High frequency amplifier circuit

A high frequency amplifier circuit includes the voltage source supplying the power source voltage on the output side of the high frequency amplifying active element. The high frequency amplifier is constructed with a voltage detector detecting a difference frequency voltage of the input signal at a frequency lower than the input signal of the high frequency amplifier circuit and control portion for attenuating the difference frequency voltage from the output signal by controlling the power source voltage on the basis of the difference frequency voltage detected by the voltage detector. Therefore, distortion due to modulating the input signal with the difference frequency component of the input signal can be reduced.

BACKGROUD OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a high frequency amplifier 
circuit. More particularly, the invention relates to a high frequency 
amplifier circuit for a cellular phone. 
2. Description of the Related Art 
One example of the conventional amplifier circuit of 900 MHz band for 
cellular phone employing a source-grounded MOSFET is illustrated in FIG. 
19. On the other hand, a spectrum of an output signal of the amplifier is 
shown in FIG. 20. 
The amplifier circuit is constructed with an input matching circuit 101, a 
gate bias circuit 102, an amplifying MOSFET (Metal-Oxide Semiconductor 
Field Effect Transistor) 103, a drain power supply circuit 104, an output 
matching circuit 105 and an output DC block capacitor 106. 
Due to a second order non-linearity of the MOSFET 103, a difference 
frequency current 2 with a difference frequency of an input signal 1 is 
generated at a drain terminal of the MOSFET 103. 
The difference frequency represents a difference of arbitrary two 
frequencies within a range of frequency (fc-fB) to (fc+fB) in the case 
where an input signal is a signal having a band of .+-.fB centered at a 
frequency fc, for example. 
The difference frequency current 2 does not flow through the output DC 
blocking capacitor 106 for low frequency, and instead flows to a drain 
power supply circuit 104. 
When the difference frequency current 2 flows to the drain power supply 
circuit 104, a difference frequency voltage is generated at the drain 
terminal 3 due to impedance of the drain power supply circuit 104. 
A difference frequency spectrum shown in FIG. 20 corresponds to the 
difference frequency voltage and an input signal spectrum 4 corresponds to 
a voltage of a signal to be amplified. By modulating the input signal 1 
with the difference frequency signal, a distortion output is generated in 
a frequency band in the vicinity of the input signal 1. As a result, an 
adjacent channel leakage power 7 is generated as shown in FIG. 20. 
For restricting occurrence of strain in such mechanism, it is effective to 
lower an impedance of the drain power supply circuit 104 with respect to 
the difference frequency for shorting the difference frequency. 
As a publication to be made reference to with respect to the distortion 
generating mechanism, 1996 General Conference of The institute of 
Electronics, Information and Communication Engineer, Lecture No. C-2-23, 
"Influence of Power Source Circuit in Mutual Modulation Distortion 
Characteristics of Power MOSFET" (Matsuno, et al.) may be made reference 
to. 
However, to the drain power supply circuit 104, it is required to set high 
an impedance in the band of the input signal 1. 
Particularly, when a modulation signal having wide band width is to be 
handled, since difference frequency becomes high, a difficulty is 
encountered in simultaneously optimizing two conditions that the impedance 
with respect to the difference frequency is low, and the impedance at the 
band of the input signal 1 is high. 
Another example of the prior art has been disclosed in Japanese Unexamined 
Patent Publication No. Heisei 5-235646. In the disclosed prior art, 
distortion of the amplifier is reduced by providing a distortion 
compensator having inverted distortion characteristics at a preceding 
stage of the amplifier containing distortion. 
However, what can be compensated by the prior art is only distortion for 
generated by the odd-order non-linearity of the amplifier. 
The distortion generated by modulating the input signal with the difference 
frequency component of the input signal which is generated by the second 
order non-linearity of the amplifier cannot be lowered. 
