Abstract:
There is disclosed a feedforward amplifier for compensating for distortion produced in an amplifier. The feedforward amplifier controls the phase in a vector adjuster effectively. The feedforward amplifier has a first variable phase shifter PH 1   —   1  or PH 2   —   1  for varying the phase of a signal passed through the first variable phase shifter and a second variable phase shifter PH 1   —   2  or PH 2   —   2  for varying the signal passed through the first variable phase shifter in either or both of a distortion detection loop for detecting the distortion and a distortion compensation loop for compensating for the distortion. A phase control portion controls the amount of variation in phase in the first variable phase shifter and values of the amount of variation in phase are concentrated toward either one of relatively-larger directions or relatively-smaller directions, the amount of variation in phase in the second phase shifter is controlled according to the concentrated values.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The present application claims priority from Japanese application JP2007-106689 filed on Apr. 16, 2007, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a feedforward (FF) amplifier for compensating for distortion produced in an amplifier and, more particularly, to a feedforward amplifier for effectively providing phase control in a vector adjuster. 
         [0003]    In a wireless communication system such as a mobile communication system, distortion produced when a signal to be sent is amplified by an amplifier in a base station unit or the like is compensated for by a feedforward type distortion compensator. 
         [0004]      FIG. 4  shows an example of configuration of a fundamental circuit of a feedforward amplifier that compensates for distortion by a feedforward method. For convenience of illustration, configuration portions of  FIG. 4  which are similar to their counterparts of  FIG. 1  that will be referenced in an embodiment described later are indicated by the same reference numerals as in  FIG. 1 . It is to be understood, however, this does not restrict the scope of the present invention unnecessarily. 
         [0005]    The feedforward amplifier shown in  FIG. 4  has three directional couplers (combiner/splitter devices) HYB 1 , HYB 2 , and HYB 3 . Two routes are present between the directional couplers HYB 1  and HYB 2 . One of the two routes has a variable attenuator AT 1 _ 1 , a variable phase shifter PH 1 _ 1 , and a main amplifier AMP 1 . The other route has a coaxial delay line D 1 . Similarly, two routes are present between the directional couplers HYB 2  and HYB 3 . One of these two routes has a coaxial delay line D 2 . The other route has a variable attenuator AT 2 _ 1 , a variable phase shifter PH 2 _ 1 , and an auxiliary amplifier AMP 2 . The feedforward amplifier further includes a control portion  11  for controlling the two variable attenuators AT 1 _ 1  and AT 2 _ 1  and the two variable phase shifters PH 1 _ 1  and PH 2 _ 1 . 
         [0006]    The feedforward amplifier is composed of two loops, i.e., a distortion detection loop L 1  and a distortion compensation loop L 2 . The detection loop L 1  is made up of two directional couplers HYB 1 , HYB 2  and intervening components, i.e., variable attenuator AT 1 _ 1 , variable phase shifter PH 1 _ 1 , main amplifier AMP 1 , and coaxial delay line D 1 . The compensation loop L 2  is made up of two directional couplers HYB 2 , HYB 3  and intervening components, i.e., coaxial delay line D 2 , variable attenuator AT 2 _ 1 , variable phase shifter PH 2 _ 1 , and auxiliary amplifier AMP 2 . 
         [0007]    In each of the loops L 1  and L 2 , the gain can be varied by the variable attenuator AT 1 _ 1  or AT 2 _ 1  such that the amplifier side route and the delay line route are identical in amount of delay and gain but are 180° out of phase with each other for a signal to be treated. The phase can be varied by the variable phase shifter PH 1 _ 1  or PH 2 _ 1 . Such variations in gain and phase are controlled by the control portion  11 . Generally, the control portion  11  monitors the output levels from the directional couplers HYB 2  and HYB 3  and controls the variable attenuators and variable phase shifters so as to maximize or minimize the output levels. This is known as adaptive control. 
         [0008]    In each of the loops L 1  and L 2 , the gain or phase is adjusted by the variable attenuator AT 1 _ 1  or AT 2 _ 1  or by the variable phase shifter PH 1 _ 1  or PH 2 _ 1 . This is known as vector adjustment. 
       SUMMARY OF THE INVENTION 
       [0009]    If the amount of variation in phase due to absorption of moisture into the substrate and coaxial delay lines, the amount of variation in phase due to temperature, and the amounts of variations in phase due to various other factors are totalized, the total amount of variation in phase may not be sufficiently compensated for only by one phase shifter. 
         [0010]    One conceivable method of solving this problem is to increase the number of variable phase shifters (hereinafter referred to as the first example of improvement). Another conceivable method is that the number of variable phase shifters is increased and that each individual phase shifter is controlled independently (hereinafter referred to as the second example of improvement). 
