JPH0530335B2 - - Google Patents

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. When performing high-density digital transmission, linear modulation methods such as 4-phase PSK and 16-value QAM are used, but signals modulated by such modulation methods inevitably suffer from transmission spectrum degradation due to amplifier nonlinearity. . To compensate for such deterioration, distortion suppression using a negative feedback circuit is one of the methods commonly used. 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 to demodulate and feed back the amplifier output,
2nd International Conference on Radio Spectrum Convergence Technics
Conference on Radio Spectrum Conversion
There is a method published in ``Techniques''. Figure 2 shows an example of this method. Signals x(t) and y(t) input from terminals 201 and 202 are respectively subtracted by subtracter 2.
01,215 to the low range wave generators 230 and 2
Enter 35. A multiplier 240 multiplies the output of the low frequency filter LPF 230 and a carrier wave (angular frequency is ω _c ) which is the output of the sine wave generator 290, and the output of the low frequency filter LPF 235 and the phase shifter 295 have a 90° phase. After multiplying by the changed sine wave generator 290 output in the multiplier 245, the multiplier 240
and 245 outputs are added by an adder 250. The output of the adder 250 is a signal that is orthogonally modulated using the baseband signals input from the 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. amplifier 2
Attenuator 270 with an attenuation amount R receiving a part of the 60 output
The transmitted signal attenuated by is input to multipliers 280 and 285 after adjusting its phase by 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 subtracted from the input signal from the terminal 201. The multiplier 285 demodulates the signal by multiplying it with the 90° phase-shifted sine wave generator output, and the demodulated signal is input to the subtracter 215 and sent to the terminal 20.
Subtract from the input signal from 2. LPF230
and 235 are for limiting the band of the feedback circuit, and it is desirable that the two characteristics are approximately equal. Furthermore, the phase shifter 275 is provided to prevent deterioration of the round-trip gain due to delay. For example, the amplifier 26
Assume that the delay from the 0 input to the output of the attenuator 270 is Δτ, and that there is no phase shifter. At this time, if the output of the adder 250 is Z ₀ (t), then Z ₀ (t)=x(t)cosω _c t+y(t)sinω _c t (1), then the output of the attenuator 270 Z ₁ (t) is , Z ₁ (t)=G・R・{x(t−△τ) cosω _c (t−△
τ)+y(t-△τ) sinω _c (t-△τ)} (2), and the multiplier circuits 280 and 285 output x ₁
(t) _and y _{1 (} t) _{are respectively} becomes. Therefore, the gain for the signal component during demodulation is
It will take cosω _c △τ. This means that the signal is attenuated in the demodulator. The gain _GF of the negative feedback circuit for the signal at this time is as follows.

G _F =G/1+G・R・cosω _c △τ (4) This means the following.

When cosω _c △τ > 0, the circuit gain increases by 1/cosω _c △τ, the amplifier output and the odd-order cross modulation added to it increase, and the loop gain decreases by a factor of cosω _c △τ, which improves distortion characteristics. deteriorates. In other words, nonlinear distortion increases.

It oscillates when cosω _c △τ<0.

Therefore, a phase shifter 275 is used to solve this problem. When the phase of the attenuator output is shifted by △τ and cos(ω _c △τ−△θ)=1 (5), the outputs of multipliers 280 and 285 are transformed into x(t−△τ), y(t− △τ). If Δθ is determined to satisfy Equation (5) in this way, the above (1) and (2) can be solved.

However, if such a transmitter is used in an FDM system and the delay of the amplifier 260 has a frequency characteristic, such a circuit in which the amount of phase change of the phase shifter 275 is fixed to a constant Δθ will not be able to switch the line. Sometimes the problems (1) and (2) above occur again. Also,
When the input signal level of a circuit becomes high, the power of the circuit output may also 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; a quadrature modulator that receives the first and second subtracter outputs and a sine wave generator output; a power amplifier that amplifies and outputs the quadrature modulator output; an attenuator that receives and attenuates a part of the output of the power amplifier; demodulates first and second baseband signals by receiving the output of the attenuator and the output of the sine wave generator, and demodulates both of these signals. In a negative feedback amplifier comprising a band-limiting circuit in a loop circuit having at least a quadrature demodulator outputting to the first and second subtracters: the average value of the power amplifier output is a certain fixed value; a control circuit that outputs a control signal indicating whether the value is greater than or equal to a value; a control circuit that is located in a negative feedback path from the power amplifier to the first and second subtracters and receives the output of the control circuit; It is characterized by the addition of a variable gain circuit that adjusts the negative feedback path gain so that the average power is always constant.

