Abstract:
An amplifying device including: a first amplifier configured to generate a first output signal by amplifying an input signal, a second amplifier configured to generate a second output signal by amplifying the first output signal, and a processor configured to perform a first compensation by compensating a distortion for the second amplifier in accordance with the first output signal and the second output signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-006657, filed on Jan. 17, 2013, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to an amplifying device, a distortion compensating device, and an amplifying method. 
       BACKGROUND 
       [0003]    For example, from the viewpoint of power conversion efficiency, a transmission unit such as a base station in a mobile communication system may be designed to operate in a nonlinear region where back-off from saturation power is small. Thus, when a modulating signal for high-speed wireless communication is inputted, out-of-band radiation due to nonlinear distortion occurs, thereby affecting adjacent transmission channels. 
         [0004]    Distortion compensation technology for reducing such out-of-band radiation caused by a transmission amplifier includes predistortion (PD) compensation in which the reverse characteristic of the transmission amplifier is added in advance to transmission signals. In the predistortion compensation, a distortion compensation coefficient is adaptively updated so as to reduce the difference between a transmission signal and a signal obtained by feeding back part the output signal of the transmission amplifier. In addition, digital predistortion is known in which predistortion is performed in a digital region. 
         [0005]    A transmission amplifier such as a base station may be implemented by multi-stage connection of a plurality of amplifiers. When the transmission amplifier having such a multi-stage structure is operated with high power conversion efficiency, nonlinear distortion occurs also in an amplifier other than the final stage amplifier, and thus a signal distorted in a former stage amplifier (for example, a driver stage amplifier) is inputted to the final stage amplifier, thereby causing complicated nonlinear distortion in the final stage amplifier. For this problem, a distortion compensation circuit is known in which a predistortion compensation unit for compensating the former stage amplifier, and a predistortion compensation unit for compensating the final stage amplifier are connected in cascade (for example, see Japanese Laid-Open Patent Publication No. 2006-279633). 
       SUMMARY 
       [0006]    According to an aspect of the invention, an amplifying device includes a first amplifier configured to generate a first output signal by amplifying an input signal, a second amplifier configured to generate a second output signal by amplifying the first output signal, and a processor configured to perform a first compensation by compensating a distortion for the second amplifier in accordance with the first output signal and the second output signal. 
         [0007]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a diagram illustrating an example of a configuration of an amplifying device according to a first embodiment; 
           [0010]      FIG. 2  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 1 ; 
           [0011]      FIG. 3  is a diagram illustrating an example of a signal flow when a coefficient of a latter stage PD unit is set; 
           [0012]      FIG. 4  is a diagram illustrating an example of a signal flow when a coefficient of a former stage PD unit is set; 
           [0013]      FIG. 5  is a diagram illustrating another example of a signal flow when a coefficient of the former stage PD unit is set; 
           [0014]      FIG. 6  is a flow chart illustrating an example of a coefficient setting operation performed by the amplifying device; 
           [0015]      FIG. 7  is a diagram illustrating an example of a hardware configuration of the amplifying device; 
           [0016]      FIG. 8  is a diagram illustrating an example of a configuration of an amplifying device according to a second embodiment; 
           [0017]      FIG. 9  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 8 ; 
           [0018]      FIG. 10  is a flow chart illustrating an example of a coefficient setting operation for a former stage PD unit according to the second embodiment; 
           [0019]      FIG. 11  is a diagram illustrating an example of a configuration of an amplifying device according to a third embodiment; 
           [0020]      FIG. 12  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 11 ; 
           [0021]      FIG. 13  is a flow chart illustrating an example of a coefficient setting operation for a former stage PD unit according to the third embodiment; 
           [0022]      FIG. 14  is a diagram illustrating an example of a configuration of an amplifying device according to a fourth embodiment; 
           [0023]      FIG. 15  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 14 ; 
           [0024]      FIG. 16  is a flow chart illustrating an example of a coefficient setting operation for a former stage PD unit according to the fourth embodiment; and 
           [0025]      FIG. 17  is a graph illustrating an example of a CCDF of an input amplitude of the latter stage PD unit before and after the former stage PD unit is updated. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    With the above-described conventional technology, however, when distortion remains at the output of the former stage amplifier due to e.g. a change in the input signal for example, the characteristic of a signal changes which is fed back to the predistortion compensation unit corresponding to the latter stage amplifier, and thus distortion compensation of the latter stage amplifier may not be performed with high accuracy. 
         [0027]    In order to solve the problem of the above-described conventional technology, the present disclosure provides an amplifying device, a distortion compensating device, and a distortion compensation method that allow distortion compensation accuracy to be improved. 
         [0028]    Hereinafter, an embodiment of an amplifying device, a distortion compensating device, and a distortion compensation method according to the present disclosure will be described in detail with reference to the accompanying drawings. 
