Abstract:
A push-pull amplifier comprising at least one pair of transistors wherein the transistors comprising the pair of transistors are transistors having a control terminal, a first terminal, and a second terminal, and the current that flows between the first terminal and the second terminal is controlled in accordance with signals applied to the control terminal, this push-pull amplifier being characterized in that when the amount of current flowing between the first terminal and the second terminal of one of the pair of transistors is within a predetermined range, the high-frequency component of the signals input to the control terminal of one of the pair of transistors is amplified in comparison to the case when this current is outside the predetermined range.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention pertains to a power amplifying apparatus and relates in particular to a power amplifying apparatus comprising a push-pull amplifier.  
       DISCUSSION OF THE BACKGROUND ART  
       [0002]     A push-pull amplifier is a typical power amplifying means. A push-pull amplifier is an amplifier made from a pair of transistors and is such that the output from each of the transistors in the pair is synthesized to obtain a signal output (for instance, refer to JP (Kokai) 2003-060,451 (pages 4 and 5, FIG. 1)).  
         [0003]     A circuit diagram of a typical power amplifying apparatus comprising a push-pull amplifier is shown in  FIG. 1 . A power amplifying apparatus  10  in  FIG. 1  comprises an input terminal X 1 , an operational amplifier A 11 , a push-pull amplifier  100 , and an output terminal Y 1 . Input terminal X 1  is connected to the inverting input of operational amplifier A 11 . The output signals of operational amplifier A 11  are input to a push-pull amplifier  100 . The output signals of push pull amplifier  100  are applied to output terminal Y 1  and to the non-inverting input of operational amplifier A 11 . It should be noted that a load Z is connected to output terminal Y 1  in order to facilitate understanding of the description that follows. Moreover, load Z is a resistor.  
         [0004]     Push-pull amplifier  100  is a bipolar push-pull amplifier that uses MOS-type field effect transistors. Push-pull amplifier  100  comprises an input terminal S 1 , an output terminal T 1 , a P channel MOS-type field effect transistor Q 11 , and an N channel MOS-type field effect transistor Q 12 . The source of field effect transistor Q 11  is connected to a positive power source Vcc. The source of field effect transistor Q 12  is connected to a negative power source Vee. The output signals of operational amplifier A 11  are applied through a constant-voltage source E 11  to the gate of field effect transistor Q 11 . Moreover, the output signals of operational amplifier A 11  are applied through a constant-voltage source E 12  to the gate of field effect transistor Q 12 . The drain of field effect transistor Q 11  and the drain of field effect transistor Q 12  are connected together. Moreover, these drains are connected to output terminal Y 1 . In short, push-pull amplifier  100  is a drain output-type push-pull amplifier that outputs signals that have been inverted with respect to signals received at S 1 . Constant-voltage source E 11  and constant-voltage source E 12  are used for bias. Push-pull amplifier  100  can be operated as a class A, class AB, or class B amplifier by adjusting the magnitude of the bias.  
         [0005]     Moreover, operational amplifier A 11  acts in such a way that there is no difference between the voltage at input terminal X 1  and the voltage at output terminal Y 1 . As a result, power amplifying apparatus  10  power-amplifies signals input from input terminal X 1  and outputs them from output terminal Y 1 .  
         [0006]     The loop gain (dB) of power amplifying apparatus  10  is the sum of the amplification factor (dB) of operational amplifier A 11  and the amplification factor (dB) of push-pull amplifier  100 . In general, the amplification factor of operational amplifier A 11  is set so that it decreases with an increase in frequency, while the amplification factor of push-pull amplifier  100  is set so that it is constant regardless of frequency. Moreover, the amplification factor of push-pull amplifier  100  decreases as the drain current of field effect transistor Q 11  or Q 12  becomes smaller. The reason for this is as follows.  
