JPS6034852B2 - Protection circuit for gain control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for protecting a gain control circuit from destruction at the time of excessive input, and particularly to provide a protection circuit for an input limiting type gain control circuit.

There are various gain control circuits, but they can be broadly classified into two methods: one that keeps the output voltage constant by changing the gain of the amplifier circuit, and one that keeps the output current constant by limiting the input signal. exists.

The present invention relates to the improvement of a gain control circuit using the latter method of limiting input signals, and will be described below based on embodiments and with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is an input terminal to which an input signal is applied, and 2 is a pair of transistors 3 and 4 of the same conductivity type that amplifies the signal applied to the input terminal 1.
5 is a differential amplifier circuit consisting of the pair of transistors 3
and 4, a constant current transistor inserted between the common emitter and ground; 6, an output transistor whose base is connected to the collector of one transistor 3 of the differential amplifier circuit 2; and 7, an output transistor connected to the emitter of the output transistor 6; 8 is a gain control circuit that operates according to the gain control signal, and the gain control circuit 8 includes first and second control transistors 9 and 10. and a current equal to the collector current of the first control transistor 9 is passed through the second control transistor 1.
It is composed of a current mirror circuit 11 that supplies a current to the collector of 0, and four diodes 12, 13, 14 and 15.

Further, 16 is a protection circuit, and the protection circuit 16 connects the second mirror transistor 8 commonly connected to the base of the first mirror transistor 17 of the current mirror circuit 11, and the collector of the second mirror transistor 18 and the ground. a resistor 19 connected to the base of the second mirror transistor 1;
8, and a third control transistor 20 connected from the collector to the collector. Therefore, the four diodes 12, 13
. The collector of the third control transistor 20 is connected to the base of the constant current transistor 5.

Next, the operation will be explained.

If the input signal applied to the input terminal 1 is small, the gain control signal generation circuit 7 will not generate a gain control signal, and therefore,
The first and second control transistors 9 and 10 of the gain control circuit 8 are in a non-conducting state, and at the same time, the third control transistor 20 is also in a non-conducting state. Therefore, the differential amplifier circuit 2 amplifies the input signal without being gain controlled. When the input signal increases, the gain control signal generation circuit 7
A gain control signal is generated by which the first and second control transistors 9 and 10 are rendered conductive.

If the collector currents of the first and second control transistors 9 and 10 are respectively 1, 12, then 11=12.
．． ．． ．． ．． ．． ．． ．． ．． [11], and if the collector current of the first mirror transistor 17 is 13, then 1,=13. ．．・・・． ...'21.

Therefore, the current flowing into the four diodes 12, 13, 14 and 15 and the current flowing into the four diodes 12, 13, 14
and the current 12 flowing out from 15 are equal. Said 4
The internal impedances of the three diodes 12, 13, 14 and 15 change depending on the collector current 12. The input signal is then attenuated according to the change in impedance. Therefore, the gain of the differential amplifier circuit 2 changes substantially, and the output signal of the amplifier circuit 2 can be maintained at a constant level. At that time, as described above, the collector current 13 of the first mirror transistor 17 and the collector current 12 of the second control transistor 10 become equal, so the base bias current of the differential amplifier circuit 2 does not change. A current equal to the collector current 13 of the first mirror transistor 17 is obtained at the collector of the second mirror transistor 18 of the protection circuit 16. If the collector current of the second mirror transistor 18 is L and the value of the resistor 19 is R, then L.R>VB8...[3l (However, VBE is the base of the third control transistor 20.
When the third control transistor 20 becomes conductive (emitter rising voltage), the third control transistor 20 starts conducting.

The collector of the third control transistor 20 is connected to the base of the constant current transistor 5. Therefore, when the third control transistor 20 becomes conductive, the base voltage and base current of the constant current transistor 5 decrease, and the collector current 15 also becomes small. Therefore, the gain of the differential amplifier circuit 2 decreases, and the collector current 1 of the first control transistor 9 stops increasing. If the gain of the differential amplifier circuit 2 were to be controlled only by limiting the input, a very large control current would be required when the input is excessive.

Therefore, the control current flows into the collector of the second control transistor 10, and a similar large current flows into the collector of the first control transistor 9, so that the first and second control transistors 9 and 10 are destroyed. There is a danger.
However, in the circuit of FIG. 1 according to the present invention,
Since the third control transistor 20 is driven by the collector electric current 84 of the second mirror transistor 18 determined by the formula [3', and the gain of the differential amplifier circuit 2 is controlled, the first
and the collector current 1 of the second control transistors 9 and 10
, 12 may not exceed a predetermined value, thereby protecting the first and second control transistors 9 and 10 from destruction.