         [0011]    The above-described first example of improvement is now described.  FIG. 5  shows an example of configuration of a feedforward amplifier of this first example of improvement. The number of phase shifters in each of loops L 1  and L 2  is increased. For convenience of illustration, similar components are indicated by identical reference numerals in both  FIGS. 4 and 5 . 
         [0012]    The feedforward amplifier shown in  FIG. 5  is similar to the configuration shown in  FIG. 4  except that another variable phase shifter PH 1 _ 2  is added behind the variable phase shifter PH 1 _ 1  in the distortion detection loop L 1  and that another variable phase shifter PH 2 _ 2  is added behind the variable phase shifter PH 2 _ 1  in the distortion compensation loop L 2 . A control portion  12  controls the 2 variable attenuators AT 1 _ 1 , AT 2 _ 1  and the 4 variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2 . 
         [0013]    The control portion  12  controls each variable phase shifter by the same control signal in each of the loops L 1  and L 2 . As a result, the amount by which the phase can be adjusted within each of the loops L 1  and L 2  is doubled compared with the configuration shown in  FIG. 4 . 
         [0014]    As a specific example, assuming that the amount by which the phase can be varied in each of the loops L 1  and L 2  of the configuration shown in  FIG. 4  is 60°, the amount by which the phase can be varied in each of the loops L 1  and L 2  in the configuration shown in  FIG. 5  is doubled to about 120°. 
         [0015]    However, in the configuration of  FIG. 4 , if the phase in each phase shifter is controlled in units of  8  bits, for example, the phase can be controlled in units of 0.23°/bit (=600/256 bits) when a phase variation is caused by the variable phase shifter PH 1 _ 1 , PH 2 _ 1 . In contrast, in the configuration of  FIG. 5 , if the phase in each phase shifter is controlled in increments of 8 bits, it is possible to provide control in increments of only 0.47°/bit (=120°/256 bits) when a phase variation occurs in the variable phase shifters PH 1 _ 1  and PH 1 _ 2  or variable phase shifters PH 2 _ 1  and PH 2 _ 2 . The amount of variation in phase per bit is doubled. Hence, it is impossible to provide fine control. 
         [0016]    In this way, the amount of variation in phase can be doubled but the unit of control is also doubled. Consequently, there is the problem that it is impossible to provide finer control. 
         [0017]    The above-described second example of improvement is described. An example of configuration of a feedforward amplifier of this second example of improvement is shown in  FIG. 1 , in which the number of phase shifters in each of the loops L 1  and L 2  is increased to control each phase shifter independently. For convenience of illustration, similar components are indicated by the same reference numerals in both  FIGS. 1 and 5 .  FIG. 1  will be referenced in an embodiment described later for convenience of illustration. It is to be understood, however, this does not restrict the scope of the present invention unnecessarily. 
         [0018]    The feedforward amplifier shown in  FIG. 1  is similar to the configuration shown in  FIG. 4  except that another variable phase shifter PH 1 _ 2  is added behind the variable phase shifter PH 1 _ 1  in the distortion detection loop L 1  and that another variable phase shifter PH 2 _ 2  is added behind the variable phase shifter PH 2 _ 1  in the distortion compensation loop L 2 . The control portion  1  controls the 2 variable attenuators AT 1 _ 1  and AT 2 _ 1  and the 4 variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2 . 
         [0019]    The control portion  1  controls the two variable phase shifters PH 1 _ 1  and PH 1 _ 2  separately (i.e., independently), the two phase shifters being in the distortion detection loop L 1 . Also, the control portion controls the two variable phase shifters PH 2 _ 1  and PH 2 _ 2  separately, the phase shifters being in the distortion compensation loop L 2 . 
         [0020]    That is, the phases in the variable phase shifters PH 1 _ 1  and PH 2 _ 1  are allowed to vary invariably such that the phases are optimized in the same way as in the prior art. The phases in the added variable phase shifters PH 1 _ 2  and PH 2 _ 2  are made semifixed without applying the above-described adaptive control. 
         [0021]    For instance, temperature correction can be made, for example, by varying a control value for the phase in the variable phase shifter PH 1 _ 2  according to the temperature. Furthermore, the control value for the phase in the variable phase shifter PH 1 _ 2  can be set by the input level to the amplifier AMP 1  as well as by temperature. 
         [0022]      FIG. 6  shows one example of a control value for the variable phase shifter PH 1 - 2 , the control value being plotted against the temperature or input level. In this graph, the horizontal axis indicates the temperature or input level. The vertical axis indicates the control value. 
         [0023]    In the configuration of this second example of improvement, it is possible to provide control in increments of 0.23°/bit (=60°/256 bits) when the phase in each of the variable phase shifters PH 1 _ 1  and PH 2 _ 1  varies in a case where the phase in each phase shifter is controlled in increments of 8 bits. For example, fine control can be provided in the same way as in the prior-art example shown in  FIG. 4 . 