         [0007]     The amplification factor GM of push-pull amplifier  100  is the sum of amplification factor gm 1  of field effect transistor Q 11  and amplification factor gm 2  of field effect transistor Q 12 . The AC amplification factor of a field effect transistor is represented by mutual conductance gm. It should be noted that mutual conductance gm is also represented as forward transmission admittance |Y fs |. The mutual conductance of a field effect transistor is virtually zero as long as the gate voltage is the threshold voltage or less. The drain current increases as gate voltage rises. As a result, mutual conductance gm increases. Therefore, it is assumed that push-pull amplifier  100  operates as a class AB amplifier. In general, the drain current of field effect transistor Q 11  is large and the amplification factor gm 1  of field effect transistor Q 11  is also large when push-pull amplifier  100  outputs a large positive voltage. In this case, gm 1  dominates the amplification factor GM; therefore, GM is large, as well as gm 1 . The drain current of field effect transistor Q 11  decreases and gm 1  also becomes smaller as the output voltage of push-pull amplifier  100  gradually decreases. The drain current of field effect transistor Q 12  at this time is small and the amplification factor gm 2  of field effect transistor Q 12  is also small. There is a gradual increase in gm 2  as the output voltage of push-pull amplifier  100  falls closer to zero. However, the amplification factor GM is quite small at this time. Thereafter, amplification factor gm 2  of field effect transistor Q 12  increases with a further reduction in the output voltage of push-pull amplifier  100 . The amplification factor GM is now dominated by gm 2  at this time; therefore, GM is large, as well as gm 2 . Similarly, the overall amplification factor GM changes dependent on the drain current of each field effect transistor and the output voltage of push-pull amplifier  100 .  
         [0008]     Power amplifying apparatus  10  has a finite frequency band; therefore, the frequency band of power amplifying apparatus  10  becomes narrower as the AC amplification factor of push-pull amplifier  100  decreases. Moreover, the frequency band of power amplifying apparatus  10  changes considerably as the change in the drain current of field effect transistor Q 11  or Q 12  becomes larger. Power amplifying apparatus  10  tends to vibrate and outputs an unwanted ringing waveshape as the amount of change in the frequency band increases. Furthermore, there is also a problem with this type of power amplifying apparatus  10  in that when there is high-speed change in the size of the load, it is difficult to maintain a constant output level.  
         [0009]     The current amplification factor hfe of a bipolar transistor decreases with a reduction in the collector current. Consequently, the above-mentioned problem similarly occurs in a push-pull amplifier made from a pair of bipolar transistors and a push-pull amplifier made from a pair comprised of a bipolar transistor and a field effect transistor. Moreover, this problem becomes more obvious as the electrical efficiency of the push-pull amplifier improves. This is because the idle drain current and collector current are set at zero or at a relatively small value.  
         [0010]     The present invention solves the above-mentioned problems, an object thereof being to provide a push-pull amplifier that prevents the reduction in the AC amplification factor that is attributed to changes in the collector current or the source current of the transistors. Moreover, an object of the present invention is to provide a power amplifying apparatus comprising this type of push-pull amplifier.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides that the amplification factor in the high-frequency band of a push-pull amplifier made of a pair comprised of a push-side transistor and a pull-side transistor is compensated when the collector current or source current of the transistor on the push side or of the transistor on the pull side is within a predetermined range. In order to compensate the amplification factor, the push-pull amplifier of the present invention manipulates signals applied to the base or the gate of the transistor on the push side or of the transistor on the pull side, or directly supplements output signals.  
         [0012]     That is, the present invention is a push-pull amplifier comprising at least one pair of transistors wherein the transistors comprising the pair of transistors are transistors having a control terminal, a first terminal, and a second terminal and the current that flows between the first terminal and the second terminal is controlled in accordance with signals applied to the control terminal, this push-pull amplifier being characterized in that when the amount of current flowing between the first terminal and the second terminal of one of the pair of transistors is within a predetermined range, the high-frequency component of the signals input to the control terminal of one of the pair of transistors is amplified in comparison to the case when this current is outside the predetermined range. The control terminal here is a base or a gate. The first and second terminals are a collector, emitter, drain, or source. For instance, when the transistor is a typical field effect transistor, the control terminal is a gate and the first and second terminals are a drain and a source.  
         [0013]     Moreover, the present invention is further characterized in that it comprises a detector for detecting the current that flows between the first and second terminals of one of the pair of transistors and a filtration means for extracting a high-frequency component from the input signals and outputting the filtered signal only when the current detected by the detector is within a predetermined range.  
