JPH08204774A - Negative feedback amplifier - Google Patents

Abstract

PURPOSE: To provide a Cartesian type negative feedback amplifier in which stable distortion compensation characteristic is realized by keeping an input to the amplifier constant. CONSTITUTION: A basic circuit of the Cartesian type negative feedback amplifier is made up of subtractors 103, 104, band limit circuits 105, 106, an oscillator 107, an orthogonal modulator 108, a nonlinear amplifier 110, an attenuator 111, and an orthogonal demodulator 112. An automatic gain control circuit 109 provided between the orthogonal modulator 108 and the nonlinear amplifier 110 is used to conduct gain control, then even when an output level of the orthogonal modulator 108 is changed, the input to the nonlinear amplifier 110 is kept constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a negative feedback amplifier for compensating for non-linear distortion of an amplifier for amplifying a quadrature modulation signal,
In particular, it relates to a Cartesian-type negative feedback amplifier that compensates for distortion by demodulating an amplifier output and performing negative feedback in the form of a baseband signal.

[0002]

2. Description of the Related Art A conventional negative feedback amplifier of this type has a 4-phase P-type.
When performing high-density wireless digital transmission using a linear modulation method such as SK or 16-valued QAM, a method for compensating for nonlinear distortion of a power amplifier in a transmitter, especially a high loopback for a modulation signal in a high frequency band It is known as a method for obtaining a gain.

The negative feedback amplifier disclosed in Japanese Patent Publication No. 5-58283 will be described below with reference to the block diagram of the conventional negative feedback amplifier shown in FIG.

In this negative feedback amplifier, terminals 101 and 102 receive an input baseband signal I represented by a function x (t) and an input baseband signal Q represented by a function y (t), respectively. The subtractors 103 and 104 subtract the feedback baseband signals FIx and FQx from the input baseband signals I and Q, respectively, and generate subtracted baseband signals Iax and Qax, respectively. The subtracted baseband signals Iax and Qax are band-limited by the band limiting circuits 105 and 106, respectively, and the function x
Modulation signal Ibx represented by 1 (t) and function y1 (t)
The modulated signal Qbx represented by

The quadrature modulator 108 quadrature-modulates the carrier signal L1 of the angular frequency ωc generated by the oscillator 107 with the modulation signals Ibx and Qbx, and the function z (t) = x1.
The quadrature modulation signal Max represented by (t) · cosωct + y1 (t) · sinωct is generated. This quadrature modulation system uses QPSK (4 phase shift keying), π / 4
It is shift QPSK or 16-value QAM modulation. Further, the oscillator 107 has a frequency f (= ωc / 2 if the application of the negative feedback amplifier is, for example, a current automobile telephone system.
π) is 900MHz band, 14 if it is a commercial wireless system
A carrier signal L1 in the 0 MHz band is generated.

The quadrature modulation signal Max is supplied to the nonlinear amplifier 11
By 0, it is amplified to the quadrature modulation signal Mbx. Most of the quadrature modulation signal Mbx is radiated from the antenna, and part of this signal Mbx is branched by a directional coupler or the like and supplied to the attenuator 111. The attenuator 111 uses the quadrature modulation signal M
bx is attenuated to a predetermined level, and its quadrature modulation signal M
cx is supplied to the quadrature demodulator 112. Quadrature demodulator 112
Is a carrier signal L2 (signal L1
(The same signal as) is phase-shifted (phase-shifted) by the phase shifter 302 and the orthogonal modulation signal Mcx is demodulated in response to the orthogonal modulation signal Mcx and the feedback baseband corresponding to the input baseband signal I. Feedback baseband signal FQx corresponding to signal FIx and input baseband signal Q
And

The feedback baseband signals FIx and FQx output from the quadrature demodulator 112 are subjected to amplitude distortion and phase distortion due to the non-linear distortion of the non-linear amplifier 110. The feedback base-band signals FIx and FQx are subtracted as described above. The negative feedback amplifier compensates the non-linear distortion of the non-linear amplifier 110 by supplying the non-linear distortion component of the non-linear amplifier 110 with negative feedback to the amplifiers 103 and 104.

