JPH10247826A - Distortion compensation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate the distortion of a high output amplifier with simple constitution by selecting the passing loss of a microwave passing through an amplifier part and an attenuator to be larger than that of the microwave passing through a transmission circuit. SOLUTION: An amplifier part is constituted of a semiconductor element 6, an input circuit 5 and an output circuit 7, which are respectively connected to a gate terminal G and a drain terminal D of the semiconductor element 6, and a gate bias circuit 9 and a drain bias circuit 14, which apply DC bias to the semiconductor element 6. An attenuator 8 sets the passing loss of the microwave (b), passing through the amplifier part and the attenuator 8 to be larger than that of the microwave (a) passing through the transmission circuit 19. Passing phase which passes through the transmission circuit 19 and that passing through the amplifiers part, except for the semiconductor 6 and the attenuator 8, are selected to be equal. Since the microwave passing through the amplifier part and the attenuator 8 is restricted by an output voltage and therefore it shows the characteristic which saturates with the increase in input power.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for compensating nonlinear distortion generated in an output signal of a high-power amplifier.
The present invention relates to a distortion compensation circuit.

[0002]

2. Description of the Related Art In a high-output amplifier in a high frequency band, a distorted wave is generated in an output signal due to nonlinearity of a semiconductor element or the like. For this reason, a distortion compensation circuit that compensates for this nonlinear distortion is often used. There are various types of distortion compensating circuits having different circuit configurations. Roughly speaking, a pre-distortion type in which an output signal of the distortion compensating circuit is provided as an input signal of the amplifier, and is provided in a stage preceding an amplifier for which distortion is to be compensated, There are two feedforward types that compensate for distortion by combining the output signal of an amplifier to compensate for distortion with the output signal of a distortion compensation circuit. Of these, the former predistortion type is often used in terms of power efficiency.

[0003] For example, FIG.
CITIC MicrowaveConference
FIG. 7 is an equivalent circuit diagram of a conventional pre-distortion type distortion compensation circuit shown in (pp567-570). In this distortion compensation circuit, one output terminal 34 of the first coupler 30
, A distortion generating amplifier 26, a first attenuator 28, and a first equal power distributor 40 are sequentially connected, and a second attenuator 29 and a linear amplifier 27 are connected to the other output terminal 35. When,
The second equal power distributors 41 are sequentially connected.

An output terminal of the first equal power divider 40 and one output terminal of the second equal power divider 41 are connected to input terminals 36 and 37 of the second coupler 31, respectively. The output terminal of the second coupler 31 has a first variable attenuator 42
, The input terminal 38 of the fourth coupler 33 is connected. Further, the other output terminal of the second equal power divider 41 is connected to the other input terminal 39 of the fourth coupler 33 via the second variable attenuator 43 and the third coupler 32. It is connected to the. This distortion compensating circuit is formed by using a microwave integrated circuit technology on a dielectric substrate. For the distortion generating amplifier 26 and the linear amplifier 27, FETs, HEMTs and the like having substantially the same gate width are used as semiconductor elements. Have been.

Next, the operation will be described. The signal input from the input terminal 1 of the first coupler 30 is output to the output terminal 3
4 and 35, and are supplied to a distortion generating amplifier 26 and a linear amplifier 27 to be amplified. In the distortion generating amplifier 26, the first attenuator 28 is connected to the output side, whereas in the linear amplifier 27, the second attenuator 29 is connected to the input side. Amplifier 26
Saturates first. Therefore, the amplitude characteristics of the output signal of the distortion generating amplifier 26 and the output signal of the linear amplifier 27, that is, the gain characteristics with respect to the increase of the input signal, are curves A and B in FIG.
It becomes as shown in. These characteristics are shown as relative values based on the value when the input power is small.

[0006] These two output signals are equal power splitter 4
0, 41, input terminals 36, 3 of the second coupler 31.
7, where it is synthesized in reverse phase. The level of the synthesized signal is adjusted by the first variable attenuator 42 and reaches the input terminal 38 of the fourth coupler 33. Figure 12
The signal of the amplitude characteristic shown by the curve C of FIG. Further, as a signal at the other output terminal of the second equal power divider 41, a signal having the same amplitude characteristic as that of the input terminal 1 appears, and after the level is adjusted by the second variable attenuator 43, Coupler 3
The signal is supplied to the input terminal 39 of the fourth coupler 33 through the line 2. The signals of these input terminals 38 and 39 are combined in phase in the fourth coupler 33, and the signal of the curve D
As shown in (1), an output signal having a warpage amplitude characteristic can be obtained.