         [0014]     The present invention also pertains to a push-pull amplifier that comprises: an input, an output, and at least one pair of transistors, wherein the pair of transistors amplifies signals received at the input terminal and outputs the amplified signals to the output terminal and the transistors making up this transistor pair are transistors having a control terminal, a first terminal, and a second terminal that controls the current flowing between the first terminal and the second terminal in accordance with the signals applied to the control terminal, this push-pull amplifier being characterized in that it comprises an amplifier for amplifying only the high-frequency component extracted from signals received at the input terminal and outputting the amplified signal to the output terminal only when the amount of current flowing between the first and second terminals of one of the pair of transistors is within a predetermined range.  
         [0015]     The present invention is further characterized in that the amplifier comprises a detector for detecting the current flowing between the first and second terminals of one of the pair of transistors; a filter for extracting and outputting the high-frequency component from signals received as the input terminal only when the current detected by the detector is within a predetermined range, and a transistor that is disposed parallel to one of the pair of transistors and operates in response to the output signals of the filter.  
         [0016]     The filter preferably has a signal amplifier.  
         [0017]     A power amplifying apparatus comprising a push-pull amplifier comprising at least one pair of transistors wherein the transistors comprising the pair of transistors are transistors having a control terminal, a first terminal, and a second terminal and the current that flows between the first terminal and the second terminal is controlled in accordance with signals applied to the control terminal, this push-pull amplifier being characterized in that when the amount of current flowing between the first terminal and the second terminal of one of the pair of transistors is within a predetermined range, the high-frequency component of the signals input to the control terminal of one of the pair of transistors is amplified in comparison to the case when this current is outside the predetermined range.  
         [0018]     A power amplifying apparatus comprising a push-pull amplifier that comprises: an input, an output, and at least one pair of transistors, wherein the pair of transistors amplifies signals received at the input terminal and outputs the amplified signals to the output terminal and the transistors making up this transistor pair are transistors having a control terminal, a first terminal, and a second terminal that controls the current flowing between the first terminal and the second terminal in accordance with the signals applied to the control terminal, this push-pull amplifier being characterized in that it comprises an amplifier for amplifying only the high-frequency component extracted from signals received at the input terminal and outputting the amplified signal to the output terminal only when the amount of current flowing between the first and second terminals of one of the pair of transistors is within a predetermined range.  
         [0019]     By means of the present invention, it is possible to control the changes in the AC amplification factor that are attributed to changes in the collector current or source current of one of the pair of transistors of a push-pull amplifier comprising at least one pair of transistors. Moreover, by means of the present invention, it is possible to control the reduction in the AC amplification factor when the collector current or the source current of one of a pair of transistors is small in a push-pull amplifier comprising at least one pair of transistors. As a result, a power amplifying apparatus that uses this type of push-pull amplifier can amplify with stability signals of a higher frequency than in the past. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a drawing showing a power amplifying apparatus  10  comprising a conventional push-pull amplifier  100 .  
         [0021]      FIG. 2  is a drawing showing a power amplifying apparatus  20  comprising a push-pull amplifier  200  of the present invention.  
         [0022]      FIG. 3  is a drawing showing a power amplifying apparatus  30  comprising a push-pull amplifier  300  of the present invention.  
         [0023]      FIG. 4  is a drawing showing a power amplifying apparatus  40  comprising a push-pull amplifier  400  of the present invention.  
         [0024]      FIG. 5  is a drawing showing a power amplifying apparatus  50  comprising a push-pull amplifier  500  of the present invention.  
         [0025]      FIG. 6  is a drawing showing a power amplifying apparatus  60  comprising a push-pull amplifier  600  of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]     The present invention will be explained in detail based on the embodiments shown in the attached drawings. By means of the present embodiment, the amplification factor of the push-pull amplifier is compensated by manipulating the signals input to the gate of the transistor on the push side. The first embodiment of the present invention is a power amplifying apparatus  20  shown in  FIG. 2 .  
         [0027]     Power amplifying apparatus  20  in the figure comprises an input terminal X 2 , an output terminal Y 2 , an operational amplifier A 21 , and a push-pull amplifier  200 . Signals at input terminal X 2  and signals at output terminal Y 2  are applied to operational amplifier A 21 . Operational amplifier A 21  inputs to push-pull amplifier  200  signals such that the difference between the two signals to be input is zero. Push-pull amplifier  200  power-amplifies input signals and outputs the product to output terminal Y 2 .  