(Function) The present invention performs the following two controls. The first is to detect the amplifier output and obtain the overall gain of the negative feedback circuit.
This is a control that adjusts the amplifier gain so that _GF always becomes the desired gain G ₀ . In other words, when attenuation occurs in the demodulator due to delay and the output of the power amplifier has not reached the saturation region, the gain of the negative feedback circuit is 1/cosω _c △τ(> 1) Double. 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 described 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. The signal input from terminal 101 is input to subtracter 110, and the signal input from terminal 1
The signal input from 02 is input to subtracter 115. The outputs of subtracter 110 and subtracter 115 are input to quadrature modulator 130 through low-pass filters 120 and 125, respectively. Quadrature modulator 130
Then, the output of the sine wave generator 131 is modulated. 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 . In the quadrature demodulator 135, a variable gain attenuator 145
Sine wave generator 13 with output and passed through phase shifter 132
1 output is received and demodulated to obtain first and second baseband signals. The variable attenuator 145 input is also input to a control circuit 150 to obtain a gain control signal for the variable gain attenuator 145 so that the attenuator 145 input takes a desired value. Each baseband signal is input to subtracters 110 and 115. Control circuit 1
50 output is input to attenuator 145 and amplifier 14
When the 0 output 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 a full-wave rectifier circuit 310, smoothed by an LPF 320, and an effective value is obtained. A subtracter 330 subtracts between the LPF output and the desired effective value (ref), a comparator 340 determines whether the subtracter output is positive or negative, and if it is positive (LPF output is greater than ref), an attenuator 145 It generates a pulse that lowers the attenuation of , and if it is negative (LPF output is smaller than ref), it generates a pulse that increases it.

An example of the configuration of the variable gain attenuator 145 is shown in FIG. Up-down converter (up-down counter) that counts pulses from control circuit 150
410, a DA converter 420 that converts it into an analog voltage, and a double balanced mixer 43 connected so that the DA converter output is input to the IF terminal.
It can be configured with 0. In the present embodiment, the low frequency wave generator is provided after the subtracter, but in the present invention, it may be provided between the subtracter and the orthogonal demodulator 135. Furthermore, a similar band-limiting effect can be obtained by providing a band wave generator in place of the low band wave generator 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 drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional example.
3 is a block diagram showing a specific example of the control circuit shown in FIG. 1, and FIG. 4 is a block diagram showing a specific example of the variable gain attenuator shown in FIG. In the figure, 101, 102, 201, 202
are input terminals, 110, 115, 210, 2
15,330 is a subtractor, 120,12
5,230, 235, 320 are low frequency wave generators, 130 is a quadrature modulator, 131,290
is a sine wave generator, 132, 275, 295
is a phase shifter, 135 is a quadrature demodulator, and 1
40, 260 is a power amplifier, 145 is a variable gain attenuator, 150 is a control circuit, 2
40,245,280,285 are multipliers,
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, and 4
20 is a digital to analog converter, 430
is a voltage-controlled variable attenuator.

Claims (1)

[Claims] 1 with the first and second baseband signals as input;
first and second subtracters receiving the first and second input signals, respectively; a quadrature modulator receiving the first and second subtracter outputs and the sine wave generator output; and the quadrature modulator output. a power amplifier that amplifies and outputs a power amplifier; an attenuator that receives and attenuates a part of the output of the power amplifier; and an attenuator that receives the output of the attenuator and the output of the sine wave generator and outputs first and second bases; In the negative feedback amplifier, the negative feedback amplifier includes a band-limiting circuit in a loop circuit having at least a quadrature demodulator that demodulates a band signal and outputs both signals to the first and second subtracters; a control circuit that outputs a control signal indicating whether the average value of the output is above or below a certain predetermined value; 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 receives a circuit output and adjusts a negative feedback path gain so that the average power input to the control circuit is always constant.