JPH0785523B2 - Non-linear distortion compensation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] In an amplifier using a transistor, an electron tube, or the like, the occurrence of non-linear distortion due to imperfect linearity between input and output often poses a problem. .

The present invention relates to the configuration of a non-linear distortion compensation circuit that suppresses the occurrence of such non-linear distortion.

[Conventional technology]

As an input / output non-linear compensation method for an amplifier effective in a high frequency band such as microwaves, there is a feedforward type distortion compensation circuit shown in FIG. 1 (hereinafter, an amplifier equipped with this compensation circuit is also referred to as a feedforward amplifier).

The feedforward distortion compensation circuit is basically composed of two loops. One is a distortion detection loop, and the other is a distortion removal loop 17.

The distortion detection loop 16 includes a main amplifier signal path 12 and a linear signal path.
The distortion elimination loop 17 is composed of a main amplifier output signal path 14 and a distortion injection path 15. further,
The main amplifier signal path 12 is composed of the main amplifier 6, and the linear signal path 13 is composed of a variable attenuator 8 and a variable delay line 9. The main amplifier output signal path 14 is composed of a transmission line, and the distortion injection path 15 is composed of a variable attenuator 10, a variable delay line 11 and an auxiliary amplifier 7.

Here, because there is no big difference in characteristics,
The variable attenuator 8 and the variable delay line 9 may be provided in the main amplifier signal path 12 both or only in one form.

Similarly, variable attenuator 10 and variable delay line 11 may optionally be included in main amplifier output signal path 14.
The power distributor 3 and the power combiners 4 and 5 are simple lossless power distributors / combiners composed of a transformer circuit, a hybrid circuit, or the like.

The operation of this circuit will be described below.

The input signal applied to the input terminal 1 is first distributed to the main amplifier signal path 12 and the linear signal path 13 by the power distributor 3, and then power-combined by the power combiner 4. Here, the variable attenuator 8 and the variable delay line 9 are connected to the power combiner 4
The signal components of the two paths 12 and 13 output to the distortion injection path 15 side are adjusted so that their amplitudes and delay amounts are equal and their phases are opposite to each other. However, the condition of the negative phase is realized by appropriately setting the amount of phase shift between the input and output terminals of the power divider 3 or the power combiner 4, or whether the phase inversion in the main amplifier 6 is used. Alternatively, as shown in FIG. 2, it is realized by inserting a phase inverting circuit having a short-circuit termination 19 at one terminal of the circulator 18 into either of the paths 12 or 13.

Since the distortion detection loop is configured in this way, the output on the side of the path 15 of the power combiner 4 is ultimately the two paths 12
The difference component of the two signals of and 13 will be detected.

The component of this difference is exactly the distortion component generated by the main amplifier, and for this reason, this loop is called the distortion detection loop.

Next, the variable attenuator 10 and the variable delay line 11 transmit the two paths 14 and 15 from the input terminal 4-1 of the power combiner 4 for the path 12 to the output terminal 5-2 of the power combiner 5. The functions are adjusted so that they are equal in amplitude and delay and opposite in phase with respect to each other. Since the input signal of the path 15 is the distortion component of the main amplifier detected by the distortion detection loop, the path 15 reverses the distortion component to the output signal of the main amplifier at the output 5-2 of the power combiner 5. Since they are injected with equal amplitude, the distortion components in the output of the entire circuit are canceled out.

The above is the ideal operation of the feedforward amplifier configuration.

In brief, the principle of operation is to detect only the distortion component generated by the main amplifier with the distortion detection loop, increase its level with the auxiliary amplifier, and reinject it into the main amplifier output with anti-phase equal amplitude by the distortion injection loop. It suppresses distortion and realizes an amplifier with good linearity.

Here, from the output terminal of the main amplifier to the power combiner 4 and the path 14
Also, the transmission path loss up to the output terminal 2 via the power combiner 5 is made as small as possible in order to avoid a decrease in the output level of the feedforward amplifier. For this purpose, the power combiner 4 and the power combiner 5
Is configured so that the coupling loss from the input terminals 4-1 to 4-3 and the coupling loss from the output terminals 5-1 to 5-2 are as small as possible.

Here, since the power combiners 4 and 5 are lossless circuits, the coupling attenuation amount from the input terminals 4-1 to 4-4 and the coupling attenuation amount from the output terminals 5-3 to 5-2 are eventually You will have to take a large value.

For example, the input terminals 4-1 to 4-3 and the output terminal 5-
In order to keep the loss from 1 to 5-2 within 0.1 dB, the combined attenuation amount from 4-1 to 4-4 and 5-3 to 5-2 needs to be 20 dB or more.