The above-described negative feedback amplifier obtained by removing the phase difference detection circuit 301 from the circuit of FIG. 8 generally produces a quadrature modulation signal Max due to the loop length of the negative feedback circuit and the band-limited frequency characteristic of the non-linear amplifier 110. In comparison, the quadrature modulation signal Mcx is delayed and the carrier wave phases of both signals are different from each other. For example, the amount of phase shift of the phase shifter 302 is fixed, and the quadrature demodulator 11 is output from the output end of the quadrature modulator 108 due to temperature characteristic variations of the non-linear amplifier 110, load variations of the antenna, and the like.
When the delay amount up to the input terminal of 2 changes, the quadrature modulation signal I
The feedback baseband signal vector, which is a combined vector of the feedback baseband signals FI and FQ, makes a phase rotation with respect to the input baseband signal vector that is a combined vector of Q and Q. As a result, the distortion suppression characteristic of this negative feedback amplifier deteriorates.

Therefore, in the negative feedback amplifier disclosed in the above publication, the phase difference detection circuit 301 receives the quadrature modulation signals Max and Mcx and outputs the phase control signal PCx representing the phase difference Δθ between the two. This phase control signal PC
x is supplied to the phase shifter 302, and the phase shifter 302 receives the phase difference Δ
The phase shift amount is changed so that θ becomes a minimum. As a result of this control, the vector phases of the quadrature modulation signal Max and the quadrature modulation signal Mcx substantially match, and this negative feedback amplifier can reduce the deterioration of the non-linear distortion due to the above-described phase rotation characteristic.

[0010]

However, the above-described negative feedback amplifier according to the prior art has the quadrature modulation signals Max and Mc.
There is a drawback in that the effect of reducing the distortion deterioration due to the phase shift is weakened due to the delay with respect to x and the delay from the signal detection to the update of the phase shift amount of the phase shifter 302.

[0011]

SUMMARY OF THE INVENTION In order to solve the above-mentioned drawbacks of the present invention, a negative feedback amplifier according to claim 1 uses a first and a second input baseband signals to obtain a first and a second feedback baseband signals. First and second subtractors for subtracting the signals to produce first and second subtracted baseband signals, respectively.
First and second band limiting circuits for respectively band limiting the first and second subtracted baseband signals to generate first and second modulated signals, an oscillator for generating a carrier signal, and first and second A quadrature modulator that modulates a carrier signal by the modulation signal of 1 to generate a quadrature modulated signal, an automatic gain control circuit that amplifies the quadrature modulated signal, an amplifier that amplifies the output of the automatic gain control circuit, a carrier signal and an amplifier First demodulating the quadrature modulated signal in response to a part of the quadrature modulated signal;
And a quadrature demodulator that produces the second feedback baseband signal corresponding to the second input baseband signal.

By performing gain control in the automatic gain control circuit, the input level to the amplifier is kept constant even if the input level to the automatic gain control circuit changes.

The negative feedback amplifier according to claim 2 is
From the first and second input baseband signals, first and second
And subtracting the feedback baseband signal to generate first and second subtraction baseband signals, respectively, and receiving and band limiting the first and second subtraction baseband signals, respectively. First and second band limiting circuits that generate first and second modulated signals, an oscillator that generates a carrier signal, and quadrature modulation that modulates the carrier signal by the first and second modulated signals to generate a quadrature modulated signal. , An automatic gain control circuit that amplifies a quadrature modulation signal, an amplifier that amplifies the output of the automatic gain control circuit, a phase shifter that receives a carrier signal and changes its phase, and a phase shifter output and an amplifier In response to a portion of the quadrature modulated signal, the quadrature modulated signal is demodulated to generate a first feedback baseband signal corresponding to the first input baseband signal and a second feedback vector corresponding to the second input baseband signal. Composed of the quadrature demodulator to produce a baseband signal. Then, in the phase shifter, the phases of the two-dimensional vector represented by the first and second subtraction baseband signals and the two-dimensional vector represented by the first and second feedback baseband signals match. By controlling the amount of phase change and performing the gain control in the automatic gain control circuit, the input level to the amplifier is kept constant even if the input level to the automatic gain control circuit changes.

[0014]

As shown in FIG. 1, the automatic gain control circuit 109 changes the gain in accordance with the input to the signal M of a constant level.
Since p is output and is input to the amplifier 110, the input to the amplifier 110 is kept constant. As a result, a stable distortion improving characteristic can be realized without using complicated control such as phase control in the negative feedback amplifier.

[0015]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of a negative feedback amplifier according to the present invention.

