Abstract:
[Problem] To provide an amplifier and an amplification method in which degradation in the quality of the output signal is reduced. [Solution] This amplifier is characterized in having: an amplification unit; a power supply modulator for determining the modulation voltage applied to the amplification unit according to an input signal inputted into the amplification unit; a first predistorter for modeling the characteristics of the amplification unit and performing distortion compensation with respect to the amplification unit; a first controller for controlling the parameters of the first predistorter on the basis of an input signal inputted into the first predistorter and an output signal from the amplification unit; a second predistorter for modulating an input signal inputted into the power supply modulator; and a second controller for controlling the second predistorter on the basis of the input signal inputted into the power supply modulator and a signal from which the RF component of the drain voltage of an FET of the amplification unit has been removed; the amplifier performing a correction such that the signal from which the RF component of the drain voltage of the FET has been removed is linear.

Description:
This application is a National Stage Entry of PCT/JP2013/002112 filed on Mar. 28, 2013, which claims priority from Japanese Patent Application 2012-081595 filed on Mar. 30, 2012, the contents of all of which are incorporated herein by reference, in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an amplifier and an amplification method. 
     BACKGROUND ART 
     Modulation formats such as QPSK (Quadrature Phase Shift Keying) and Multilevel QAM (Quadrature Amplitude Modulation) are adopted as a digital modulation scheme used in today&#39;s wireless communications. In these modulation formats, transition between symbols entails amplitude modulation. Accordingly, the amplitude of transmission signals modulated by these modulation formats changes with time. A transmission signal combined with a carrier signal is referred to as an input signal. Research and development on amplifiers amplifying an input signal when the input signal is transmitted is being conducted. Note that the input signal is sometimes referred to as an RF (Radio Frequency) signal. 
     Polar modulation is known as a scheme that amplifies an input signal with high efficiency so that the output signal intensity widely changes in a dynamic range. Known examples of polar modulation schemes are EER (Envelope Elimination and Restoration) and ET (Envelope Tracking). In the EER, first a transmission signal is split into a phase component and an amplitude component. The phase component that has a certain amplitude is input into an amplification unit. The amplification unit operates around the saturation point at which the maximum efficiency is achieved. On the other hand, the amplitude component is input into a power supply modulator, where the amplitude component is amplified. The output voltage from the power supply modulator is used as the power supply for the amplification unit. The amplification unit having such a configuration acts as a multiplier that combines the phase and amplitude components of a transmission signal. 
     In the ET scheme, on the other hand, the amplitude component of a transmission signal is amplified by a power supply modulator and the output voltage from the power supply modulator is used as the power supply for the amplification unit. Only a phase-modulated signal that has a certain amplitude is input into the amplification unit in the EER scheme whereas a transmission signal including both of the amplitude component and the phase component is input in the amplification unit in the ET scheme. The ET scheme is less efficient in amplification than the EER scheme. However, the ET scheme does not require high accuracy of timing of combining the amplitude and phase components in the amplification unit. Accordingly, the ET scheme can be implemented more easily than the EER scheme. 
     As a related technique, an ET amplifier is disclosed in Patent Literature 1. 
     An amplifier according to the related technique and voltage waveforms around a power supply modulator and an amplitude circuit will be described with reference to  FIG. 8 . The amplifier includes an amplification unit  100 , a power supply modulator  300  and a load  1900 . The amplification unit  100  includes a choke coil  101 , an amplification circuit  102  and a matching circuit  103 . A configuration in which an FET (field effect transistor) is used as the amplification circuit  102  will be described as an example. 
     The power supply modulator  300  amplifies an input signal and output a voltage. The choke coil  101  inhibits high-frequency components included in current input in the amplification unit  100  that has a carrier frequency from passing through the choke coil  101 . The amplification circuit  102  allows current amplified in proportion to current input at the gate terminal on the basis of power supplied from the power supply modulator  300  to flow from the drain terminal to the source terminal. The matching circuit  103  performs impedance matching for the amplification circuit  102 . 
     An operation of the amplifier according to the related technique will be described. 
