JPS61257006A - Agc circuit - Google Patents

Abstract

PURPOSE:To prevent storage of electric charge in a rectifier capacitor of an AGC circuit by connecting an output of a power supply detecting circuit to an AGC control circuit and using an output of the AGC control circuit so as to control the rectifier circuit in the AGC circuit. CONSTITUTION:Transistors 37, 38 are conducted during a period T when a waveform at application of power detected by the power supply detecting circuit 25 and no electric charge is stored in the rectifier capacitor 19. Thus, in setting the period T longer slightly than the DC voltage fluctuation (leading of amplifier circuit) time of an amplifier circuit 4 at power power application and interruption, the storage of the electric charge into the rectifier capacitor 19 attended with an output DC voltage fluctuation of the amplifier circuit 4 at power application and interruption, and malfunction such that the AGC circuit is operative independently of the input signal level is prevent at application of power.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an AGC circuit, and particularly to a transient response when power is turned off and turned on in an AGC circuit.

[Background of the invention]

The prior art will be explained using a block diagram of an AGC circuit shown in FIG. 6, an input/output characteristic diagram shown in FIG. 7, and an example of a conventional specific circuit shown in FIG.

FIG. 6 is a block diagram of the AGC circuit, in which the signal v of signal source 1
1 is input from input terminal 2, passes through resistor 3, and goes to amplifier circuit 4.
After being amplified by the amplifier circuit 4 (or passing through a buffer amplifier with an AC gain of 1), it is output to the output terminal 5 as an output signal v0. On the other hand, the output signal v0 is also input to the rectifier circuit 7 in the AGC circuit 6 shown by the dashed line, and the resistance value is set to decrease when the rectified DC voltage vDo of the rectifier circuit 7 exceeds a certain potential. The variable resistance element 8 is driven, and the input signal level of the amplifier circuit 4 is kept at approximately - constant by resistance division between the resistance 3 and the variable resistance element 8. As a result, the input/output characteristics of the AGC circuit in FIG.
The result will be as shown in the figure. In FIG. 7, the horizontal axis represents the input signal V, and the vertical axis represents the output signal v0.

FIG. 8 is a specific circuit example of a conventional AGC circuit. Capacitors 9, 12, and 13 are DC blocking capacitors, and resistor 10 is a bias resistor that applies a bias voltage from bias power supply 11. The output signal v0 of the amplifier circuit 4 is output to the output terminal 5, and is also input to the base of the transistor 14 via the capacitor 13. A resistor 16 is further connected between the base of the transistor 14 and a power supply 15, and a resistor 17 is connected between the base of the transistor 14 and the ground. By setting , the AGC operation start level is selected. Transistor 14 has emitter resistor 18
and the rectifying capacitor 19 constitute an emitter follower detection circuit, which rectifies the AC signal amplified by the amplifier circuit 4 to obtain a DC voltage vDC. The resistor 20 is for overcurrent protection of the transistor 14.

Furthermore, by using an emitter follower circuit composed of the transistor 21 and the resistor 22, the input impedance of the transistor 21 is made high, and the base current of the transistor 21 can be ignored. It has a structure that can be determined by constants.

The emitter of the transistor 21 is connected to the base protection resistor 23.
The on-resistance of the transistor 24 and the resistor 3 keep the input signal level of the amplifier circuit 4 constant.
Configure the GC circuit.

In this type of AGC circuit, if the power is repeatedly turned on and off in a time shorter than the discharge time constant made up of the resistor 18 and the capacitor 19, the amplifier circuit 4 when the power is turned on and off is Due to the DC voltage fluctuation, a capacitance of 191C charge is accumulated. Therefore, consideration has been given to the fact that if the power is turned on after the power is turned on and off repeatedly, the AGC circuit will be activated and the discharge time constant will be long, usually about several tens of seconds, which may give the operator a sense of discomfort. There wasn't.

Incidentally, the above-mentioned conventional example includes Japanese Patent Application Laid-Open No. 1915-1983.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an AGC circuit that eliminates the drawbacks of the prior art described above and does not cause charge accumulation in the rectifying capacitor of the AGC circuit even after repeated power-on and power-off.