       First Embodiment 
     Configuration of Amplifying Device According to First Embodiment 
       [0029]      FIG. 1  is a diagram illustrating an example of a configuration of an amplifying device according to a first embodiment.  FIG. 2  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 1 . An amplifying device  100  according to the first embodiment is an amplifying device that amplifies an input signal and outputs the amplified signal. For example, the amplifying device  100  may be applied to an amplifying device to amplify a signal to be transmitted in a transmitter to transmit a signal such as a radio signal. 
         [0030]    As illustrated in  FIGS. 1 and 2 , the amplifying device  100  includes a former stage PD unit  101 , a latter stage PD unit  102 , a digital/analog converter (DAC)  103 , an up converter  104  (Up Cony), a driver stage amplifier  105 , a final stage amplifier  106 , a down converter  107  (Down Cony), an analog/digital converter (ADC)  108 , a selector  109  (SEL1), a down converter  110  (Down Cony), an ADC  111 , a selector  112  (SEL2), a updating PD unit  113 , and a coefficient update unit  114 . 
         [0031]    A signal inputted to the amplifying device  100  is inputted to the former stage PD unit  101 . The signal inputted to the former stage PD unit  101  is, for example, a digital electrical signal at the baseband frequency. The former stage PD unit  101  performs predistortion to provide the input signal with the reverse characteristic of nonlinear distortion which occurs in the signal in the final stage amplifier  106 . Thus, the nonlinear distortion which occurs in the signal in the final stage amplifier  106  may be compensated. The reverse characteristic with which the former stage PD unit  101  provides the input signal is controlled by the coefficient which is set to the former stage PD unit  101 . The former stage PD unit  101  outputs the signal having the reverse characteristic to the latter stage PD unit  102  and the selector  109 . 
         [0032]    The latter stage PD unit  102  performs predistortion to provide the signal outputted from the former stage PD unit  101  with the reverse characteristic of nonlinear distortion which occurs in the signal in the driver stage amplifier  105 . Thus, the nonlinear distortion which occurs in the signal in the driver stage amplifier  105  may be compensated. The reverse characteristic with which the latter stage PD unit  102  provides the output signal is controlled by the coefficient which is set to the latter stage PD unit  102 . The latter stage PD unit  102  outputs the signal having the reverse characteristic to the DAC  103  and the selector  109 . 
         [0033]    The DAC  103  converts an output signal from a digital form to an analog form, and outputs the converted signal to the up converter  104 , the output signal being outputted from the latter stage PD unit  102 . The up converter  104  converts the signal outputted from the DAC  103  to a radio frequency signal, and outputs the converted signal to the driver stage amplifier  105 . 
         [0034]    The driver stage amplifier  105  amplifies the signal outputted from the up converter  104 , and outputs the amplified signal to the final stage amplifier  106 . The final stage amplifier  106  amplifies the signal outputted from the driver stage amplifier  105 , and outputs the amplified signal. 
         [0035]    Part of the signal outputted from the driver stage amplifier  105  to the final stage amplifier  106  is fed back and inputted to the down converter  107 . The down converter  107  converts the frequency of the inputted signal, and outputs the converted signal to the ADC  108 . The ADC  108  converts an output signal from an analog form to a digital form, and outputs the converted signal to the selector  109  and the selector  112 , the output signal being outputted from the down converter  107 . 
         [0036]    A signal ( 1 ) outputted from the former stage PD unit  101 , a signal ( 2 ) outputted from the latter stage PD unit  102 , and a signal ( 3 ) outputted from the ADC  108  are inputted to the selector  109 . The selector  109  selects one of the inputted signals, and outputs the one as a reference signal to the coefficient update unit  114 . The signal switching with the selector  109  is performed, for example, by a control circuit (not illustrated) of the amplifying device  100 . 
         [0037]    Part of the signal outputted from the final stage amplifier  106  is fed back and inputted to the down converter  110 . The down converter  110  converts the frequency of the inputted signal, and outputs the converted signal to the ADC  111 . The ADC  111  converts an output signal from an analog form to a digital form, and outputs the converted signal to the selector  112 , the output signal being outputted from the down converter  110 . 
         [0038]    A signal ( 1 ) outputted from the ADC  108  and a signal ( 2 ) outputted from the ADC  111  are inputted to the selector  112 . The selector  112  selects one of the inputted signals, and outputs the one as a feedback signal to the updating PD unit  113 . The signal switching with the selector  112  is performed, for example, by a control circuit (not illustrated) of the amplifying device  100 . 