     First, the power supply modulator  300  amplifies the voltage  3000  of an envelope signal extracted from a transmission signal and outputs an amplified voltage  3100 . In the amplification circuit  102 , on the other hand, current amplified in proportion to an input signal input into the gate terminal on the basis of power supplied from the power supply modulator  300  flows from the drain terminal to the source terminal. In the load  1900 , a voltage proportional to the current flowing from the drain terminal to the source terminal is output. The voltage  3200  at the drain terminal is equal to the sum of a voltage  3400   a  having a frequency component nearly equal to that of the amplified voltage  3100  and a voltage  3400   b  having a carrier frequency component. 
     As shown in  FIG. 8 , the power supply modulator  300  amplifies the voltage  3000  of the envelope signal and outputs the amplified voltage  3100 . If the value of the impedance of the choke coil  101  can be assumed to be negligible in the frequency band of the amplified voltage  3100 , the amplified voltage  3100  and the voltage  3400   a  will have the same waveform. In other words, the voltage  3000  of the envelope signal is proportional to the voltage  3400   a .  FIGS. 9A and 9B  show the relationship between the voltage  3000  of the envelope signal and the voltage  3400   a . Ideally, the voltage  3000  of the envelope signal is proportional to the voltage  3400   a  as in  FIG. 9A . However, a certain degree of deviation from the proportionality was expected since the gain of an operational amplifier or the like that constitutes the power supply modulator  300  is frequency dependent. 
     CITATION LIST 
     Patent Literature 
     
         
         [Patent Literature 1] Japanese Laid-open Patent No. 2010-74679 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, experiments conducted by the inventor have found that more measurement values of the voltage  3000  of the envelope signal and the voltage  3400   a  deviated from the proportionality than expected.  FIG. 9B  shows the distribution of measurement values of the voltage  3000  of the envelope signal and the voltage  3400   a . The deviation from the proportionality has prevented proper control of the voltage  3400   a  corresponding to the input signal. There was the problem of degradation of the quality of an output signal of the amplification circuit  102 . 
     An object of the present invention is to solve the above-mentioned problem and provide an amplifier and an amplification method by which degradation of the quality of an output signal is reduced. 
     Solution to Problem 
     An amplifier according to the present invention includes an amplification unit, a power supply modulator determining a modulation voltage to be applied to the amplification unit according to an input signal input into the amplification unit, a first predistorter modeling a characteristic of the amplification unit and compensating for distortion in the amplification unit, a first controller controlling a parameter of the first predistorter on the basis of an input signal input into the first predistorter and an output signal from the amplification unit, a second predistorter modulating an input signal input into the power supply modulator, and a second controller controlling the second predistorter on the basis of a signal from which a radio frequency component of a drain voltage of a field effect transistor in the amplification unit has been removed and the input signal input into the power supply modulator, wherein correction is performed such that the signal from which the radio frequency component of the drain voltage of the field effect transistor has been removed is linearized. 
     An amplification method according to the present invention is the amplification method for amplifying an input signal input into an amplification unit, the amplification method including determining, in a power supply modulator, a modulation voltage to be applied to the amplification unit according to an input signal input into the amplification unit, controlling a parameter of a first predistorter which models a characteristic of the amplification unit to compensate for distortion in the amplification unit, on the basis of an input signal input into the first predistorter and an output signal from the amplification unit, modulating, in a second predistorter, an input signal input into the power supply modulator, controlling the second predistorter on the basis of a signal from which a radio frequency component of a drain voltage of an field effect transistor in the amplification unit has been removed and the input signal input into the power supply modulator, and performing correction such that the signal from which the radio frequency component of the drain voltage of the field effect transistor has been removed is linearized. 