Then, the distortion has been reporting in 1997 General Conference of The 
institute of Electronics, Information and Communication Engineer, Lecture 
No. C-2-23, "Influence of Power Source Circuit in Mutual Modulation 
Distortion Characteristics of Power MOSFET" (Matsuno, et al.). 
A further example of the prior arts have been disclosed in Japanese 
Unexamined Patent Publication No. Heisei 7-154169 and Japanese Unexamined 
Patent Publication No. Heisei 6-125230. In these prior art, depending upon 
an amplitude of an input signal of the amplifier, a power source voltage 
input to the amplifier is varied to lower distortion with maintaining 
efficiency of the amplifier high. 
However, these prior art only controls the output amplitude in linear 
operation of the amplifier depending upon amplification of the input 
signal of the amplifier, and cannot reduce distortion to be generated by 
modulation of the input signal by the difference frequency component of 
the input signal caused due to the second order non-linearity of the 
amplifier. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide with high 
frequency amplifier circuit which can lower distortion generated by 
modulation of the input signal by a difference frequency component of the 
input signal. 
In accordance with the present invention, a high frequency amplifier 
circuit including a voltage supply means for supplying a power source 
voltage to an output side of a high frequency amplifying active element, 
comprises: 
signal detecting means for detecting a difference frequency signal of an 
input signal at a frequency lower than the input signal of the high 
frequency amplifier circuit; and 
signal attenuation means for attenuating the difference frequency signal 
from an output signal by controlling a power source voltage on the basis 
of the difference frequency signal detected by the signal detecting means. 
With the present invention set forth above, on the basis of the difference 
frequency signal detected by the signal detecting means, the signal 
attenuation means adjusts the power source voltage for canceling the 
difference frequency signal.

DESCRPTION OF THE PREFERRED EMBODIMENT 
The present invention will be discussed hereinafter in detail in terms of 
the preferred embodiment of the present invention with reference to the 
accompanying drawings. In the following description, numerous specific 
details are set forth in order to provide a thorough understanding of the 
present invention. It will be obvious, however, to those skilled in the 
art that the present invention may be practiced without these specific 
details. In other instance, well-known structures are not shown in detail 
in order to avoid unnecessarily obscure the present invention. 
FIG. 1 is a block diagram showing a construction of the first embodiment of 
a high frequency amplifier circuit according to the present invention, 
which constitutes the best mode of the present invention. 
A high frequency amplifier circuit includes an input matching circuit 101, 
a gate bias circuit 102, a high frequency amplifying MOSFET 103, a drain 
choke inductance 18, a voltage source 17 supplying a drain power source of 
the MOSFET 103, a voltage detector 14 detecting a voltage at a drain, a 
control portion 16 controlling a voltage of the voltage source 17 
depending upon a voltage detected by the voltage detector 14. 
The drain choke inductance 18 is a sufficiently high inductance with 
respect to a signal which is amplified (a signal corresponding to an input 
signal spectrum 4, see FIG. 20). The inductance represents low impedance 
with respect to a signal having a frequency lower than a signal to be 
amplified as a difference frequency (a signal corresponding to a 
difference frequency spectrum 6, see FIG. 20). 
It should be noted that the voltage detector 14 is a conventional detector 
which can detect the voltage having relative low frequency, namely a 
voltage corresponding to the difference frequency spectrum of the present 
invention. Accordingly, a voltage corresponding to the input signal 
spectrum 4 far higher than the difference frequency 6 cannot be detected. 
On the other hand, a low-pass filter may be inserted for inputting an only 
voltage corresponding to the difference frequency spectrum 6 on an input 
side of the voltage detector 14. 
On the other hand, the voltage output from the voltage detector 14 is an 
instantaneous value instead of an effective value. Accordingly, the 
control portion 16 controls a voltage of the voltage source 17 depending 
upon a voltage of the instantaneous value. 
Next, operation will be discussed. 