Subtractors 103 and 104 subtract the feedback baseband signals FI and FQ from the input baseband signals I and Q to produce subtracted baseband signals Ia and Qa, respectively. The subtracted baseband signals Ia and Qa are
Band limiting circuits 105 and 106 respectively band limit the modulated signals Ib and Qb. The quadrature modulator 108 receives the carrier signal L1 generated by the oscillator 107.
Is quadrature-modulated by the modulation signals Ib and Qb to generate a quadrature-modulation signal Ma. The quadrature modulation signal Ma is input to the automatic gain control circuit 109, and the automatic gain control circuit 109 outputs the quadrature modulation signal Mp. The power of the quadrature modulation signal Mp is amplified by the non-linear amplifier 110 to become a quadrature modulation signal Mb. Most of the quadrature modulation signal Mb is radiated by the antenna, and part of this signal Mb is branched by the directional coupler or the like and supplied to the attenuator 111. The attenuator 111 attenuates the quadrature modulation signal Mb to a predetermined level and supplies the quadrature modulation signal Mc to the quadrature demodulator 112. The quadrature demodulator 112 demodulates the quadrature modulation signal Mc in response to the carrier signal L2 generated by the oscillator 107 and the quadrature modulation signal Mc, and returns the feedback baseband signal FI and the input baseband signal I corresponding to the input baseband signal I. A feedback baseband signal FQ corresponding to Q is produced.

The block diagram of FIG. 1 shows a nonlinear amplifier 1
The configuration is the same as that of a normal negative feedback amplifier, except for the automatic gain control circuit 109 provided in the preceding stage of 10.

An automatic gain control circuit 1 which is a feature of this embodiment
At 09, the magnitude of the output is made constant by changing the gain by the input, and the magnitude of the input to the nonlinear amplifier 110 is kept constant.

There are two problems of phase rotation: (1) increase in distortion due to increase in level to the input of the nonlinear amplifier, and (2) loop oscillation due to phase rotation of 45 ° or more.
There is one.

For example, as shown in FIG. 2B, it is assumed that the phase of the feedback baseband signal vector is delayed by θ from the input baseband signal vector. At this time, the subtraction signal vector becomes larger than that when there is no phase rotation in FIG. This complex subtracted signal is the band limiting circuits 105 and 10
After passing through 6, the signal is input to the quadrature modulator 108 and output as the quadrature modulated signal Ma. Since the quadrature modulation signal Ma is input to the non-linear amplifier 110 in the conventional negative feedback amplifier, if a phase rotation occurs, the input Ma of the non-linear amplifier 110 is increased.
Becomes larger and the nonlinear distortion becomes larger. Further, if the phase rotation exceeds 45 °, positive feedback occurs and the loop oscillates.

Conventionally, it has been attempted to solve the problem by correcting the phase difference between the vectors due to the circuit delay by the phase control as described in the section "Prior Art".

However, although the phase control is indispensable for the problem (2), the phase control is necessary for the problem (1) if the increase of the input level to the nonlinear amplifier 110 can be suppressed. There is no. Therefore, when the phase rotation is large enough not to make the loop unstable, it is sufficient to suppress the increase in the input level to the nonlinear amplifier 110.

In the configuration of this embodiment, the quadrature modulation signal Ma is input to the automatic gain control circuit 109, and the automatic gain control circuit 109 changes the gain according to the input and outputs the signal Mp of a constant level, which is Since it is input to the nonlinear amplifier 110, the input to the nonlinear amplifier 110 is kept constant. Therefore, the negative feedback amplifier of the present embodiment can realize stable distortion improving characteristics without using complicated control such as phase control.

The automatic gain control circuit 109 can be sufficiently realized by a generally used method, and for example, FIG.
It is possible to use a variable attenuator 120 and a monitoring circuit 150 as shown in FIG. Variable attenuator 120
The amount of attenuation is determined by the control output 630 of the monitoring circuit 150 that monitors the DC current supplied from the stabilized power supply circuit 600 of the amplifier 110.

Variable attenuator 120 and monitoring circuit 150
May be, for example, circuits having the characteristics of FIGS. 4 and 5, respectively. Variable attenuator 120 having the characteristics of FIG.
Can be obtained by applying a DC voltage from the IF terminal and using a double balanced mixer using the BF terminal as an input terminal and the LO terminal as an output terminal.

