JPS6161568B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gain control circuit that can suppress the occurrence of distortion in an output signal.

For example, the contrast or volume of a television receiver is adjusted by changing the value of a variable element in the gain control circuit using a knob attached to the front of the television receiver. There is a considerable distance between the gain control circuit and the variable element mounted on the front of the television receiver. By the way, if this gain control circuit is an AC control method in which an AC signal flows through the variable element, noise may be induced from the peripheral circuitry into the wiring connecting between the gain control circuit and the variable element, or the wiring may be prevented. There is a risk of inconveniences such as the radiation of harmful waves, and it is necessary to avoid wiring such as using shielded wires and redundant wiring. Therefore, instead of such an AC control system, a DC control type gain control circuit that indirectly performs gain control through DC control has come into widespread use in recent years.

FIG. 1 is a circuit diagram showing the configuration of a known DC-controlled gain control circuit. As shown in the figure, a signal is applied to the base of the emitter circuit of transistors 1 and 2 that constitute a differential amplifier. A circuit configuration in which transistors 3 are connected is adopted, and gain control is performed by changing the differential base input. In the figure, 4 is a fixed DC power supply, 5 is a variable DC power supply, 6 is an output load resistance, 7 is an output terminal, 8 is a signal application terminal, 9 is a coupling capacitor, 10 is a base power supply, 11 is a base resistor, and 12 is a Emitsuta resistance,
And 13 is a collector power supply terminal.

In the gain control circuit configured as described above, when a signal is applied to the input terminal 8, this signal is applied to the base of the transistor 3 via the coupling capacitor 9, is amplified by the transistor 3, and its collector current is used for gain control. This becomes the signal source current for the differential amplifier. This signal source current is divided into transistors 1 and 2 according to the differential base input by changing the voltage of the variable DC power supply 5, and the current divided into transistor 2 is taken from the output terminal 7 as the output of the gain control circuit. issued. In such a gain control circuit,
Since the gain is controlled by changing the voltage of the variable DC power supply 5 as described above, all of the inconveniences that occur in the AC control system are completely eliminated. By the way, as is well known, there is a series resistance known as a base spread resistance r _bb ' at the bases of transistors 1 and 2, and it has been revealed that this causes distortion in the output current, and this inconvenience can be eliminated. As shown in FIG.
4 is connected, and the collector of transistor 1 and the base of transistor 2 as well as the collector of transistor 2 and the base of transistor 1 are connected by resistors 15 and 16 to form a feedback loop, and the bases of transistors 1 and 2 are also connected. A gain control circuit has been proposed in which resistors 17 and 18 are connected to form a feedback circuit. Note that 19 is an emitter bias power supply. In such a gain control circuit, a voltage equal to the voltage generated by the base currents of transistors 1 and 2 is fed back, and this feedback reduces the effect of the base spread resistance.

This gain control circuit is superior to the circuit shown in FIG. 1, but since the amount of feedback changes depending on the current amplification factors of transistors 1 and 2, it is necessary to make the current amplification factors of transistors 1 and 2 the same. , since the feedback signal is based on the collector current signal, its properties are different from the base currents of transistors 1 and 2, and the correction effect is not necessarily sufficient;
Furthermore, a load resistor 1 is connected to the collector of transistor 1.
Therefore, it became necessary to connect 4, which caused problems such as difficulty in designing DC bias conditions.

The present invention has been made in view of the problems of the conventional gain control circuit described above, and includes connecting transistors in cascade to each of transistors 1 and 2 constituting the gain control differential amplifier, and The base circuit of the transistor is provided with a circuit for amplifying a feedback signal corresponding to the base current, and the output signal of this circuit is fed back to the base of the transistor forming the differential amplifier.

The gain control circuit of the present invention will be explained in detail below with reference to the drawings.

FIG. 3 is a circuit diagram showing a first embodiment of the gain control circuit of the present invention, and as shown in the figure, transistors 20 and 21 are connected in cascade to transistors 1 and 2, respectively, which constitute a gain control differential amplifier. is,
Furthermore, the base circuit of these transistors includes a transistor 22, a diode 23, and a resistor 2.
4, as well as transistor 25 and diode 2
6. A current mirror circuit composed of a resistor 27 is connected so that the collector currents of the transistors 22 and 25 are fed back to the feedback base resistors 17 and 18. Note that 28 is the transistor 2
0 and 21, and circuit elements and circuit portions given the same numbers as the circuit of FIG. 2 are the same as those of FIG.

In the gain control circuit of the present invention having the above configuration, when a signal is applied to the input terminal 8, this signal is applied to the base of the transistor 3 via the coupling capacitor 9, is amplified by the transistor 3, and its collector current is This becomes the signal source current for the gain control differential amplifier. This signal source current is divided into transistors 1 and 2 according to the differential base input based on the voltage change of the variable DC power supply 5, and the current divided into transistor 2 flows through the transistor 21 and resistor 6.
The signal is taken out from the output terminal 7 attached to the collector of the transistor 21 as the output of the gain control circuit.

