Patent Publication Number: US-2021175853-A1

Title: Power amplifier

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2019-223209 filed on Dec. 10, 2019, and claims priority from Japanese Patent Application No. 2020-133611 filed on Aug. 6, 2020. The contents of these applications are incorporated herein by reference in their entireties. 
     BACKGROUND 
     The present disclosure relates to a power amplifier. 
     Doherty amplifiers are known as highly efficient power amplifier circuits. A typical Doherty amplifier includes a carrier amplifier and a peak amplifier which are connected in parallel. The carrier amplifier operates regardless of the power level of an input signal. The peak amplifier is switched off when the power level of an input signal is low, and is switched on when the power level of the input signal is high. When the power level of an input signal is high, the carrier amplifier operates while maintaining saturation at the saturated output power level. Doherty amplifiers may improve the efficiency compared with typical power amplifier circuits. 
     For example, Japanese Unexamined Patent Application Publication No. 2016-19228 discloses a Doherty amplifier, as a modified example of Doherty amplifier, which does not include X/4 lines used in a typical Doherty amplifier. 
     In the Doherty amplifier disclosed in Japanese Unexamined Patent Application Publication No. 2016-19228, a signal from the carrier amplifier goes through an inductor, and merges with a signal from the peak amplifier. When the amplified frequency is 3 GHz or higher, the inductance of the inductor needs to be made small, resulting in a reduction in the size of the inductor. A small inductor is susceptible to influence from the parasitic component. In this case, amplification for obtaining desired characteristics is difficult to achieve due to influence from the parasitic component. 
     BRIEF SUMMARY 
     The present disclosure provides a Doherty amplifier which achieves high efficiency and a reduction in size and which reduces influence from the parasitic component. 
     A power amplifier according to one aspect the present disclosure includes a splitter, a first amplifier, a second amplifier, and a hybrid coupler. The splitter splits a first signal into a second signal and a third signal which lags behind the second signal by 90°. The first amplifier amplifies the second signal and outputs a fourth signal in a range in which a power level of the first signal is equal to or higher than a first level. The second amplifier amplifies the third signal and outputs a fifth signal in a range in which the power level of the first signal is equal to or higher than a second level. The second level is higher than the first level. The hybrid coupler includes a first transmission line and a second transmission line. The first transmission line receives the fourth signal at its first terminal. The second transmission line receives the fifth signal at its first terminal. The second transmission line is open at its second terminal. The first transmission line outputs, from its second terminal, an amplified signal of the first signal. The amplified signal is obtained by combining the fourth signal with the fifth signal. 
     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. 
     According to the present disclosure, it is provided a Doherty amplifier which achieves high efficiency and a reduction in size and which reduces influence from the parasitic component. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the configuration of a power amplifier; 
         FIG. 2  is a diagram illustrating exemplary operating characteristics of a carrier amplifier and a peak amplifier; 
         FIG. 3  is an equivalent circuit diagram illustrating a configuration part of a power amplifier; 
         FIG. 4  is a diagram illustrating a simulation result of loss in a power amplifier; 
         FIG. 5  is a diagram illustrating a simulation result of the phase difference in a power amplifier; 
         FIG. 6  is a diagram illustrating a simulation result of isolation in a power amplifier; 
         FIG. 7  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a peak operation; 
         FIG. 8  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a backoff operation; 
         FIG. 9  is a diagram for describing the efficiency of a power amplifier; 
         FIG. 10  is a diagram illustrating another form of a hybrid coupler; 
         FIG. 11  is a diagram illustrating the configuration of a power amplifier according to a reference example; 
         FIG. 12  is an equivalent circuit diagram illustrating a configuration part of a power amplifier according to the reference example; 
         FIG. 13  is a diagram illustrating a simulation result of loss in a power amplifier according to the reference example; 
         FIG. 14  is a diagram illustrating a simulation result of the phase difference in a power amplifier according to the reference example; 
         FIG. 15  is a diagram illustrating a simulation result of isolation in a power amplifier according to the reference example; 
         FIG. 16  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a peak operation of a power amplifier according to the reference example; 
         FIG. 17  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a backoff operation of a power amplifier according to the reference example; 
         FIG. 18  is a diagram illustrating the configuration of a power amplifier according to a second embodiment; 
         FIG. 19  is a diagram illustrating a simulation result of loss in a power amplifier according to the second embodiment; 
         FIG. 20  is a diagram illustrating a simulation result of the phase difference in a power amplifier according to the second embodiment; 
         FIG. 21  is a diagram illustrating a simulation result of isolation in a power amplifier according to the second embodiment; 
         FIG. 22  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a peak operation according to the second embodiment; 
         FIG. 23  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a backoff operation according to the second embodiment; 
         FIG. 24  is a diagram illustrating a simulation result of loss in a power amplifier according to a reference example of the second embodiment; 
         FIG. 25  is a diagram illustrating a simulation result of the phase difference in a power amplifier according to the reference example of the second embodiment; 
         FIG. 26  is a diagram illustrating a simulation result of isolation in a power amplifier according to the reference example of the second embodiment; 
         FIG. 27  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a peak operation of a power amplifier according to the reference example of the second embodiment; 
         FIG. 28  is a diagram illustrating a locus of the impedance on the load side, as seen from the output port of a carrier amplifier, in a backoff operation of a power amplifier according to the reference example of the second embodiment; and 
         FIG. 29  is a diagram for describing an example of the efficiency of a power amplifier according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiment of the present disclosure will be described in detail below by referring to the drawings. Identical components are designated with identical reference numerals, and repeated description will be avoided as much as possible. 