In FIG. 3, A of the stabilized power supply circuit 600 is shown.
Since the potential at the point is always constant, the potential at the point B increases due to the resistance R ₁ as the direct current I supplied to the amplifier 110 increases. Therefore, when the potential at the point B becomes equal to or higher than a certain reference voltage V _R (that is, the current I is equal to or higher than a constant value I _o ), the comparator 6 which outputs the potential difference between the point B and the reference voltage V _R.
10 is provided, and the value obtained by subtracting the difference from the voltage V _o in FIGS. 4 and 5 may be output as the control voltage 630 by the comparator 620. The comparators 610 and 6 in FIG.
20 assumes a high input impedance.

FIG. 6 is a block diagram showing a second embodiment of the negative feedback amplifier according to the present invention. This block diagram has the same configuration as the conventional negative feedback amplifier shown in FIG. 8 except for the automatic gain control circuit 109 provided in the preceding stage of the non-linear amplifier 110.

When the phase rotation is 45 ° or more, it is necessary to deal with both the problems (1) and (2) due to the phase rotation. Therefore, phase control is essential. However, the requirement for the accuracy of the phase control for the problem (1) is stricter than the requirement for the problem (2). Therefore, if both the problems are addressed only by the phase control, the circuit becomes complicated. Therefore,
As in the present embodiment, the problem (1) is dealt with by keeping the input level of the nonlinear amplifier 110 constant, and the problem (2) is dealt with by the phase control to loosen the requirement for the accuracy of the phase control. Can be configured more easily.

An automatic gain control circuit 1 which is a feature of this embodiment
At 09, the magnitude of the output is made constant by changing the gain by the input, and the magnitude of the input to the nonlinear amplifier 110 is kept constant. Further, since the phase detection circuit 301 detects the phase rotation and updates the value of the phase shifter 302 to correct the phase rotation, it is possible to prevent loop oscillation due to the phase rotation. Therefore, the negative feedback amplifier of the present embodiment can realize stable distortion improving characteristics with a simple configuration.

FIG. 7 is a block diagram showing a third embodiment of the negative feedback amplifier according to the present invention. In this embodiment, the phase shift signal is output from the phase shift difference detection circuit 401 to the phase shifter 302 inserted between the oscillator 107 and the quadrature demodulator 112 to control the amount of phase shift.

That is, the phase detection circuit 401 first detects the phase difference between the input baseband signal vector which is a combined vector of the input baseband signals I and Q and the modulation signal vector which is a combined vector of the modulation signals Ib and Qb. .
When the phase of the modulation signal vector is behind the phase of the input baseband signal vector, the phase shift amount of the phase shifter 302 is increased, and conversely, the phase of the modulation signal vector leads the phase of the input baseband signal vector. If so, the phase shift amount of the phase shifter 302 is decreased.

In this embodiment as well, in the same way as the second embodiment, the problem (1) is solved by keeping the input level of the non-linear amplifier 110 constant in coping with the problems (1) and (2) due to the phase rotation. And the problem (2) is dealt with by phase control. Compared with the second embodiment (FIG. 6), the modulation signal I
A phase control signal PC corresponding to a phase lag advance of a modulation signal vector (feedback baseband signal response vector) that is a combined vector of b and Qb and an input baseband signal vector that is a combined vector of input baseband signals I and Q is generated. The difference from the second embodiment is that the phase shifter 302 is controlled using the phase control circuit 401.

In the example of FIG. 7, the phase difference detection circuit 401 is
The phase information of the feedback baseband signals FI and FQ is obtained from the modulation signals Ib and Qb which are the outputs of the band limiting circuits 105 and 106 and respond to the feedback baseband signals FI and FQ. And FQ may be used directly. In that case, when the phase of the feedback baseband signal vector is behind the phase of the input baseband signal vector, the phase shift amount of the phase shifter 302 is decreased, and the phase of the feedback baseband signal vector is changed to the input baseband signal vector. The phase shift amount of the phase shifter 302 may be increased when the phase is advanced from the phase.

[0034]

As described above, according to the present invention,
Since the input to the amplifier is kept constant by the automatic gain control circuit, a stable distortion improving characteristic can be easily realized.

[Brief description of drawings]

FIG. 1 is a block diagram of a first embodiment of a negative feedback amplifier according to the present invention.

FIG. 2 is a vector diagram for explaining the first embodiment, where (a) is an input baseband signal vector, (b) is a feedback baseband signal vector, and (c) is a subtraction signal vector.

