JPS6367925A - Negative feedback amplifier - Google Patents

Abstract

PURPOSE:To prevent an output signal spectrum from deterioration due to the delay of a power amplifier or the like by adding a control circuit for outputting a control signal indicating whether the average value of outputs obtained from the power amplifier is >= or <=a fixed value in case of demodulating the output of the power amplifier and feeding back the demodulated signal as a base band signal and a variable gain circuit for adjusting negative feedback gain at the time of receiving the output of the control circuit so that average power inputted to the control circuit is always fixed. CONSTITUTION:The 1st and 2nd base band signals are inputted to a DC/AC converter 130 through subtractors 110, 115 and LPFs 120, 125. A signal modulated by the converter 130 is amplified by a power amplifier 140. A part of the output of the amplifier 140 is attenuated by a variable gain attenuator 145 and inputted to a DC/AC demodulator 135. The demodulator 135 receives the output of the attenuator 145 and the output of a sine wave generator 131 through a phase shifter 132 and demodulates the inputted signals to obtained a base band signal. The input to the attenuator 145 is also inputted to a control circuit 150 and a gain control signal is obtained from the attenuator 145 so that a required value is obtained from said input. The output of the control circuit 150 controls the attenuator 145 so that the attenuation of the attenuator 145 is reduced when the output of the amplifier 140 exceeds a required value, and when the output is smaller than the required value, the attenuation is increased. Consequently, a negative feedback circuit can be prevented from the deterioration of the whole characteristics due to the change of delay characteristics from a designed value.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a negative feedback amplifier that demodulates a power amplifier output and feeds it back in the form of a baseband signal.

In wireless communications, the relationship between power amplifier linearity and power supply efficiency in a transmitter is always an issue. If an amplifier with high power supply efficiency is used, nonlinear distortion will increase. For high-density digital transmission, 4-phase P
Linear modulation methods such as SK and 16-value QAM are used, but
A signal modulated by such a modulation method inevitably suffers from deterioration of the transmission spectrum due to nonlinearity of the amplifier. To compensate for such deterioration, distortion suppression using a negative feedback circuit is one of the commonly used methods. As one method for obtaining a high loop gain especially for signals in a high frequency band, a method is known in which the output of a power amplifier is demodulated and fed back in the form of a baseband signal.

(Prior art and its problems) As a method for demodulating and feeding back the amplifier output, the Second International Conference on Radio Spectrum Conversion Technics (2n
d International Conference
e onRadio Spectrum Converter
There is a method announced in sion 1:echniques). Figure 2 shows an example of this method. Terminal 201
, 202 input signals x (t> , y (t
) are input to low-pass filters 230 and 235 through subtracters 201 and 215, respectively. Low-pass filter LP
F 230 output is multiplied by a carrier wave (angular frequency is ωt) which is the sine wave generator 290 output in a multiplier 240, and the low pass filter L P F 235 output and a phase shifter 29 are used.
After the multiplier 245 multiplies the output of the sine wave generator 290 whose phase has changed by 90° in step 5, the adder 250 adds the outputs of the multipliers 240 and 245. Adder 25
The output of 0 is a signal that has been orthogonally modulated by the baseband signals input from terminals 201 and 202. The output of the adder 250 is amplified by an amplifier 260 with a gain of G and is transmitted from the terminal 203.

