JPS6347066Y2 - - Google Patents

Description

[Detailed description of the invention] [Purpose of the invention] (Field of industrial application) The invention relates to an AGC (automatic gain control) circuit.
In particular, it relates to an AGC circuit that suppresses the occurrence of DC offset in the gain control voltage and performs stable gain control operation.

(Prior art) Figure 2 is a circuit diagram showing a conventional AGC circuit.
An input signal is input to terminal a. The input signal has its gain controlled by the variable gain amplifier 1, and is output to terminal b.

Here, the output of the variable gain amplifier 1 is applied to an absolute value circuit 2, which detects the potential difference between the absolute value of the output of the variable gain amplifier 1 and a predetermined reference voltage.

The potential difference with the reference potential detected by the absolute value circuit 2 is applied to a low pass filter (LPF) 3, and then applied to a gain control circuit 4 which generates a control voltage used to control the gain of the variable gain amplifier 1. Added.

As a result, the gain of the variable gain amplifier 1 is
The gain is controlled according to the control voltage of the gain control circuit 4, and the gain for the input signal is automatically controlled by a feedback loop consisting of the absolute value circuit 2, the LPF 3, the gain control circuit 4, and the variable gain amplifier 1.

(Problem to be solved by the invention) In the conventional AGC circuit described above, when the DC offset component generated at the output of the variable gain amplifier 1 is input to the absolute value circuit 2, the absolute value When comparing the absolute value of the output of the variable gain amplifier 1 and a predetermined potential in the circuit 2, DC offset is promoted.

As a result, an unnecessary ripple occurs in the output of the absolute value circuit 2, and accordingly, a ripple component also occurs in the output of the LPF 3, which affects the gain control of the variable gain amplifier 1.

Therefore, a DC offset occurs in the feedback loop, and the signal obtained at the output terminal b is also affected by the DC offset. In some cases, this DC offset may exhibit a pulsating current instead of a constant DC potential, and this pulsating current may cause the system to become unstable.

Furthermore, the above DC offset is determined by the variable gain amplifier 1.
This amount also depends on circuit-specific performance, such as the temperature characteristics of the active elements that make up the circuit, and it is desirable to suppress the effects of DC offset from the standpoint of system stability.

[Structure of the idea]

(Means for solving the problem) In this invention, the output signal of the variable gain amplifier 1 whose input is the signal applied to the input signal terminal a is
The configuration is such that it is applied to the output signal terminal b via the HPF 5 and is also supplied to the voltage comparison means 2.

This suppresses the generation of DC offset within the AGC loop and performs AGC operation stably.

In view of the above points, this idea suppresses the DC offset of the system, achieves stable feedback operation, and achieves stable output through optimal control.
The purpose is to provide circuits.

Hereinafter, embodiments of this invention will be described with reference to the drawings.

FIG. 1 is a circuit block diagram showing an embodiment of the AGC circuit according to the invention, and the same reference numerals are given to the functions corresponding to those in FIG. 2 above.

In Fig. 2, a high pass filter (HPF) 5 is installed between the variable gain amplifier 1 and the absolute value circuit 2.
to suppress DC offset in the AGC loop system.

A DC offset is superimposed on the output of a variable gain amplifier 1 which receives a signal applied from an input terminal a and obtains a gain corresponding to a control voltage generated by a gain control circuit 4 as appropriate.

The causes of this DC offset include the free-route characteristics of the variable gain amplifier 1 and the DC level shift, as described above. This DC offset is
This is blocked by the HPF 5, and a signal from which the DC offset has been removed is applied to the output terminal b and the absolute value circuit 2.

Therefore, a half-wave rectified signal is obtained at the output of the absolute value circuit 2, and thereby a voltage from which the DC offset has been removed is obtained at the output of the LPF 3.

This means that the DC offset that causes the AGC operation to become unstable is removed, and the gain of the variable gain amplifier 1 is stably controlled, so that the AGC operation can be performed without compromising the stability of the system. means to be

The AGC circuit is originally controlled to obtain an optimal gain that suppresses output distortion and does not degrade the noise figure, and in order to perform this AGC operation stably, it is necessary to remove DC offset. Although the DC offset itself, which causes system instability, is caused by the circuit characteristics of the variable gain amplifier 1 itself, the DC level shift operation, etc., the generated DC offset potential is removed by the HPF 5.

Here, the HPF 5 may be of any type as long as it can substantially block DC offset or pulsating flow components and transmit only the information to be controlled.

[Effect of idea]

As described above, according to this invention, it is possible to provide an AGC circuit that can suppress the occurrence of DC offset in the AGC loop and provide a stable output.

Note that the AGC circuit according to the present invention is an analog type,
It can be applied to any digital method.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an AGC circuit according to the present invention, and FIG. 2 is a conventional AGC circuit diagram. a...Input signal terminal, b...Output signal terminal, 1
...Variable gain amplifier, 2...Voltage comparison means, 3...
...Low-pass wave generator, 4...Gain control circuit, 5...High-pass wave generator.

Claims (1)

[Scope of utility model registration request] an input signal terminal to which an input signal is applied; a variable gain amplifier that controls the gain of the signal applied to the input terminal according to a gain control signal; and an output signal of the variable gain amplifier to which the output signal of the variable gain amplifier is input. A high-pass waveform generator that outputs a signal with the DC offset component removed from its output; an output signal terminal that derives the output of the high-pass waveform; the output of the high-pass waveform is inputted, and the input signal voltage comparison means for comparing the potential of the voltage with a predetermined potential; a low-pass waveform generator connected to the output side of the voltage comparison means; and a gain for the variable gain amplifier according to the output of the low-pass waveform generator. An AGC circuit comprising at least a gain control circuit that generates a control signal.