacitance between the base and emitter of the upper-stage transistor  112 . A capacitor  301  represents capacitance connected to the emitter of the upper-stage transistor  112 . The capacitance connected to the emitter of the transistor  112  includes, for example, the capacitor  142  illustrated in  FIG. 2 , the parasitic capacitance between the base and collector of the lower-stage transistor  111 , the parasitic capacitance between the base and emitter of the transistor  111 , and the like. For convenience of illustration, the lower-stage transistor  111  is indicated by a current source  310 . 
       FIG. 3B  illustrates an equivalent circuit of the transistor illustrated in  FIG. 3A . In  FIG. 3B , the upper-stage transistor  112  is indicated by a current source  330  and a resistance element  320  representing input impedance. It is assumed that the capacitance value of the parasitic capacitor  300  is denoted by C BE , the capacitance value of the capacitor  301  by C P , the input impedance of the transistor  112  by rπ, the transconductance by g m , and the angular frequency of the signal input to the transistor  112  by ω. The real part Re(Zin) of input impedance Zin seen from the base of the transistor  112  is given by Equation (1) below. 
     
       
         
           
             
               
                 
                   
                     Re 
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                       Zin 
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                         ω 
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                                 m 
                               
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                                 C 
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                           ( 
                           
                             
                               g 
                               m 
                             
                             + 
                             
                               1 
                               
                                 r 
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                         2 
                       
                       + 
                       
                         
                           
                             ω 
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                             ( 
                             
                               
                                 C 
                                 BE 
                               
                               + 
                               
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                         2 
                       
                     
                   
                 
               
               
                 