In the above description, the amplitude characteristic has been described, but the same applies to the phase characteristic. That is, the phase characteristics of the output signal with respect to the input signal of the distortion generating amplifier 26 and the linear amplifier 27 are as shown by curves E and F in FIG.
In, a signal having a phase characteristic indicated by a curve G is obtained. Further, by combining these signals G and F in phase in the fourth coupler 33, an output signal having a downwardly warped phase characteristic as shown by a curve H can be obtained.

In general, the amplitude of the output signal of a high power amplifier is
As shown by a curve I in FIG. 14A, the gain gradually decreases as the input signal increases. On the other hand, the distortion compensation circuit has a characteristic that the gain gradually increases as the input signal increases, as shown by the curve D in FIG.
Therefore, by connecting this distortion compensation circuit to the input side of the high-power amplifier, an ideal amplitude characteristic shown by the curve J can be obtained. That is, by using this distortion compensation circuit, it is possible to suppress a distortion wave generated in the high-output amplifier, and to obtain an output signal having excellent linearity.

The same applies to the phase. The phase characteristic of the high-output amplifier is represented by a curve K in FIG.
Since the phase characteristic of the distortion compensation circuit becomes as shown by a curve H,
By using the output signal of the distortion compensation circuit as the input signal of the high-output amplifier, it is possible to obtain a characteristic having excellent linearity as shown by a curve L.

[0010]

The conventional distortion compensating circuit is constructed as described above, and includes a large number of couplers 30, 31, and
Since the 32, 33 and attenuators 28, 29, 42, 43 are used, there is a problem that the entire distortion compensation circuit is very large and expensive.

Also, according to the characteristics of the amplifier whose distortion is to be compensated, the amplifiers 26 and 27 and the attenuators 28, 29, 42 and 4 are used.
It is necessary to adjust characteristics such as 3 and the like, and there is also a problem that fine adjustment of the output signal is complicated and much time is required for the adjustment.

Furthermore, since many attenuators 28, 29, 42, 43 and equal power distributors 40, 41 are combined,
The loss of the entire distortion compensation circuit becomes very large, and it is necessary to increase the gain of the high-output amplifier connected to the subsequent stage. Accordingly, it is necessary to increase the number of stages of the high-output amplifier, and there is a problem that the power efficiency of the entire amplifier is reduced.

Furthermore, the couplers 30, 31, 32,
33 and the attenuators 28, 29, 42, and 43 have frequency characteristics.
There is also a problem that the frequency band of the distortion compensation circuit becomes narrow.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide a wide-band distortion compensating circuit which is small in size, low in loss, easy to adjust an output signal, and wide.

[0015]

A distortion compensating circuit according to the present invention is configured such that a microwave passing through a cascade circuit of an amplifier and an attenuator and a microwave passing through a transmission circuit are out of phase by a T branch circuit. It is intended to be synthesized.

Further, the distortion compensating circuit according to the present invention uses a variable attenuator as the attenuator so that the amplitude of the microwave passing through the amplifier and the attenuator can be adjusted.

Further, in the distortion compensation circuit according to the present invention, a transmission circuit is provided with a phase variable function so that the phase of a microwave passing through the transmission circuit can be adjusted.

In the distortion compensating circuit according to the present invention, an attenuator is provided between the input circuit and the T-branch circuit so that the amplitude of the microwave input to the amplifier unit can be set.

Further, in the distortion compensation circuit according to the present invention, the bias voltage applied to the gate terminal of the semiconductor element is made lower as the temperature becomes higher.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. 1A is a configuration diagram showing Embodiment 1 of the present invention,
FIG. 1B is a diagram showing output power characteristics with respect to the input power of the microwave passing through the transmission circuit, the amplifier section, and the attenuator of FIG. 1A, respectively. In FIG. 1A,
1 is an input terminal, 2 is an output terminal, 3 and 4 are T branch circuits, 5
Is an input circuit, 6 is a semiconductor element, 7 is an output circuit, 8 is an attenuator, 9 is a gate bias circuit, 10 is an inductor, 11
Is a capacitor, 12 and 13 are resistors, 14 is a drain bias circuit, 15 is an inductor, 16 is a capacitor,
Reference numerals 7 and 18 denote DC power supplies, and reference numeral 19 denotes a transmission circuit.