         [0028]     Push-pull amplifier  200  is a bipolar push-pull amplifier that uses an MOS field effect transistor. Push-pull amplifier  200  comprises an input terminal S 2 , an output terminal T 2 , a push-side P channel MOS field effect transistor Q 21 , a pull-side N channel MOS field effect transistor Q 22 , a constant-voltage source E 21 , and a constant-voltage source E 22 . The drain of field effect transistor Q 21  and the drain of field effect transistor Q 22  are connected to output terminal T 2 . The source of field effect transistor Q 21  is connected to a positive power source Vcc. The source of field effect transistor Q 22  is connected to a negative power source Vee. Signals received at input terminal S 2 , that is, output signals of operational amplifier A 21 , are biased by constant-voltage source E 21  and input to the gate of field effect transistor Q 21 . Moreover, signals received at input terminal S 2  are biased by constant-voltage source E 22  and input to the gate of field effect transistor Q 22 . The pair of field effect transistors Q 21  and Q 22  operates as a class A, class AB, or class B amplifier in accordance with the voltage of constant-voltage sources E 21  and E 22 . The structure described above is the basic part of push-pull amplifier  200 .  
         [0029]     Furthermore, push-pull amplifier  200  comprises a resistor R 21  and an operational amplifier A 22 . Resistor R 21  is disposed between the source of field effect transistor Q 21  and positive power source Vcc. Operational amplifier A 22  detects the potential difference between the two terminals of resistor R 21  and outputs signals that represent the amount of current flowing between the source and drain of field effect transistor Q 21 .  
         [0030]     Push-pull amplifier  200  comprises a high-pass filter F 2 , an amplifier A 23 , a capacitor C 2 , a resistor R 22 , and a resistor R 23 . High-pass filter F 2  extracts the high-frequency component of signals received at input terminal S 2  and outputs the filtered product to amplifier A 23 . The cut-off frequency of high-pass filter F 2  is set such that it includes the band that should be compensated. Amplifier A 23  is the amplifier that amplifies input signals by a predetermined amplitude and then outputs the amplified signal. Amplifier A 23  operates in accordance with output signals of operational amplifier A 22 . In further detail, amplifier A 23  outputs signals only when the amount of current flowing between the source and drain of field effect transistor Q 21  is a predetermined value or less. Capacitor C 2  is used in order to obtain the alternating current component from the output signals of amplifier A 23 . Resistors R 22  and R 23  operate as a signal adding means.  
         [0031]     Consequently, only signals that have been biased by constant-voltage source E 21  are applied at the gate of field effect transistor Q 21  when the amount of current flowing between the source and the drain of field effect transistor Q 21  is larger than a predetermined value. On the other hand, signals obtained by adding signals that have been biased by constant-voltage source E 21  and output signals of amplifier A 23  are applied at the gate of field effect transistor Q 21  when the amount of current flowing between the source and drain of field effect transistor Q 21  is a predetermined value or less. In short, if the amount of current flowing between the source and drain of field effect transistor Q 21  is a predetermined value or less, the high-frequency component of signals input to the gate of field effect transistor A 21  are amplified in comparison to when the amount of current is greater than a predetermined value. The amplification factor of push-pull amplifier  200  is thereby compensated.  
         [0032]     Next, a second embodiment of the present invention will be described. By means of the present embodiment, signals input to the gate of the pull-side transistor are manipulated to compensate the amplification factor of the push-pull amplifier.  FIG. 3  shows a power amplifying apparatus  30 , the second embodiment of the present invention.  
         [0033]     Power amplifying apparatus  30  in the figure comprises an input terminal X 3 , an output terminal Y 3 , an operational amplifier A 31 , and a push-pull amplifier  300 . Signals at input terminal X 3  and signals at output terminal Y 3  are applied to operational amplifier A 31 . Operational amplifier A 31  inputs to push-pull amplifier  300  signals such that the difference between two signals to be input is zero. Push-pull amplifier  300  power-amplifies input signals and outputs amplified signals to output terminal Y 3 .  