         [0024]    In the configuration of the second example of improvement, the added variable phase shifters P 1 _ 2  and P 2 _ 2  can vary their phases if the parameter such as temperature or input level can be converted into an electrical signal and measured. However, if the parameter cannot be converted into an electrical signal such as phase variations due to moisture absorption into the substrate or due to phase variations caused by aging, it is still necessary that the variable phase shifters P 1 _ 1  and P 2 _ 1  take account of such variations in the same way as in the prior art. For example, as the frequency is increased, the amount of variation in phase increases. In addition, if the amplifier is stocked in high-temperature, high-humidity environments or the product is operated in totally moisture-free environments, there is the problem that a sufficient amount of phase variation cannot be achieved simply by the variable phase shifters P 1 _ 1  and P 2 _ 1 . 
         [0025]    As described previously, the feedforward amplifier cannot yet sufficiently control the phase in the vector adjuster (i.e., control of phases in the variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2 ). There is a demand for further development. 
         [0026]    The present invention has been made in view of the foregoing circumstances in the prior art. It is an object of the present invention to provide a feedforward amplifier capable of effectively controlling the phase in a vector adjuster. 
         [0027]    The above-described object is achieved in accordance with the teachings of the present invention by a feedforward amplifier for compensating for distortion produced in an amplifier, the feedforward amplifier being characterized in that at least one of the amplification route in a distortion detection loop for detecting the distortion and the distortion amplification route of a distortion compensation loop for compensating for the distortion is configured as follows. 
         [0028]    A first variable phase shifter varies the phase of a signal by a variable amount. A second variable phase shifter varies the phase of the signal transmitted through the first variable phase shifter by a variable amount. A phase control portion controls the amount of variation in phase of the first variable phase shifter. When values of the amount of variation in phase are concentrated toward either one of relatively-larger direction(s) and relatively-smaller direction(s), the amount of variation in phase in the second phase shifter is controlled according to whether the values are concentrated toward either one of relatively-larger direction(s) and relatively-smaller direction(s). 
         [0029]    Therefore, when the amount of variation in phase in the first variable phase shifter is controlled and values of the amount of variation in phase are concentrated toward either one of relatively-larger direction(s) and relatively-smaller direction(s), the adjustable range of phase can be extended by controlling the amount of variation in phase in the second variable phase shifter according to whether the values are concentrated toward either one of relatively-larger direction(s) and relatively-smaller direction(s). The phases in vector adjusters in the distortion detection loop and distortion compensation loop can be controlled effectively. With respect to the amount of variation in phase in the second phase shifter, the amount of variation is controlled only when values of the amount of variation in phase in the first variable phase shifter are unevenly distributed and control is required. Hence, the control operation is efficiently carried out. 
         [0030]    The present invention can be applied to either one or both of the amplification route in the distortion detection loop and the distortion amplification route in the distortion compensation loop. 
         [0031]    The first and second variable phase shifters can have the same or different characteristics. 
         [0032]    The amounts of variation in phase in the first and second variable phase shifters can be controlled in various manners. In one embodiment, the amount of variation in phase in the first variable phase shifter is varied continuously or in small increments, while the amount of variation in phase in the second variable phase shifter is varied discretely (e.g., in larger increments). 
         [0033]    Where values of the amount of variation in phase are concentrated toward either one of relatively-larger value(s) and relatively-smaller value(s) (e.g., when a value (control value) for controlling the amount of variation in phase has a proportional or inversely proportional relationship with the resulting amount of variation in phase), the control value may be concentrated toward either one of relatively-larger value(s) and relatively-smaller value(s). An example of the case in which values of the amount of variation in phase are concentrated toward either one of relatively-larger direction(s) and relatively-smaller direction(s) is that a given number of values of the amount of variation are concentrated toward either ones of relatively-larger value(s) and relatively-smaller value(s). 
         [0034]    As an example, where values of the amount of variation in phase in the first variable phase shifter are mainly distributed on the larger side, a control is provided to increase the amount of variation in phase in the second variable phase shifter. Meanwhile, where values of the amount of variation in phase in the first variable phase shifter are mainly distributed on the smaller side, a control is provided to reduce the amount of variation in phase in the second variable phase shifter. Consequently, the apparent total range of amounts of variation in phase can be made wider. 
         [0035]    Where the power supply of the feedforward amplifier is turned off and turned on the next time, the amount of variation in phase, for example, in the first variable phase shifter is initialized at a given value (e.g., 0). In a further example of configuration, the previous amount of variation in phase is stored in a memory, and a control is started from the stored value of amount of variation in phase. 
         [0036]    Where the power supply for the feedforward amplifier is turned of and turned on the next time, with respect to the amount of variation in phase in the second variable phase shifter, for example, the previous amount of variation in phase is stored in a memory, and a control is started from the stored value of the amount of variation in phase. As a further example of configuration, the amount of variation in phase may be initialized at a given value (e.g., zero (0)). 