This distortion compensating circuit comprises a source grounded FE.
A semiconductor element 6 such as T, HEMT, etc., and an input circuit 5, an output circuit 7, and a gate bias circuit 9 for applying a DC bias to the semiconductor element 6 connected to the gate terminal G and the drain terminal D of the semiconductor element 6, respectively. An amplifier section including the drain bias circuit 14, an attenuator 8 connected to the output side of the amplifier section, and a cascade connection of the amplifier section and the attenuator 8 and a transmission circuit 19 including a transmission line, a capacitor, and the like. And T branch circuits 3 and 4 connecting the input circuit 5 and the input terminal of the transmission circuit 19 and the attenuator 8 and the output terminal of the transmission circuit 8, respectively. Are connected to an input terminal 1 and an output terminal 2, respectively.

A gate bias circuit 9 for applying a desired negative bias voltage to the gate terminal G of the semiconductor device 6 includes an inductor 10, a capacitor 11, and resistors 12 and 13. And 13 make it possible to divide the pressure to a desired value. A drain bias circuit 14 for applying a positive bias voltage to the drain terminal D of the semiconductor element 6 includes an inductor 15 and a capacitor 16. Are directly applied with a bias voltage.

The input circuit 5 and the output circuit 7 used in the distortion compensating circuit are used to match the input impedance and the power supply impedance of the semiconductor element 6 with the output impedance and the load impedance, respectively.

Further, the attenuator 8 is for setting the passage loss of the microwave b passing through the amplifier section and the attenuator 8 larger than the passage loss of the microwave a passing through the transmission circuit 19. In addition, the passing phase passing through the transmission circuit 19 and the passing phase passing through the amplifier section and the attenuator 8 except for the semiconductor element 6 are selected to be substantially equal.

Next, the operation will be described. Input terminal 1
Is equally distributed by the T-branch circuit 3, and one of the equally distributed microwaves a passes through the transmission circuit 19 and appears at the output terminal of the transmission circuit 19. The other microwave b appears at the input terminal of the input circuit 5, the semiconductor element 6, the output circuit 7, and the output terminal of the attenuator 8. Further, the microwaves a and b appearing at the output terminal of the transmission circuit 19 and the output terminal of the attenuator 8 are combined by the T branch circuit 4 and supplied to the output terminal 2.

FIG. 1B shows the output power characteristics of the microwaves a and b passing through each path with respect to the input power. As shown by the solid line, the output power of the microwave a passing through the transmission circuit 19 formed of a passive element such as a transmission line or a capacitor increases linearly with an increase in input power. On the other hand, the microwave b passing through the amplifier section and the attenuator 8 is limited by the output power by the semiconductor element 6, and therefore increases almost linearly at a place where the input power is small as indicated by a broken line.
It shows a characteristic that saturates as the input power increases.

FIG. 2 shows microwaves a and b passing through each path.
2A and 2B are diagrams showing how the combined microwave appears at the output terminal 2 in the T branch circuit 4. FIG. 2A is a vector diagram, and FIG. 2B is a diagram showing gain and phase characteristics with respect to input power. is there. Microwave b and synthesized microwave c when each input power is normalized by microwave a
Can be represented as shown in FIG. That is, the amplifier section and the attenuator 8 excluding the transmission circuit 19 and the semiconductor element 6
Are selected to be substantially equal, and the phase is shifted by approximately 180 degrees between the gate terminal G and the drain terminal D of the semiconductor element 6, so that the microwave b is shifted 180 degrees with respect to the microwave a. Can be represented as being out of phase by degrees. Therefore, assuming that the amplitude of the microwave a is A and the amplitude of the microwave b is B, in the region where the input power is small, that is, in the linear region, the synthesized microwave c
Can be represented as C. On the contrary,
In the region where the input power is large, that is, in the non-linear region, the amplitude of the microwave b becomes relatively small from B to B1.
The amplitude of the synthesized microwave c shifts from C to C1.