         [0034]     Push-pull amplifier  300  is a bipolar push-pull amplifier that uses an MOS field effect transistor. Push-pull amplifier  300  comprises an input terminal S 3 , an output terminal T 3 , a push-side P channel MOS field effect transistor Q 31 , a pull-side N channel MOS field effect transistor Q 32 , a constant-voltage source E 31 , and a constant-voltage source E 32 . The drain of field effect transistor Q 31  and the drain of field effect transistor Q 32  are connected to output terminal T 3 . The source of field effect transistor Q 31  is connected to a positive power source Vcc. The source of field effect transistor Q 32  is connected to a negative power source Vee. Signals received at input terminal S 3 , that is, output signals of operational amplifier A 31 , are biased by constant-voltage source E 31  and input to the gate of field effect transistor Q 31 . Moreover, signals received at input terminal S 3  are biased by constant-voltage source E 32  and input to the gate of field effect transistor Q 32 . The pair of field effect transistors Q 31  and Q 32  operates as a class A or class AB amplifier in accordance with the voltage of constant-voltage sources E 31  and E 32 . The structure described above is the basic part of push-pull amplifier  300 .  
         [0035]     Furthermore, push-pull amplifier  300  comprises a resistor R 31  and an operational amplifier A 32 . Resistor R 31  is disposed between the source of field effect transistor Q 31  and positive power source Vcc. Operational amplifier A 32  detects the potential difference between the two terminals of resistor R 31  and outputs signals that represent the amount of current flowing between the source and drain of field effect transistor Q 31 .  
         [0036]     Push-pull amplifier  300  comprises a high-pass filter F 3 , an amplifier A 33 , a capacitor C 3 , a resistor R 32 , and a resistor R 33 . High-pass filter F 3  extracts the high-frequency component of signals received at input terminal S 3  and outputs the filtered signal to amplifier A 33 . The cut-off frequency of high-pass filter F 3  is set such that it includes the band that should be compensated. Amplifier A 33  is the amplifier that amplifies input signals by a predetermined amplitude and then outputs an amplified signal. Amplifier A 33  operates in accordance with the output signals of operational amplifier A 32 . In further detail, amplifier A 33  outputs signals only when the amount of current flowing between the source and the drain of field effect transistor Q 31  is a predetermined value or less. Capacitor C 3  is used in order to obtain the alternating current component from the output signals of amplifier A 33 . Resistors R 32  and R 33  operate as a signal adding means.  
         [0037]     Consequently, only signals that have been biased by constant-voltage source E 32  are applied at the gate of field effect transistor Q 32  when the amount of current flowing between the source and the drain of field effect transistor Q 31  is greater than a predetermined value. On the other hand, signals obtained by adding signals that have been biased by constant-voltage source E 32  and output signals of amplifier A 33  are applied at the gate of field effect transistor Q 32  when the amount of current flowing between the source and the drain of field effect transistor Q 31  is a predetermined value or less. In short, if the amount of current flowing between the source and the drain of field effect transistor Q 31  is a predetermined value or less, the high-frequency component of signals input to the gate of field effect transistor Q 32  is amplified in comparison to when the amount of current is greater than a predetermined value. The amplification factor of push-pull amplifier  300  is thereby compensated.  
         [0038]     By means of the first and second embodiments, signals applied to the gate of a field effect transistor are manipulated in order to compensate the amplification factor. Embodiments whereby the amplification factor is compensated by directly supplementing output signals will be described next.  
         [0039]     The third embodiment of the present invention is a power amplifying apparatus  40  shown in  FIG. 4 .  
         [0040]     Power amplifying apparatus  40  in the figure comprises an input terminal X 4 , an output terminal Y 4 , an operational amplifier A 41 , and a push-pull amplifier  400 . Signals at input terminal X 4  and signals at output terminal Y 4  are applied to operational amplifier A 41 . Operational amplifier A 41  inputs to push-pull amplifier  400  signals such that the difference between two signals to be input is zero. Push-pull amplifier  400  power-amplifies input signals and outputs the amplified signal to output terminal Y 4 .  
         [0041]     Push-pull amplifier  400  is a bipolar push-pull amplifier that uses an MOS field effect transistor. Push-pull amplifier  400  comprises an input terminal S 4 , an output terminal T 4 , a push-side P channel MOS field effect transistor Q 41 , a pull-side N channel MOS field effect transistor Q 42 , a constant-voltage source E 41 , and a constant-voltage source E 42 . The drain of field effect transistor Q 41  and the drain of field effect transistor Q 42  are connected to output terminal T 4 . The source of field effect transistor Q 41  is connected to a positive power source Vcc. The source of field effect transistor Q 42  is connected to a negative power source Vee. Signals received at input terminal S 4 , that is, output signals of operational amplifier A 41 , are biased by constant-voltage source E 41  and input to the gate of field effect transistor Q 41 . Moreover, signals received at input terminal S 4  are biased by constant-voltage source E 42  and input to the gate of field effect transistor Q 42 . The pair of field effect transistors Q 41  and Q 42  operates as a class A, class AB, or class B amplifier in accordance with the voltage of constant-voltage sources E 41  and E 42 . The structure described above is the basic part of push-pull amplifier  400 .  