     A power amplifier  100  according to a first embodiment will be described.  FIG. 1  is a circuit diagram of the power amplifier  100  according to the first embodiment. The power amplifier  100  includes a first-stage amplifier  101 , a 3-dB coupler  102 , a carrier amplifier  103 , a peak amplifier  104 , a hybrid coupler  105 , matching circuits  106  and  107 , an inductor  108 , and a capacitor  109 . The components of the power amplifier  100  may be formed on the same substrate, or may be formed on multiple substrates. 
     The first-stage amplifier  101  amplifies signal RF IN  which is received through the matching circuit  107 , and outputs signal RF 1 . The frequency of signal RF IN  is, for example, on the order of several GHz. 
     The carrier amplifier  103 , the peak amplifier  104 , and the hybrid coupler  105  form a second-stage amplifier circuit which amplifies signal RF 1  which is output from the first-stage amplifier  101 , and has a configuration similar to a typical Doherty amplifier. 
     The 3-dB coupler  102  (splitter) splits signal RF 1  (a first signal), which is output from the first-stage amplifier  101 , into signal RF 2  (a second signal) for the carrier amplifier  103  and signal RF 3  (a third signal) for the peak amplifier  104 . Signal RF 3  lags behind signal RF 2  by about 90°. The 3-dB coupler  102  is grounded through a termination resistor  1021 . The 3-dB coupler  102  may have a splitter and a phase shifter which perform 3-dB splitting and phase shifting by about 90°. 
     The carrier amplifier  103  amplifies signal RF 2  from the 3-dB coupler  102 , and outputs signal RF 4  (a fourth signal). The carrier amplifier  103  is supplied with the power supply voltage Vcc through an inductor  1031 . 
     The peak amplifier  104  amplifies signal RF 3  from the 3-dB coupler  102 , and outputs signal RF 5  (a fifth signal). The peak amplifier  104  is supplied with the power supply voltage Vcc through an inductor  1041 . 
     An example of operating characteristics of the carrier amplifier  103  and the peak amplifier  104  will be described by referring to  FIG. 2 . In  FIG. 2 , the horizontal axis represents the voltage of signal RF IN , and the vertical axis represents currents through the amplifiers. The change in a current through the carrier amplifier  103  is indicated by straight line I C . The change in a current through the peak amplifier  104  is indicated by straight line I P . 
     The carrier amplifier  103  operates regardless of the voltage level of signal RF IN . That is, the carrier amplifier  103  operates regardless of the power level of RF IN . In other words, the carrier amplifier  103  operates at such levels that the power level of RF IN  is higher than zero (a first level). 
     The peak amplifier  104  operates in a range of the voltage level of signal RF IN  which is equal to or higher than V back  that is lower than maximum level V max  by a determined level. The peak amplifier  104  operates in a range of the power level of RF IN  which is equal to or higher than a low level (a second level) that is lower than the maximum level, for example, by a determined level of 3 dB. 
     An operation performed when only the carrier amplifier  103  is turned on is called a backoff operation. An operation performed when the carrier amplifier  103  and the peak amplifier  104  are turned on is called a peak operation. 
     The hybrid coupler  105  illustrated in  FIG. 1  includes a transmission line  1051  and a transmission line  1052 . The transmission line  1051  and the transmission line  1052  are, for example, strip lines or microstrip lines disposed on or in a substrate. 
     Both of the transmission line  1051  and the transmission line  1052  are formed so as to extend in a certain direction when the power amplifier  100  is viewed in plan. 
     The transmission line  1051  is connected, at its first end, to the output of the carrier amplifier  103 . The transmission line  1051  is connected, at its second end, to the matching circuit  106 . The transmission line  1052  is connected, at its first end, to the output of the peak amplifier  104 . The transmission line  1052  is open at its second end. “To be open” herein encompasses a state, in which nothing is physically connected to an end of a transmission line, and also encompasses a case in which a transmission line is connected to a resistor or a passive device whose impedance is equal to or more than a hundredfold of the characteristic impedance of the transmission line. 
     The hybrid coupler  105  outputs, from the second end of the transmission line  1051 , signal RF 6  as an amplified signal which is obtained by combining signal RF 4  from the carrier amplifier  103  with signal RF 5  from the peak amplifier  104 . 
     The matching circuit  107  matches the impedance between the input port of the power amplifier  100  and the first-stage amplifier  101 . The matching circuit  106  matches the impedance between the second end of the transmission line  1051  and the output port of the power amplifier  100 . Signal RF 6  is output as signal RF OUT  to the outside of the power amplifier  100  through the matching circuit  106 . 
     The inductor  108  is supplied, at its first end, with the power supply voltage Vcc, and is connected, at its second end, to the output of the first-stage amplifier  101 . The capacitor  109  is connected, at its first end, to the output of the first-stage amplifier  101 , and is connected, at its second end, to the 3-dB coupler  102 . 
     Referring to  FIG. 3 , the formulation of the impedance in the power amplifier  100  will be described.  FIG. 3  is a circuit diagram illustrating circuits obtained by modeling the power amplifier  100  as a current source model. The circuits illustrated in  FIG. 3  include current sources  301  and  302 , a load resistance  303 , an inductor  304 , a capacitor  305 , a capacitor  306 , and an inductor  307 . 
     The output of the current source  301  is connected to a first end of the inductor  304  and a first end of the capacitor  306 . The output of the current source  302  is connected to a first end of the capacitor  305  and a first end of the inductor  307 . 
     The capacitor  306  is connected, at its second end, to a second end of the inductor  307 . The inductor  304  is connected, at its second end, to a second end of the capacitor  305  and a first end of the load resistance  303 . The load resistance  303  is connected, at its second end, to the current source  301  and the current source  302 . 
     The current source  301  corresponds to the carrier amplifier  103 . The current source  302  corresponds to the peak amplifier  104 . The load resistance  303  indicates the resistance of a load to which the resulting signal, which is amplified by the power amplifier  100 , is output. 
     The inductor  304  indicates the inductance of the transmission line  1051 . The inductor  307  indicates the inductance of the transmission line  1052 . The capacitor  305  and the capacitor  306  indicate the capacitance produced between the transmission line  1051  and the transmission line  1052 . Mutual inductance is produced between the inductor  304  and the inductor  307 . 
     The transmission line  1051  and the transmission line  1052  are disposed so that inductance L of each of the inductor  304  and the inductor  307 , capacitance C of each of the capacitor  305  and the capacitor  306 , and mutual inductance M between the inductor  304  and the inductor  307  satisfy Expression (1). 
     