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                   1 
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     Equation (1) indicates that the real part Re(Zin) of the input impedance Zin becomes negative depending on the condition of the transistor. In this case, a negative resistance is generated, which may impair the stability of the transistor  112 . As a result, oscillation may occur. To address this, a load whose impedance has a positive real part is intentionally inserted in the adjustment circuit  130 A according to this embodiment to cancel the negative resistance. Specifically, the resistance element  221  connected in series with the capacitor  200  is included in the adjustment circuit  130 A. Thus, the negative resistance that appears in the input impedance Zin (i.e., in  FIG. 2 , the impedance on the transistor  112  side seen from the output end of the bias circuit  122 ) is canceled, and the stability of the transistor  112  is improved. The resistance value of the resistance element  221  is not limited to any specific value, and can be a resistance value that makes the real part of the input impedance Zin be 0 or more, for example. 
     With the configuration described above, the power amplifier circuit  100 A can provide larger maximum output power than a configuration in which power is amplified using a single transistor, while providing improved stability. 
     In the adjustment circuit  130 A, the resistance element  220  and the inductor  210  are connected in series in this order from the bias circuit  122  to the supply point Y 1 , and the capacitor  200  and the resistance element  221  are connected in series in this order from the supply point Y 1  to ground, by way of example but not limitation. The order of the series-connected elements may be reversed. This also applies to the following modifications. In addition, depending on the input impedance Zin, the adjustment circuit  130 A may not include one of the resistance element  220 , the inductor  210 , the capacitor  200 , and the resistance element  221 . 
     The power amplifier circuit  100 A is configured such that two transistors are vertically connected in the power-stage amplifier. Alternatively, three or more transistors may be vertically connected. 
     The power amplifier circuit  100 A includes two-stage amplifiers. However, the number of stages of amplifiers is not limited to two, and one or three or more stages of amplifiers may be used. In a power amplifier circuit including three or more stages of amplifiers, the configuration of the power-stage amplifier illustrated in  FIG. 2  may be applied to any of the amplifiers. 
       FIG. 4  illustrates an example configuration of a power amplifier circuit  100 B including a first modification of the adjustment circuit illustrated in  FIG. 2 . In this modification and the following modifications, features common to the embodiment described above will not be described, and only the differences will be described. In particular, similar operational effects achieved with similar configurations will not be described again in the individual modifications. 
     As illustrated in  FIG. 4 , in the power amplifier circuit  100 B, unlike the power amplifier circuit  100 A, an adjustment circuit  130 B further includes a resistance element  400 . 
     The resistance element  400  (second resistance element) is connected in series with the ground path X 1  between the base of the upper-stage transistor  112  and the supply point Y 1 . In other words, the resistance element  400  is connected in series with the resistance element  221  (third resistance element). Accordingly, the resistance element  400  has both a function of adjusting the bias current or bias voltage, like the resistance element  220 , and a function of a load that consumes energy, like the resistance element  221 . 
     With the configuration described above, the power amplifier circuit  100 B can also achieve advantages similar to those of the power amplifier circuit  100 A. The adjustment circuit  130 B is capable of separately adjusting the respective constants of the capacitor  200 , the resistance element  221 , and the resistance element  400 , and is thus more flexible in design that the adjustment circuit  130 A. In the adjustment circuit  130 B, furthermore, the load on the ground path X 1  is divided into the resistance element  221  and the resistance element  400 , and thus the resistance value of each resistance element can be smaller than that in the adjustment circuit  130 A. The ground path X 1  is defined as a path whose initial point (node) is the base of the transistor  112  and whose end point is a point (node) connected to the capacitor  200  or the inductor  210 . 
     The adjustment circuit  130 A may not include the inductor  210 , for example. When the adjustment circuit  130 A does not include the inductor  210 , the resistance element  220  also functions as a load that consumes energy, in addition to the capacitor  200 , the resistance element  221 , and the resistance element  400 . If the resistance value of the resistance element  220  is large to some extent, the resistance element  220  has small consumption of energy. By setting the respective constants of the capacitor  200  and the resistance elements  221  and  400 , as appropriate, appropriate consumption can be obtained. 
       FIG. 5  illustrates an example configuration of a power amplifier circuit  100 C including a second modification of the adjustment circuit illustrated in  FIG. 2 . As illustrated in  FIG. 5 , in the power amplifier circuit  100 C, unlike the power amplifier circuit  100 B, an adjustment circuit  130 C includes a capacitor  500  in place of the inductor  210 . 
     The capacitor  500  (third capacitor) has an end connected to the output end of the bias circuit  122  and another end connected to the supply point Y 1  so that the capacitor  500  is connected in parallel with the resistance element  220 . The capacitance value of the capacitor  500  can be smaller than the capacitance value of the capacitor  142 , for example. In the adjustment circuit  130 C, in addition to the capacitor  200 , the capacitor  500  also contributes to the adjustment of the impedance seen from the base of the transistor  112 . 