Therefore, the output level of the auxiliary amplifier 7 needs to be 20 dB higher at the output terminal 5-3 than at the output terminal 5-2.

The spectrum diagram shown in FIG. 3 represents an example of a spectrum at each point of the feedforward constituent element when two signals of equal amplitude are input in such a situation.

First, FIG. 3A shows the output spectrum of the main amplifier.

In the figure, f ₁ and f ₂ are fundamental wave output components in which the input signal is linearly amplified, 2f ₁ −f ₂ and 2f ₂ −f ₁ are third-order intermodulation distortion components, 3f
₁ -2f ₂ and 3f ₂ -2f ₁ is a fifth-order intermodulation distortion component.

Here, assuming that the output level of the main amplifier is near the maximum output, the level difference between the basic output component f ₁ or f ₂ and the third-order intermodulation distortion component 2f ₁ −f ₂ or 2f ₂ −f ₁ is It is usually less than 20 dB.

Next, FIG. 3 (b) is an output spectrum of the distortion detection loop and shows a state in which a distortion component in which the fundamental wave component is sufficiently suppressed is obtained.

Furthermore Figure 3 (c) is an output spectrum of the auxiliary amplifier 7, showing a state where the third-order intermodulation distortion component having the frequency of 5f ₁ -4f ₂ and 5f ₂ -4f ₁ is newly generated by the auxiliary amplifier .

The distortion component shown in FIG. 3 (c) is injected into the output of the main amplifier in the distortion elimination loop, and the cancellation of the distortion component of FIG. 3 (a) is achieved.

However, as shown in FIG. 3 (d), the components of 5f ₁ −4f ₂ and 5f ₂ −4f ₁ , which are the third-order intermodulation distortion components generated by the auxiliary amplifier, do not decrease in level and the feedforward amplifier Remains as the distortion of. Here, as described above, the output level of the auxiliary amplifier must be higher than the output terminal 2 by 20 dB or more, so that the level difference between the fundamental wave component and the third-order intermodulation distortion component in the main amplifier output is 20 dB. For example, the output level of the auxiliary amplifier is almost the same as the output level of the main amplifier.

Therefore, even if the auxiliary amplifier having the same maximum output as that of the main amplifier is used, 2f ₁ −f ₂ and 2f ₂ −
Since the level difference between the f ₁ component and the 5f ₁ −4f ₂ and 5f ₂ −4f ₁ components can be only 20 dB at the maximum, the fundamental wave components f ₁ , f ₂ and the third-order intermodulation at the feedforward output are eventually obtained. The maximum level difference between the distortion components 5f ₁ −4f ₂ and 5f ₂ −4f ₁ is 40 dB. If the amount of distortion improvement by the feedforward configuration is evaluated by the residual distortion level difference between when the auxiliary amplifier is not operated and when the auxiliary amplifier is operated, the distortion improvement amount is only 20 dB at most.

Further, when the maximum output level of the auxiliary amplifier is lower than that of the main amplifier, the third-order intermodulation distortion level generated by the auxiliary amplifier becomes higher, so that the distortion improvement amount decreases.

In the above description, the compensation operation for the third-order intermodulation distortion has been described, but basically the same applies to the fifth-order distortion.

Further, in the spectrum example of FIG. 3 (b), the distortion component that drops to the fundamental frequency is omitted for simplification of description.
In reality, a distortion component whose frequency is the same as that of the fundamental frequency is generated at a level higher than the levels of 2f ₁ −f ₂ and 2f ₂ −f ₁ .

[Problems to be Solved by the Invention]

As described above, the feedforward amplifier operates as an amplifier having better linearity than the main amplifier used, but in order to achieve a distortion improvement amount of 20 dB or more at an operating point with a high output level, the feedforward amplifier serves as an auxiliary amplifier. There is a drawback in that a power amplifier having a maximum output characteristic equal to or higher than that is required.

It is an object of the present invention to provide a feedforward type non-linear compensation circuit that can realize a good distortion compensation characteristic even if an auxiliary amplifier whose maximum output level is smaller than the main amplifier is used.

[Means for Solving the Problems]

According to the invention, the above mentioned objects are achieved by the means recited in the claims.

That is, according to the present invention, in the feedforward amplifier, a predistortion compensating circuit (hereinafter referred to as a predistortion circuit) capable of adjusting the amplitude and phase of the nonlinear distortion to be injected is provided in the input signal path of the main amplifier. The main feature is that the third-order intermodulation distortion component generated by the main amplifier is reduced in advance.