         [0039]    The updating PD unit  113  performs predistortion for imparting a characteristic according to the set coefficient to the signal which has been outputted as a feedback signal from the selector  112 . The coefficient set to the updating PD unit  113  is updated by the coefficient update unit  114 . The updating PD unit  113  outputs the signal having the characteristic to the coefficient update unit  114 . When the update by the coefficient update unit  114  is completed, the coefficient set to the updating PD unit  113  is copied to the former stage PD unit  101  or the latter stage PD unit  102 . The copy of the coefficient to the former stage PD unit  101  or the latter stage PD unit  102  is made, for example, by a control circuit (not illustrated) of the amplifying device  100 , the coefficient being set to the updating PD unit  113 . 
         [0040]    The coefficient update unit  114  updates the coefficient of predistortion in the updating PD unit  113  based on the difference between the signal outputted as a reference signal from the selector  109  and the signal outputted from the updating PD unit  113 . For example, the coefficient update unit  114  updates the coefficient of predistortion in the updating PD unit  113  so as to reduce the difference between the signal outputted from the selector  109  and the signal outputted from the updating PD unit  113 . 
         [0041]    In this manner, the amplifying device  100  has an indirect learning distortion compensation circuit including the updating PD unit  113  in a feedback system, the updating PD unit  113  being similar to the former stage PD unit  101  and the latter stage PD unit  102  in a feed-forward system. 
         [0042]    A distortion compensating device may be achieved by excluding the driver stage amplifier  105  and the final stage amplifier  106  from the amplifying device  100 . In this case, predistortion of the driver stage amplifier  105  and the final stage amplifier  106  may be performed by coupling the distortion compensating device to the driver stage amplifier  105  and the final stage amplifier  106 . 
         [0043]    (Signal Flow when Coefficient of Latter Stage PD Unit is Set) 
         [0044]      FIG. 3  is a diagram illustrating an example of a signal flow when the coefficient of the latter stage PD unit is set. In  FIG. 3 , a portion similar to that illustrated in  FIGS. 1 and 2  is labeled with the same symbol, and description is omitted. When the coefficient of the latter stage PD unit  102  is updated, the control circuit of the amplifying device  100  sets the input of the selector  109  to “2” as illustrated in  FIG. 3 , thereby outputting the signal from the latter stage PD unit  102  as a reference signal to the coefficient update unit  114 . In addition, the control circuit of the amplifying device  100  sets the input of the selector  112  to “1”, thereby outputting the signal outputted from the ADC  108  as a feedback signal to the updating PD unit  113 . 
         [0045]    In this manner, by using the signal outputted from the latter stage PD unit  102  as a reference signal and the signal outputted from the driver stage amplifier  105  as a feedback signal, the coefficient of the updating PD unit  113  may be updated by the coefficient update unit  114 . When the coefficient of the updating PD unit  113  is updated, the control circuit of the amplifying device  100  copies the updated coefficient of the updating PD unit  113  to the latter stage PD unit  102 . In this manner, the coefficient updated by using the signal outputted from the latter stage PD unit  102  as a reference signal and the signal outputted from the driver stage amplifier  105  as a feedback signal may be set to the latter stage PD unit  102 . 
         [0046]    (Signal Flow when Coefficient of Former Stage PD Unit is Set) 
         [0047]      FIG. 4  is a diagram illustrating an example of a signal flow when the coefficient of the former stage PD unit is set. In  FIG. 4 , a portion similar to that illustrated in  FIGS. 1 and 2  is labeled with the same symbol, and description is omitted. When the coefficient of the former stage PD unit  101  is updated, the control circuit of the amplifying device  100  sets the input of the selector  109  to “3” as illustrated in  FIG. 4 , thereby outputting the signal from the ADC  108  as a reference signal to the coefficient update unit  114 . In addition, the control circuit of the amplifying device  100  sets the input of the selector  112  to “2”, thereby outputting the signal outputted from the ADC  111  as a feedback signal to the updating PD unit  113 . 
         [0048]    In this manner, by using the signal outputted from the driver stage amplifier  105  as a reference signal and the signal outputted from the final stage amplifier  106  as a feedback signal, the coefficient of the updating PD unit  113  may be updated by the coefficient update unit  114 . When the coefficient of the updating PD unit  113  is updated, the control circuit of the amplifying device  100  copies the updated coefficient of the updating PD unit  113  to the former stage PD unit  101 . In this manner, the updated coefficient may be set to the former stage PD unit  101 , the updated coefficient being obtained by using the signal outputted from the driver stage amplifier  105  as a reference signal and the signal outputted from the final stage amplifier  106  as a feedback signal. 
         [0049]    Consequently, the coefficient of the updating PD unit  113  may be updated by using the input signal and the output signal of the final stage amplifier  106 , thus the coefficient of the former stage PD unit  101  may be set without any influence of residual distortion in the driver stage amplifier  105 . 