         [0037]    Furthermore, when a given time is not yet passed since the power supply for the feedforward amplifier has been turned on, when the temperature is varying rapidly, or when the input level is varying rapidly, a configuration in which the amount of variation in phase in the second variable phase shifter is not controlled can also be used. 
         [0038]    As described so far, according to the feedforward amplifier associated with the present invention, when a vector adjuster in a distortion detection loop or distortion compensation loop is equipped with two variable phase shifters and values of the amount of variation in the phase in the first variable phase shifter are mainly distributed on larger or smaller side, the amount of variation in phase in the second variable phase shifter is controlled. Therefore, the phase in the vector adjuster in the distortion detection loop or distortion compensation loop can be controlled effectively. With respect to the amount of variation in phase in the second variable phase shifter, a control is provided only when values of the amount of variation in phase in the first variable phase shifter are unevenly distributed and a necessity arises. In consequence, the control is provided efficiently. 
         [0039]    Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a diagram showing an example of configuration of a feedforward amplifier associated with one embodiment of the present invention. 
           [0041]      FIG. 2  is a flowchart illustrating a sequence of control operations performed in a feedforward amplifier associated with a first embodiment of the invention. 
           [0042]      FIG. 3  is a flowchart illustrating a sequence of control operations performed for a feedforward amplifier associated with a second embodiment of the invention. 
           [0043]      FIG. 4  is a diagram showing an example of configuration of a prior-art feedforward amplifier. 
           [0044]      FIG. 5  is a diagram showing an example of configuration of a feedforward amplifier. 
           [0045]      FIG. 6  is a graph showing one example of control value used for a variable phase shifter. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0046]    Embodiments of the present invention are hereinafter described with reference to the drawings. 
         [0047]      FIG. 1  shows an example of circuit configuration of a feedforward amplifier associated with one embodiment of the present invention. The feedforward amplifier of the present embodiment has three directional couplers (combiner/splitter devices) HYB 1 , HYB 2 , and HYB 3 . Two routes are present between the directional couplers HYB 1  and HYB 2 . One of the two routes has a variable attenuator AT 1 _ 1 , two variable phase shifters PH 1 _ 1  and PH 1 _ 2 , and a main amplifier AMP 1 . The other route has a coaxial delay line D 1 . Similarly, two routes are present between the directional couplers HYB 2  and HYB 3 . One of these two routes has a coaxial delay line D 2 . The other route has a variable attenuator AT 2 _ 1 , two variable phase shifters PH 2 _ 1  and PH 2 _ 2 , and an auxiliary amplifier AMP 2 . The feedforward amplifier further includes a control portion  1  for controlling the two variable attenuators AT 1 _ 1  and AT 2 _ 1  and the four variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2 . 
         [0048]    Instead of the coaxial delay lines D 1  and D 2 , filters capable of realizing a certain amount of delay may be used. 
         [0049]    The feedforward amplifier of the present embodiment is composed of two loops, i.e., distortion detection loop L 1  and distortion compensation loop L 2 . The detection loop L 1  is made up of two directional couplers HYB 1 , HYB 2  and intervening components therebetween. The intervening components are a variable attenuator AT 1 _ 1 , two variable phase shifters PH 1 _ 1  and PH 1 _ 2 , a main amplifier AMP 1 , and a coaxial delay line D 1 . The compensation loop L 2  is made up of two directional couplers HYB 2 , HYB 3  and intervening components therebetween. The intervening components are a coaxial delay line D 2 , a variable attenuator AT 2 _ 1 , two variable phase shifters PH 2 _ 1  and PH 2 _ 2 , and an auxiliary amplifier AMP 2 . 
         [0050]    In each of the loops L 1  and L 2 , the gain can be varied by the variable attenuator AT 1 _ 1 , AT 2 _ 1  such that the amplifier side route and the delay line route are identical in amount of delay and gain but are 180° out of phase with each other. The phases can be varied by the variable phase shifters PH 1 _ 1 , PH 2 _ 1 , PH 1 _ 2 , and PH 2 _ 2 . Such variations in gain and phase can be controlled by the control portion  1 . 
         [0051]    The gain and phase are adjusted by the variable attenuators AT 1 _ 1 , AT 2 _ 1  and variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2  in the loops L 1  and L 2 . Because of these functions, the functions of the vector adjusters are achieved. 
         [0052]    The control portion  1  controls the two variable phase shifters PH 1 _ 1  and PH 1 _ 2  in the distortion detection loop L 1  independently (separately), and controls the two variable phase shifters PH 2 _ 1  and PH 2 _ 2  in the distortion compensation loop L 2  independently (separately). 
         [0053]    One example of operation performed in the feedforward amplifier of the present embodiment is described now. A signal to be amplified is applied to the directional coupler HYB 1 . The coupler HYB 1  splits the input signal into two parts. One part is output to the variable attenuator AT 1 _ 1 , while the other part is output to the coaxial delay line D 1 . 
