JPH0311939Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to amplifiers in electronic equipment, and in particular, the operation of the amplifier when the power supply voltage is turned on by turning on the power switch or when the power switch is repeatedly turned on and off. Regarding stabilization circuits.

(B) Prior art In general, an AGC circuit includes an AGC detection circuit that detects the level of the output signal of the amplifier from the output terminal to the input terminal of the amplifier, and an AGC drive circuit that operates according to the output level of the detection circuit. and an AGC loop that automatically controls the gain of the amplifier.

As an example, there is an example of an applied circuit for the LA7040 IC published in "Sanyo Semiconductor News" No. 1139A published by Tokyo Sanyo Electric Co., Ltd.'s Semiconductor Division, the main part of which is shown in Figure 2.

In FIG. 2, 1 is an amplifier, 2 is an AGC detection circuit 7 consisting of capacitors 3 and 4 and diodes 5 and 6, is an AGC drive circuit, 8 is an input terminal, 9 is an output terminal, 10 and 11 are coupling capacitors, When an input signal is applied to the input terminal 8, the input signal is amplified by the amplifier 1 via the coupling capacitor 10.
An output signal is obtained from the output terminal 9 via a coupling capacitor 11.

At this time, depending on the level of the input signal,
Voltage doubler rectification is performed by capacitors 3, 4 and diodes 5, 6 of AGC detection circuit 2, and AGC
Feedback is applied to the amplifier 1 by a drive circuit 7,
Its gain control is performed. For example, when recording audio on a tape recorder, the recording signal level (recording level)
AGC control called ALC (automatic level control) is used to keep the level within a certain range.

However, the AGC detection circuit 2 is inoperative when the power switch is off, and therefore the charge in the charging/discharging capacitor 4 is zero. If you turn on the power switch in this state,
Since the input signal applied to the input terminal 8 is amplified by the maximum gain of the amplifier 1, an abnormally large output signal is generated, causing a shock noise. On the other hand, when the power switch is turned off while a large input signal is applied, and then immediately turned on again, the charge accumulated in the charging/discharging capacitor 4 is actually applied to the input terminal 8. Greater than the output of the AGC detection circuit 2 corresponding to the level of the input signal, that is, the level of the terminal voltage of the charging/discharging capacitor 4,
Along with this, the amount of feedback from the AGC drive circuit 7 to the amplifier 1 increases, and as a result, the operation becomes equivalent to when a large input signal is applied, and the output signal from the output terminal 9 becomes equal to the input signal. The level will be lower than the corresponding level.

(c) Problems to be solved by the invention As mentioned above, the invention prevents abnormalities in the gain of the amplifier caused by turning the power switch on and off, that is, turning on or cutting off the power supply voltage, and stabilizes the operation of the amplifier. The purpose is to

(d) Means for solving the problem This invention uses a Schottky diode to set the voltage across the charging/discharging capacitor of the AGC detection circuit connected to the output terminal of the amplifier to a predetermined level when the power supply voltage is turned on. The Schottky diode is turned on when the power supply voltage is turned on.

(E) Effect When the above-mentioned power supply voltage is applied to the Schottky diode of the present invention, the voltage across the charge/discharge capacitor is immediately reduced to the forward voltage V of the Schottky diode by the operation of the control voltage generation circuit and the switching transistor. Since it can be equivalent to _SF ,
If a large signal is applied when the power supply voltage is turned on,
Even if the charged charge of the AGC detection voltage charging/discharging capacitor is high, it is lowered to the above-mentioned V _SF .

(F) Embodiment To explain the present invention according to the drawings, Fig. 1 shows an embodiment of the amplifier operation stabilizing circuit of the present invention, and the same elements as in Fig. 2 are given the same figure numbers. be. In the drawing, 12 is a switching transistor, 13 is a Schottky diode, 14 is a power supply transistor, 15 is a ripple filter capacitor, 16 is a switching drive transistor, 17 is a DC power supply terminal, 18 is a charging resistor,
19 is an emitter resistance, 19 is a load resistance, and 20 is a base resistance.

