JPH0488711A - Agc amplifier circuit - Google Patents

Abstract

PURPOSE:To increase the gain at reduction in a power supply voltage Vcc (in operation of AGC interrupt circuit) and to make the gain constant by constituting the AGC interrupt circuit to make the gain constant at a drop of the power supply voltage with an operational amplifier. CONSTITUTION:When a power supply voltage Vcc is a normal voltage (or the normal voltage or over), an L level is inputted to an AGC interrupt circuit control signal input 4, AGC interrupt circuits 2,3 are stopped and the circuit acts like the usual AGC amplifier circuit. When the power supply voltage Vcc drops, an H level is inputted to the AGC interrupt circuit control signal input 4, the AGC interrupt circuits 2,3 are operated, an AGC input to an operational amplifier 1 of the AGC amplifier circuit is constant (Vref) to make the gain of the AGC amplifier circuit constant. An operational amplifier 2 is employed for the AGC interrupt circuit section to decrease the remaining voltage at the operation of the AGC interrupt circuit is as small as an offset voltage (nearly several mV) of the operational amplifier and a leakage current at the stop of the AGC interrupt circuit is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an AGC amplifier circuit that shuts off an AGC (automatic gain control) circuit to maintain a constant gain when the power supply voltage drops.

(Prior Art) A partial circuit diagram of a conventional general AGC amplifier circuit is shown in FIG. This AGC amplifier circuit turns the AGC cutoff circuit into NP.
It consists of one N transistor and one PNP transistor.

Next, FIG. 3 will be explained. The figure partially shows an example of an AGC amplifier circuit. In the figure, l is AGC
Amplifier circuit output stage operational amplifier, 3.10 is an AGC cutoff circuit, 5 to 9 are AGC circuit output sections, and 8.9 is an AGC circuit time constant setting resistor and capacitor. 4 is AGC
This is a cutoff circuit control signal input terminal. Here, the case when the power supply voltage drops will be explained. In this AGC amplifier circuit, when the power supply voltage Vcc decreases to a certain voltage or less, the AG
“H” is manually input to the C cutoff circuit control signal input terminal 4.

As a result, the AGC cutoff circuit 3.10 operates, the signals of the AGC circuit output sections 5 to 9 are cut off, and the gain of the AGC amplifier circuit output stage operational amplifier 1 becomes constant. However, this A
In the GC amplifier circuit, the AGC cutoff circuit 3. IO is composed of an NPN transistor and a PNP transistor, and when the AGC cutoff circuit operates, the
Because of the emitter-collector voltage of , the gain (constant) of the AGC amplifier circuit becomes small.

This is sometimes noticeable in small signal AGC amplifier circuits. Also, when the AGC cutoff circuit is in a stopped state, the AGC
There is a leakage current between the emitter and collector of the PNP transistor IO in the cutoff circuit, which adversely affects the AGC circuit operation.

[The invention tries to solve i! Title]

Since the conventional AGC amplifier circuit is configured as described above, there is a residual voltage in the AGC cutoff circuit section (emitter-collector voltage of the transistor in the AGC cutoff circuit section) when the AGC cutoff circuit operates, and the gain of the AGC amplifier circuit ( constant)
There is a problem that the current is small, and even when the AGC cutoff circuit is stopped, leakage current of the AGC cutoff circuit becomes a problem.

This invention was made to solve the above-mentioned problems, and when the AGC cutoff circuit operates when the power supply voltage drops, the
There is no drop in the pressure (constant) of the GC amplifier circuit, and
To provide an AGC amplifier circuit that has no leakage current in an AGC cutoff circuit even when the GC cutoff circuit is stopped and does not adversely affect the operation of the AGC circuit.

[Means for Solving the Problems] In the AGC amplifier circuit according to the present invention, an AGC cutoff circuit for keeping the gain constant when the power supply voltage drops is configured using an operational amplifier.

(Function) In the AGC amplifier circuit of the present invention, by configuring the AGC cutoff circuit with an operational amplifier, it is possible to almost eliminate residual voltage when the AGC cutoff circuit is in operation, and almost eliminate leakage TL flow when the AGC cutoff circuit is stopped. I can do it. As a result, the AGC circuit is cut off when the power supply voltage drops and the A
Even if the gain of the GC amplifier circuit is constant, it is possible to prevent the gain from becoming small (especially noticeable in small-signal AGC amplifier circuits), and it is possible to eliminate the influence of leakage current on the AGC circuit when the AGC cutoff circuit is stopped. can.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure is a circuit diagram of an AGC amplifier circuit that is an embodiment of the present invention. In the figure, 1 is an operational amplifier at the output stage of the AGC amplifier circuit, 2 and 3 are AGC cutoff circuits, 4 is an AGC cutoff circuit control signal manually operated, and 5 to 9 is the AGC circuit output section, 8 and 9
are a resistor and a capacitor for setting the AGC circuit time constant.

Next, the operation will be explained. When the power supply voltage Vcc is a normal voltage (a certain voltage or higher), "L" is input to the AGC cutoff circuit control signal 4, the AGC cutoff circuit 2.3 stops, and the normal AGC amplifier circuit operates. ing. When the power supply voltage Vcc decreases, AGC cutoff circuit control signal 4
"H" is input to , the AGC cutoff circuit 2.3 operates,
The AGC input of the AGC amplifier circuit output stage operational amplifier 1 is constant (Vref), and the gain of the AGC amplifier circuit is constant. By using operational amplifier 2 in the AGC cutoff circuit section, the remaining voltage when the AGC cutoff circuit is operating can be reduced to about the offset voltage of the operational amplifier (about several mV), and leakage occurs when the AGC cutoff circuit is stopped. Electric current can also be eliminated.

The above embodiment is particularly effective in a small signal AGC amplifier circuit, and a circuit having the characteristics shown in FIG. 2 will be briefly described.

When the AGC cutoff circuit operates, the gain remains constant, but A
When the remaining voltage of the GC cutoff circuit is large, the gain becomes constant at point B in FIG. 2, and the gain becomes small. However, if the remaining voltage becomes small, the gain at point A becomes constant, and the gain can be fixed as large.

(Effects of the Invention) As described above, according to the present invention, the A
Since the GC cutoff circuit is configured with an operational amplifier, in an AGC amplifier circuit that keeps the gain constant when the power supply voltage Vcc is below a certain voltage, the gain must be increased and kept constant when the power supply voltage Vcc drops (when the AGC cutoff circuit operates). I can do it. This is particularly effective in small signal AGC amplifier circuits, and also in AGC! ! It is possible to eliminate the leakage TL flow when the circuit is stopped, and there are effects such as being able to eliminate the adverse effect on the AGC circuit operation.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an AGC amplifier circuit according to an embodiment of the present invention, FIG. 2 is an input-gain characteristic curve diagram of the small signal AGC amplifier circuit in FIG. 1, and FIG. 3 is a circuit diagram of a conventional AGC amplifier circuit. It is a diagram. In the figure, l is the AGC amplifier circuit output stage oven amplifier,
2 is an AGC cutoff circuit (operational amplifier), 3 is an AGC cutoff circuit (NPN transistor), 4 is an AGC cutoff circuit control signal input, 5 and 6°7 are AGC circuit output transistors, 8 is a resistor for setting the AGC circuit time constant, 9 is a capacitor for setting the AGC circuit time constant, 10 is an AGC cutoff circuit (PNP
transistor). In addition, in the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] An AGC amplifier circuit comprising an AGC cutoff circuit for stopping (blocking) an AGC circuit.