         [0054]    The variable attenuator AT 1 _ 1  attenuates the signal entered from the directional coupler HYB 1  by an amount of attenuation controlled by the control portion  1  and outputs the attenuated signal to the variable phase shifter PH 1 _ 1 . 
         [0055]    The variable phase shifter PH 1 _ 1  varies the phase of the signal entered from the variable attenuator AT 1 _ 1  by an amount of variation controlled by the control portion  1  and outputs the varied phase to the variable phase shifter PH 1 _ 2 . 
         [0056]    The variable phase shifter PH 1 _ 2  varies the phase of the signal entered from the variable phase shifter PH 1 _ 1  by an amount of variation controlled by the control portion  1  and outputs the varied phase to the main amplifier AMP 1 . 
         [0057]    The main amplifier AMP 1  amplifies the signal entered from the variable phase shifter PH 1 _ 2  and outputs the amplified signal to the directional coupler HYB 2 . In the main amplifier AMP 1 , distortion to be compensated for is produced. 
         [0058]    The coaxial delay line D 1  delays the signal entered from the directional coupler HYB 1  and outputs the delayed signal to the directional coupler HYB 2 . 
         [0059]    The directional coupler HYB 2  outputs the signal entered from the main amplifier AMP 1  to the coaxial delay line D 2 , combines the signal entered from the main amplifier AMP 1  and the signal entered from the coaxial delay line D 1 , and outputs the resulting signal to the variable attenuator AT 2 _ 1 . The signal output to the variable attenuator AT 2 _ 1  contains the component of distortion (ideally, only distortional component) produced in the main amplifier AMP 1 . 
         [0060]    The coaxial delay line D 2  delays the signal entered from the directional coupler HYB 2  and outputs the signal to the directional coupler HYB 3 . The variable attenuator AT 2 _ 1  attenuates the signal entered from the directional coupler HYB 2  by an amount of attenuation controlled by the control portion  1  and outputs the attenuated signal to the variable phase shifter PH 2 _ 1 . 
         [0061]    The variable phase shifter PH 2 _ 1  varies the phase of the signal entered from the variable attenuator AT 2 _ 1  by an amount of variation controlled by the control portion  1  and outputs the varied phase to the variable phase shifter PH 2 _ 2 . The phase shifter PH 2 _ 2  varies the phase of the signal entered from the variable phase shifter PH 2 _ 1  by an amount of variation controlled by the control portion  1  and outputs the varied phase to the auxiliary amplifier AMP 2 . The auxiliary amplifier AMP 2  amplifies the signal entered from the variable phase shifter PH 2 _ 2  and outputs the amplified signal to the directional coupler HYB 3 . 
         [0062]    The directional coupler HYB 3  combines the signal entered from the coaxial delay line D 2  and the signal entered from the auxiliary amplifier AMP 2 , and outputs the resulting signal as a signal indicating the result of distortion compensation. 
         [0063]    Ideally, the signal entered from the coaxial delay line D 2  includes the main signal (i.e., obtained by amplifying the original input signal) and a distortional component produced in the main amplifier AMP 1 . The signal entered from the auxiliary amplifier AMP 2  contains the distortional component produced in the main amplifier AMP 1 . These signals are combined, whereby the distortional component is canceled out. As a result, a distortionless amplifier output signal is produced from the directional coupler HYB 3 . 
         [0064]    In the feedforward amplifier of the present embodiment, the main amplifier AMP 1  is an amplifier for which distortion is compensated. In the distortion detection loop L 1 , the route having variable attenuator AT 1 _ 1 , variable phase shifters PH 1 _ 1 , PH 1 _ 2 , and main amplifier AMP 1  is an amplification route. In the distortion compensation loop L 2 , the route having variable attenuator AT 2 _ 1 , variable phase shifters PH 2 _ 1 , PH 2 _ 2 , and auxiliary amplifier AMP 2  is a distortion amplification route. 
         [0065]    In the feedforward amplifier of the present embodiment, the distortion detection loop L 1  has the first variable phase shifter PH_ 1  used for control and the second variable phase shifter PH 1 _ 2  used for adjustment. The distortion compensation loop L 2  has the first variable phase shifter PH 2 _ 1  used for control and the second variable phase shifter PH 2 _ 2  used for adjustment. The control portion  1  has the function of controlling the variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2 . This function constitutes a phase control portion. 
       First Embodiment 
       [0066]    A first embodiment of the present invention is described.  FIG. 2  illustrates one example of a sequence of operations for controlling the variable phase shifters PH 1 _ 1  and PH 1 _ 2  within the distortion detection loop L 1  in the feedforward amplifier of the present embodiment shown in  FIG. 1 . 