     Advantageous Effects of Invention 
     The present invention can provide an amplifier and an amplification method by which degradation of the quality of an output signal is reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of functional blocks of an amplifier according to a first embodiment; 
         FIG. 2  shows an example of functional blocks of an amplifier according to a second embodiment; 
         FIG. 3  shows an example of functional blocks of an amplifier according to a third embodiment; 
         FIG. 4  shows functional blocks of a modified example of the amplifier according to the third embodiment; 
         FIG. 5  shows functional blocks of a modified example of the amplifier according to the third embodiment; 
         FIG. 6  Shows an example of circuit of an LPF  1500 ; 
         FIG. 7  shows an example of functional blocks of an amplifier according to a fourth embodiment; 
         FIG. 8  shows an amplifier according to a related technique and voltage waveforms near a power supply modulator and an amplification circuit; 
         FIG. 9A  shows an example of the relationship between the voltage  3300  of an envelope signal and voltage  3400   a;    
         FIG. 9B  shows an example of the relationship between the voltage  3300  of an envelope signal and voltage  3400   a;    
         FIG. 10A  shows a relational expression relating to an example of control by a controller; 
         FIG. 10B  shows a relational expression relating to an example of control by the controller; 
         FIG. 10C  shows a relational expression relating to an example of control by the controller; 
         FIG. 10D  shows a relational expression relating to an example of control by the controller; 
         FIG. 11A  shows a relational expression relating to an example of control by a shaping unit; 
         FIG. 11B  shows a relational expression relating to an example of control by the shaping unit; and 
         FIG. 12  is a flowchart showing an example of operation of the amplifier according to the first embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The best mode for carrying out the present invention will be described in detail with reference to drawings. However, the present invention is not limited to the embodiments described below. 
     The First Embodiment 
     Referring to  FIG. 1 , a configuration of an amplifier according to the first embodiment will be described. 
     The amplifier according to this embodiment includes an amplification unit  100 , a power supply modulator  300 , a first predistorter  600 , a first controller  700 , a second predistorter  200 , and a second controller  400 . The power supply modulator  300  determines a modulation voltage to be applied to the amplification unit according to an input signal input into the amplification unit. The first predistorter  600  models a characteristic of the amplifier and compensates for distortion in the amplification unit. The first controller  700  controls parameters of the first predistorter on the basis of the input signal input into the first predistorter and the output signal from the amplification unit. The second predistorter  200  modulates an input signal input into the power supply modulator. The second controller  400  controls the second predistorter  200  on the basis of a signal from which the RF component of the drain voltage of an FET (Field Effect Transistor)  102  in the amplification unit  100  has been removed and the input signal input into the power supply modulator  300 . The amplifier according to this embodiment performs a correction such that the signal from which the RF component of the drain voltage of the FET  102  has been removed is linearized. 
     The amplifier according to this embodiment reduces degradation of the quality of an output signal. 
     The Second Embodiment 
     Referring to  FIG. 2 , a configuration of an amplifier according to a second embodiment will be described. 
     The amplifier includes an amplification unit  100 , a predistorter  200 , a power supply modulator  300  and a controller  400 . 
     An input signal is input into the amplification unit  100 . The amplification unit  100  amplifies the input signal and outputs the amplified input signal. 
     The amplifier according to this embodiment further includes a filter  500 . The amplification unit  100  includes a choke coil  101  and an amplification circuit  102 . A configuration using an FET as the amplification circuit  102  will be described as an example. 
     The amplification unit  100  according to this embodiment further includes a matching circuit  103 . 
     The amplifier according to this embodiment further includes a predistorter  600 , a controller  700 , an up-converter  800 , a down-converter  900 , a shaping unit  1000 , an envelope extractor  2000 , a branching unit  2100 , and a load  1900 . The amplification unit  100  further includes a matching circuit  104 , a choke coil  105  and a power supply  106 . 
     The predistorter  200  adds distortion to an input electrical signal and outputs the electrical signal to which the distortion has been added. In this embodiment, an envelope signal extracted from a transmission signal is input into the predistorter  200 . The predistorter  200  adds distortion to the envelope signal. The predistorter  200  outputs the envelope signal to which the distortion has been added. The input signal is a signal obtained by combining the transmission signal and a carrier signal. The distortion of the electrical signal includes a change in the amplitude or phase of the signal at a given time instant. 
     The power supply modulator  300  amplifies an input signal and outputs a voltage. In this embodiment, the envelope signal to which distortion has been added by the predistorter  200  is input into the power supply modulator  300 . The power supply modulator  300  amplifies the envelope signal to which distortion has been added and outputs a voltage. The power supply modulator  300  supplies power to the amplification unit  100  on the basis of the envelope signal to which distortion has been added. The power supply modulator  300  may be implemented by a combination of an operational amplifier and switching, for example. 