       
         
           
             
               
                 
                   
                     L 
                     = 
                     
                       
                         R 
                         L 
                       
                       
                         ω 
                         0 
                       
                     
                   
                   , 
                   
                     C 
                     = 
                     
                       1 
                       
                         2 
                          
                         
                           R 
                           L 
                         
                          
                         
                           ω 
                           0 
                         
                       
                     
                   
                   , 
                   
                     M 
                     = 
                     
                       
                         R 
                         L 
                       
                       
                         ω 
                         0 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In Expression (1), R L  represents the impedance of the load resistance  303 ; wo represents the angular frequency corresponding to the center frequency of signal RF IN . 
     The voltage at the output of the current source  301  is represented by V C . The voltage at the output of the current source  302  is represented by V P . The current flowing from the current source  301  is represented by i C . The current flowing from the current source  302  is represented by i P . In this case, impedance Z C , as seen from the output of the current source  301 , and impedance Z P , as seen from the output of the current source  302 , are derived as follows. 
     The current flowing from the capacitor  306  to the inductor  307  is represented by ix. In this case, Expression (2) described below holds. 
     
       
         
           
             
               
                 
                   
                     
                       V 
                       C 
                     
                     - 
                     
                       V 
                       P 
                     
                   
                   = 
                   
                     
                       
                         
                           i 
                           C 
                         
                          
                         j 
                          
                         
                             
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         L 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               i 
                               C 
                             
                             + 
                             
                               i 
                               P 
                             
                           
                           ) 
                         
                          
                         
                           R 
                           L 
                         
                       
                       - 
                       
                         
                           
                             i 
                             P 
                           
                           + 
                           
                             i 
                             X 
                           
                         
                         
                           j 
                            
                           ω 
                            
                           C 
                         
                       
                       - 
                       
                         
                           ( 
                           
                             
                               i 
                               C 
                             
                             + 
                             
                               i 
                               P 
                             
                           
                           ) 
                         
                          
                         
                           R 
                           L 
                         
                       
                     