When a signal 1 is input to a gate of the MOSFET 103, due to the second 
order non-linearity of the MOSFET 103, a difference frequency current 2 
with a difference frequency of the input signal 1 is generated at a drain 
terminal 3 of the MOSFET 103. Since the difference frequency current 2 has 
low frequency, the difference frequency current 2 flows into the voltage 
source 17 of the drain via the drain choke inductance 18. 
At this time, in the amplifier circuit employing the prior art, the 
difference frequency voltage is generated by the impedance of the power 
source supply circuit 104 (see FIG. 19). The difference frequency voltage 
causes fluctuation of the voltage at the drain terminal 3 to cause 
distortion by modulation of the input signal 1 from the gate. 
In contrast to this, in the preferred embodiment of the present invention, 
a voltage of the difference frequency at the drain terminal 3 is detected 
by the voltage detecting portion 14 to control a voltage generated by the 
power source 17 by the control portion 16 so that the voltage of the 
difference frequency becomes zero. 
Namely, the control portion 16 generates the voltage having different phase 
over 180.degree. with respect to the difference frequency voltage to 
cancel only difference frequency voltage component among the power source 
voltage. 
It should be noted that while the voltage corresponding to the input signal 
spectrum is also generated together with the difference frequency voltage, 
the conventional voltage detector can not detect the input signal since 
the frequency of the input signal spectrum is much higher than the 
difference frequency, therefore, an influence of the voltage different in 
the phase of 180.degree. relative to the difference frequency voltage 
generated from the voltage source 17 would hardly occur to the frequency 
of the input signal spectrum. 
By this, since the difference voltage is not generated at the drain 
terminal 3, distortion to be generated by modulation of the input signal 1 
by the difference frequency voltage of the input signal 1 can be 
restricted. 
Next, embodiments of the present invention will be discussed. At first, the 
first embodiment will be discussed with reference to FIG. 1. 
The input signal 1 is a signal having a band width of 1.23 MHz centered at 
a frequency of 950 MHz. The voltage of the drain terminal 3 is detected by 
the voltage detector 14 to input the detected data to the control portion 
16. The control portion 16 controls a voltage to be generated by a voltage 
source 17 so that the difference frequency voltage of the input signal 1 
generated at the drain terminal 3, in the shown case, the voltage less 
than or equal to 1.23 MHz, namely an integer multiple of the difference 
frequency voltage and the difference frequency and the voltage of the 
frequency component of less than or equal to 1.23 MHz, becomes zero. 
By this, the difference frequency voltage of the input signal 1 generated 
at the drain terminal 3 can be restricted. Thus, distortion can be reduced 
irrespective of the impedance of the drain power supply circuit. 
Next, the embodiments of the control portion 16 will be discussed. FIGS. 2 
to 4 are block diagrams of the first to third embodiments of the control 
portion 16. It should be noted that, for convenience in FIGS. 2 to 4, the 
voltage detector 14 and the voltage source 17 are illustrated. 
At first, the first embodiment of the control portion 16 will be discussed 
with reference to FIG. 2. The control portion 16 comprises an AD converter 
71 performing digital conversion of an analog data, an arithmetic portion 
17 performing arithmetic operation of the data output from the AD 
converter 72 and a DA converter 73 converting the result of arithmetic 
operation into the analog data. 
Next, operation will be discussed. The analog data of the instantaneous 
value of the voltage output from the voltage detector 14 is converted into 
the digital data by the AD converter 71. Also, in the arithmetic portion 
72, the control signal is determined. The control signal after 
determination is converted into the analog data again by the DA converter 
73, and thereafter, is input to the voltage source 17. The voltage source 
17 is controlled by the analog data. 
Next, the second embodiment of the control portion will be discussed with 
reference to FIG. 3. The control portion 16 comprises the AD converter 71 
performing digital conversion of the analog data and the arithmetic 
portion 72 performing arithmetic operation of the data output from the AD 
converter 71. 