As described above, the amplitude of the synthesized microwave c increases and the phase rotates in a counterclockwise direction as the linear region changes to the nonlinear region. Therefore, the gain of the microwave c synthesized as shown in FIG. 2B increases as the input power increases, and the phase is delayed. With these characteristics, almost the same gain and phase characteristics as those of the distortion compensation circuit shown in FIG. 11 can be obtained, and the distortion of the high-output amplifier can be compensated.

As described above, in this distortion compensating circuit, a large number of couplers 30, 31, 3 used in the conventional distortion compensating circuit are used.
Since the components 2, 33 and the attenuators 28, 29, 42, 43, etc. are not required, the circuit configuration becomes very simple, the circuit size can be reduced, and the band can be widened. Further, since only a DC bias needs to be applied to only one semiconductor element 6, a remarkable reduction in power consumption can be achieved.

Second Embodiment FIG. 3 is a configuration diagram of a distortion compensation circuit according to a second embodiment of the present invention. This distortion compensation circuit uses a variable attenuator 20 that can continuously vary the amount of attenuation using a pin diode, an FET, or the like, instead of the attenuator 8 shown in FIG.

FIG. 4 is a diagram showing a state where the microwaves a and b passing through the transmission circuit 19 and the amplifier section and the variable attenuator 20, respectively, are combined as in FIG. FIG.
5A, the variable attenuator 2 is set so that the amplitude of the microwave b passing through the amplifier section and the variable attenuator 20 becomes B.
When the attenuation amount is set to 0, the amplitude approaches zero from B as it changes from the linear region to the nonlinear region. Therefore, the synthesized microwave c moves from C to A. on the other hand,
Microwave b passing through amplifier section and variable attenuator 20
Is set to B1, the amplitude approaches from B1 to zero and the synthesized microwave c shifts from C1 to A as the region changes from the linear region to the non-linear region. As described above, the amplitude and the phase change depending on the attenuation amount of the variable attenuator 20.

Therefore, as shown in FIG. 4B, by reducing the attenuation of the variable attenuator 20, that is, by increasing the amplitudes of the amplifier section and the variable attenuator 20,
The warpage of the gain and the phase lag can be set large with respect to the input power.

As described above, in this distortion compensating circuit, since the gain and phase change width can be set with respect to the input power, even if the gain and phase of the high-r output amplifier to be distortion-compensated have variations. Distortion compensation becomes possible.

Third Embodiment FIG. 5 is a configuration diagram of a distortion compensation circuit according to a third embodiment of the present invention. This distortion compensation circuit is the transmission circuit 1 shown in FIG.
Instead of 9, a transmission circuit 21 having a built-in phase shifter, a line length adjusting mechanism and the like and capable of adjusting a passing phase is used.

FIG. 6 is a diagram showing how the microwaves a and b passing through the transmission circuit 21 and the amplifier section and the attenuator 8 are combined by the T-branch circuit 4. FIG. 6 (a)
In the case where the phase of the transmission circuit 21 is set such that the phase difference between the microwave a passing through the transmission circuit 21 and the microwave b passing through the amplifier unit and the attenuator 8 becomes larger than 180 degrees, The amplitude approaches zero from B as the region approaches, and the synthesized microwave shifts from C to A. When the phase difference is selected to be 180 degrees, the amplitude of the microwave b passing through the amplifier section and the attenuator 8 approaches zero from B1 and the synthesized microwave c becomes C1.
To A. Further, when the amplitude is selected to be smaller than 180 degrees, the amplitude of the microwave b passing through the amplifier unit and the attenuator 8 approaches zero from B2, and the synthesized microwave c shifts from C2 to A.

Accordingly, as shown in FIG. 6B, the phase difference between the microwave a passing through the transmission circuit 21 and the phase of the microwave b passing through the amplifier unit and the attenuator 8 is one.
When the phase of the transmission circuit 21 is set so as to be smaller than 80 degrees, the amplitude and the phase of the synthesized microwave c are warped. When the phase difference is set to 180 degrees, the phase is flat, and only the amplitude becomes a large warpage. Further, when the phase difference is set to be larger than 180 degrees, the phase is downwardly warped and the amplitude is slightly upwardly warped.