                     = 
                     
                       
                         
                           i 
                           x 
                         
                         
                           j 
                            
                           ω 
                            
                           C 
                         
                       
                       + 
                       
                         
                           i 
                           c 
                         
                          
                         j 
                          
                         ω 
                          
                         M 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The symbol j represents the imaginary unit. 
     The center component of Expression (2) indicates calculation of V C −V P  from the difference between V C  and V P  in the following manner: V C  is obtained by using a voltage decrease obtained through the current source  301 , the inductor  304 , and the load resistance  303  in this sequence; V P  is obtained by using a voltage decrease obtained through the current source  302 , the capacitor  305 , and the load resistance  303  in this sequence. 
     The right-side component of Expression (2) indicates calculation of V C −V P  by using a voltage decrease obtained through the current source  301 , the capacitor  306 , the inductor  307 , and the current source  302  in this sequence. Thus, ix is expressed in Expression (3) by using the relation between the center component and the right-side component in Expression (2). 
     
       
         
           
             
               
                 
                   
                     i 
                     X 
                   
                   = 
                   
                     - 
                     
                       
                         i 
                         P 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Therefore, voltages V C  and V P  are expressed in Expression (4) and Expression (5). 
     
       
         
           
             
               
                 
                   
                     V 
                     C 
                   
                   = 
                   
                     
                       
                         i 
                         C 
                       
                        
                       j 
                        
                       
                           
                       
                        
                       ω 
                        
                       
                           
                       
                        
                       L 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             i 
                             c 
                           
                           + 
                           
                             i 
                             p 
                           
                         
                         ) 
                       
                        
                       
                         R 
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     P 
                   
                   = 
                   
                     
                       
                         i 
                         P 
                       
                       
                         j 
                          
                         
                             
                         
                          
                         2 
                          
                         
                             
                         
                          
                         ω 
                          
                         
                             
                         
                          
                         C 
                       
                     
                     + 
                     
                       
                         ( 
                         
                           
                             i 
                             c 
                           
                           + 
                           
                             i 
                             p 
                           
                         
                         ) 
                       
                        
                       
                         R 
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Assume that ω=ω 0  in Expression (4) and Expression (5). 
     
       
         
           
             
               
                 
                   
                     Z 
                     C 
                   
                   = 
                   
                     
                       
                         V 
                         C 
                       
                       
                         i 
                         C 
                       
                     
                     = 
                     
                       
                         j 
                          
                         
                             
                         
                          
                         
                           R 
                           L 
                         
                       
                       + 
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 i 
                                 P 
                               
                               
                                 i 
                                 C 
                               
                             
                           
                           ) 
                         
                          
                         
                           R 
                           L 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
             
               
                 
                   
                     Z 
                     P 
                   
                   = 
                   
                     
                       
                         V 
                         P 
                       
                       
                         i 
                         P 
                       
                     
                     = 
                     
                       
                         
                           - 
                           j 
                         
                          
                         
                           R 
                           L 
                         
                       
                       + 
                       
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 i 
                                 C 
                               
                               
                                 i 
                                 P 
                               
                             
                           
                           ) 
                         
                          
                         
                           R 
                           L 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Assume that the power amplifier  100  performs a peak operation. That is, assume that both the carrier amplifier  103  and the peak amplifier  104  operate. The amplitude of i C  and that of i P  are represented by I C  and I P , respectively. In consideration of the phase difference produced by the 3-dB coupler  102 , i C =I C  and i P =−jI P  hold. The final impedance is expressed in Expression (8) and Expression (9) by using Expression (6) and Expression (7). 
     
       
         
           
             
               
                 
                   
                     Z 
                     C 
                   
                   = 
                   
                     
                       R 
                       L 
                     
                     + 
                     
                       j 
                        
                       
                           
                       
                        
                       
                         
                           R 
                           L 
                         
                          
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 I 
                                 P 
                               
                               
                                 I 
                                 C 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     Z 
                     P 
                   
                   = 
                   
                     
                       R 
                       L 
                     
                     + 
                     
                       j 
                        
                       
                           
                       
                        
                       
                         
                           R 
                           L 
                         
                          
                         
                           ( 
                           
                             
                               
                                 I 
                                 C 
                               
                               
                                 I 
                                 P 
                               
                             
                             - 
                             1 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     As a reference example, the impedance in a power amplifier  1100  illustrated in  FIG. 11  will be described. The power amplifier  1100  is different from the power amplifier  100  in that the power amplifier  1100  includes a phase shifter  1101  having an inductor  1102 , a phase shifter  1103  having a capacitor  1104 , and a combining unit  1105 . The power amplifier  1100  functions as a Doherty amplifier. 
       FIG. 12  illustrates a calculation model of the impedance of the power amplifier  1100 . Inductance L of the inductor  1102  and capacitance C of the capacitor  1104  satisfy Expression (10). 
     