Next, operation will be discussed. The analog data of the instantaneous 
value of the voltage output from the voltage detector 14 is converted into 
the digital data by the AD converter 71. Then, the control signal is 
determined by the arithmetic portion 72. The control signal after 
determination is input to the voltage source 17 in a form of the digital 
data. The voltage source 17 is controlled by the digital data. 
Next, the third embodiment of the control portion will be discussed with 
reference to FIG. 4, the control portion 16 comprises the arithmetic 
portion 72 performing arithmetic operation of the digital data output from 
the voltage detector 14 and the DA converter 73 converting the result of 
arithmetic operation in the arithmetic portion 72. 
Next, operation will be discussed. The digital data of the instantaneous 
value of the voltage output from the voltage detector 14 is input to the 
arithmetic portion 72. Then, the control signal is determined. The control 
signal thus determined is converted into the analog signal by the DA 
converter 73. Thereafter, the analog control signal is input to the 
voltage source 17 to control the voltage source 17 by the analog data. 
Next, the second embodiment of the high frequency amplifier circuit 
according to the present invention will be discussed with reference to 
FIG. 5. FIG. 5 is a block diagram of the second embodiment of the high 
frequency amplifier circuit according to the present invention, It should 
be noted that the like portions to those in FIG. 1 will be represented by 
like reference numerals to avoid necessity of redundant discussion to 
maintain the disclosure simple enough for facilitating clear understanding 
of the present invention. 
The second embodiment of the high frequency amplifier circuit comprises the 
input matching circuit 101, the gate bias circuit 102, the high frequency 
amplifying MOSFET 103, the drain choke inductance 18, a current detector 
21 for detecting a drain current, the voltage source 17 for supplying a 
power source to the drain of the MOSFET 103, a control portion controlling 
the voltage of the voltage source 17 depending upon a current detected by 
the current detector 21, and the output matching circuit 105. 
Next, operation will be discussed. 
A current flowing through the drain power source supply circuit is detected 
by the current detector 21. The detected data is input to the control 
portion 22. The control portion 22 predicts a difference frequency voltage 
component of the input signal 1 generated at the drain terminal 3 from the 
current flowing through the drain power supply circuit. 
A method for predicting the difference frequency voltage component from the 
current input to the current detector 21 is similar to the method for 
detecting the difference frequency voltage component from the output 
voltage set forth above. 
Namely, the current detector 21 is a typical current detector which can 
detect a current of a signal having relative low frequency, such as the 
difference frequency. By the current detector 21, only a direct current 
voltage of the power source and difference frequency current are detected, 
and a current corresponding to the input signal spectrum 4 is not 
detected. 
The control portion 22 separates the difference frequency current from the 
direct current voltage and predicts the difference frequency voltage from 
the difference frequency current. 
Then, the control portion 22 controls the voltage generated in the voltage 
source 17 so that the difference frequency voltage component becomes zero. 
By this, since the difference frequency voltage of the input signal 1 
generated at the drain terminal 3 is restricted, distortion can be reduced 
irrespective of the impedance of the drain power source supply circuit. 
Accordingly, in the present invention, the drain choke inductance 18 is 
required to have sufficiently high impedance for the input signal to be 
amplified. Thus, it becomes possible to avoid trade off f the impedance 
with respect to the amplifying signal band and the difference frequency 
band. 
Next, embodiments of the control portion 22 will be discussed. FIGS. 6 to 8 
are block diagrams showing a construction of the fourth to sixth 
embodiment of the control portion. It should be noted that, for 
convenience, the current detector 21 and the voltage source 17 are 
illustrated in FIGS. 6 to 8. 
At first, the fourth embodiment of the control portion will be discussed 
with reference to FIG. 6. The control portion 22 comprises an AD converter 
81 digital converting the analog signal, an arithmetic portion 82 
performing arithmetic operation for the data output from the AD converter 
81 and a DA converter 83 converting the result of arithmetic operation in 
the arithmetic portion 82 into an analog data. 