In the conventional technology, since the third-order distortion level generated by the main amplifier is high, it is necessary to correspondingly increase the level of the output signal of the auxiliary amplifier (distortion component of the detected / amplified main amplifier). In order to reduce the residual distortion component and enhance the distortion improving effect, there is a drawback that an auxiliary amplifier having a maximum output comparable to that of the main amplifier is required.

On the other hand, according to the present invention, the required maximum output of the auxiliary amplifier can be significantly reduced without deteriorating the distortion improvement amount, and the maximum output level is as high as that of the main amplifier. In the case of using, it is possible to realize a higher amount of distortion improvement, and this point is greatly different from the prior art.

〔Example〕

 FIG. 4 is a diagram showing the configuration of the first embodiment of the present invention.

The present invention is configured by providing a predistortion circuit 20 in the input path of the main amplifier in the basic configuration of the feedforward amplifier shown in FIG. And FIG. 5 is a diagram showing the configuration and principle of the predistortion circuit.

First, the operation of the circuit of FIG. 5 (a) will be outlined.

The signal is input from the input terminal 21, and is distributed by the power distributor 23 to the two signal paths with equal amplitude or an appropriate level relationship. One is the power combiner via the variable delay line 28.
Entered in 24. The other distribution signal is a third-order distortion generator
25, the phase and amplitude are adjusted by the variable phase shifter 26 and the variable attenuator 27, and then input to the power combiner 24, which is linearly combined with the linear signal component of the variable delay line in an appropriate phase-amplitude relationship. . The output signal of the predistortion circuit into which the third-order distortion component is injected is input to the post-main amplifier 6.

The spectral relationship between the linear signal component and the distortion component at this time is conceptually shown in FIGS. 2B and 2C, taking two signals of equal amplitude as an example.

The input signal spectrum is the input signal spectrum of the predistortion circuit and is composed of pure two frequencies f ₁ and f ₂ as shown in FIG. On the other hand, in the output signal spectrum of the main amplifier, distortion components (frequency) 2f ₁ −f ₂ and 2f ₂ −f ₁ injected by the predistortion circuit (PD) 20 ( There are a dashed line in the figure and the letter D) and a distortion component generated by the main amplifier (PA) (indicated by the solid line and letter A in the figure).

Here, as conceptually shown in the figure, these two distortion components are adjusted by the PD circuit so that they are in the opposite phase with equal amplitude, so that the PA output 22 eventually cancels the third-order distortion, A linear output signal without distortion is obtained. Here, the third-order distortion generator is the sixth
As shown in the diagram, the pair diode 30 is connected in antiparallel to the two equal amplitude distribution ports of the 90 ° hybrid circuit 29.
It can be realized by a circuit that terminates. The applied voltage and conduction current characteristics of the pair diodes connected in anti-parallel are similar to the third-order characteristics.
Third-order distortion occurs in the diode loads 30 of and 34. These two third-order distortion components are due to the properties of the 90 ° hybrid circuit,
The input end 31 can be set to output in opposite phases, and the output end 32 can be set to output in phase.

On the other hand, for the linear component, since the terminals 31 and 32 are isolated from each other, the impedance in the linear operation region of the diode circuit is matched with the terminal impedance of the hybrid so that the output terminal 32 does not output. Can be configured. This suppression amount matches the isolation amount of the terminals 31 and 32, and it is practically possible to secure 30 dB or more. Therefore, it is possible to generate a third-order distortion component having a small linear component.

Regarding the predistortion circuit described above, the detailed operation principle is described in Japanese Patent No. 1107681 (Showa 57.8.13).

As described above, since the predistortion circuit 20 is provided, the third-order distortion component in the output of the main amplifier 6 can be greatly reduced. Therefore, in the configuration shown in FIG. The signal spectrum at that time is as shown in FIGS. 7 (a) to 7 (d).

That is, as shown in FIG. 7 (a), the third-order distortion (2f ₁ −f ₂ and 2f ₂ −f ₁ ) in the output of the main amplifier is significantly reduced, so that the distortion detection loop output is also shown in FIG. 7 (b). As shown in
The spectrum has a reduced third-order distortion component.

For example, if the characteristics of the auxiliary amplifier are the same as those in the configuration shown in FIG. 1 and the amount of distortion improvement due to predistortion is 20 dB, the third-order distortion level will be 20 dB lower than that in FIG. 3B.