         [0050]      FIG. 5  is a diagram illustrating another example of a signal flow when the coefficient of the former stage PD unit is set. In  FIG. 5 , a portion similar to that illustrated in  FIGS. 1 and 2  is labeled with the same symbol, and description is omitted. When the coefficient of the former stage PD unit  101  is updated with a small amount of the residual distortion of the driver stage amplifier  105 , updating the coefficient is not affected so much even if the update loop of the former stage PD unit  101  includes the latter stage PD unit  102  and the driver stage amplifier  105 . 
         [0051]    For this reason, in this case, the control circuit of the amplifying device  100  may set the input of the selector  109  to “1” as illustrated in  FIG. 5  so as to output the signal from the former stage PD unit  101  as a reference signal to the coefficient update unit  114 . Also in this case, the control circuit of the amplifying device  100  sets the input of the selector  112  to “2”, thereby outputting the signal outputted from the ADC  111  as a feedback signal to the updating PD unit  113 . 
         [0052]    In this manner, the digital output signal of the former stage PD unit  101  may be used as a reference signal, thus the coefficient update may be made with increased accuracy. 
         [0053]    (Coefficient Setting Operation by Amplifying Device) 
         [0054]      FIG. 6  is a flow chart illustrating an example of a coefficient setting operation performed by the amplifying device. The amplifying device  100  performs, for example, the following operations as a coefficient setting operation for the former stage PD unit  101  and the latter stage PD unit  102 . Each of the following operations is performed, for example, by a control circuit (not illustrated) of the amplifying device  100 . 
         [0055]    S 601  to S 605  correspond to the coefficient setting operation for the latter stage PD unit  102 , which has been described with reference to  FIG. 3 . First, the amplifying device  100  copies the coefficient set in the latter stage PD unit  102  to the updating PD unit  113  (S 601 ). Next, the amplifying device  100  sets the input of the selector  109  (SEL1) to “2” and sets the input of the selector  112  (SEL2) to “1” (S 602 ). 
         [0056]    Next, the amplifying device  100  updates the coefficient of the updating PD unit  113  by the coefficient update unit  114  (S 603 ). Next, the amplifying device  100  determines whether or not the coefficient of the updating PD unit  113  has converged by the update in S 603  (S 604 ). When the coefficient has not converged (No in S 604 ), the operational flow of the amplifying device  100  returns to S 603 . 
         [0057]    In S 604 , when the coefficient has converged (Yes in S 604 ), the amplifying device  100  copies the coefficient of the updating PD unit  113  to the latter stage PD unit  102  (S 605 ). Thus, the amplifying device  100  completes the coefficient setting operation for the latter stage PD unit  102 , and the operational flow proceeds to the coefficient setting operation for the former stage PD unit  101 . 
         [0058]    S 606  to S 609  correspond to the coefficient setting operation for the former stage PD unit  101 , which has been described with reference to  FIG. 4 . First, the amplifying device  100  copies the coefficient set in the former stage PD unit  101  to the updating PD unit  113  (S 606 ). Next, the amplifying device  100  sets the input of the selector  109  (SEL1) to “3” and sets the input of the selector  112  (SEL2) to “2” (S 607 ). 
         [0059]    Next, the amplifying device  100  updates the coefficient of the updating PD unit  113  by the coefficient update unit  114  (S 608 ). Next, the amplifying device  100  determines whether or not the coefficient of the updating PD unit  113  has converged by the update in S 608  (S 609 ). When the coefficient has not converged (No in S 609 ), the operational flow of the amplifying device  100  returns to S 608 . 
         [0060]    In S 609 , when the coefficient has converged (Yes in S 609 ), the amplifying device  100  completes the coefficient setting operation for the former stage PD unit  101 , which has been described with reference to  FIG. 4 , and proceeds to the coefficient setting operation for the former stage PD unit  101 , which has been described with reference to  FIG. 5 . In the example illustrated in  FIG. 6 , the coefficient of the updating PD unit  113  that has converged in S 609  is not copied to the former stage PD unit  101 . However, the coefficient of the updating PD unit  113  that has converged in S 609  may be copied to the former stage PD unit  101 . 
         [0061]    S 610  to S 613  correspond to the coefficient setting operation for the former stage PD unit  101 , which has been described with reference to  FIG. 5 . First, the amplifying device  100  sets the input of the selector  109  (SEL1) to “1” and sets the input of the selector  112  (SEL2) to “2” (S 610 ). 
         [0062]    Next, the amplifying device  100  updates the coefficient of the updating PD unit  113  by the coefficient update unit  114  (S 611 ). Next, the amplifying device  100  determines whether or not the coefficient of the updating PD unit  113  has converged by the update in S 611  (S 612 ). When the coefficient has not converged (No in S 612 ), the operational flow of the amplifying device  100  returns to S 611 . 