     The controller  400  controls distortion added by the predistorter  200  on the basis of an output signal from the amplification unit  100  and the envelope signal to which distortion has been added by the predistorter  200 . In this embodiment, voltage  3200  and/or voltage  3500  in  FIG. 8  is referred to as the output signal from the amplification unit  100 . 
     The choke coil  101  is connected between the output terminal of the power supply modulator  300  and the drain terminal of the amplification circuit  102 . The choke coil  101  inhibits high-frequency components included in current flowing from the power supply modulator  300  to the amplification circuit  102  that has a carrier frequency from passing through the choke coil  101 . In this way, the power supply modulator  300  supplies power to the amplification circuit  102  through the choke coil.  101 . 
     The amplification circuit  102  is supplied with power from the power supply modulator  300  through the choke coil  101 . The amplification circuit  102  amplifies an input signal. The amplification circuit  102  generates an output signal on the basis of the amplified input signal and outputs the output signal. Specifically, current amplified in proportion to current corresponding to an input signal input at the gate terminal flows from the drain terminal to the source terminal. A voltage proportional to the current flowing from the drain terminal to the source terminal is output in a load not depicted. 
     The filter  500  is connected between the drain terminal of the amplification circuit  102  and the controller  400 . The filter  500  attenuates the voltage of a component that has a carrier frequency contained in the output signal output from the amplification circuit  102 . The filter  500  outputs the output signal from which the above component has been attenuated to the controller  400 . 
     The matching circuit  103  is connected to the output terminal of the amplification circuit  102  and the load not depicted. The matching circuit  103  performs impedance matching for the amplification circuit  102 . The amplification circuit  102  outputs an output signal to the load not depicted, through the matching circuit  103 . The controller  400  controls distortion added by the predistorter  200  on the basis of the output signal input from the amplification circuit  102  into the matching circuit  103  and an envelope signal that has been extracted from a transmission signal and to which distortion has been added by the predistorter  200 . Specifically, the output signal input from the amplification circuit  102  into the matching circuit  103  is input into the controller  400  through the filter  500 . The controller  400  controls distortion added by the predistorter  200  on the basis of the output signal from the filter  500  and the envelope signal to which distortion has been added by the predistorter  200 . 
     Next, a procedure for controlling the predistorter  200  performed by the controller  400  on the basis of an output signal from the amplification unit  100  and an envelope signal that has been extracted from a transmission signal and to which distortion has been added by the predistorter  200  will be described as a specific example. Here, an example will be described with use of an indirect learning algorithm. 
     An envelope signal input into the predistorter  200  is denoted by x i , the envelope signal to which distortion has been added by the predistorter  200  is denoted by u i , an output signal output from the amplification unit  100  is denoted by y i , the order of a polynomial model is denoted by j and a time-series tap is denoted by i. Here, the predistorter  200  adds distortion to the envelope signal input into the predistorter  200  according to a relational expression including a parameter g shown in  FIG. 10A . Note that the parameter g is determined so that the parameter g satisfies the relational expression shown in  FIG. 10B . The parameter g can be obtained by using the least-squares method, the gradient method or the like. The controller  400  determines the parameter in this way. The controller  400  uses the determined parameter to control distortion added by the predistorter  200 . The predistorter  200  adds the inverse of distortion of the output signal that occurs in a circuitry subsequent to the power supply modulator  300  to the envelope signal by the above processing. 
     Note that the predistorter  200  may control distortion according to a relational expression shown in  FIG. 10C , for example, in order to compensate for distortion due to a memory effect of the amplification unit  100 . The relational expression shown in  FIG. 10C  is the relational expression to which the contribution of the tap is added to the relational expression shown in  FIG. 10A . The parameter g can be determined so that the parameter g satisfies a relational expression shown in  FIG. 10D , for example. 
     An operation of the amplifier according to this embodiment will be described with reference to a flowchart in  FIG. 12 . 