An attenuator 27 having an attenuation amount R receives a part of the output of the amplifier 260.
The transmitted signal attenuated by 0 is input into multipliers 280 and 285 after adjusting its phase with a phase shifter 275. The multiplier 280 demodulates the signal by multiplying it by the output of the sine wave generator 290, and the demodulated signal is input to the subtracter 210 and sent to the terminal 2.
Subtract from the input signal from 01. The multiplier 285 demodulates the signal by multiplying it by the 90° phase-shifted sine wave generator output, and the demodulated signal is input to the subtracter 215 and subtracted from the input signal from the terminal 202. L P F 2
30 and 235 are for limiting the band of the feedback circuit, and it is desirable that the two characteristics be approximately equal. Furthermore, the phase shifter 275 is provided to prevent deterioration of the round-trip gain due to delay. For example, from amplifier 260 to attenuator 270
Assume that the delay to the output is −τ and that there is no phase shifter. At this time, if the output of the adder 250 is z, (t), then z e(t) = x (t)cos(I)11t+
y (t) sinωtt (1), the attenuator 270 output z, (t) is z, (t) = G-R・(
x(t-xr)cosωc(t-6τ)+y(-Δτ
) sinωc(t−μτ)) (2), and the multiplication circuits 280 and 285 outputs XI(t)1ybat) are respectively x*(t)=x(tt−τ)cosω2Δτ or,
7x (t> −y(−Δτ)cosω6muτ
(3) becomes. Therefore, the gain cltAω1Δτ is applied to the signal component during demodulation. This means that the signal is attenuated in the demodulator. At this time, the gain G of the negative feedback circuit that corresponds to the reliability is the following value:

When C
Yozensa J, ``Monkai Nezumi'': Since the gain is reduced by cO3ω, Δτ times, the distortion improvement characteristics deteriorate. In other words, nonlinear distortion increases.

■cosω. When Δτ<0 oscillate.

Therefore, a phase shifter 275 is used to solve this problem. The phase of the attenuator output is shifted by Δθ, and cos(ω.Δτ−mu0)=1
(5), the outputs of multipliers 280 and 285 are x
(t-Δτ), y(t-Δτ). In this way, if θ is determined so as to satisfy Equation (9), the above (1) and (2) can be solved.

However, if such a transmitter is used in an FDM system and the delay of amplifier 260 has a frequency characteristic,
In such a circuit in which the amount of phase change of the phase shifter 275 is fixed to a fixed part, the above problem (1) (2) occurs again when the line is switched. Furthermore, when the input signal level of the circuit becomes high, the output power of the circuit may exceed its limit.

An object of the present invention is to provide a negative feedback amplifier that overcomes these drawbacks.

(Means for Solving the Problems) Means provided by the present invention for solving the above-mentioned problems uses first and second baseband signals as inputs:
and a first and a second input signal, each receiving a second input signal.
a subtracter; a quadrature modulator that receives the outputs of the first and second subtracters and the output of the sine wave generator; a power amplifier that amplifies and outputs the output of the quadrature modulator; and one of the outputs of the power amplifier. an attenuator that receives and attenuates the output of the attenuator; demodulates first and second baseband signals by receiving the output of the attenuator and the output of the sine wave generator; In a negative feedback amplifier comprising at least a quadrature demodulator that outputs an output to a subtracter and a band limiting circuit in the circuit: whether the average value of the power amplifier output is above or below a certain predetermined value; a control circuit that outputs a control signal indicating: a control circuit located in a negative feedback path from the power amplifier to the first and second subtracters and receiving the control circuit output so that the average power of the control circuit input is always constant; The present invention is characterized in that a variable gain circuit is added to adjust the negative feedback path gain so that the gain of the negative feedback path is adjusted.

(Function) The present invention performs the following two controls. The first is control that detects the amplifier output and adjusts the amplifier gain so that the gain G2 of the entire negative feedback circuit is always the desired gain G. That is, when attenuation due to delay occurs in the demodulator and the output of the power amplifier has not reached the saturation region, the equation (4) is obtained. The amplifier output is monitored and the amplifier gain is adjusted so that the amplifier output does not exceed the desired output. As a result, the negative feedback amplifier maintains the open circuit gain at the time of design.