     With the configuration described above, the power amplifier circuit  100 C can also achieve advantages similar to those of the power amplifier circuits  100 A and  100 B. Like the adjustment circuits  130 A and  130 B, the adjustment circuit  130 C may include an inductor corresponding to the inductor  210 . 
       FIG. 6  illustrates an example configuration of a power amplifier circuit  100 D including a third modification of the adjustment circuit illustrated in  FIG. 2 . As illustrated in  FIG. 6 , in the power amplifier circuit  100 D, unlike the power amplifier circuit  100 C, the base of the upper-stage transistor  112  is not connected to ground, and an adjustment circuit  130 D does not include the capacitor  200  or the resistance element  221 . 
     As described previously, like the capacitor  200  illustrated in  FIG. 5 , the capacitor  500  enables the AC component to be grounded in a state close to the ideal. The resistance element  400  has a function similar to that of the resistance element  221  illustrated in  FIG. 5 . 
     Thus, the adjustment circuit  130 D can achieve advantages similar to those of the adjustment circuits  130 A to  130 C without necessarily including the capacitor  200  and the resistance element  221  illustrated in  FIG. 5 . 
     With the configuration described above, the power amplifier circuit  100 D can also achieve advantages similar to those of the power amplifier circuits  100 A to  100 C. Like the adjustment circuits  130 A and  130 B, the adjustment circuit  130 D may include an inductor corresponding to the inductor  210 . 
       FIG. 7  illustrates an example configuration of a power amplifier circuit  100 E in which each of the upper- and lower-stage transistors illustrated in  FIG. 2  includes a plurality of unit transistors. 
     Specifically, the power amplifier circuit  100 E includes n (n is an integer greater than or equal to 2) unit transistors  111 - 1  to  111 - n  and  112 - 1  to  112 - n , n adjustment circuits  130 A- 1  to  130 A-n, n capacitors  141 - 1  to  141 - n , and n resistance elements  161 - 1  to  161 - n  in place of the transistors  111  and  112 , the adjustment circuit  130 A, the capacitor  141 , and the resistance element  161  of the power amplifier circuit  100 A. The circuit configuration of each of the n adjustment circuits  130 A- 1  to  130 A-n is similar to that of the adjustment circuit  130 A described above and will not be described in detail. 
     The n unit transistors  111 - 1  to  111 - n  and the n unit transistors  112 - 1  to  112 - n  are each a minimum element that functions as a transistor. These unit transistors are connected in parallel with each other to operate as a single transistor as a whole. In the power amplifier circuit  100 E, for example, in the lower stage, the unit transistor  111 - 1 , the capacitor  141 - 1 , and the resistance element  161 - 1  are formed into a single unit. In the upper stage, the unit transistor  112 - 1  and the adjustment circuit  130 A- 1  are formed into a single unit. The formed n units are connected in parallel with each other. 
     Since the power amplifier circuit  100 E includes an adjustment circuit for each unit transistor, the constants of the elements included in the adjustment circuits  130 A- 1  to  130 A-n can be set in accordance with the condition of each unit transistor. Thus, for example, it is possible to reduce deterioration of stability caused by the parasitic capacitance of wiring or the like. 
     In the power amplifier circuit  100 E illustrated in  FIG. 7 , the configuration of the adjustment circuit  130 A is applied as an example of an adjustment circuit. However, the configuration of an adjustment circuit is not limited to this, and the configuration of any of the other adjustment circuits  130 B to  130 D may be applied. This also applies to  FIGS. 8 to 10  described below. 
     Next, a specific example of a bias circuit that supplies bias currents or bias voltages to the upper- and lower-stage transistors will be described. 
       FIG. 8  illustrates an example configuration of a power amplifier circuit  100 F, in which bias circuits illustrated in  FIG. 2  are specifically illustrated. As illustrated in  FIG. 8 , unlike the power amplifier circuit  100 A, the power amplifier circuit  100 F includes a single bias circuit  121 A in place of the two bias circuits  121  and  122 . 
     In the power amplifier circuit  100 F, the single bias circuit  121 A functions as a bias circuit of the upper-stage transistor and a bias circuit of the lower-stage transistor. The bias circuit  121 A includes, for example, transistors  600   a  to  603   a  and a capacitor  610   a.    
     The collector and base of the transistor  600   a  are connected to each other (hereinafter referred to as diode-connected). A control current Iref is supplied to the collector of the transistor  600   a , and the emitter of the transistor  600   a  is connected to the collector of the transistor  601   a . The transistor  601   a  is diode-connected, and the emitter of the transistor  601   a  is connected to ground. Accordingly, a voltage having a predetermined level (for example, about 2.6 V) is generated at the collector of the transistor  600   a . The diode-connected transistors  600   a  and  601   a  may be replaced by diodes. 
     The capacitor  610   a  has an end connected to the bases of the transistor  602   a  and the transistor  603   a  and another end connected to ground. The capacitor  610   a  suppresses changes in the base voltages of the transistor  602   a  and the transistor  603   a.    
     The transistor  602   a  and the transistor  603   a  are connected in parallel to each other. The collector of the transistor  602   a  is supplied with a battery voltage Vbatt, the base of the transistor  602   a  is connected to the collector of the transistor  600   a , and the emitter of the transistor  602   a  is connected to an end of the resistance element  161 . The collector of the transistor  603   a  is supplied with the battery voltage Vbatt, the base of the transistor  603   a  is connected to the collector of the transistor  600   a , and the emitter of the transistor  603   a  is connected to an end of the resistance element  220 . Accordingly, bias currents each corresponding to the control current Iref are output from the emitters of the transistor  602   a  and the transistor  603   a . In this embodiment, the bias circuit  121 A is controlled by the control current Iref, by way of example. The bias circuit may be controlled by a control voltage in place of a control current. 