       
         
           
             
               
                 
                   
                     L 
                     = 
                     
                       
                         R 
                         L 
                       
                       
                         ω 
                         0 
                       
                     
                   
                   , 
                   
                     C 
                     = 
                     
                       1 
                       
                         
                           R 
                           L 
                         
                          
                         
                           ω 
                           0 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     In this case, impedance Z C , as seen from the current source  301  toward the load resistance  303 , and impedance Z P , as seen from the current source  302  toward the load resistance  303 , have the same results in Expression (8) and Expression (9) through calculation similar to that in the model of the power amplifier  100 . 
     In addition to the calculation using the model, simulation results will be described by comparing  FIGS. 4 to 8  with  FIGS. 13 to 17 . The simulation uses an ideal model without necessarily consideration of the resistance in the transmission line  1051  and the transmission line  1052 . 
       FIG. 4  is a graph indicating loss in the power amplifier  100 . Curve L 1  indicates the insertion loss of the output of the carrier amplifier  103 . It shows that, as the frequency increases, the loss increases due to the inductance of the transmission line  1051 . For example, the value of the insertion loss at a frequency of 3.75 GHz in curve L 1  is −3.010 dB. Curve L 2  indicates the insertion loss of the output of the peak amplifier  104 . It shows that, as the frequency increases, the loss decreases due to the capacitance, which behaves like a lead wire, between the transmission line  1051  and the transmission line  1052 . For example, the value of the insertion loss at a frequency of 3.70 GHz is −3.069. 
     The reference example corresponding to  FIG. 4  is the graph illustrating loss in the power amplifier  1100  in  FIG. 13 . Curve L 3  indicates the insertion loss of the output of the carrier amplifier  103  in the power amplifier  1100 . Curve L 4  indicates the insertion loss of the output of the peak amplifier  104  in the power amplifier  1100 . Curve L 3  behaves like curve L 1 , and curve L 4  behaves like curve L 2 . 
       FIG. 5  is a graph illustrating the phase difference in the power amplifier  100  which is obtained by subtracting the phase of a signal, which is output from the carrier amplifier  103  through the hybrid coupler  105 , from the phase of a signal which is output from the peak amplifier  104  through the hybrid coupler  105 . 
     The phase difference in the graph in  FIG. 5  is 90°. That is, the hybrid coupler  105  functions as a 90° hybrid coupler. Signal RF 5 , which is output from the peak amplifier  104 , lags behind signal RF 4 , which is output from the carrier amplifier  103 , by about 90°. Thus, the phase difference of 90° indicates that the signals are output as an in-phase signal from the hybrid coupler  105 . 
     The reference example corresponding to  FIG. 5  is the graph illustrating the phase difference, which is calculated similarly in the power amplifier  1100 , in  FIG. 14 . The phase difference in  FIG. 14  is also 90°. The output from the combining unit  1105  is in phase. 
       FIG. 6  is a graph illustrating the isolation of signal RF 4 , which is input to the transmission line  1051 , in the power amplifier  100 . That is,  FIG. 6  is a graph illustrating the degree of leakage of signal RF 4  to the transmission line  1052 . In  FIG. 6 , the isolation is −6.021 dB at a frequency of 3.75 GHz. 
     The reference example corresponding to  FIG. 6  is the graph illustrating the isolation, which is calculated similarly, in the power amplifier  1100  in  FIG. 15 . In  FIG. 15 , the isolation changes as in  FIG. 6 . 
       FIG. 7  is a Smith chart for describing the impedance as seen from the carrier amplifier  103  toward the output side in the case where the power amplifier  100  performs a peak operation.  FIG. 8  is a Smith chart for describing the impedance as seen from the carrier amplifier  103  toward the output side in the case where the power amplifier  100  performs a backoff operation. Curve S 1  in  FIG. 7  and curve S 2  in  FIG. 8  indicate loci obtained when the frequency changes from 500 MHz to 20 GHz. 