Next, operation will be discussed. An analog data of the instantaneous 
value of the current output from the current detector 21 is converted into 
the digital data by the AD converter 81. Then, the control signal is 
determined in the arithmetic portion 82. After determination, the control 
signal is converted into the analog data again by the DA converter 83 to 
be input to the voltage source 17. The voltage source 17 is controlled by 
the analog data. 
Next, the fifth embodiment of the control portion will be discussed with 
reference to FIG. 7. The control portion 22 comprises the AD converter 81 
digital converting the analog data and the arithmetic portion 82 
performing arithmetic operation of the data output from the AD converter 
81. 
Next, operation will be discussed. The analog data of the instantaneous 
value of the current output from the current detector 21 is converted into 
the digital data by the AD converter 81. Then, the control signal is 
determined by the arithmetic portion 82. After determination, the control 
signal is input to the voltage source 17 in a form of the digital data. 
The voltage source 17 is controlled by the digital data. 
Next, the sixth embodiment of the control portion will be discussed with 
reference to FIG. 8. The control portion 22 comprises the arithmetic 
portion 82 performing arithmetic operation of the digital data output from 
the current detecting portion 21, and the DA converter 83 converting the 
result of arithmetic operation of the arithmetic portion 82 into the 
analog data. 
Next, operation will be discussed. The digital data of the instantaneous 
value of the current output from the current detector 21 is input to the 
arithmetic portion 82. Then, in the arithmetic portion 82, the control 
signal is determined. After determination, the control signal is converted 
into the analog signal by the DA converter 83 and then input to the 
voltage source 17. Thus, the voltage source 17 is controlled by the analog 
data. 
Next, the third embodiment of the present invention will be discussed with 
reference to FIG. 9. FIG. 9 is a block diagram showing a construction of 
the third embodiment of the high frequency amplifier circuit according to 
the present invention. It should be noted that like portions to those of 
FIG. 1 will be represented by like reference numerals to avoid necessity 
of redundant discussion to maintain the disclosure simple enough for 
facilitating clear understanding of the present invention. 
The third embodiment of the high frequency amplifier circuit according to 
the present invention includes the input matching circuit 101, the gate 
bias circuit 102, a high frequency amplifying bipolar transistor 111, a 
collector choke inductance 29, the voltage source 17 for supplying a power 
source to a collector of the bipolar transistor 111, a control portion 26 
controlling the output voltage of the voltage source 17 on the basis of 
the input signal 1, and the output matching circuit 105. 
Next, operation will be discussed. 
To the control portion 26, the input signal 1 is input to predict a 
difference frequency component of the input signal 1 generated at a 
collector terminal 27. The prediction is performed by calculating a 
difference signal of arbitrary two frequencies in the frequency band of 
the input signal 1 set forth above. 
The control portion 26 predicts the difference frequency voltage component 
and controls the voltage source 17 for canceling the difference frequency 
voltage component. 
By this, the difference frequency voltage of the input signal 1 generated 
at the collector terminal 27 is restricted. Thus, distortion can be 
reduced irrespective of the impedance of the drain power supply circuit. 
Accordingly, in the present invention, the collector choke inductance 29 
is required to have sufficiently high impedance for the input signal to be 
amplified. A trade off of the impedance for the amplifying signal band and 
the difference frequency band as required in the prior art. 
Next, discussion will be given for the embodiments of the control portion 
26. FIGS. 10 to 12 are block diagrams of the seventh to ninth embodiments 
of the control portion. It should be noted that for convenience, the 
voltage source 17 is also illustrated. 
At first, the seventh embodiment of the control portion will be discussed 
with reference to FIG. 10. The control portion 26 comprises a spectrum 
analyzer 91 analyzing a spectrum of the input signal 1, a difference 
frequency component predictor 92 for predicting a difference frequency 
component from the spectrum analyzed by the spectrum analyzer 91 and a 
control signal generator 93 generating a control signal on the basis of 
the difference frequency component predicted by the difference frequency 
component predictor 92. 