FIG. 3 is a block diagram showing a configuration example of an automatic gain control circuit.

FIG. 4 is a diagram showing characteristics of the variable attenuator of FIG.

5 is a diagram showing characteristics of the monitoring circuit of FIG.

FIG. 6 is a block diagram of a second embodiment of the negative feedback amplifier according to the present invention.

FIG. 7 is a block diagram of a third embodiment of the negative feedback amplifier according to the present invention.

FIG. 8 is a configuration diagram of a conventional negative feedback circuit.

[Explanation of symbols]

 101, 102 Input terminals 103, 104 Subtractor 105, 106 Band limiting circuit 107 Oscillator 108 Quadrature modulator 109 Automatic gain control circuit 110 Non-linear amplifier 111 Attenuator 112 Quadrature demodulator 301 Phase difference detection circuit 302 Phase shifter 401 Phase difference detection circuit

Claims (6)

[Claims] 1. A first subtraction of first and second feedback baseband signals from first and second input baseband signals to produce a first
And a first subtracted baseband signal that produces a second subtracted baseband signal, respectively.
And a second subtracter, first and second band limiting circuits for receiving and band limiting the first and second subtracted baseband signals, respectively, to generate first and second modulated signals, respectively, and a carrier signal For generating the quadrature modulated signal by modulating the carrier signal with the first and second modulated signals, an amplifier for amplifying the quadrature modulated signal, the carrier signal and the amplifier In response to a part of the quadrature modulated signal, the quadrature modulated signal is demodulated to correspond to the first feedback baseband signal corresponding to the first input baseband signal and the second input baseband signal. In a negative feedback amplifier including a quadrature demodulator that generates the second feedback baseband signal, an automatic gain control circuit is provided between the quadrature modulator and the amplifier, and the automatic gain control circuit is provided. A negative feedback amplifier characterized in that the gain control is performed in a gain control circuit to keep the input to the amplifier constant even if the output level of the quadrature modulator changes. 2. A first subtraction of the first and second feedback baseband signals from the first and second input baseband signals
And a first subtracted baseband signal that produces a second subtracted baseband signal, respectively.
And a second subtracter, first and second band limiting circuits for receiving and band limiting the first and second subtracted baseband signals, respectively, to generate first and second modulated signals, respectively, and a carrier signal For generating a quadrature modulated signal by modulating the carrier signal with the first and second modulated signals, an amplifier for amplifying the quadrature modulated signal, and a phase for receiving the carrier signal. In response to the phase shifter output and the part of the quadrature modulation signal from the amplifier, and demodulates the quadrature modulation signal to correspond to the first input baseband signal. One feedback baseband signal and a quadrature demodulator for producing the second feedback baseband signal corresponding to the second input baseband signal, and the first and second subtraction bases. And a phase change amount in the phase shifter so that the phases of the two-dimensional vector represented by the two-dimensional vector signal and the two-dimensional vector represented by the first and second feedback baseband signals match. In a negative feedback amplifier, an automatic gain control circuit is provided between the quadrature modulator and the amplifier, and gain control is performed in the automatic gain control circuit, so that the amplifier does not change even when the output level of the quadrature modulator changes. Negative feedback amplifier characterized by keeping the input to the constant. 3. A phase difference which receives the output of the quadrature modulator and the input of the quadrature demodulator and outputs a phase control signal corresponding to the lag or advance of the phases of the two as means for controlling the phase amount in the phase shifter. The negative feedback amplifier according to claim 2, further comprising a detection circuit, wherein the phase control signal controls the phase amount in the phase shifter. 4. A phase of a feedback baseband signal response vector, which is a composite vector of baseband signals respectively responsive to the first and second feedback baseband signals, as means for controlling the phase amount in the phase shifter. 3. The negative feedback amplifier according to claim 2, further comprising a phase difference detection circuit for generating a phase control signal corresponding to a phase advance of an input baseband signal vector which is a combined vector of the first and second input baseband signals with respect to. 5. In the phase difference detection circuit, the first
The negative feedback amplifier according to claim 4, wherein the baseband signals respectively responsive to the first and second feedback baseband signals are the first and second modulated signals. 6. The phase difference detection circuit according to claim 1, wherein
The negative feedback amplifier according to claim 4, wherein the baseband signals respectively responsive to the first and second feedback baseband signals are the first and second feedback baseband signals themselves.