As described above, in the distortion compensating circuit of the present invention, by using the transmission circuit 21 having the phase adjusting function, it is possible to vary the phase in a wide range from downward warpage to upward warpage while keeping the amplitude upward. Can be. For this reason, even when there is a large variation in the phase of the high-output amplifier that performs distortion compensation, good distortion compensation can be performed.

Fourth Embodiment FIG. 7 is a configuration diagram of a distortion compensation circuit according to a fourth embodiment of the present invention. This distortion compensating circuit corresponds to the input circuit 5 shown in FIG.
And an attenuator 22 for level adjustment is provided between the A and T branch circuits 3. By providing the attenuator 22, the level of the microwave input to the amplifier unit can be set to a desired value. That is, since the amplitude level of the microwave at which the semiconductor element 6 shifts from the linear region to the non-linear region is constant,
By using the attenuator 22 having a large attenuation, the level of the microwave to the input terminal 1 for operating the semiconductor element 6 in a non-linear manner can be set high. On the other hand, when the attenuator 22 having a small attenuation is used, the semiconductor element 6 operates nonlinearly at a low level.

Accordingly, as shown in FIG. 8, when the attenuation of the attenuator 22 is large, the gain warpage and phase warpage characteristics can be obtained with high input power. On the other hand, the attenuator 22
When the amount of attenuation is small, gain warpage and phase warpage characteristics can be obtained at a low input power level. As described above, in this distortion compensation circuit, by setting the amount of attenuation of the attenuator 22, it is possible to set the input power level at which the gain is warped and the phase is warped. Therefore, there is an advantage that the distortion can be compensated even when the gain of the high-output amplifier to be compensated for varies. The same effect is obtained even when the attenuator 22 is connected to the input terminal 1.

Fifth Embodiment FIG. 9 is a block diagram of a distortion compensation circuit according to a fifth embodiment of the present invention. In this circuit, a posistor 23 is connected in parallel with the resistor 13 of the gate bias circuit 9 of the distortion compensation circuit shown in FIG. The resistance of this posistor 23 increases as the temperature increases.
The combined resistance of the resistor 13 and the posistor 23 increases. For this reason, even if the voltage of the DC power supply 17 is constant, the bias voltage applied to the gate terminal of the semiconductor element 6 becomes low at a high temperature.

In general, semiconductor devices 6 such as FETs and HEMTs
Decreases in gain at high temperatures. Accordingly, the attenuation of the microwave passing through the amplifier section and the attenuator 8 increases.
When the amount of attenuation increases in this manner, both the warpage of the gain and the warpage of the phase decrease as shown in FIG. 4 of the second embodiment. Therefore, as shown in FIG. 10A, the higher the temperature becomes, the smaller the gain and phase warpage become. To compensate for this, the distortion compensating circuit lowers the gate terminal of the semiconductor element 6 as the temperature increases. That is, by lowering the gate voltage, the transconductance of the semiconductor element 6 increases, and the gain increases.
Therefore, as shown in FIG. 10B, the warpage of the gain and the phase becomes large.

As described above, by lowering the gate voltage of the semiconductor element 6 as the temperature increases, it is possible to compensate for the reduction in the warpage of the gain and the phase warpage at the high temperature. Therefore, it is possible to obtain almost constant gain and phase characteristics with respect to temperature, and it is possible to perform distortion compensation for each temperature by using this distortion compensation circuit. As a means for lowering the gate voltage of the semiconductor element 6 at a high temperature, the case where the posistor 23 is connected in parallel with the resistor 13 has been described. However, the case where a thermistor whose resistance value decreases at a high temperature is connected in parallel with the resistor 12. The effect is the same even if the voltage of the DC power supply 17 is lowered.

[0043]

The present invention comprises a semiconductor device, an input circuit and an output circuit respectively connected to a gate terminal and a drain terminal of the semiconductor device, and a bias circuit for applying a desired bias voltage to the semiconductor device. An amplifier unit, an attenuator connected to the output side of the amplifier unit, a transmission circuit, and a T-branch circuit connecting the input circuit to the input terminal of the transmission circuit and connecting the attenuator to the output terminal of the transmission circuit. It is possible to compensate for the distortion of a high-power amplifier with a very simple configuration by selecting the microwave transmission loss that passes through the amplifier section and the attenuator to be larger than the microwave transmission loss that passes through the transmission circuit. it can.