         [0063]    In S 612 , when the coefficient has converged (Yes in S 612 ), the amplifying device  100  copies the coefficient of the updating PD unit  113  to the former stage PD unit  101  (S 613 ). Thus, the amplifying device  100  completes the coefficient setting operation for the former stage PD unit  101 , which has been described with reference to  FIG. 5 , and terminates a series of the coefficient setting operations. 
         [0064]    (Hardware Configuration of Amplifying Device) 
         [0065]      FIG. 7  is a diagram illustrating an example of a hardware configuration of the amplifying device. In  FIG. 7 , a portion similar to that illustrated in  FIG. 1  is labeled with the same symbol, and description is omitted. As illustrated in  FIG. 7 , the former stage PD unit  101 , the latter stage PD unit  102 , the selector  109 , the selector  112 , the updating PD unit  113 , and the coefficient update unit  114  of the amplifying device  100  may be implemented, for example, by a digital circuit  700 . 
         [0066]    The above-mentioned control circuit of the amplifying device  100  may be also implemented by the digital circuit  700 . In the digital circuit  700 , various types of digital circuits may be used, such as a field programmable gate array (FPGA) or a digital signal processor (DSP). 
         [0067]    Although a case has been described where digital predistortion is performed using the digital circuit  700  in the former stage PD unit  101  and the latter stage PD unit  102 , at least part of the digital circuit  700  may be replaced by an analog circuit. 
         [0068]    In this manner, with the amplifying device  100  according to the first embodiment, the coefficient (a first coefficient) of the former stage PD unit  101  (a first compensation unit) may be set based on the difference between the signal inputted to the final stage amplifier  106  (a second amplifier) and the signal outputted from the final stage amplifier  106 . Consequently, the coefficient of the former stage PD unit  101  may be set without any influence of the residual distortion in the driver stage amplifier  105  (a first amplifier), thus accuracy of distortion compensation may be improved. 
       Second Embodiment 
     Configuration of Amplifying Device According to Second Embodiment 
       [0069]      FIG. 8  is a diagram illustrating an example of a configuration of an amplifying device according to a second embodiment.  FIG. 9  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 8 . In  FIGS. 8 and 9 , a portion similar to that illustrated in  FIGS. 1 and 2  is labeled with the same symbol, and description is omitted. 
         [0070]    As illustrated in  FIGS. 8 and 9 , the amplifying device  100  according to the second embodiment includes a bandwidth determination unit  801  and a selector control unit  802  in addition to the configuration illustrated in  FIGS. 1 and 2 . The bandwidth determination unit  801  and the selector control unit  802  may be implemented, for example, by the digital circuit  700  illustrated in  FIG. 7 . 
         [0071]    The bandwidth determination unit  801  and the selector control unit  802  are detection units which detect a change in the characteristic of the input signal to the amplifying device  100 . The bandwidth determination unit  801  determines the bandwidth (for example, the bandwidth of frequency) of the input signal to the amplifying device  100 . For example, the bandwidth determination unit  801  determines the bandwidth by performing Fast Fourier Transform (FFT) on the input signal to the amplifying device  100 . Optionally, the bandwidth determination unit  801  may determine a bandwidth based on the information from a higher level device (for example, transmitting device) of the amplifying device  100 . The bandwidth determination unit  801  then outputs bandwidth information, which indicates the determined bandwidth, to the selector control unit  802 . 
         [0072]    The selector control unit  802  changes the signal to be outputted by the selector  109  based on the bandwidth information outputted from bandwidth determination unit  801  (for example, see  FIG. 10 ). 
         [0073]    (Coefficient Setting Operation of Former Stage PD Unit According to Second Embodiment) 
         [0074]      FIG. 10  is a flow chart illustrating an example of a coefficient setting operation for a former stage PD unit according to the second embodiment. The amplifying device  100  according to the second embodiment performs the coefficient setting operation of the latter stage PD unit  102  based on, for example, S 601  to S 605  illustrated in  FIG. 6 , and then repeats and performs, for example, each of the following operations as the coefficient setting operation of the former stage PD unit  101 . 
         [0075]    First, the selector control unit  802  calculates the amount of change in the bandwidth of the input signal to the amplifying device  100  based on the bandwidth information outputted from the bandwidth determination unit  801  (S 1001 ). Next, the selector control unit  802  determines whether or not the amount of change in the bandwidth which has been calculated in S 1001  is greater than or equal to a threshold value (S 1002 ). 
         [0076]    In S 1002 , when the amount of change in the bandwidth is greater than or equal to the threshold value (Yes in S 1002 ), it is determined that change in nonlinear distortion in the final stage amplifier  106  is large. In this case, the selector control unit  802  sets the input of the selector  109  (SEL1) to “3” (S 1003 ). Thus, the output signal (output signal of the driver stage amplifier  105 ) to the selector  109  from the ADC  108  is outputted to the coefficient update unit  114 , thereby allowing the coefficient setting operation of the former stage PD unit  101  described with reference to  FIG. 4  to be performed. 