[Summary of the invention]

In order to achieve this object, the present invention operates (ON) and deactivates (OFF) a transistor installed at the base of an emitter follower transistor in an AGC circuit that uses an emitter follower detection circuit for signal rectification.
A transistor is also installed at the emitter of the emitter follower transistor, and the transistor is operated/deactivated in synchronization with the transistor installed at the base, and the operation/deactivation of the installed transistor is controlled by a signal detected when the power is turned on or off. As a result, the electric charge of the rectifying capacitor is always discharged when the power is turned on and off, thereby preventing the Ar0 circuit from malfunctioning when the power is turned on after the power is turned on and off repeatedly.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an AGC circuit according to the present invention, in which 6 is an AGC circuit, 15 is a power supply, 25 is a power supply detection circuit, and 26 is an AGC control circuit. Note that other parts that are the same as those in FIG. 6 are given the same reference numerals.

In FIG. 1, in contrast to the conventional block diagram of FIG. 6, a power supply 15 is connected to a power supply detection circuit 25, and its output is further connected to an AGC control circuit 26, and the output of the AGC control circuit is used to control the AGC control circuit. It is configured to control the rectifier circuit 7 in the circuit.

A specific circuit example of the power supply detection circuit 25 will be explained based on FIG. 2. In the figure, a resistor 27 has one end connected to the power supply 15, the other end of which is connected to a capacitor 2 and the base of a transistor 29, and the other end of the capacitor 28 is grounded. The collector of the transistor 29 is connected to the power supply 15, and the emitter is connected to the N
The emitter of the PN transistor 31 and the constant current source 50 are connected to each other. The collector of NPN) transistor 1 is connected to the resistor 34 and the base of PNP transistor 35,
Further, its base is connected to a resistor 32 and ssK@, and the other ends of the resistors 34 and 32 are connected to the power supply 15, and the other end of the resistor 33 is grounded. Further, the emitter of the PNP transistor 35 is connected to the power supply 15, and the collector is connected to ground via a resistor 36.

As shown in FIG. 3, when the power is first turned on, the waveforms of the base of the NPN transistor 29 are as shown in FIG.
Capacitor 28 is charged with a time constant determined by , and this charging is performed at power supply potential vcct. On the other hand, as shown above, when the power is turned on, the waveform of the transistor 310 instantly becomes the potential v1 determined by the resistors 32 and 33. In the differential circuit configured by the NPN transistors 29 and 31, the NPN transistor 31 becomes conductive and the voltage drop across the resistor 54 occurs during the period T until the potential of the waveform A reaches the potential V of the waveform A. Yo, 9, P
The NP transistor 35 is forward-biased and becomes conductive, and the waveform C at the collector of the transistor 35 becomes approximately the power supply voltage vCc.

Furthermore, when the potential of A exceeds V, the NPN transistor 29 becomes conductive and the transistor 31 becomes non-conductive, so the PNP transistor 354 becomes non-conductive and the waveform C becomes the ground potential.

In FIG. 4 showing the overall embodiment of the present invention, NPN)
The bases of transistors 14 and 21 are connected to the collectors of transistors 37 and 38, respectively. A malfunction prevention resistor 39 and a base current protection resistor 40 are connected between the bases of these transistors 37 and 38 and ground, respectively, thereby forming the AGC control circuit 26. Furthermore, resistance 40
The other end is the PNP transistor 35 of the power supply detection circuit 25.
and is connected to the resistor 36.

With the above configuration, when the power is turned on, the transistors 37 and 38 are conductive during the period T when the waveform C in FIG. 3 detected by the power supply detection circuit 25 is "H#". Therefore, if the period T is set to be slightly longer than the DC voltage fluctuation time (rise of the amplifier circuit) of the output of the amplifier circuit 4 when the power is turned on and off, It is possible to prevent charge from accumulating in the rectifying capacitor 19 due to fluctuations in the output DC voltage of the amplifier circuit 4 when the power is turned on and off, and it is possible to prevent malfunction of the AGC circuit where the AGC operates when the power is turned on.