According to the present invention, by using a variable attenuator as the attenuator connected to the output side of the amplifier section, it is possible to adjust the warpage of the gain and the warpage of the phase,
Even if the amplitude and phase characteristics of the high power amplifier vary, distortion compensation can be performed.

According to the present invention, by providing the transmission circuit with the phase adjustment function, the phase can be adjusted over a wide range, and more favorable distortion compensation than in the second embodiment can be performed.

According to the present invention, by providing an attenuator between the input circuit and the T-branch circuit, it is possible to adjust the level of the input power at which the warpage of the gain and the warpage of the phase can be obtained. Even if there is, distortion compensation is possible.

According to the present invention, by setting the bias voltage of the gate terminal of the semiconductor element to be low at a high temperature, it is possible to obtain almost constant gain and phase characteristics with respect to temperature. Thus, a high-output amplifier with good temperature characteristics and low distortion can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration of a first embodiment of a distortion compensation circuit according to the present invention and an output power with respect to an input power of a microwave passing through each path.

FIG. 2 is a vector diagram for explaining the operation principle of the distortion compensation circuit according to the first embodiment, and a diagram showing gain and phase characteristics with respect to input power.

FIG. 3 is a diagram showing a configuration of a distortion compensation circuit according to a second embodiment of the present invention;

FIG. 4 is a diagram illustrating a principle of operation of the distortion compensation circuit according to the second embodiment, and a diagram illustrating gain and phase characteristics with respect to input power;

FIG. 5 is a diagram showing a configuration of a distortion compensation circuit according to a third embodiment of the present invention.

FIG. 6 is a vector diagram for explaining the operation principle of the distortion compensation circuit according to the third embodiment, and a diagram showing gain and phase characteristics with respect to input power.

FIG. 7 is a diagram illustrating a configuration of a distortion compensation circuit according to a fourth embodiment of the present invention;

FIG. 8 is a diagram illustrating gain and phase characteristics with respect to input power of a distortion compensation circuit according to a fourth embodiment.

FIG. 9 is a diagram showing a configuration of a fifth embodiment of the distortion compensation circuit according to the present invention.

FIG. 10 is a diagram illustrating gain and phase characteristics with respect to input power of a distortion compensation circuit according to a fifth embodiment.

FIG. 11 is a diagram showing an equivalent circuit of a conventional distortion compensation circuit.

FIG. 12 is a diagram illustrating gain characteristics with respect to input power for explaining the operation principle of a conventional distortion compensation circuit.

FIG. 13 is a diagram illustrating phase characteristics with respect to input power for explaining the operation principle of a conventional distortion compensation circuit.

FIG. 14 is a diagram illustrating amplitude and phase characteristics with respect to input power for explaining the principle of distortion compensation of a high-output amplifier using a conventional distortion compensation circuit.

[Explanation of symbols]

 Reference Signs List 1 input terminal 2 output terminal 3 T branch circuit 4 T branch circuit 5 input circuit 6 semiconductor element 7 output circuit 8 attenuator 9 gate bias circuit 10 inductor 11 capacitor 12 resistance 13 resistance 14 drain bias circuit 15 inductor 16 capacitor 17 DC power supply 18 DC power supply 19 transmission circuit 20 variable attenuator 21 transmission circuit 22 attenuator 23 posistor

Claims (5)

[Claims] 1. A semiconductor device having a gate terminal, a source terminal, and a drain terminal, an input circuit and an output circuit respectively connected to the gate terminal and the drain terminal of the semiconductor device, and for applying a desired bias voltage to the semiconductor device. An attenuator connected to the output side of the amplifier, a transmission circuit for connecting the amplifier and the attenuator in parallel to a cascade circuit, the input circuit and the transmission circuit And a T-branch circuit connecting the attenuator and the output terminal of the transmission circuit with each other, and wherein the passage loss of the cascaded circuit is larger than that of the transmission circuit. And a distortion compensation circuit. 2. The distortion compensation circuit according to claim 1, wherein a variable attenuator is used as said attenuator. 3. The distortion compensation circuit according to claim 1, wherein the transmission circuit has a variable phase function. 4. The distortion compensation circuit according to claim 1, wherein an attenuator is provided between the input circuit and the T branch circuit. 5. The distortion compensation circuit according to claim 1, wherein the bias voltage applied to the gate terminal of the semiconductor element is reduced as the temperature increases.