Next, operation will be discussed. The spectrum of the input signal 1 is 
analyzed by the spectrum analyzer 91. Then, on the basis of the result of 
analysis, the difference frequency component predictor 92 predicts the 
difference frequency component. The control signal generator 93 controls 
the voltage source 17 on the basis of the difference frequency component 
predicted by the difference frequency component predictor 92. 
The eighth embodiment of the control portion will be discussed with 
reference to FIG. 11. The eighth embodiment is illustrated in terms that a 
signal which is generated by modulating a carrier wave with a base band 
signal is used as the input signal 1. 
The control portion 26 comprises a down converter 94 dropping the input 
signal to the base band, an AD converter 95, the difference frequency 
component predictor 92 and the control signal generator 93. 
Next, operation will be discussed. The input signal 1 is dropped to the 
base band by the down converter 94. The base band signal is sampled by the 
AD converter 95. On the basis of the result of sampling, the difference 
frequency component predictor 92 predicts the difference frequency. On the 
basis of the result of prediction, the control signal generator 93 
generates the control signal. By the control signal, the output voltage of 
the voltage source 17 can be controlled. 
It should be noted that once a string of the signal of the base band is 
determined, the waveform of the input signal can be predicted on the basis 
of the string. When the waveform of the input signal can be predicted, the 
difference frequency signal can be predicted as set forth above. 
Next, the ninth embodiment of the control portion will be discussed with 
reference to FIG. 12. The ninth embodiment also illustrates the case where 
a signal which is generated by modulating a carrier wave with a base band 
signal is used as the input signal 1. 
The control portion 26 comprises the down converter 94 dropping the input 
signal 1 to the base band, the spectrum analyzer 91, the difference 
frequency predictor 92 and the control signal generator 93. 
Next, operation will be discussed, the input signal 1 is dropped to the 
base band by the down converter 94. The base band signal is then analyzed 
by spectrum analysis by the spectrum analyzer 91. On the basis of the 
result of spectrum analysis, the difference frequency component predictor 
92 predicts the difference frequency spectrum. On the basis of the result 
of prediction, the control signal generator 93 generates the control 
signal. By this control signal, the output voltage of the voltage source 
17 is controlled. 
Next, the fourth embodiment of the present invention will be discussed with 
reference to FIG. 13. FIG. 13 is a block diagram showing a construction of 
the high frequency amplifier circuit according to the present invention. 
It should be noted that the like portions to those in FIG. 1 will be 
represented by like reference numerals to avoid necessity of redundant 
discussion to maintain the disclosure simple enough for facilitating clear 
understanding of the present invention. 
The fourth embodiment of the high frequency amplifier circuit according to 
the present invention comprises the matching circuit 101, the gate bias 
circuit 102, the high frequency amplifying MOSFET 103, the drain choke 
inductance 18, the voltage source 17 supplying the power to the drain of 
the MOSFET 103, a voltage detector 31 constructed with a high resistance 
resistor 32 and a current detector 33, a low-pass filter (L.P.F.) 34, a 
transfer function circuit 35, and the matching circuit 105. 
The voltage detector 31 detects the voltage by detecting a fine current 
flowing through the high resistance resistor 32 by the current detector 
33. The detected voltage value is input only difference frequency of the 
input signal and the frequency component of integer multiple of the 
difference frequency to the transfer function circuit 35 through the 
low-pass filter 34. Through the transfer function circuit 35, a signal 
made up the frequency characteristics of the intensity and phase controls 
the output voltage of the voltage controlled voltage source 17. 
At this time, the frequency characteristics of the transfer function 
circuit 35 is selected that the difference frequency of the input signal 1 
and the voltage having frequency of integer multiple of the difference 
frequency at the drain terminal 3 become zero. 