         [0067]    In the present embodiment, processing for controlling the variable phase shifters PH 1 _ 1  and PH 1 _ 2  within the distortion detection loop L 1  is described. Processing for controlling the variable phase shifters PH 2 _ 1  and PH 2 _ 2  within the distortion compensation loop L 2  can be performed similarly. 
         [0068]    In the present embodiment, two counters assume values i and j, respectively. Furthermore, in the present embodiment, it is assumed that the control portion  1  controls the amounts of attenuation of the variable attenuators AT 1 _ 1  and AT 2 _ 1  and the amounts of variations in phase in the variable phase shifters PH 1 _ 1 , PH 1 _ 2 , PH 2 _ 1 , and PH 2 _ 2 , using a control signal of 8 bits indicating a control value from 0 to 255. 
         [0069]    In addition, in the present embodiment, a control is provided so that as the value of the control signal indicating the control value from 0 to 255 decreases, the controlled amount (such as amount of attenuation and amount of variation in phase) is reduced, and vice versa. The relationship in magnitude between the value of the control signal (control value) and the controlled amount may be reversed as compared with the present embodiment. 
         [0070]    First, when the power supply for the feedforward amplifier is switched from OFF state to ON state, the values of the counters i and j are initialized at 0 (step S 1 ). Processing for optimizing the feedforward is started (step S 2 ). The amount of attenuation of the variable atteuator AT 1 _ 1  and the amount of variation in phase in the variable phase shifter PH 1 _ 1  are adaptively controlled to optimize the gain and phase of the distortion detection loop L 1 . 
         [0071]    At this time, with respect to the distortion compensation loop L 2 , too, the amount of attenuation of the variable attenuator AT 2 _ 1  and the amount of variation in phase in the variable phase shifter PH 2 _ 1  are varied to optimize the gain and phase in the distortion compensation loop L 2 . Because the operation is the same as for inside the distortion detection loop L 1 , its description is omitted below. 
         [0072]    In processing for feedforward optimization, if the control value for the variable phase shifter P 1 _ 1  assumes a minimum value of 0 (step S 3 ), the value of the counter i is incremented by 1 (step S 11 ). If the value of the counter i reaches +3 before control settles down (step S 12 ), a correction is made such that the control value α for the variable phase shifter P 1 _ 2  is varied to (α−100) (step S 13 ). Control returns to the processing in which the values of the counters i and j are set to 0 (step S 1 ). If the value of the counter i has not reached +3 (step S 12 ), the processing for feedforward optimization is continued (step S 2 ). 
         [0073]    Where the control value for the variable phase shifter P 1 _ 1  assumes a maximum value of 255 during processing for feedforward optimization (step S 4 ), the value of the counter j is incremented by 1 (step S 14 ). If the value of the counter j reaches +3 before the control settles down (step S 15 ), a correction is made such that the control value α for the variable phase shifter P 1 _ 2  is varied to (α+100) (step S 16 ). Control returns to the processing for resetting the values of the counters i and j to 0 (step S 1 ). If the value of the counter j has not reached +3 (step S 15 ), the processing for feedforward optimization is continued (step S 2 ). 
         [0074]    A decision is made as to whether the control has settled down by the processing for feedforward optimization (step S 5 ). If the control has settled down, the processing is terminated. Meanwhile, if the control has not settled down, control returns to the processing for resetting the values of the counters i and j to 0 (step S 1 ). The processing for feedforward optimization is again performed (step S 2 ). Alternatively, after step S 13 , the control value for the variable phase shifter P 1 _ 1  may be increased by 100. Still alternatively, after step S 16 , the control value may be reduced by 100. 
         [0075]    Any arbitrary technique can be used to determine whether the control has settled down. For example, with respect to the distortion detection loop L 1 , if a control is provided so that the level of the signal output to the variable attenuator AT 2 _ 1  from the directional coupler HYB 2  is detected and that the level is reduced (i.e., only distortional component is contained in the signal), a technique making it possible to determine that the control has settled down when the level has been equal to or less than a given threshold value can be used. With respect to the distortion compensation loop L 2 , if a control is provided so that the level of distortion contained in a signal output from the directional coupler HYB 3  is detected and that the level is reduced, a technique making it possible to determine that the control has settled down when the level has been equal to or less than a given threshold value can be used. 
         [0076]    If the control value for the variable phase shifter PH 1 _ 1  becomes “0” or “255” three times before the control has settled down as in the present embodiment, the phase in the variable phase shifter PH 1 _ 2  is automatically varied by an apparatus. Thus, the control range of the variable phase shifter P 1 _ 1  seems to have become wider. 