     The reference example corresponding to  FIG. 7  is  FIG. 16 , and the reference example corresponding to  FIG. 8  is  FIG. 17 .  FIG. 16  is a Smith chart for describing the impedance which is obtained similarly in the case where the power amplifier  1100  performs a peak operation.  FIG. 17  is a Smith chart for describing the impedance which is obtained similarly in the case where the power amplifier  1100  performs a backoff operation. Curve S 3  in  FIG. 16  and curve S 4  in  FIG. 17  indicate loci obtained when the frequency changes from 500 MHz to 20 GHz. 
     It is shown that, in the Smith charts in  FIG. 16  and  FIG. 17 , the loci change as in the Smith charts in  FIG. 7  and  FIG. 8 . 
       FIGS. 4 to 8  and  FIGS. 13 to 17  indicate that the power amplifier  100  operates like the power amplifier  1100  which functions as a Doherty amplifier. Therefore, the power amplifier  100  functions as a Doherty amplifier. 
       FIG. 9  illustrates the efficiency in the power amplifier  100 , which is obtained in the case where the horizontal axis represents the decibel value relative to the peak power of an input signal, that is, the backoff value.  FIG. 9  shows that the power amplifier  100  performs a peak operation when the backoff amount exceeds 3 dB, functioning as a Doherty amplifier which maintains high efficiency. 
       FIG. 10  is a diagram illustrating a modified example of the hybrid coupler  105 . A hybrid coupler  105 A includes a dielectric  1001  between the transmission line  1051  and the transmission line  1052 . The dielectric  1001  is, for example, a dielectric layer which has a high relative permittivity. The hybrid coupler  105 A having a metal-insulator-metal (MIM) structure may shorten the length of the transmission line  1051  and the transmission line  1052  for achieving a determined capacitance between the transmission line  1051  and the transmission line  1052 . 
     A second embodiment will be described. Points common to the first embodiment will not be described in the second embodiment, and only different points will be described. Especially, similar effects caused by similar configurations will not be described in each embodiment.  FIG. 18  illustrates a circuit diagram of a power amplifier  1800  according to the second embodiment. 
     The power amplifier  1800  is different from the power amplifier  100  in that a splitter  1801  is included between the first-stage amplifier  101 , and the carrier amplifier  103  and the peak amplifier  104 . 
     The splitter  1801  includes capacitors  1802 ,  1803 ,  1804 , and  1805 , a resistance device  1806 , and inductors  1807  and  1808 . 
     The capacitor  1802  is connected, at its first end, to the output of the first-stage amplifier  101 , and is connected, at its second end, to a first end of the resistance device  1806 . The capacitor  1803  is connected, at its first end, to the output of the first-stage amplifier  101 , and is connected, at its second end, to a second end of the resistance device  1806 . The resistance device  1806  is disposed between the capacitor  1802  and the capacitor  1803 . 
     The capacitor  1804  is connected, at its first end, to the second end of the capacitor  1802 , and is connected, at its second end, to the input of the carrier amplifier  103 . The inductor  1807  is connected, at its first end, between the capacitor  1804  and the second end of the capacitor  1802 , and is connected, at its second end, to the ground. The capacitor  1804  and the inductor  1807  function as a phase shifter. 
     The capacitor  1805  is connected, at its first end, to the second end of the capacitor  1803 , and is connected, at its second end, to the ground. The inductor  1808  is connected, at its first end, to the first end of the capacitor  1805 , and is connected, at its second end, to the input of the peak amplifier  104 . The capacitor  1805  and the inductor  1808  function as a phase shifter. 
     The splitter  1801  splits signal RF 1  from the first-stage amplifier  101  into signal RF 2  and signal RF 3 . 
     The capacitor  1805  and the inductor  1808  change the phase of signal RF 3  so that the change amount of the phase of signal RF 2  and the positive-negative direction of signal RF 2 , which are produced by the capacitor  1804  and the inductor  1807 , are symmetric to those of signal RF 3 . 
     The parameters of the circuit devices in the splitter  1801  are set so that signal RF 3  lags behind signal RF 2  by −2θ° where θ, which is expressed in Expression (11), represents a change in the phase of signal RF 2  which is produced by the capacitor  1804  and the inductor  1807 . The value, 2θ, changes in a range which is equal to or greater than 90° and less than 180°. 
     