FIG. 5 is an overall circuit diagram showing another embodiment of the AGC circuit according to the present invention, in which 41 is a PNP transistor, 42
is a transistor, 45.44 is a resistor, 45 is a constant current source,
46 and 47 are transistors, 48.degree. 49 is a resistor, and 50 is a resistor, and parts corresponding to those in FIG. 4 are given the same reference numerals and redundant explanations will be omitted.

In another embodiment of the invention shown in FIG.
0 is input to the PNP transistor 41, and is output to the collector of the transistor 42 through a differential amplifier circuit with the transistor 42. The output level is determined by a resistor 43 and a resistor 44, and determines the level at which the AGC starts operating. Reference numeral 45 is a constant current source of the differential amplifier circuit. Furthermore, in the circuit of this embodiment, the transistor 21 operates as a voltage/current conversion circuit, and the transistors 46 and 47 and the resistor 4
The current is made constant by a current mirror circuit consisting of 8 and 49, and drives the transistor 24, which is a variable resistance element. The resistor 50 is for preventing leakage of the transistor 47. In this circuit as well, during the period when the transistors 37 and 3B of the AGC control circuit 26 are conductive due to the signal from the power supply detection circuit 25, no charge is accumulated in the rectifying capacitor 19, and the fourth
It is clear that the circuit works similarly to the circuit shown in the figure. Furthermore, in the circuit example of FIG. 5, a diode 51 is inserted, with the base of the transistor 29 of the power supply detection circuit 25 serving as an anode, and the base of the transistor 31 serving as a cathode. This diode 51 is the transistor 29
The base potential of the transistor 31 is clamped at a voltage higher than the base potential of the transistor 31 by one base-emitter voltage VB, which is useful for accelerating the discharge time of the capacitor by two days when the power is turned off. By selecting an arbitrary number of diodes 51, the base of the transistor 29 can be set to an arbitrary potential.

In the above embodiment, the power supply detection circuit is composed of transistors, resistors, and capacitors, but a system control circuit using a microcomputer creates the period T shown in FIG. It is clear that the same behavior can be obtained by controlling .

〔Effect of the invention〕

As explained above, according to the present invention, when the power is turned on after repeatedly turning on and off the power, it is possible to prevent the malfunction in which the AGC circuit enters the operating state regardless of the input signal level. An AGC circuit with excellent functionality can be provided without the drawbacks of the prior art.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the AGC circuit according to the present invention, FIG. 2 is a circuit diagram showing a specific example of the power supply detection circuit of FIG. 1, FIG. 3 is a waveform diagram thereof, and FIG. The overall circuit diagram of the embodiment of the AGC circuit according to the present invention shown in FIG.
6 is a block diagram of a conventional AGC circuit, FIG. 7 is its input/output characteristic diagram, and FIG. 8 is the AGC circuit of FIG. 6. FIG. 1... Signal source 3... Resistor 4... Amplifier circuit 6... AGC circuit 7... Rectifier circuit 8...・Variable resistance element 19... Rectifying capacitor 14.21.37.38... Transistor 25
...Power supply detection circuit 26...AGC control circuit.

Claims (2)

[Claims] (1) Input the signal from the signal source to the amplifier circuit via the resistor,
While extracting the output of the amplifier circuit as an output signal,
The output of the amplifier circuit is also input to a rectifier circuit that combines a rectifier transistor and a smoothing capacitor, and the output of the rectifier circuit drives a variable resistance element, and the variable resistance element is connected between the resistance and the amplifier circuit. 1. An AGC circuit connected to the rectifier circuit, comprising: a power supply detection circuit; and an AGC control circuit driven by the power supply detection circuit, the AGC control circuit being connected to the rectification circuit. (2) A first transistor for switching is provided at the base of the rectifying transistor, and a second transistor for switching is provided at the emitter, and the first and second transistors are driven by the power supply detection circuit. An AGC circuit according to claim 1.