         [0077]    In S 1002 , when the amount of change in the bandwidth is less than the threshold value (No in S 1002 ), it is determined that change of the nonlinear distortion of the final stage amplifier  106  is small and residual distortion in the output signal of the driver stage amplifier  105  is small, the residual distortion being generated by the update of the coefficient of the former stage PD unit  101 . In this case, the selector control unit  802  sets the input of the selector  109  (SEL1) to “1” (S 1004 ). Thus, the output signal (output signal of the former stage PD unit  101 ) to the selector  109  from the former stage PD unit  101  is outputted to the coefficient update unit  114 , thereby allowing the coefficient setting operation of the former stage PD unit  101  described with reference to  FIG. 5  to be performed. 
         [0078]    Subsequent to S 1003  or S 1004 , the coefficient update unit  114  updates the coefficient of the updating PD unit  113  (S 1005 ). Next, the coefficient update unit  114  determines whether or not the coefficient of the updating PD unit  113  has converged in S 1005  (S 1006 ). When the coefficient has not converged (No in S 1006 ), the operational flow of the amplifying device  100  returns to S 1005 . 
         [0079]    In S 1006 , when the coefficient has converged (Yes in S 1006 ), the control circuit of the amplifying device  100  copies the coefficient of the updating PD unit  113  to the former stage PD unit  101  (S 1007 ), and completes the coefficient setting operation of the former stage PD unit  101 . 
         [0080]    By performing the above operations, when the change of the nonlinear distortion of the final stage amplifier  106  is large, the coefficient of the former stage PD unit  101  may be set without any influence of the residual distortion in the driver stage amplifier  105 . Thus, accuracy of distortion compensation may be improved (a first control state). 
         [0081]    When the change of the nonlinear distortion of the final stage amplifier  106  is small and the residual distortion in the output signal of the driver stage amplifier  105  is small, the digital output signal of the former stage PD unit  101  may be used as a reference signal (a second control state). Thus, the coefficient update may be made with increased accuracy. 
         [0082]    Thus, in addition to the effect provided by the first embodiment, the amplifying device  100  according to the second embodiment allows the digital output signal of the former stage PD unit  101  to be used as a reference signal when the residual distortion in the output signal of the driver stage amplifier  105  is small. Thus, accuracy of distortion compensation may be improved. 
       Third Embodiment 
     Configuration of Amplifying Device According to Third Embodiment 
       [0083]      FIG. 11  is a diagram illustrating an example of a configuration of an amplifying device according to a third embodiment.  FIG. 12  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 11 . In  FIGS. 11 and 12 , a portion similar to that illustrated in  FIGS. 8 and 9  is labeled with the same symbol, and description is omitted. As illustrated in  FIGS. 11 and 12 , the amplifying device  100  according to the third embodiment includes an average power calculation unit  1101  instead of the bandwidth determination unit  801  illustrated in  FIGS. 8 and 9 . The average power calculation unit  1101  may be implemented, for example, by the digital circuit  700  illustrated in  FIG. 7 . 
         [0084]    The average power calculation unit  1101  calculates the average power (for example, the moving average value) of the input signal to the amplifying device  100 . The average power calculation unit  1101  then outputs power information to the selector control unit  802 , the power information indicating the calculated average power. The selector control unit  802  changes the signal outputted by the selector  109  based on the power information outputted from the average power calculation unit  1101  (for example, see  FIG. 13 ). 
         [0085]    (Coefficient Setting Operation of Former Stage PD Unit According to Third Embodiment) 
         [0086]      FIG. 13  is a flow chart illustrating an example of a coefficient setting operation for a former stage PD unit according to the third embodiment. The amplifying device  100  according to the third embodiment performs the coefficient setting operation of the latter stage PD unit  102  based on, for example, S 601  to S 605  illustrated in  FIG. 6 , and then repeats and performs, for example, each of the following operations as the coefficient setting operation of the former stage PD unit  101 . 
         [0087]    First, the selector control unit  802  calculates the amount of change in the average power of the signal inputted to the amplifying device  100  based on the power information outputted from the average power calculation unit  1101  (S 1301 ). Next, the selector control unit  802  determines whether or not the amount of change in the average power which has been calculated in S 1301  is greater than or equal to a threshold value (S 1302 ). 