     First, an input signal which is a transmission signal combined with a carrier signal, and an envelope signal extracted from the input signal are input into the amplifier. The envelope signal is input into the predistorter  200 . The predistorter  200  adds distortion to the input envelope signal (step S 1  of  FIG. 12 ). The predistorter  200  outputs the envelope signal to which the distortion has been added. The power supply modulator  300  increases or decreases a power supply voltage to be applied to the amplification unit  100  in accordance with the amplitude of the envelope signal to which the distortion has been added by the predistorter  200  (step S 2 ). The amplification unit  100  amplifies the input signal (step S 3 ). Specifically, the amplification unit  100  amplifies the input signal on the basis of the power supply voltage supplied from the power supply modulator  300 . The envelope signal to which distortion has been added by the predistorter  200  and an output signal from the amplification unit  100  are input into the controller  400 . The controller  400  controls distortion added by the predistorter  200  on the basis of the output signal from the amplification unit  100  and the envelope signal (step S 4 ). 
     Causes of degradation of the quality of an output signal will be discussed below. 
     Deviations from the proportionality of relationship between the voltage  3000  of the envelope signal and the voltage  3400   a  shown in  FIG. 9B  result from characteristics of the power supply modulator  300  and the subsequent circuitry. For example, the gain of the power supply modulator  300  is frequency dependent and therefore the amplitude characteristics of the power supply modulator  300  vary depending on frequency. Furthermore, the impedance of the choke coil  101  is not zero in the frequency band of the amplified voltage  3100 . As the communication bandwidths increase, the frequency bands of envelope signals are expanding. Deviations from the proportionality caused by the characteristics described above are becoming more and more significant. 
     A transmission signal is input into the predistorter  600 . The predistorter  600  adds distortion to the input transmission signal. The predistorter  600  outputs the transmission signal to which the distortion has been added to the branching unit  2100 . Specifically, the predistorter  600  adds distortion which is controlled by the controller  700  to the input transmission signal. A specific example of procedure for adding distortion by the predistorter  600  will be described later. 
     The branching unit  2100  splits the transmission signal to which the distortion has been added by the predistorter  600  into two and outputs one of the signals to the envelope extractor  2000  and the other signal to the up-converter  800 . 
     The up-converter  800  combines the transmission signal to which the distortion has been added by the predistorter  600  with a carrier signal not depicted, to generate an input signal. The up-converter  800  outputs the input signal. The transmission signal is up-converted according to the procedure described above. 
     The down-converter  900  combines an output signal from the amplification unit  100  with a carrier signal not depicted. The down-converter  900  down-converts the output signal according to the procedure described above. The down-converter  900  outputs the down-converted signal to the controller  700 . The up-converter  800  and the down-converter  900  are implemented by commonly-used mixers. 
     The envelope extractor  2000  extracts an envelope component from the transmission signal that has been input from the branching unit  2100  and to which distortion has been added. The envelope extractor  2000  outputs the extracted envelope component to the shaping unit  1000  as an envelope signal. 
     The shaping unit  1000  performs shaping processing on a voltage-waveform in accordance with the amplitude of the voltage of the envelope signal. 
     The processing at the shaping unit  1000  will be described below in detail. It is assumed that a shaping function is a function expressed by a relational expression shown in  FIG. 11A . The transmission signal to which distortion has been added by the predistorter  600  is denoted by u i . An output signal from the shaping unit  1000  can be expressed by a relational expression shown in  FIG. 11B . 
     The same signal as the transmission signal input into the predistorter  600  is input into the controller  700 . The controller  700  obtains a distortion characteristic of the output signal from the amplification unit  100  or its inverse on the basis of the same signal as the transmission signal input into the predistorter  600  and a signal input from the down-converter  900 . The controller  700  controls the predistorter  600  on the basis of the obtained result. 
     An output signal from the shaping unit  1000  is input into the predistorter  200 . The predistorter  200  adds distortion to the output signal from the shaping unit  1000 . The predistorter  2000  outputs the signal to which distortion has been added. 
     The controller  400  controls distortion added by the predistorter  200  on the basis of the output signal from the shaping unit  1000  and the output signal from the amplification circuit  102 . The filter  500  reduces the voltage of components that has the carrier frequency band contained in a signal input at the drain terminal of the amplification circuit  102 . The voltage resulting from the reduction of the voltage of the components in the carrier frequency band by the filter  500  and applied to the drain terminal of the amplification circuit  102  is input into the controller  400 . The controller  400  controls distortion added by the predistorter  200  on the basis of the output signal from the shaping unit  1000  and the voltage input from the filter  500 . 