         [0042]     Furthermore, push-pull amplifier  400  comprises resistor R 4  and operational amplifier A 42 . Resistor R 4  is disposed between the source of field effect transistor Q 41  and positive power source Vcc. Operational amplifier A 42  detects the potential difference between the two terminals of resistor R 4  and outputs signals that represent the amount of current flowing between the source and the drain of field effect transistor Q 41 .  
         [0043]     Push-pull amplifier  400  comprises a high-pass filter F 4 , an amplifier A 43 , and a boost transistor Q 43 . Boost transistor Q 43  is a P channel MOS field effect transistor. High-pass filter F 4  extracts the high-frequency component of signals received at input terminal S 4  and outputs the filtered signal to amplifier A 43 . The cut-off frequency of high-pass filter F 4  is set such that it includes the band that should be compensated. Amplifier A 43  is the amplifier that amplifies input signals by a predetermined amplitude and then outputs the amplified signal. The output signals of amplifier A 43  are applied to the gate of boost transistor Q 43 . The source of boost transistor Q 43  is connected to positive power source Vcc. The drain of boost transistor Q 43  is connected to output terminal T 4 . Amplifier A 43  operates in accordance with output signals of operational amplifier A 42 . In further detail, amplifier A 43  amplifies input signals and outputs the amplified signal when the amount of current flowing between the source and the drain of field effect transistor Q 41  is a predetermined value or less. Boost transistor Q 43  is turned on by signals input to the gate at that time. On the other hand, amplifier A 43  outputs signals such that boost transistor Q 43  is turned off when the amount of current flowing between the source and the drain of field effect transistor Q 41  is greater than a predetermined value. In addition, amplifier A 43  outputs signals such that boost transistor Q 43  is turned off when the amount of current flowing between the source and the drain of field effect transistor Q 41  is zero, that is, when field effect transistor Q 41  is off.  
         [0044]     Consequently, boost transistor Q 43  amplifies only the high-frequency component extracted from signals received at input terminal S 4  and outputs the amplified signal to output terminal T 4  only when the amount of current flowing between the source and the drain of field effect ransistor Q 41  is within a predetermined range. The amplification factor of push-pull amplifier  400  is thereby compensated.  
         [0045]     A fourth embodiment of the present invention is a power amplifying apparatus  50  shown in  FIG. 5 .  
         [0046]     Power amplifying apparatus  50  in the figure comprises an input terminal X 5 , an output terminal Y 5 , an operational amplifier A 51 , and a push-pull amplifier  500 . Signals at input terminal X 5  and signals at output terminal Y 5  are applied to operational amplifier A 51 . Operational amplifier A 51  inputs to push-pull amplifier  500  signals such that the difference between two signals to be input is zero. Push-pull amplifier  500  power-amplifies input signals and outputs the amplified to output terminal Y 5 .  
         [0047]     Push-pull amplifier  500  is a bipolar push-pull amplifier that uses an MOS field effect transistor. Push-pull amplifier  500  comprises an input terminal S 5 , an output terminal T 5 , a push-side P channel MOS field effect transistor Q 51 , a pull-side N channel MOS field effect transistor Q 52 , a constant-voltage source E 51 , and a constant-voltage source E 52 . The drain of field effect transistor Q 51  and the drain of field effect transistor Q 52  are connected to output terminal T 5 . The source of field effect transistor Q 51  is connected to a positive power source Vcc. The source of field effect transistor Q 52  is connected to a negative power source Vee. Signals received at input terminal S 5 , that is, output signals of operational amplifier A 51 , are biased by constant-voltage source E 51  and input to the gate of field effect transistor Q 51 . Moreover, signals received at input terminal S 5  are biased by constant-voltage source E 52  and input to the gate of field effect transistor Q 52 . The pair of field effect transistors Q 51  and Q 52  operates as a class A, class AB, or class B amplifier in accordance with the voltage of constant-voltage sources E 51  and E 52 . The structure described above is the basic part of push-pull amplifier  500 .  