To summarize the above operation, no gain control is performed while the input signal is small, and as the input signal increases, input limiting type gain control is performed, and when the input signal increases further, the input limiting type gain control is performed. This means that gain control is stopped and gain control that directly controls the gain of differential amplifier circuit 2 is performed instead. Therefore, the circuit shown in FIG. 1 can achieve the effects of preventing destruction of the control transistor, expanding the gain control range, and limiting the power supply current used.

FIG. 2 shows another embodiment of the invention, in which the resistor 19 and third control transistor 20 of FIG.
The collector of the mirror transistor 18 is directly connected to the emitter of the constant current transistor 5, and the collector current of the constant current transistor 5 is controlled by the collector current of the second mirror transistor 18. Second
In the figure, the input signal increases and the first control transistor 9
When the collector current 1, flows, the same electric log 52
One control transistor performs input-limiting gain control. At the same time, the collector current 14 of the second mirror transistor 18, which is equal to the collector current 1 of the first control transistor 9, flows, and the collector current 15 of the constant current transistor 5 decreases by the amount that the collector current L flows. The gain of the dynamic amplifier circuit 2 is directly controlled. Therefore, different types of gain control are performed at the same time, which has the advantage of increasing control sensitivity.

It also has the advantage that the amount of change in power supply current can be kept to a minimum. In other words, at the time of weak input,
Since the first and second control transistors 9 and 10 are non-conductive, only the collector current 15 of the constant current transistor 5 is flowing, and when an extremely large input signal is applied, the first and second control transistors 9 The collector currents 1, 12 of 10 and the collector current L of the second mirror transistor 18 flow, and the collector currents of the constant current transistor 5 become approximately zero. Therefore, the initial emitter current of the constant current transistor 5 is expressed as lo. If so, the maximum current increase will be only 2L. FIG. 3 shows still another embodiment of the present invention, in which a resistor 21 is inserted between the collector of the second mirror transistor 18 and the ground, and the collector is connected to the constant current transistor 5 via a diode 22. It is characterized by a circuit configuration connected to the emitter.

According to the circuit configuration shown in FIG. 3, even if the input signal strength becomes large, the first and second control transistors 9 and 10 become conductive, and input limiting type gain control is performed, the diode 22 does not become conductive. During this period, gain control of the differential amplifier circuit 2 is not performed. Then, when the input signal further increases, the collector current of the second mirror transistor 18 increases, and the voltage across the resistor 21 reaches a predetermined level, the diode 22 becomes conductive and directly controls the gain of the differential amplifier circuit 2. Gain control is started.

With the start of the gain control, the collector current 1 of the first control transistor 9 stops increasing, thereby achieving breakdown protection. Therefore, the circuit shown in FIG. 3 also provides many advantages such as expansion of the gain control range and protection from destruction of the control transistor.

As described above, the protection circuit for the gain control circuit according to the present invention is excellent in that it can achieve reliable protection operation and has various other advantages.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the invention, FIG. 2 is a circuit diagram showing another embodiment of the invention, and FIG. 3 is a circuit diagram showing still another embodiment of the invention. Explanation of main drawing numbers, 2...Differential amplifier circuit, 5...
... Constant current transistor, 8... Gain control circuit, 9, 10... Control transistor, 16...
・Protection circuit. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. A differential amplifier circuit consisting of a pair of transistors of the same conductivity type, a constant current transistor inserted between the common emitter of the pair of transistors of the same conductivity type and the ground, and a constant current transistor inserted between the bases of the pair of transistors of the same conductivity type. means for applying an input signal; first to fourth diodes connected in a bridge type; a connection point between the cathode of the first diode and the anode of the third diode; and a connection point between the cathode of the second diode and the fourth diode; means for connecting the connection points with the anodes to the bases of the pair of transistors of the same conductivity type, first control means for supplying a control current to the common anodes of the first and second diodes, It consists of a second control means for attracting a current of the same magnitude as the control current flowing out from the common cathode, and means for supplying a variable voltage to the first and second control means, which flows to the first to fourth diodes. a gain control circuit that controls the gain of the differential amplifier circuit by attenuating the input signal by varying the impedance of the first to fourth diodes according to the current; and means for reducing the emitter current of the differential amplifier circuit when the current becomes large, and the first and second controls are controlled by operating the means for reducing the emitter current of the differential amplifier circuit. A protection circuit for a gain control circuit, characterized in that it protects circuit elements included in the means from destruction.