         [0088]    In S 1302 , when the amount of change in the average power is greater than or equal to the threshold value (Yes in S 1302 ), it is determined that the change of the nonlinear distortion of the final stage amplifier  106  is large. In this case, the operational flow of the amplifying device  100  proceeds to S 1303 . When the amount of change in the average power is less than the threshold value (No in S 1302 ), it is determined that the change of the nonlinear distortion of the final stage amplifier  106  is small and the residual distortion in the output signal of the driver stage amplifier  105  is small, the residual distortion being generated by the update of the coefficient of the former stage PD unit  101 . In this case, the operational flow of the amplifying device  100  proceeds to S 1304 . S 1303  to S 1307  illustrated in  FIG. 13  are the same as the S 1003  to S 1007  illustrated in  FIG. 10 . 
         [0089]    By performing the above operations, when the change of the nonlinear distortion of the final stage amplifier  106  is large, the coefficient of the former stage PD unit  101  may be set without any influence of the residual distortion in the driver stage amplifier  105 . Thus, accuracy of distortion compensation may be improved. 
         [0090]    When the change of the nonlinear distortion of the final stage amplifier  106  is small and the residual distortion in the output signal of the driver stage amplifier  105  is small, the residual distortion being generated by the update of the coefficient of the former stage PD unit  101 , the digital output signal of the former stage PD unit  101  may be used as a reference signal. Thus, the coefficient update may be made with increased accuracy. 
         [0091]    Thus, in addition to the effect provided by the first embodiment, the amplifying device  100  according to the third embodiment allows the digital output signal of the former stage PD unit  101  to be used as a reference signal when the residual distortion in the output signal of the driver stage amplifier  105  is small. Thus, accuracy of distortion compensation may be improved. 
       Fourth Embodiment 
     Configuration of Amplifying Device According to Fourth Embodiment 
       [0092]      FIG. 14  is a diagram illustrating an example of a configuration of an amplifying device according to a fourth embodiment.  FIG. 15  is a diagram illustrating an example of a signal flow in the amplifying device illustrated in  FIG. 14 . In  FIGS. 14 and 15 , a portion similar to that illustrated in  FIGS. 8 and 9  is labeled with the same symbol, and description is omitted. As illustrated in  FIGS. 14 and 15 , the amplifying device  100  according to the fourth embodiment includes an average power calculation unit  1401  instead of the bandwidth determination unit  801  illustrated in  FIGS. 8 and 9 . The average power calculation unit  1401  may be implemented, for example, by the digital circuit  700  illustrated in  FIG. 7 . 
         [0093]    The coefficient update unit  114  outputs an error signal to the average power calculation unit  1401 , the error signal indicating an error between the reference signal outputted from the selector  109  and the signal outputted from the updating PD unit  113 . The average power calculation unit  1401  calculates the average power of the error signal which is outputted from the coefficient update unit  114 . The average power calculation unit  1401  then outputs power information to the selector control unit  802 , the power information indicating the calculated average power. 
         [0094]    The selector control unit  802  changes the signal outputted by the selector  109  based on the power information outputted from the average power calculation unit  1401  (for example, see  FIG. 16 ). 
         [0095]    (Coefficient Setting Operation of Former Stage PD Unit According to Fourth Embodiment) 
         [0096]      FIG. 16  is a flow chart illustrating an example of a coefficient setting operation for a former stage PD unit according to the fourth embodiment. The amplifying device  100  according to the fourth embodiment performs the coefficient setting operation of the latter stage PD unit  102  based on, for example, S 601  to S 605  illustrated in  FIG. 6 , and then repeats and performs, for example, each of the following operations as the coefficient setting operation of the former stage PD unit  101 . 
         [0097]    First, the selector control unit  802  calculates the amount of change in the average power of the error signal outputted from the coefficient update unit  11  based on the power information outputted from the average power calculation unit  1401  (S 1601 ). Next, the selector control unit  802  determines whether or not the amount of change in the average power which has been calculated in S 1601  is greater than or equal to a threshold value (S 1602 ). 
         [0098]    In S 1602 , when the amount of change in the average power is greater than or equal to the threshold value (Yes in S 1602 ), it is determined that the change of the nonlinear distortion of the final stage amplifier  106  is large. In this case, the operational flow of the amplifying device  100  proceeds to S 1603 . When the amount of change in the average power is less than the threshold value (No in S 1602 ), it is determined that the change of the nonlinear distortion of the final stage amplifier  106  is small and the residual distortion in the output signal of the driver stage amplifier  105  is small, the residual distortion being generated by the update of the coefficient of the former stage PD unit  101 . In this case, the operational flow of the amplifying device  100  proceeds to S 1604 . S 1603  to S 1607  illustrated in  FIG. 16  are the same as the S 1003  to S 1007  illustrated in  FIG. 10 . 
         [0099]    By performing the above operations, when the change of the nonlinear distortion of the final stage amplifier  106  is large, the coefficient of the former stage PD unit  101  may be set without any influence of the residual distortion in the driver stage amplifier  105 . Thus, accuracy of distortion compensation may be improved. 