       
         
           
             
               
                 
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     The impedance of the power amplifier  1800  will be described. The model for describing the impedance of the power amplifier  1800  is the same as that in  FIG. 3 . Like the description about the power amplifier  100 , impedance Z C , as seen from the output of the current source  301 , and impedance Z P , as seen from the output of the current source  302 , are expressed in Expression (6) and Expression (7). 
     When a phase lag which is expressed by using Expression (11) occurs, i C  and i P  are expressed in Expression (12) where the amplitudes of i C  and i P  are represented by I C  and I P , respectively. 
     
       
         
           
             
               
                 
                   
                     
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     By using Expression (12), Expression (6), and Expression (7), the impedance is expressed finally in Expression (13) and Expression (14). 
     
       
         
           
             
               
                 
                   
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       FIGS. 19 to 23  illustrate simulation results obtained when θ=60°, that is, when signal RF 3  lags behind signal RF 2  by 120°.  FIGS. 19 to 23  will be described by referring to  FIGS. 24 to 28 . 
     The results in  FIGS. 19 to 23  are obtained when, like the power amplifier  1100  in  FIG. 11 , the phase shifter  1101 , the phase shifter  1103 , and the combining unit  1105 , instead of the hybrid coupler  105 , are used to combine the outputs in the power amplifier  1800  illustrated in  FIG. 18 . The parameters of the phase shifter  1101  and the phase shifter  1103  are values different from those used in the case where the phase lag is 90°. 
       FIG. 19  is a graph illustrating loss in the power amplifier  1800 . Curve L 5  indicates the insertion loss of the output of the carrier amplifier  103 . It shows that, as the frequency increases, the loss increases due to the inductance of the transmission line  1051 . For example, the value of the insertion loss at a frequency of 3.75 GHz in curve L 5  is −4.090 dB. Curve L 6  indicates the insertion loss of the output of the peak amplifier  104 . It shows that, as the frequency increases, the capacitance between the transmission line  1051  and the transmission line  1052  behaves like a lead wire and the loss decreases. For example, the value of the insertion loss at a frequency of 3.70 GHz is −3.42. 
     The reference example corresponding to  FIG. 19  is the graph illustrating the loss in  FIG. 24 . Curve L 7  indicates the insertion loss of the output of the carrier amplifier  103 . Curve L 8  indicates the insertion loss of the output of the peak amplifier  104 . 
       FIG. 20  is a graph illustrating the phase difference obtained by subtracting the phase of a signal, which is output from the carrier amplifier  103  through the hybrid coupler  105 , from the phase of a signal which is output from the peak amplifier  104  through the hybrid coupler  105 , in the power amplifier  1800 . 
     In the graph in  FIG. 20 , the phase difference is approximately 120° in the frequency range between 2 GHz and 6 GHz. That is, the hybrid coupler  105  functions as a coupler which combines signals whose phase difference is 120°. It is shown that, since signal RF 5 , which is output from the peak amplifier  104 , lags behind signal RF 4 , which is output from the carrier amplifier  103 , by about 120°, the phase difference of about 120° causes an almost in-phase signal to be output from the hybrid coupler  105 . 
     The reference example corresponding to  FIG. 20  is the graph, in  FIG. 25 , illustrating the phase difference calculated similarly by using a power amplifier which does not include the hybrid coupler  105 . Also, in  FIG. 25 , the phase difference is approximately 120°, and the output is substantially in phase. 
       FIG. 21  is a graph illustrating the isolation of signal RF 4  which is input to the transmission line  1051 , in the power amplifier  1800 . That is,  FIG. 21  is a graph illustrating the degree of leakage of signal RF 4  to the transmission line  1052 . In  FIG. 21 , the isolation is −7.910 dB at a frequency of 3.75 GHz. 
     The reference example corresponding to  FIG. 21  is the graph about the isolation in  FIG. 26 . In  FIG. 26 , the isolation changes as in  FIG. 21 . 
       FIG. 22  is a Smith chart for describing the impedance as seen from the carrier amplifier  103  toward the output side in the case where the power amplifier  1800  performs a peak operation.  FIG. 23  is a Smith chart for describing the impedance as seen from the carrier amplifier  103  toward the output side in the case where the power amplifier  1800  performs a backoff operation. Curve S 5  in  FIG. 22  and curve S 6  in  FIG. 23  indicate loci obtained when the frequency changes from 500 MHz to 20 GHz. 
     The reference example corresponding to  FIG. 22  is  FIG. 27 , and the reference example corresponding to  FIG. 23  is  FIG. 28 .  FIG. 27  is a Smith chart for describing the impedance obtained in a peak operation.  FIG. 28  is a Smith chart for describing the impedance obtained in a backoff operation. Curve S 7  in  FIG. 27  and curve S 8  in  FIG. 28  are loci obtained when the frequency changes from 500 MHz to 20 GHz. 
     It is shown that, in the Smith charts in  FIG. 27  and  FIG. 28 , the loci change as in the Smith charts in  FIG. 22  and  FIG. 23 . 
       FIGS. 19 to 23  and  FIGS. 24 to 28  indicate that the power amplifier  1800  operates like a power amplifier which functions as a Doherty amplifier. Therefore, the power amplifier  1800  functions as a Doherty amplifier. 