Namely, the frequency characteristics of the transfer function circuit 35 
is that derived by converting the difference frequency voltage and the 
voltage having frequency of integer multiple of the difference frequency 
into the voltage different for 180.degree. phase. 
Next, the operation of the transfer function circuit 35 is supplemented. 
The output of the transfer function circuit 35 is an analog voltage 
signal. The current detector 33 generates a voltage signal proportional to 
the voltage of the drain terminal 3. By passing the voltage signal to the 
low-pass filter 34, the voltage signal proportional to the component of 
low frequency including the difference frequency voltage among the voltage 
of the drain terminal 3 can be obtained. 
When this voltage signal is input to the voltage source 17 as is, it is 
expected that the difference frequency voltage at the drain terminal 3 
would not become zero due to influence of the choke inductance 18. In 
order to avoid the influence of the chock inductance 18, intensity and 
phase of the control voltage signal is adjusted through the transfer 
function circuit 35. 
The transfer function circuit 35 is an analog circuit, in which some of LRC 
filter circuits and amplifiers are combined. In the alternative, the 
transfer function circuit 35 is a circuit converting the signal from the 
low-pass filter 34 into the digital data by the AD converter and after 
passing a digital filter, obtaining an analog output by the DA converter. 
By this, since the difference frequency voltage of the input signal 1 
generated at the drain terminal 3 is restricted, distortion can be reduced 
irrespective of the impedance of the drain power supply circuit. 
Accordingly, in the present invention, the drain choke inductance 18 is 
merely require to have sufficiently high impedance for the input signal to 
be amplified. Thus trade off of the impedance of the amplifying signal 
band and the difference frequency band as that required in the prior art 
can be resolved. 
Next, the fifth embodiment of the present invention will be discussed with 
reference to FIG. 14. FIG. 14 is a block diagram showing the fifth 
embodiment of a high frequency amplifier circuit according to the present 
invention. It should be noted that the like portions to those in FIG. 1 
will be represented by like reference numerals to avoid necessity of 
redundant discussion to maintain the disclosure simple enough for 
facilitating clear understanding of the present invention. 
This circuit is constructed with a gate-grounded MOSFET 103 while the 
MOSFET 103 in FIG. 1 is source-grounded. Other construction is the same as 
that of FIG. 1. 
On the other hand, the operation and effect subsequent to detection of the 
voltage of the drain terminal is similar to the circuit of FIG. 1. 
Therefore, discussion will be neglected. 
In the present invention, foregoing and later discussed source grounding 
circuit of the MOSFET 103 can be all modified to gate grounding circuit 
similar to FIG. 14. 
Next, the sixth embodiment of the present invention will be discussed with 
reference to FIG. 15. FIG. 15 is a block diagram showing a construction of 
the sixth embodiment of the high frequency amplifier circuit according to 
the present invention. It should be noted that the like portions to those 
in FIG. 5 will be represented by like reference numerals to avoid 
necessity of redundant discussion to maintain the disclosure simple enough 
for facilitating clear understanding of the present invention. 
While the high frequency amplifying active element of FIG. 5 is MOSFET 103, 
the shown circuit employs the bipolar transistor 111 as the high frequency 
amplifying active element. Other construction is the same as that shown in 
FIG. 1 except for the point where DC block capacitors 124 and 125 are 
added. 
On the other hand, operation and effect subsequent to detection of the 
current of the collector terminal 27 will be neglected for similar to 
those of the circuit of FIG. 5. 
In the present invention, the source grounding circuit of the MOSFET 103 
set forth above and later can be all modified with the bipolar transistor 
111 similarly to FIG. 15. 
The seventh embodiment of the present invention will be discussed with 
reference to FIG. 16. FIG. 16 is a block diagram showing the high 
frequency amplifier circuit according to the present invention. It should 
be noted that the like portions to those in FIG. 9 will be represented by 
like reference numerals to avoid necessity of redundant discussion to 
maintain the disclosure simple enough for facilitating clear understanding 
of the present invention. 