     The amplifier according to this embodiment includes the predistorter  200  and the controller  400 . An envelope signal that has been extracted from a transmission signal and to which distortion has been added by the predistorter  200  is input into the predistorter  200 . The predistorter  200  adds distortion to the envelope signal. The controller  400  controls distortion added by the predistorter  200  on the basis of an output signal from the amplification unit  100  and the envelope signal. The inverse of distortion of the output signal that occurs in a circuitry subsequent to the power supply modulator  300  is added to the envelope signal. Distortion that occurs in the power supply modulator  300  and the subsequent circuitry is added to the envelope signal to which the inverse of distortion has been added by the predistorter  200 . This inhibits deviation from the proportionality of relationship between the voltage  3000  of the envelope signal and the voltage  3400   a  as shown in  FIG. 9B . As a result, proper control of the voltage  3400   a  corresponding to the input signal is enabled and degradation of the quality of the output signal from the amplification unit  100  is reduced. 
     In the amplifier according to this embodiment, the controller  400  controls distortion added by the distorter  200  on the basis of the envelope signal to which distortion has been added by the predistorter  200  and the output signal from the amplification unit  100 . 
     In the amplifier according to this embodiment, the controller  400  controls distortion added by the predistorter  200  on the basis of the output signal output from the choke coil  101  and the envelope signal. 
     Thus, the controller  400  more properly determines the inverse of distortion of the output signal that occurs in the power supply modulator  300  and the circuitry that succeeds the power supply modulator  300  and precedes the drain terminal of the amplification circuit  102 . 
     Suitably, the controller  400  controls distortion added by the predistorter  200  on the basis of the output signal from the filter  500  and the envelope signal. 
     Since impedance matching is performed by the matching circuit  103  in the amplifier according to this embodiment, distortion of the output signal that occurs in the amplification circuit  102  can be inhibited. 
     The matching circuit  104  matches the output impedance of the stage preceding the up-converter  800  and the input impedance of the amplification circuit  102 . 
     The choke coil  105  transmits a bias voltage of the power supply  106  to the amplification circuit  102  to determine the gate bias of the amplification circuit  102 . 
     The amplifier according to this embodiment includes two predistorters, namely the predistorter  600  and the predistorter  200 . The compensation for distortion contained in the output signal from the amplification circuit  102  is handled and shared by the predistorter  600  and the predistorter  200 . Since the load of compensation on each of the predistorter  600  and the predistorter  200  is reduced, distortion in a wider range can be compensated for by the predistorter  600  than in a configuration where only one predistorter compensates for distortion. Consequently, the predistorter  200  can further compensate for distortion of the output signal from the amplification unit  100 . Accordingly, the quality of the output signal is improved. 
     The Third Embodiment 
     An amplifier according to a third embodiment will be described with reference to  FIG. 3 . 
     The amplifier according to the third embodiment further includes a DSP (Digital Signal Processor)  1800 , a DAC (Digital to Analog Converter)  1100 , an ADC (Analog to Digital Converter)  1700 , LPFs (Low Pass Filters)  1300 ,  1600 , and a BPF (Band Pass Filter)  1200 . The DSP  1800  includes logic outputs O1, O2 and logic inputs I1, I2. Note that DSP, DAC, ADC, LPF and BPF are abbreviated word of Digital Signal Processor, Digital to Analog Converter, Analog to Digital Converter, Low Pass Filters, and Band Pass Filter, respectively. 
     The DSP  1800  outputs a digital signal of a baseband signal through output terminal O2. The digital signal of the baseband signal is input into the DAC  1100 . The DAC  1100  converts the input digital signal to an analog signal and outputs the analog signal as a transmission signal. The transmission signal is input into the up-converter  800  through the LPF  1300 . The up-converter  800  combines a carrier signal output from an oscillator not depicted, with the transmission signal to generate an input signal. The up-converter  800  outputs the generated input signal. The input signal is input into the amplification unit  100  through the BPF  1200 . 