     In the power amplifier circuit  100 F, bias currents can be supplied to both the lower-stage transistor  111  and the upper-stage transistor  112  by using the single bias circuit  121 A. Thus, it is possible to achieve a smaller circuit size than that in a configuration in which a bias circuit is provided for each transistor. 
       FIG. 9  illustrates an example configuration of a power amplifier circuit  100 G including a first modification of the bias circuit illustrated in  FIG. 8 . As illustrated in  FIG. 9 , unlike the power amplifier circuit  100 F, the power amplifier circuit  100 G includes a bias circuit  121 B in place of the bias circuit  121 A. 
     Unlike the bias circuit  121 A, the bias circuit  121 B does not include the transistor  603   a . In the bias circuit  121 B, a bias current output from the emitter of the transistor  602   a  is divided and supplied to both the transistor  111  and the transistor  112 . 
     With the configuration described above, the power amplifier circuit  100 G can also achieve advantages similar to those of the power amplifier circuit  100 F. 
       FIG. 10  illustrates an example configuration of a power amplifier circuit  100 H including a second modification of the bias circuit illustrated in  FIG. 8 . As illustrated in  FIG. 10 , the power amplifier circuit  100 H includes bias circuits  121 C and  122 A as a specific example of the bias circuits  121  and  122  in the power amplifier circuit  100 A. 
     Unlike the bias circuit  121 B illustrated in  FIG. 9 , the bias circuit  121 C (second bias circuit) further includes a transistor  604   a . The collector of the transistor  604   a  is connected to the emitter of the transistor  602   a , the base of the transistor  604   a  is connected to the base of the transistor  601   a , and the emitter of the transistor  604   a  is connected to ground. The transistor  604   a  functions to extract current from the transistor  602   a  to stably supply a bias current from the transistor  602   a  to the transistor  111 . 
     The bias circuit  122 A (first bias circuit) includes, for example, transistors  600   b  to  602   b  and a capacitor  610   b . The specific configuration of the bias circuit  122 A is similar to that of the bias circuit  121 B and will not be described in detail. The operation of the bias circuit  121 C is controlled by a control current Iref 1 , and the operation of the bias circuit  122 A is controlled by a control current Iref 2 . 
     In the power amplifier circuit  100 H, since a bias circuit is provided for each of the upper- and lower-stage transistors, it is possible to provide a suitable bias current to each transistor. In addition, the bias circuit  121 C and the bias circuit  122 A have different configurations and are controlled by the different control currents Iref 1  and Iref 2 , respectively. Accordingly, the bias circuits  121 C and  122 A can control a bias current to be supplied to the lower-stage transistor  111  and a bias current to be supplied to the upper-stage transistor  112  to have different characteristics. Thus, the following advantages are achieved. 
     In the power amplifier circuit  100 H, the adjustment circuit  130 A is connected to the upper-stage transistor  112 , and impedance is adjusted by the adjustment circuit  130 A. This may cause an imbalance between the lower-stage transistor  111  and the upper-stage transistor  112 , which are to synchronously operate, depending on the impedance of the adjustment circuit  130 A. In this embodiment, bias currents having different characteristics are supplied to the lower-stage transistor  111  and the upper-stage transistor  112 , thereby making the bias conditions for these transistors different. For example, the power amplifier circuit  100 H is configured such that in at least one region of the output power in the power-stage amplifier, the bias circuit  121 C supplies a larger amount of bias current than the bias circuit  122 A. Accordingly, the imbalance between the lower-stage transistor  111  and the upper-stage transistor  112  can be compensated for, and the characteristics of the power-stage amplifier are improved. 
     The method for making the bias conditions for the lower-stage transistor  111  and the upper-stage transistor  112  different is not limited to that described above. For example, the bias circuit  121 C may supply a bias current to the upper-stage transistor  112 , and the bias circuit  122 A may supply a bias current to the lower-stage transistor  111 . Alternatively, both bias circuits may have the same configuration, and generate bias currents having different characteristics in accordance with control currents or control voltages. 
     As illustrated in  FIGS. 8 and 9 , when a bias circuit is shared by the lower-stage transistor  111  and the upper-stage transistor  112 , for example, the resistance element  161  and the resistance element  220  may have different resistance values to make the bias conditions different. 
     Further, in the power amplifier circuit  100  illustrated in  FIG. 1 , the configuration of any of the bias circuits  121 A to  121 C and  122 A may be applied to the bias circuit  120  that operates the drive-stage transistor  110 . 