Therefore, 3 auxiliary amplifier of the present invention generates distortion (5f ₁ -4
f ₂ and 5f ₂ −4f ₁ component) level is 3 of the output level.
Since it is proportional to the power and is 60 dB lower than the case of the configuration of FIG. 1, it is ignored as a minute term in FIG. 7 (c).

Therefore, the output spectrum of the feedforward amplifier is obtained by subtracting the injection distortion component given in FIG. 7 (c) from the output of the main amplifier in FIG. 7 (a), as shown in FIG. 7 (d). Therefore, an output spectrum with extremely little residual distortion can be obtained. In this case, it was assumed that the maximum output of the auxiliary amplifier was equal to that in the configuration of FIG. 1, but for example, the residual distortion level is shown in FIG. 7 (d).
If the same level as in the above is satisfied, the auxiliary amplifier output level required by the present invention is reduced by 20 dB as described above, and thus the required maximum output level can be reduced by 20 dB.

That is, it becomes possible to reduce the required maximum output level of the auxiliary amplifier by the amount of dB equal to the amount of improvement of the third-order distortion due to predistortion.

For example, if the third-order distortion improvement amount of predistortion is 20 dB and the maximum output of the main amplifier is 1 kW as in the above description, the required maximum output of the auxiliary amplifier, which was required in the configuration of FIG. 1, is 1 kW. It can be reduced to 10 W by the present invention.

FIG. 8 is a diagram showing the configuration of the second embodiment of the present invention.

Compared with the first embodiment of FIG. 4, the present invention is characterized in that the input signal path of the auxiliary amplifier is also provided with the predistortion circuit 20.

The basic operation is almost the same as that in the first embodiment, but since the third-order distortion level generated by the auxiliary amplifier is also reduced by such a configuration, the residual distortion as a feedforward amplifier is reduced. It is possible to further reduce the amount as compared with the first embodiment of the invention.

〔The invention's effect〕

As described above, according to the present invention, it is possible to reduce the non-linear distortion of the main amplifier having a much larger output by using the auxiliary amplifier having a smaller maximum output.
The present invention can be applied as an economical and compact nonlinear distortion compensating circuit for compensating the nonlinear distortion of a linear amplifier not only to a high output amplifier for transmission in wireless broadcasting but also to a wire communication repeater, an audio device and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of the basic configuration of a feedforward type distortion guarantee circuit, FIG. 2 is a diagram showing an example of the configuration of a phase inverting circuit using a circulator, and FIG. 3 is a diagram illustrating the operating principle of a feedforward amplifier. FIG. 4 is a diagram showing the configuration of the first embodiment of the present invention, FIG. 5 is a diagram showing the configuration and principle of the predistortion circuit, and FIG. 6 is a diagram showing the third-order distortion generator. FIG. 7 is a diagram showing an example of the configuration, FIG. 7 is a signal spectrum diagram for explaining the operating principle of the present invention, and FIG. 8 is a diagram showing the configuration of the second embodiment of the present invention. 1 ... Input terminal, 2 ... Output terminal, 3,23 ... Power distributor, 4,5,24 ... Power combiner, 6 ... Main amplifier, 7 ... Auxiliary amplifier, 8,10,27 ... … Variable attenuator, 9, 11, 28 …… Variable delay line, 12 …… Main amplifier signal path, 13 …… Linear signal path, 14 …… Main amplifier output signal path, 15 …… Distortion injection path,
16 ... Distortion detection loop, 17 ... Distortion removal loop, 18 ... Circulator, 19 ... Short-circuit termination, 20 ... Predistortion circuit, 21 ... Predistortion input terminal, 22 ...
… Pre-distortion output terminal, 25 …… Third-order distortion generator, 26 …… Variable phase shifter, 29 …… 90 ° hybrid, 30…
… Inverse parallel connection diode, 31 …… Third-order distortion generator input terminal, 32 …… Third-order distortion generator output terminal, 33,34 …… Distribution terminal

Claims (2)

[Claims] 1. A feedforward type having means for detecting a non-linear distortion component of a main amplifier and means for canceling the distortion component by amplifying the detected distortion component using an auxiliary amplifier and then reinjecting it into the output of the main amplifier. In the non-linear distortion compensating circuit, the pre-distortion compensating circuit having means for adjusting the amplitude and phase of the distortion component is provided in the input signal path of the main amplifier. 2. The non-linear distortion compensating circuit according to claim 1, further comprising a predistortion compensating circuit having means for adjusting the amplitude and phase of the distortion component in the input signal path of the auxiliary amplifier. Distortion compensation circuit.