     While most part of an output signal from the amplification unit  100  is provided to the load  1900 , a part of the output signal is down-converted to a baseband signal through the down-converter  900  and the LPF  1600 . The baseband signal resulting from the down-conversion is converted to a digital signal at the ADC  1700 . The digital signal resulting from the conversion is input at input terminal I2 of the DSP  1800 . 
     The predistorter  600  and the controller  800 , which are responsible for compensation for distortion in the amplification unit  100 , are aggregated into the DSP  1800 . 
     A procedure for the controller  700  to calculate a distortion characteristic of an output signal from the amplification unit  100  or its inverse will be described. While various implementations and algorithms have been proposed, an indirect learning algorithm which uses a polynomial model will be described here as an example. 
     An input baseband signal is denoted by x i , a transmission signal to which distortion has been added by the predistorter  600  is denoted by u i  (O2 in  FIG. 3 ), a digital signal of the baseband signal obtained by down-conversion of an output signal from the amplification unit  100  is denoted by y i  (I2 in  FIG. 3 ). The order of the polynomial model is denoted by j and a time-series tap is denoted by i. The predistorter  600  adds distortion expressed by the relational expression shown in  FIG. 10A  to the transmission signal input into the predistorter  600 . The parameter g is determined so that the parameter g satisfies the relational expression shown in  FIG. 10B . The parameter g can be obtained by using the least-squares method, the gradient method or the like. The controller  700  uses the obtained parameter g to control the predistorter  600 . The inverse of distortion of the output signal from the amplification unit  100  is added to the baseband signal. 
     The baseband signal x i  synchronizes with the digital signal y i  of the baseband signal generated by down-conversion. A coefficient of correlation between the baseband signal x i  and the digital signal y i  is calculated and a signal delay in the DSP  1800  is adjusted so that the value of the coefficient of correlation is maximized. 
     The amplitude component of the transmission signal is output from I1 of the DSP  1800  and is converted to an envelope signal through the DAC  1100  and an LPF  1400 . The envelope extractor  2000  in  FIG. 2  is implemented by the DSP  1800  and the DAC  1100 . The envelope signal is input into the power supply modulator  300 . The power supply modulator  300  amplifies the input envelope signal. The power supply modulator  300  supplies power to the amplification circuit  102  through the choke coil  101 . 
     Carrier frequency band components of the output signal from the amplification circuit  102  are reduced by the LPF  1500  and the resulting signal is converted to a digital signal by the ADC  1700 . The ADC  1700  outputs the converted digital signal to I1 of the DSP  1800 . Here, the LPF  1500  is configured so that matching of the impedance of the LPF  1500  viewed from amplification circuit  102  and the impedance of the amplification circuit  102  is maintained. Further, the LPF  1500  is configured so that gain and phase have a flat frequency dependence in the baseband frequency band and gain decreases in the carrier frequency band. 
     Since the predistorter  600 , the predistorter  200  and the shaping unit  1000  in the DSP  1800  operate successively in response to temperature change characteristics and aging characteristics of the amplification unit  100 , the amplifier according to this embodiment performs stable distortion compensation. 
     As shown in  FIG. 4 , an output signal from the predistorter  600  may be input into the controller  700  instead of the same signal as the transmission signal input into the predistorter  600 . An output signal from the predistorter  600  may be input into the predistorter  200  instead of the output signal from the shaping unit  1000 .  FIG. 4  shows an example in which an output signal from the branching unit  2100  is input into the predistorter  200  as an output signal from the predistorter  600 . 
     While an example has been given in which the controller  700  uses a polynomial model to determine distortion added at the predistorter  600  in the configuration described above, the present invention is not limited to this. Modulation may be performed on the basis of corrections stored in an LUT (Look Up Table) according to the amplitude of a transmission signal input into the predistorter  600 . A model for distortion of an output signal from the amplification unit  100  may be assumed and its inverse model may be computed. The configuration of the amplifier of the present invention is not limited to the embodiments described above. 
     As shown in  FIG. 5 , one predistorter  2200  may include the function of the shaping unit  1000  and the function of the predistorter  200 . 
     A preamplifier may be disposed before the amplification unit  100 . 
     The DSP  1800  may be configured in combination with a processor such as an FPGA (Field-Programmable Gate Array). 
     Next, an example of the LPF  1500  will be described.  FIG. 6  shows an example of a circuitry of the LPF  1500  in  FIG. 3 . 