         [0048]     Furthermore, push-pull amplifier  500  comprises a resistor R 5  and operational amplifier A 52 . Resistor R 5  is disposed between the source of field effect transistor Q 51  and positive power source Vcc. Operational amplifier A 52  detects the potential difference between the two terminals of resistor R 5  and outputs signals that represent the amount of current flowing between the source and the drain of field effect transistor Q 51 .  
         [0049]     Push-pull amplifier  500  comprises a high-pass filter F 5 , an amplifier A 53 , and a boost transistor Q 53 . Boost transistor Q 53  is an N channel MOS field effect transistor. High-pass filter F 5  extracts the high-frequency component of signals received at input terminal S 5  and outputs the filtered signal to amplifier A 53 . The cut-off frequency of high-pass filter F 5  is set such that it includes the band that should be compensated. Amplifier A 53  is the amplifier that amplifies the input signals by a predetermined amplitude and then outputs the product. The output signals of amplifier A 53  are applied to the gate of boost transistor Q 53 . The source of boost transistor Q 53  is connected to negative power source Vee. The drain of boost transistor Q 53  is connected to output terminal T 5 . Amplifier A 53  operates in accordance with output signals of operational amplifier A 52 . In further detail, amplifier A 53  amplifies the input signals and outputs amplified signal when the amount of current flowing between the source and the drain of field effect transistor Q 51  is a predetermined value or less. Boost transistor Q 53  is turned on by signals input to the gate contemporaneously. On the other hand, amplifier A 53  outputs signals such that boost transistor Q 53  is turned off when the amount of current flowing between the source and the drain of field effect transistor Q 51  is greater than a predetermined value. In addition, amplifier A 53  outputs signals such that boost transistor Q 53  is turned off when the amount of current flowing between the source and the drain of field effect transistor Q 51  is zero, that is, when field effect transistor Q 51  is off.  
         [0050]     Consequently, boost transistor Q 53  amplifies only the high-frequency comonent extracted from signals received at input terminal S 5  and outputs the amplified signal to output terminal T 5  only when the amount of current flowing between the source and the drain of field effect transistor Q 51  is within a predetermined range. The amplification factor of push-pull amplifier  500  is thereby compensated.  
         [0051]     The above-mentioned embodiments compensate for the reduction in the amplification factor attributed to the transistor on the push side. A detection means for the drain current and a circuit segment that operates in response to this means can be disposed for the pull side in order to compensate for a reduction in the amplification factor attributed to the transistor on the pull side. A power amplifying apparatus  60  having this type of push-pull amplifier is described below as a fifth embodiment of the present invention while referring to  FIG. 6 . The fifth embodiment expands on the first embodiment.  
         [0052]     Power amplifying apparatus  60  in the figure comprises an input terminal X 6 , an output terminal Y 6 , an operational amplifier A 61 , and a push-pull amplifier  600 . Signals at input terminal X 6  and Signals at output terminal Y 6  are applied to operational amplifier A 61 . Operational amplifier A 61  inputs to push-pull amplifier  600  signals such that the difference between two signals to be input is zero. Push-pull amplifier  600  power-amplifies input signals and outputs the amplified signal to output terminal Y 6 .  
         [0053]     Push-pull amplifier  600  is a bipolar push-pull amplifier that uses an MOS field effect transistor. Push-pull amplifier  600  comprises an input terminal S 6 , an output terminal T 6 , a push-side P channel MOS field effect transistor Q 61 , a pull-side N channel MOS field effect transistor Q 62 , a constant-voltage source E 61 , and a constant-voltage source E 62 . The drain of field effect transistor Q 61  and the drain of field effect transistor Q 62  are connected to output terminal T 6 . The source of field effect transistor Q 61  is connected to a positive power source Vcc. The source of field effect transistor Q 62  is connected to a negative power source Vee. The signals received at input terminal S 6 , that is, the output signals of operational amplifier A 61 , are biased by constant-voltage source E 61  and input to the gate of field effect transistor Q 61 . Moreover, the signals received at input terminal S 6  are biased by constant-voltage source E 62  and input to the gate of field effect transistor Q 62 . The pair of field effect transistors Q 61  and Q 62  operates as a class A, class AB, or class B amplifier in accordance with the voltage of constant-voltage sources E 61  and E 62 . The structure described above is the basic part of push-pull amplifier  600 .  