     An exemplary embodiment of the present disclosure has been described. The power amplifier circuits  100 ,  100 A to  100 C, and  100 E to  100 H include the lower-stage transistor  111  having a first terminal, a second terminal, and a third terminal, wherein a first power supply voltage is supplied to the first terminal, the second terminal is connected to ground, and a first signal is supplied to the third terminal; the capacitor  142 ; the upper-stage transistor  112  having a first terminal, a second terminal, and a third terminal, wherein a second power supply voltage is supplied to the first terminal, a second signal obtained by amplifying the first signal is output from the first terminal, the second terminal is connected to the first terminal of the lower-stage transistor  111  via the capacitor  142 , and the third terminal is connected to ground via a ground path; the inductor  153  that connects the second terminal of the upper-stage transistor  112  to ground; and the adjustment circuits  130  and  130 A to  130 C that adjust impedance seen from the third terminal of the upper-stage transistor  112 . The adjustment circuits  130  and  130 A to  130 C include the capacitor  200  and at least one resistance element  221 , which are connected in series with the ground path between the third terminal of the upper-stage transistor  112  and ground. With this configuration, the resistance element  221  cancels the negative resistance of input impedance in the upper-stage transistor  112 . Thus, it is possible to provide large maximum output power while providing improved stability. 
     In the power amplifier circuits  100 ,  100 A to  100 C, and  100 E to  100 H, the ground path X 1  has the supply point Y 1  to which a bias current or bias voltage is supplied from the bias circuits  122 ,  121 A,  121 B, and  122 A via the bias supply path X 2 ; and the adjustment circuits  130  and  130 A to  130 C further include the resistance element  220  connected in series with the bias supply path X 2  between the bias circuits  122 ,  121 A,  121 B, and  122 A and the supply point Y 1 . With this configuration, it is possible to adjust the bias current or bias voltage to be supplied to the upper-stage transistor  112  and to reduce the increase in the temperature of the upper-stage transistor  112 . 
     In the power amplifier circuits  100 B and  100 C, the at least one resistance element includes the resistance element  400  and the resistance element  221 , the resistance element  400  is provided between the base of the upper-stage transistor  112  and the supply point Y 1 , and the resistance element  221  is provided between the supply point Y 1  and ground. With this configuration, a load on the ground path X 1  is divided into the resistance element  221  and the resistance element  400 , and, for example, the resistance value of each resistance element can be smaller than that in the adjustment circuit  130 A. 
     In the power amplifier circuit  100 C, the adjustment circuit  130 C further includes the capacitor  500  connected in parallel with the resistance element  220 . With this configuration, like the capacitor  200 , the capacitor  500  also has a function of adjusting impedance, and is thus flexible in design. 
     The power amplifier circuit  100 H includes the bias circuit  122 A that supplies a bias current or bias voltage to the base of the upper-stage transistor  112 , and the bias circuit  121 C that supplies a bias current or bias voltage to the base of the lower-stage transistor  111 . The bias circuit  122 A and the bias circuit  121 C have different configurations, and the bias currents or bias voltages that are output therefrom also have different characteristics. With this configuration, the bias conditions for the upper- and lower-stage transistors can be made different. Accordingly, the imbalance between the upper- and lower-stage transistors is compensated for, and the characteristics of the power-stage amplifier are improved. 
     The power amplifier circuit  100 D includes the lower-stage transistor  111  having a first terminal, a second terminal, and a third terminal, wherein a first power supply voltage is supplied to the first terminal, the second terminal is connected to ground, and a first signal is supplied to the third terminal; the capacitor  142 ; the upper-stage transistor  112  having a first terminal, a second terminal, and a third terminal, wherein a second power supply voltage is supplied to the first terminal, a second signal obtained by amplifying the first signal is output from the first terminal, the second terminal is connected to the first terminal of the lower-stage transistor via the capacitor  142 , and a bias current or bias voltage is supplied to the third terminal from the bias circuit  122  via the bias supply path X 2 ; the inductor  153  that connects the second terminal of the upper-stage transistor  112  to ground; and the adjustment circuit  130 D that adjusts impedance seen from the third terminal of the upper-stage transistor  112 . The adjustment circuit  130 D includes the capacitor  500  and at least one resistance element  400 , which are connected in series with the bias supply path X 2  between the third terminal of the upper-stage transistor  112  and the output end of the bias circuit  122 . By setting the respective constants of the capacitor  500  and the resistance element  400  as appropriate, it is possible to achieve advantages similar to those of the adjustment circuits  130 A to  130 C. 
     The embodiment described above is intended to help easily understand the present disclosure, and is not to be used to construe the present disclosure in a limiting fashion. Various modifications or improvements can be made to the present disclosure without necessarily departing from the gist of the present disclosure, and equivalents thereof are also included in the present disclosure. That is, the embodiment may be appropriately modified in design by those skilled in the art, and such modifications also fall within the scope of the present disclosure so long as the modifications include the features of the present disclosure. For example, the elements included in the embodiment and the arrangement, materials, conditions, shapes, sizes, and the like thereof are not limited to those described in the illustrated examples, but can be modified as appropriate. Furthermore, the elements included in the embodiment can be combined as much as technically possible, and such combinations of elements also fall within the scope of the present disclosure so long as the combinations of elements include the features of the present disclosure. 
     While embodiments of the disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without necessarily departing from the scope and spirit of the disclosure. The scope of the disclosure, therefore, is to be determined solely by the following claims.