(Example) The present invention will be explained in more detail below with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. Band-limited baseband signals are input from terminals 101 and 102, respectively. A signal input from terminal 101 is input to subtracter 110, and a signal input from terminal 102 is input to subtracter 115. Subtractor 110 and subtractor 115 outputs are connected to low pass filters 120 and 125, respectively.
The signal is input to the quadrature modulator 130 through the . Quadrature modulator 13
0 modulates the output of the sine wave generator 131. The modulated signal is amplified by power amplifier 140 and transmitted. A portion of the output of the amplifier 140 is attenuated by a variable gain attenuator 145 and input to the quadrature demodulator 135 . The orthogonal demodulator 135 receives and demodulates the output of the variable gain attenuator 145 and the output of the sine wave generator 131 that has passed through the phase shifter 132 to obtain first and second baseband signals. The power of the variable attenuator 145 is also input to a control circuit 150 to obtain a signal for controlling the gain of the variable gain attenuator 145 so that the power of the attenuator 145 takes a desired value. Each baseband signal is input to subtracters 110 and 115. The control circuit 150 outputs the attenuator 1
When the output of the amplifier 140 is larger than a desired value, the attenuation is controlled to be small, and when it is small, it is controlled to be large. By doing so, it is possible to prevent the delay characteristics from changing from the designed values and deteriorating the overall characteristics of the negative feedback circuit.

An example of the control circuit 150 shown in FIG. 1 is shown in FIG.

The input signal is full-wave rectified by the aftermath rectifier circuit 310, and L P
Smooth with F320 to obtain the effective value. Subtractor 330
subtracts between the LPF output and the desired effective value (ref), the comparator 340 determines whether the subtracter output is positive or negative, and if it is positive (the LPF output is greater than ref), the attenuation amount of the attenuator 145 If it is negative (LPF output is smaller than ref'), a pulse is generated to raise it.

An example of the configuration of the variable gain attenuator 145 is shown in FIG. Up-down-= counting pulses from control circuit 150
+ Converter (up-down counter) 4
10 and a DA converter 42 that converts it into an analog voltage.
0 and a double balanced mixer 430 connected so that the DA converter output is input to the IF terminal.

Although the low-pass filter is provided after the subtracter in this embodiment, it may be provided between the subtracter and the orthogonal demodulator 135 in the present invention. Furthermore, a similar band-limiting effect can be obtained by providing a bandpass filter in place of the low-pass filter in the circuit from the orthogonal modulator to the orthogonal demodulator.

Although the present invention has been described above with reference to the drawings, the present invention is not limited to this embodiment, and it goes without saying that changes can be made within the scope of the present invention.

(Effects of the Invention) The present invention described in detail above provides a negative feedback amplifier that prevents deterioration of the output signal spectrum caused by delays in power amplifiers and the like.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing a conventional example, and FIG. 3 is a block diagram showing a specific example of the control circuit in FIG. 4 is a block diagram showing a specific example of the variable gain attenuator in FIG. 1. In the figure, 101, 102, 201, 202 are input terminals, 110, 115, 210, 21
5, 330 are subtractors, 120, 125,
230, 235, 320 are low pass filters; 13
0 is a quadrature modulator, 131, 290 are sine wave generators, 132, 275, 295 are phase shifters, 135 is a quadrature demodulator, 140, 260 are power amplifiers, 145 is a Variable gain attenuator, 150
is a control circuit, 240, 245, 280, 2
85 is a multiplier, 250 is an adder, 270 is an attenuator, 310 is a full-wave rectifier circuit, 340 is a comparator, 410 is an up/down counter,
420 is a digital-to-analog converter, and 430 is a voltage-controlled variable attenuator.

Claims (1)

[Claims] first and second subtracters receiving first and second baseband signals; first and second subtracters receiving the first and second input signals, respectively; the first and second subtractor outputs and a sine wave generator; a quadrature modulator that receives an output; a power amplifier that amplifies and outputs the output of the quadrature modulator; an attenuator that receives and attenuates a part of the output of the power amplifier; an output of the attenuator and the sine wave generator. demodulates the first and second baseband signals in response to the
and a quadrature demodulator outputting to a second subtracter; and a negative feedback amplifier including a band-limiting circuit in a loop circuit having at least a quadrature demodulator that outputs an output to a second subtracter. a control circuit that outputs a control signal indicating whether the control circuit is located in a negative feedback path from the power amplifier to the first and second subtracters; A negative feedback amplifier further comprising a variable gain circuit that adjusts the negative feedback path gain so that the gain is always constant.