         [0077]    As described so far, in the feedforward amplifier of the present embodiment, the vector adjuster in the distortion detection loop L 1  is equipped with the two phase shifters, i.e., variable phase shifter PH 1 _ 1  used for phase control and variable phase shifter PH 1 _ 2  used for phase adjustment. The vector adjuster in the distortion compensation loop L 2  is equipped with the two phase shifters, i.e., variable phase shifter PH 2 _ 1  used for phase control and variable phase shifter PH 2 _ 2  used for phase adjustment. Furthermore, in the present embodiment, in each of the loops L 1  and L 2 , the amount of variations in phase in the two variable phase shifters PH 1 _ 1  and PH 1 _ 2  or PH 2 _ 1  and PH 2 _ 2  are controlled independently by variable amounts. In the present embodiment, in each of the loops L 1  and L 2 , if values of the amounts of variations in phase in the controlling variable phase shifters PH 1 _ 1  and PH 2 _ 1  are concentrated toward either one of relatively-larger direction(s) and relatively-smaller direction(s), the phases of the adjusting variable phases PH 1 _ 2  and PH 2 _ 2  are switched. 
         [0078]    In this way, in the feedforward amplifier of the present embodiment, the function of varying the phase in the vector adjuster in each of the loops L 1  and L 2  is achieved by two stages. The variable range of phases is substantially extended. Principally, the amount of variation in phase in one of the variable phase shifters PH 1 _ 1  and PH 2 _ 1  is adaptively controlled. When one limit of the variable range is reached, the number of times that the limit is reached is counted. If the count value reaches or exceeds a prescribed number, the amount of variation in phase in the other of the variable phase shifters PH 1 _ 2  and PH 2 _ 2  is varied. 
         [0079]    Accordingly, in the feedforward amplifier of the present embodiment, a wider range of phases can be varied in adjusting vectors in each of the loops L 1  and L 2  when the phase is varied due to moisture absorption into the substrate, due to variations of the temperature of the substrate, or due to aging. Therefore, when the phase is varied greatly due to moisture absorption into the substrate, for example, variations in phase can be suppressed accordingly and appropriately. Furthermore, in the present embodiment, if the phase is varied by a large amount due to moisture absorption into the substrate, the feedforward control range of phase can be extended, for example, without adding any phase shifter. 
         [0080]    The feature of the control method of the present embodiment shown in  FIG. 2  is described below. In the present embodiment, after the amount of variation in phase in each of the adjusting variable phase shifters PH 1 _ 2  and PH 2 _ 2  is varied, if the power supply is once turned off, the previous value of the amount of variation in phase is stored in a memory. Therefore, if a variation occurs at all, the variable phase shifters PH 1 _ 2  and PH 2 _ 2  start control from the varied value (the amount of variation in phase) when the power supply is turned on the next time. 
         [0081]    Furthermore, in the circuit of the feedforward amplifier of the present embodiment, if the power supply is activated, the ambient temperature varies rapidly, or the input level varies rapidly, it takes a long time until control settles down. In addition, the control value reaches “0” of or “ 255 ” multiple times (three times, in the present embodiment) until a focal point is found. As a result, the amounts of variations in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  may vary in a manner deviating from the intrinsic object. 
       Second Embodiment 
       [0082]    A second embodiment of the present invention is described. The present embodiment provides improvements of the features of the control method shown in  FIG. 2 , i.e., the amounts of variations in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  are stored in a memory before the power supply is turned off and, when the power supply is activated or the temperature or input level varies rapidly, the amounts of variations in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  vary in a manner different from the intrinsic object. 
         [0083]      FIG. 3  illustrates one example of a sequence of operations for controlling the variable phase shifters PH 1 _ 1  and PH 1 _ 2  in the distortion detection loop L 1  of the feedforward amplifier of the present embodiment shown in  FIG. 1 . 
         [0084]    In the present embodiment, processing for controlling the variable phase shifters PH 1 _ 1  and PH 1 _ 2  within the distortion detection loop L 1  is described. Processing for controlling the variable phase shifters PH 2 _ 1  and PH 2 _ 2  within the distortion compensation loop L 2  can be processed similarly. 
         [0085]    The control method of the present embodiment illustrated in  FIG. 3  is similar to the control method illustrated in  FIG. 2  except that processing of steps S 21 , S 22 , S 23 , and S 24  is added. For convenience of illustration, processing steps of  FIG. 3  similar to their counterparts (steps S 1 -S 5  and S 11 -S 16 ) illustrated in  FIG. 2  are indicated by the same reference numerals as in  FIG. 2 . The differences of the present embodiment with the processing illustrated in  FIG. 2  are next described in detail. 
         [0086]    In the control method of the present embodiment illustrated in  FIG. 3 , when the power supply for the feedforward amplifier is switched from OFF state to ON state, the amount of variation in phase in the adjusting variable phase shifter PH 1 _ 2  is first initialized (step S 21 ). Then, control goes to the processing of step S 1 , where the amounts of variation in phase are set to a given value (e.g., 0), for example, to initialize the amounts of variation in phase. 