         [0100]    When the change of the nonlinear distortion of the final stage amplifier  106  is small and the residual distortion in the output signal of the driver stage amplifier  105  is small, the digital output signal of the former stage PD unit  101  may be used as a reference signal, the residual distortion being generated by the update of the coefficient of the former stage PD unit  101 . Thus, the coefficient update may be made with increased accuracy. 
         [0101]    Thus, in addition to the effect provided by the first embodiment, the amplifying device  100  according to the fourth embodiment allows the digital output signal of the former stage PD unit  101  to be used as a reference signal when the residual distortion in the output signal of the driver stage amplifier  105  is small. Thus, accuracy of distortion compensation may be improved. 
         [0102]    (CCDF of Input Amplitude of Latter Stage PD Unit Before and after Update of Former Stage PD Unit) 
         [0103]      FIG. 17  is a diagram illustrating an example of a CCDF of an input amplitude of a latter stage PD unit before and after the former stage PD unit is updated. In  FIG. 17 , the horizontal axis indicates a relative power of the input signal of the latter stage PD unit  102 , and the vertical axis indicates a complementary cumulative distribution function (CCDF). 
         [0104]    A CCDF characteristic  1701  indicates the CCDF before the coefficient of the former stage PD unit  101  is updated. A CCDF characteristic  1702  indicates the CCDF after the coefficient of the former stage PD unit  101  is updated. When the nonlinear distortion of the final stage amplifier  106  varies with a change of a transmission signal, CCDFs in a signal peak area  1703  before and after the update of the former stage PD unit  101  are different from each other as indicated by the CCDF characteristics  1701  and  1702 . That is to say, the signal characteristics of the input to the latter stage PD unit  102  before and after the update of the former stage PD unit  101  are different from each. 
         [0105]    The latter stage PD unit  102  has a coefficient which cancels the distortion of the driver stage amplifier  105  before the coefficient of the former stage PD unit  101  is updated. For this reason, when the signal characteristic of the input to the latter stage PD unit  102  changes by the update of the coefficient of the former stage PD unit  101 , the distortion of the driver stage amplifier  105  may not be completely cancelled, and thus the distortion remains in the output signal of the driver stage amplifier  105 . Therefore, the distortion in the final stage amplifier having received distorted signals becomes complicated. 
         [0106]    Thus, with the conventional configuration in which the coefficient of the former stage PD unit is updated based on the difference between the output signals of the former stage PD unit and the final stage amplifier, when the distortion of driver stage amplifier is not appropriately compensated by the latter stage PD unit, compensation in the former stage PD unit becomes difficult because of the influence of the residual distortion in the input signal of the final stage amplifier. An example of the above situation may occur when the average power or the signal bandwidth of the input signal significantly changes over time. That is, a change in the input signal causes especially the distortion characteristic of the final stage amplifier to change, thus the characteristic of the input signal to the latter stage PD unit significantly changes before and after the update of the former stage PD unit. 
         [0107]    On the other hand, with the amplifying device  100  according to the above-described embodiments, the coefficient of the former stage PD unit  101  may be updated based on the difference between the input signal and the output signal of the final stage amplifier  106 . Thus, the coefficient of the former stage PD unit  101  may be set without any influence of the residual distortion in the driver stage amplifier  105 , and distortion in the final stage amplifier  106  may be compensated with high accuracy. Consequently, for example, reduction of convergence speed of the coefficient of the former stage PD unit  101  is controlled, and even when the characteristic of input signal significantly changes, stable operation may be achieved. 
         [0108]    In addition, in the amplifying device  100 , when the residual distortion of the driver stage amplifier  105  is small, by adopting a configuration that allows the digital output signal of the former stage PD unit  101  to be used as a reference signal, highly accurate compensation of distortion may be achieved. 
         [0109]    However, it is also possible to exclude the configuration which allows the output signal of the former stage PD unit  101  to be used as a reference signal. For example, in the selector  109  according to the first embodiment described above, an input path for the output signal of the former stage PD unit  101  may be excluded. Also in this case, the coefficient of the former stage PD unit  101  may be updated based on the difference between the input signal and the output signal of the final stage amplifier  106 , thus highly accurate compensation of the distortion in the final stage amplifier  106  may be achieved. 
         [0110]    As described above, with the amplifying device, the distortion compensating device, and the distortion compensation method, accuracy of distortion compensation may be improved. 
         [0111]    In this application, for example, “connected to” is able to be replaced with “coupled to”. Moreover, for example, when an element is referred to as being “connected to” or “coupled to” another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected to” or “directly coupled to” another element, there are no intervening elements present. So do “connecting to”, “coupling to”, “connection to”, “coupling to” and so on. 
         [0112]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.