Next, to explain the operation of the circuit of the present invention, first, in order to change from a non-operating state to an operating state, the power switch is turned on and a voltage (+V _CC ) is applied to the DC power supply terminal 17.
When the ripple filter is applied, the power transistor 14 constituting the ripple filter is turned off, and charging current begins to flow into the ripple filter capacitor 15 through the charging resistor 18.

At this time, the switching drive transistor 16
The base of the switching transistor 12 is almost at ground potential, and it remains on until the ripple filter capacitor 15 is almost completely charged, and the collector potential rises, so the base of the switching transistor 12 becomes high, and the switching transistor 12 becomes on and shoots. key diode 1
The cathode of No. 3 is grounded.

Therefore, if the power switch is turned off and then turned on again immediately after a large input signal is input to the terminal 8, the charging/discharging capacitor 4 receives a large voltage as a detected output corresponding to the large input signal. Although it is in a charged state, since the switching drive transistor 16 and the switching transistor 12 are turned on, the cathode of the Schottky diode 13 is grounded.
Both ends of the charging/discharging capacitor 4 are lowered to the forward voltage V _SF (approximately 0.4V) of the Schottky diode 13, and even if a small input signal is applied to the amplifier 1 when the power is turned on again, the feedback voltage remains low. is small, so the gain of amplifier 1 is not suppressed excessively.

Here, the ripple filter capacitor 15
When charging is completed and the power transistor 14 is turned on, the power supply voltage is supplied to the amplifier 1, and at the same time, the base potential of the switching drive transistor 16 rises, so the switching drive transistor 16 is turned off, and accordingly, the switching transistor 12 is turned off.

Thereby, the AGC detection circuit 2 generates a predetermined detection output according to the output signal obtained from the input terminal 8 via the amplifier 1. When a bipolar transistor is used in the AGC drive circuit 7, its base-emitter rise voltage V _BE (approximately 0.7 V) is larger than the forward voltage V _SF of the Schottky diode 13, and when the power is turned on, the charge Even if the discharge capacitor 4 is charged with a potential greater than V _BE , it is suppressed to V _SF of the Schottky diode, so the first stage transistor of the AGC drive circuit 7 is kept in an off state.

(g) Effects of the invention According to the amplifier operation stabilizing circuit of the invention, when the power supply voltage is turned on or when the power switch is repeatedly turned on and off, the terminal voltage of the charging and discharging capacitor in the AGC detection circuit is controlled by the Schottky diode. It can be clamped to the forward voltage Vsf of
The AGC loop can be operated in an appropriate state according to the input signal applied to the amplifier, and a stable amplifier can be constructed.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a circuit for stabilizing the operation of an amplifier according to the present invention, and FIG. 2 is a circuit diagram showing a conventional circuit. Explanation of main figure numbers, 1...Amplifier, 2...AGC
Detection circuit, 4... Charging/discharging capacitor, 7...
AGC drive circuit, 8...input terminal, 9...output terminal, 12...switching transistor, 13...
...Shotkey diode, 14...Power transistor, 15...Ripple filter capacitor, 16...Switching drive transistor.

Claims (1)

[Scope of utility model registration request] In an AGC circuit that automatically controls the gain of the amplifier by connecting a feedback loop from the output side of the amplifier to the input side via the AGC detection circuit, there is a The collector-emitter path of the switching transistor and the Schottky diode are connected in series, and the base of the switching transistor is connected to a pulse generation circuit that operates when the power supply voltage is turned on.
The charging voltage of a charging/discharging capacitor connected to the output terminal of the AGC detection circuit is made equal to the forward voltage of the Schottky diode at the same time when the power supply voltage is turned on, thereby stabilizing the gain of the amplifier. Amplifier operation stabilization circuit.