         [0087]    For example, depending on the state in which the feedforward amplifier is stocked, the state of the substrate is varied, for example, due to moisture absorption compared with the state in which the power supply for the amplifier was turned on the previous time. Therefore, it is desired that the initial values of the amounts of phase in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  are returned to a given value and reset to the original state. 
         [0088]    In the control method of the present embodiment, if processing for feedforward optimization is started (step S 2 ), a decision is made as to whether a given time (1 minute in the present embodiment) has passed since the power supply for the feedforward amplifier has been turned on (step S 22 ). If the given time has passed, a decision is made as to whether the temperature has varied rapidly (step S 23 ). If the temperature has not varied rapidly, a decision is made as to whether the input level has varied rapidly (step S 24 ). If the input level has not varied rapidly, control proceeds to the processing of step S 3 . Meanwhile, if the given time has not passed since the power supply has been turned on (step S 22 ), the temperature has varied rapidly (step S 23 ), or the input level has varied rapidly (step S 24 ), control goes to processing of step S 5 . If control has not settled down, control returns to the processing of step S 1 . 
         [0089]    During the processing of step S 22 , the amounts of variations in phase in the adjusting variable phase shifters PH 1 _ 2  and PH 2 _ 2  are varied after the given time has passed since the power supply has been turned on for the following reason. When the power supply is activated, it takes some time until the operation of the amplifier stabilizes. During this time interval, the amounts of variations in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  are prevented from being varied. 
         [0090]    One example of the configuration for making a decision as to whether the given time has passed since the power supply has been turned on has the function of a timer starting to count the time in response to turning on of the power supply and can determine that the given time has passed since the power supply has been turned on when the time counted by the timer has been equal to or longer than a given time or has passed beyond a given instant of time. 
         [0091]    During the processing of step S 23 , when the temperature varies violently, the amounts of variations in phase in the adjusting variable phase shifters PH 1 _ 2  and PH 2 _ 2  are prevented from being varied, for the following reason. If violent temperature variations take place, the amplifier does not operate stably. During this time interval, the amounts of variations in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  are prevented from being varied. 
         [0092]    One example of the configuration for making a decision as to whether the temperature is varying violently is equipped with a temperature detector inside or near the feedforward amplifier or at any arbitrary position to detect timewise amounts of variation (or otherwise, rate of variation) of the temperature detected by the temperature detector and can determine that the temperature is varying violently if the timewise amount of variation is in excess of a given threshold value. 
         [0093]    In the processing of step S 24 , the amounts of variations in phase in the adjusting variable phase shifters PH 1 _ 2  and PH 2 _ 2  are prevented from being varied if the input level is varying violently, for the following reason. If the input level varies violently, the amplifier does not operate stably. During this time interval, the amounts of variations in phase in the variable phase shifters PH 1 _ 2  and PH 2 _ 2  are prevented from being varied. 
         [0094]    One example of the configuration for making a decision as to whether the input level is varying violently can be equipped with a level detector in a stage preceding the feedforward amplifier, the input terminal, or other position where the level of the input signal can be grasped, detects the timewise amounts of variation (or otherwise, rate of variation) of the level detected by the level detector, and determines that the input level is varying violently if the timewise amount of variation is equal to or in excess of a given threshold value. 
         [0095]    Because of the processing of the steps S 22 , S 23 , and S 24 , the amounts of variations in phase in the adjusting variable phase shifters PH 1 _ 2  and PH 2 _ 2  are prevented from being varied when the amplifier does not operate stably. 
         [0096]    Furthermore, in the control method of the present embodiment, too, the phase can be varied within a wider range in each of the loops L 1  and L 2  by performing processing (steps S 1 -S 5  and S 11 -S 16 ) similar to the processing illustrated in  FIG. 2  and permitting the amount of variation in phase of another adjusting variable phase adjuster PH 1 _ 2  or PH 2 _ 2  to be varied automatically even if the control value for the controlling variable phase shifter PH 1 _ 1  or PH 2 _ 1  becomes “0” or “255” plural times (three times in the present embodiment). 
         [0097]    As described so far, in the feedforward amplifier of the present embodiment, when the power supply is turned on, the phases (amounts of variations in phase) of the adjusting variable phase shifters PH 1 _ 2  and PH 2 _ 2  are initialized at a given set value. Therefore, if the amplifier is affected by variations in state due to moisture absorption into the substrate during the time interval from the instant when the power supply was turned off the previous time to the instant when the power supply was turned on the present time, control can be started from a given set value (amount of variation in phase) at all times. 
         [0098]    Furthermore, in the feedforward amplifier of the present embodiment, the amounts of variations in phase in the adjusting variable phase adjusters PH 1 _ 2  and PH 2 _ 2  are prevented from being switched immediately after the power supply is turned on or when the temperature or input level is varying violently. Consequently, the amounts of variations in phase in the adjusting variable phase adjusters PH 1 _ 2  and PH 2 _ 2  can be switched, for example, after the operation has stabilized. 
         [0099]    It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claim.