     The LPF  1500  includes resistances  1501   a ,  1501   b  and capacitors  1502   a ,  1502   b . When the impedance viewed from the amplification circuit  102  is not high, a large value is set on the resistance  1501   a  so that matching of the amplification circuit  102  is not affected. Other parameters are set so that gain has a flat frequency dependence in the baseband signal frequency band and decreases in the carrier frequency band with the input impedance of the ADC  1700  as a load  1701 . 
     Inductors may be used instead of the resistances  1501   a ,  1501   b.    
     The number of resistance stages may be two or more. A Butterworth filter or a Chebyshev filter or the like may be used as the LPF  1500 . 
     The Fourth Embodiment 
     An amplifier according to a fourth embodiment will be described with reference to  FIG. 7 . 
       FIG. 7  shows a configuration in which an additional amplification circuit is added to the amplification unit  100  in  FIG. 3 . In the amplifier shown in  FIG. 7 , the two amplification circuits perform push-pull operation. While a configuration that uses FETs as the amplifiers is described in this embodiment, the type of amplifiers is not limited to FET. 
     In  FIG. 7 , an input signal passes through a matching circuit  104  and is combined with a power supply  109  through a transformer  108 . The combined signal is input into the gate terminals of FET  110   a  and FET  110   b . The power supply  109  controls the gate voltages of the FET  110   a  and FET  110   b  to determine at which operating point among class AB, class B and class C the amplifier  100  operates. Output power mod from a power supply modulator  300  is supplied to the FET  110   a  and FET  110   b  through a transformer  107 . The transformer  107  supplies output power to a load not depicted through a matching circuit  103 . 
     The voltage at the drain terminals of the FET  110   a  and FET  110   b  are divided by resistances  112   a  and  112   b  and input into the ADC  1700  through the LPF  1500 . The resistance values of the resistances  112   a  and  112   b  are twice the resistance value of the resistance  1501   a  as opposed to LPF  1500  according to the third embodiment. The other parameters are the same as those in the third embodiment. In this configuration, the LPF  1500  according to the fourth embodiment has the same characteristics as the LPF  1500  according to the third embodiment. 
     Note that the push-pull circuit is not limited to the configuration described above; a configuration of other push-pull circuit may be applied. 
     While output voltages from the two FETs are input into the LPF  1500  in the configuration described above, an output voltage from one of the FETs may be input into the LPF  1500 . 
     While the present invention has been described with preferred embodiments, the present invention is not limited to the embodiments described above and can be implemented with various modifications without departing from the technical spirit thereof. Combinations of some or all of the components of the embodiment described above also fall within the scope of the present invention. 
     For example, while embodiments have been described mainly using an FET as the amplification circuit  102  in the embodiments described above, a bipolar transistor or electron tube may be used as the amplification circuit  102 . 
     If a bipolar transistor is used as the amplification circuit  102 , the output signal from the choke coil  101  is input into the collector terminal of the bipolar transistor and the input signal is input into the base terminal of the bipolar transistor. 
     This application claims priority based upon Japanese Patent Application 2012-081595 filed on 30 Mar. 2012, the entire disclosure of which is incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The present invention is not limited to the embodiments described above and is suitably applicable to any amplifiers and amplification methods used in communication. 
     REFERENCE SIGNS LIST 
     
         
           100  Amplification unit 
           101 ,  105  Choke coil 
           102  Amplification circuit 
           103 ,  104  Matching circuit 
           106 ,  109  Power supply 
           107 ,  108  Transformer 
           110   a ,  110   b  FET 
           112   a ,  112   b ,  1501   a ,  1501   b  Resistance 
           200 ,  600 ,  2200  Predistorter 
           300  Power supply modulator 
           400 ,  700  Controller 
           500  Filter 
           800  Up-converter 
           900  Down-converter 
           1000  Shaping unit 
           1100  DAC 
           1200  BPF 
           1300 ,  1400 ,  1500 ,  1600  LPF 
           1502   a ,  1502   b  Capacitor 
           1700  ADC 
           1701 ,  1900  Load 
           1800  DSP 
           2000  Envelope extractor 
           2100  Branching unit