       FIG. 29  illustrates the efficiency obtained in the power amplifier  1800  in the case where θ takes multiple values and where the horizontal axis represents the decibel value relative to the peak power of an input signal, that is, the backoff value. Curve E 2  indicates the case in which 2θ=90°; curve E 3  indicates the case in which 2θ=109°; curve E 4  indicates the case in which 2θ=120°. The power amplifier  1800  performs a peak operation when the backoff amount is equal to or greater than 3 dB. This causes a backoff amount to be provided even when the peak-to-average power ratio (PAPR) is large, enabling the power amplifier  1800  to function as a Doherty amplifier which maintains high efficiency. 
     Exemplary embodiment of the present disclosure is described above. The power amplifier  100  includes the 3-dB coupler  102 , the carrier amplifier  103 , the peak amplifier  104 , and the hybrid coupler  105 . The 3-dB coupler  102  splits signal RF 1  into signal RF 2  and signal RF 3  that lags behind signal RF 2  by about 90°. The carrier amplifier  103  amplifies signal RF 2  in a range in which the power level of signal RF 1  is equal to or higher than the first level, and outputs signal RF 4 . The peak amplifier  104  amplifies signal RF 3  in a range in which the power level of signal RF 1  is equal to or higher than the second level that is higher than the first level, and outputs signal RF 5 . The hybrid coupler  105  includes the transmission line  1051  and the transmission line  1052 . The transmission line  1051  receives signal RF 4  at its first terminal. The transmission line  1052  receives signal RF 5  at its first terminal. The transmission line  1052  is open at its second terminal. The transmission line  1051  outputs, from its second terminal, an amplified signal of signal RF 1  obtained by combining signal RF 4  with signal RF 5 . 
     Thus, the power amplifier  100  functions as a power amplifier which amplifies signal RF 1  with high efficiency with the on state of the peak amplifier  104  being switched in accordance with the power level of an amplified signal. The power amplifier  100 , which combines power by using the hybrid coupler  105 , enables influence from the parasitic component to be reduced compared with a Doherty amplifier which includes an inductor device. 
     In the power amplifier  100 , the hybrid coupler  105  may include the dielectric  1001  between the transmission line  1051  and the transmission line  1052 . 
     This enables the capacitance value between the lines to be made higher than the case in which the dielectric  1001  is not present and in which there is a free space. When the transmission line  1051  and the transmission line  1052  are regarded as a single capacitance device, the area, which is required to obtain a certain capacitance value, of the facing surfaces of the transmission lines may be made small. If the thickness of a transmission line is constant, the length of the transmission lines may be made short, enabling a reduction in the size of the hybrid coupler  105  to be achieved. 
     The hybrid coupler  105 , the carrier amplifier  103 , and the peak amplifier  104  may be disposed on the same substrate. This enables the circuit to be made further smaller while influence from the parasitic component in wiring for connecting the circuit device is reduced. 
     The power amplifier  1800  according to the second embodiment includes the splitter  1801 , the carrier amplifier  103 , the peak amplifier  104 , and the hybrid coupler  105 . The splitter  1801  splits signal RF 1  into signal RF 2  and signal RF 3  which lags behind signal RF 2 . The carrier amplifier  103  amplifies signal RF 2  in a range in which the power level of signal RF 1  is equal to or higher than the first level, and outputs signal RF 4 . The peak amplifier  104  amplifies signal RF 3  in a range in which the power level of signal RF 1  is equal to or higher than the second level which is higher than the first level, and outputs signal RF 5 . The hybrid coupler  105  includes the transmission line  1051  and the transmission line  1052 . The transmission line  1051  receives signal RF 4  at its first terminal. The transmission line  1052  receives signal RF 5  at its first terminal. The transmission line  1052  is open at its second terminal. The transmission line  1051  outputs, from its second terminal, an amplified signal of signal RF 1  which is obtained by combining signal RF 4  with signal RF 5 . 
     This enables the phase lag of signal RF 3  to be set appropriately by using the splitter  1801 . If the phase lag is greater than about 90°, the backoff amount is 3 dB or greater. This enables the power amplifier  1800  to function as a power amplifier which reduces influence from the parasitic component, and which, at the same time, amplifies signal RF 1  with high efficiency even if signal RF 1  has a high PAPR. 
     In the power amplifier  1800 , signal RF 3  may lag behind signal RF 2  by about 120°. This causes the backoff amount to be 6 dB, and the power amplifier  1800  may amplify signal RF 1  with high efficiency even if signal RF 1  has a high PAPR. 
     The embodiments described above are provided for ease of understanding of the prevent disclosure, not for limited interpretation of the present disclosure. The present disclosure may be changed/improved without necessarily departing from the gist of the present disclosure, and encompasses its equivalence. That is, embodiments, which are obtained by those skilled in the art changing the design of the embodiments appropriately and which have the characteristics of the present disclosure, are encompassed in the scope of the present disclosure. For example, the components and their positions, the material, the condition, the shape, the size, and the like which are included in the embodiments are not limited to those illustrated, and may be changed appropriately. Needless to say, the embodiments are exemplary, and partial replacement or combination of the configurations illustrated in different embodiments may be made. Such embodiments, which have the characteristics of the present disclosure, are encompassed in the scope 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.