The basic operation described above is the same as the conventional circuit shown in FIGS. 1 and 2. By the way, in the circuit of the present invention, transistors 20 and 21 are cascade-connected to transistors 1 and 2, respectively, and the collector currents of transistors 1 and 2 flow through these transistors. By the way, if we establish the relationships h _FE1 ≒ h _FE20 and h _FE ≒ h _FE21 between the forward current amplification factors of transistors 1 and 20 and 2 and 21, then the collector currents of transistors 1, 2, 20, and 21 will be Between I _c1 , I _c2 , I _c20 and I _c21 , I _c1 ≒ I _c20 and I _c2 ≒ I _c
21 relationships hold. On the other hand, the base current I _b20 of the transistor 20 and the base current I _b21 of the transistor 21 are expressed as I _b20 = I _c20 / where the forward current amplification factors of the transistors 20 and 21 are h _FE20 and h _FE21 .
The magnitude of h _FE20 and I _b21 =I _c21 /h _FE21 . By the way, current mirror circuits are connected to the base circuits of transistors 20 and 21, respectively, and therefore, the collector currents I _c22 and I _c25 of transistors 22 and 25, which are the output currents of the current mirror circuits, are the same as those of transistors 20 and 21. The base currents I _b20 and I _b21 become equal. Further, the base currents I _b1 and I _b2 of transistors 1 and 2 are respectively I _b1 = I _c1 /h _FE1 and I _b2 = I _c2 /h _FE2 . Therefore, as above, h _FE1 ≒ h _FE20
If transistors 1 and 20 and 2 and 21 are selected so that the relationship h _FE2 ≒ h _FE21 is established, the base currents I _b1 and I _b of transistors 1 and 2 are
_2. Base current I _b of transistors 20 and 21
₂₀ , I _b21 and the collector currents I _c22 and I _c25 of transistors 22 and 25 as follows: I _b1 ≒ I _b20 = I _c22 ......(1) I _b2 ≒ I _b21 = I _c25 ......(2) A relationship is established.

I _c22 and I _c25 shown by the equations (1) and (2) are fed back to the feedback base resistors 17 and 18 connected to the bases of transistors 1 and 2, and by this feedback, The influence is almost eliminated, and a gain control operation is performed without causing distortion in the output. Note that the resistors 24 and 27 in the drawings are used because the operating currents of the transistors 22 and 25 are too small in terms of direct current with only the base currents of transistors 1 and 2.
This is a circuit element inserted for the purpose of increasing only the DC operating current when sufficient frequency characteristics cannot be exhibited, and when this is not necessary, the resistor 2 is inserted.
4 and 27 may be removed.

FIG. 4 is a diagram showing another embodiment of the gain control circuit of the present invention, and is an example of a circuit in which base feedback is applied only to the base of transistor 1. In this circuit, the collector current of transistor 25 is fed back to the base of transistor 1 via a current amplifier composed of diode 29 and transistor 30, and in the circuit of FIG. However, in the case of the configuration shown in FIG. 4, the magnitudes of the DC current sources 31 and 32 are almost canceled out at the base of the transistor 1. Note that the gain control operation by this circuit is the same as that of the circuit shown in FIG.

As is clear from the above explanation, the gain control circuit of the present invention considers a signal having the same quality as the base current at the base of the transistor constituting the gain control differential amplifier, and eliminates the influence of series resistance. and good gain control operation is achieved. Furthermore, since there is no need to connect a load resistor to the collector circuit of one of the transistors whose emitters are commonly connected, there is no risk of difficulty in designing bias conditions when designing the circuit.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing conventional gain control circuits, and FIGS. 3 and 4 are diagrams showing gain control circuits of the present invention. 1, 2...Transistor forming a differential amplifier for gain control, 3...Transistor for signal amplification, 4
... Fixed power supply for differential amplifier, 5 ... Variable power supply for differential amplifier, 6, 14 ... Load resistance, 7 ... Output terminal, 8 ... Input terminal, 9 ... Coupling capacitor, 1
0, 28...Base power supply, 11...Base resistance,
12... Emitter resistance, 13... Collector power supply terminal, 15, 16... Resistor for forming feedback loop, 1
7, 18... Base resistor for feedback, 19... Emitter bias power supply, 20, 21... Transistor for forming feedback circuit, 22, 25... Transistor for forming current mirror circuit, 23, 26... Current mirror circuit Forming diodes, 24, 27...
Resistor for increasing DC operating current, 29, 30... Diode and transistor for forming current amplifier, 31, 32... DC current source.

Claims (1)

[Claims] 1. The emitters of the first and second transistors are commonly connected, a signal current source is connected to the emitter common connection point, and a signal current source is connected between the bases of the first and second transistors. A gain control circuit to which a differential base bias is applied and controls a division ratio of the signal source current to the first and second transistors by changing the differential base bias, the gain control circuit having a third transistor applied to the first transistor. first and second transistors in which a fourth transistor is cascade-connected to the second transistor, and feedback signals corresponding to base currents of the third and fourth transistors are amplified in the base circuits of the third and fourth transistors;
a signal amplifying circuit, and supplying at least one of the outputs of the first and second signal amplifying circuits to the bases of at least one of the first and second transistors connected to the bases of the signal amplifying circuits. A gain control circuit featuring: 2. The gain control circuit according to claim 1, wherein the signal amplification circuit is a current mirror circuit. 3. Claim 1, characterized in that the output of the first signal amplification circuit is directly supplied to the base of the first transistor, and the output of the second signal amplification circuit is supplied to the base of the first transistor via a current amplifier. Gain control circuit as described.