         [0054]     Furthermore, push-pull amplifier  600  comprises a resistor R 61  and an operational amplifier A 62 . Resistor R 61  is disposed between the source of field effect transistor Q 61  and positive power source Vcc. Operational amplifier A 62  detects the potential difference between the two terminals of resistor R 61  and outputs signals that represent the amount of current flowing between the source and the drain of field effect transistor Q 61 .  
         [0055]     Push-pull amplifier  600  comprises a high-pass filter F 61 , an amplifier A 63 , a capacitor C 61 , a resistor R 62 , and a resistor R 63 . High-pass filter  61  extracts the high-frequency component of signals received at input terminal S 6  and outputs the filtered signal to amplifier A 63 . The cut-off frequency of high-pass filter F 61  is set such that it includes the band that should be compensated. Amplifier A 63  is the amplifier that amplifies input signals by a predetermined amplitude and then outputs the product. Amplifier A 63  operates in accordance with the output signals of operational amplifier A 62 . In further detail, amplifier A 63  outputs signals only when the amount of current flowing between the source and the drain of field effect transistor Q 61  is a predetermined value or less. Capacitor C 61  is used in order to obtain the alternating current component from the output signals of amplifier A 63 . Resistors R 62  and R 63  operate as a signal adding means.  
         [0056]     Moreover, push-pull amplifier  600  comprises a resistor R 64  and an operational amplifier A 64 . Resistor R 64  is disposed between the source of field effect transistor Q 62  and negative power source Vee. Operational amplifier A 64  detects the potential difference between the two terminals of resistor R 64  and outputs signals that represent the amount of current flowing between the source and drain of field effect transistor Q 62 .  
         [0057]     Push-pull amplifier  600  further comprises a high-pass filter F 62 , an amplifier A 65 , a capacitor C 62 , a resistor R 65 , and a resistor R 66 . High-pass filter F 62  extracts the high-frequency component of the signals received at input terminal S 6  and outputs filtered signal to amplifier A 65 . The cut-off frequency of high-pass filter F 62  is set such that it includes the band that should be compensated. Amplifier A 65  is the amplifier that amplifies input signals by a predetermined amplitude and then outputs the amplified signal. Amplifier A 65  operates in accordance with the output signals of operational amplifier A 62 . In further detail, amplifier A 65  outputs signals only when the amount of current flowing between the source and the drain of field effect transistor Q 62  is a predetermined value or less. Capacitor C 62  is used in order to obtain the alternating current component from the output signals of amplifier A 65 . Resistors R 65  and R 66  operate as a signal adding means.  
         [0058]     If the amount of current flowing between the source and the drain of field effect transistor Q 61  of power amplifying apparatus  60  that is constructed as described above is a predetermined current or less, the high-frequency component of signals input to the gate of field effect transistor Q 61  is amplified in comparison with the case when this amount of current is greater than the predetermined current. If the amount of current flowing between the source and the drain of field effect transistor Q 62  is a pre-determined current or less, the high-frequency component of signals input to the gate of field effect transistor Q 62  is amplified in comparison to the case when this current is greater than a predetermined current. The amplification factor of push-pull amplifier  600  is thereby compensated. This compensation is performed from both the push side and the pull side.  
         [0059]     The following modifications are possible in each of the above-mentioned embodiments.  
         [0060]     First, although the field effect transistors are connected such that a drain output-type of push-pull amplifier is formed in each of the above-mentioned embodiments, the field effect transistors can be connected such that a source follower-type of push-pull amplifier is formed.  
         [0061]     Moreover, the field effect transistors in each of the above-mentioned embodiments can be replaced by bipolar transistors and other unipolar transistors. All or some of the field effect transistors can be replaced. Field effect transistors Q 43  and Q 53  can also be replaced with active elements. For instance, at least one of field effect transistors Q 22  and Q 23  in  FIG. 2  can be replaced with bipolar transistors. Moreover, field effect transistor Q 32  in  FIG. 3  can be replaced with active elements other than transistors.