While the high frequency amplifying active element in FIG. 9 is bipolar 
transistor 111, the shown circuit employs the MOSFET 103 as the high 
frequency amplifying active element. Other construction is the same as 
that shown in FIG. 9 except for the point where DC block capacitors 124 
and 125 are added. 
On the other hand, operation and effect subsequent to inputting of the 
input signal 1 to the control portion 26 are the same as those shown in 
FIG. 9, and discussion there of will be neglected. 
Next, the eighth embodiment of the present invention will be discussed with 
reference to FIG. 17. FIG. 17 is a block diagram showing the eighth 
embodiment of the high frequency amplifier circuit according to the 
present invention. It should be noted that the like portions to those in 
FIG. 16 will be represented by like reference numerals to avoid necessity 
of redundant discussion to maintain the disclosure simple enough for 
facilitating clear understanding of the present invention. 
The eighth embodiment is similar in construction, operation and effect to 
those of the seventh embodiment of FIG. 16 except for the point where a 
base band signal 131 is input. 
Namely, the eighth embodiment predicts the input signal from the base band 
signal, and predicts the difference frequency voltage from the input 
signal. 
A signal derived by modulating the carrier wave by the base band signal 131 
is the input signal 1. For example, in case of the cellular phone, the 
base band signal is the data derived by digitizing the voice. The carrier 
is a sine wave of 1.9 GHz. The input signal to the power amplifier is the 
signal which is modulated with a .pi./4 shifted QPSK of the carrier by the 
base band signal. Since modulation system is known, once the string of the 
bit of the base band signal is known, the input signal 1 can be predicted. 
Since distortion characteristics of the amplifier is also known, the 
difference frequency voltage to be generated at the drain can be predicted 
from the input signal 1. 
Once the difference frequency voltage can be predicted, it becomes possible 
to predict how the voltage of the voltage source 17 has to be varied. On 
the basis of the result of prediction, the output of the control portion 
26 can be determined. 
Next, the embodiment of the control portion 26 of the eighth embodiment 
will be discussed with reference to FIG. 18. FIG. 18 is a block diagram 
showing the tenth embodiment of the control portion. It should be noted 
that, for convenience, the voltage source 17 is also illustrated. 
At first, the control circuit 26 with reference to FIG. 18 is constructed 
with a difference frequency component predictor 135 input the base band 
signal 131 and a control signal generator 136 generating the control 
signal on the basis of the predicted value. 
Next, the operation will be discussed. When the base band signal 131 is 
input to the difference frequency component predictor 135, the difference 
frequency component predictor 135 predicts the input signal 1 from the 
string of the bits of the base band signal 131 to predict the difference 
frequency component from the predicted input signal 1. The predicted 
difference frequency component is input to the control signal generator 
136. The control signal generator 136 outputs the control signal to the 
voltage source 17 on the basis of the predicted difference frequency 
component. 
According to the present invention, since the high frequency amplifier 
circuit including the voltage supply means supplying the power source 
voltage on the output side of the high frequency amplifying active element 
is constructed with signal detecting means detecting a difference 
frequency signal of the input signal at a frequency lower than the input 
signal of the high frequency amplifier circuit and signal attenuation 
means for attenuating the difference frequency signal from the output 
signal by controlling the power source voltage on the basis of the 
difference frequency signal detected by the signal detecting means. 
Therefore, by modulating the input signal with the difference frequency 
component of the input signal, distortion can be reduced. 
Although the present invention has been illustrated and described with 
respect to exemplary embodiment thereof, it should be understood by those 
skilled in the art that the foregoing and various other changes, omissions 
and additions may be made therein and thereto, without departing from the 
spirit and scope of the present invention. Therefore, the present 
invention should not be understood as limited to the specific embodiment 
set out above but to include all possible embodiments which can be 
embodied within a scope encompassed and equivalents thereof with respect 
to the feature set out in the appended claims.