JPS5837722B2 - variable gain amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable gain amplifier whose gain can be changed by a control signal, and which is particularly suitable for integration into an integrated circuit and can be used as a muting circuit with little variation in DC level. Regarding.

Conventionally, there is a current inverting circuit as shown in FIG. 1. This circuit consists of a transistor 1 and a diode 2, and is a circuit that can obtain an output current IOUT equal to the input current IIN supplied to the input terminal 3 at the output terminal 4. There is a variable gain amplifier using this as shown in FIG.

In FIG. 2, the base of a current inverting transistor 1 is connected to an input terminal 3, and the collector of the current inverting transistor 1 is connected to a common emitter of a pair of transistors 5 and 6 constituting a differential amplifier.

Further, a reference bias voltage is supplied to the base of one transistor 5 of the differential amplifier, a control signal is supplied to the base of the other transistor 6, and the collector of the other transistor 6 is connected to the output terminal 7. configured to be connected.

Therefore, the value of the collector current of the current inverting transistor 1 is equal to the input current, and the sum of the emitter currents of both transistors 5 and 6 of the differential amplifier is equal to the collector current of the inverting transistor 1, so that the value obtained at the output terminal 7 is The output current is obtained in response to a control signal to the base of the other transistor 6 of the differential amplifier.

However, such a variable gain amplifier has a complicated circuit configuration, and the degree of freedom in circuit configuration is greatly restricted due to the relationship between the circuits connected before and after it, and is not very suitable for integration into an integrated circuit.

The present invention has been made in view of the various points mentioned above, and it is an object of the present invention to provide a variable gain amplifier having a simplified circuit configuration and suitable for integration into an integrated circuit.

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

FIG. 3 is a circuit diagram showing the basic principle of the present invention, showing first and second transistors 8 and 9 whose bases and emitters are commonly connected to each other, and the first and second transistors 8 and 9 whose bases and emitters are commonly connected. and the same polarity as the second transistors 8 and 9, and the base is connected to the first transistor 8.
a third transistor 10 whose emitter is connected to the base of the first transistor 8; an input terminal 32 connected to the collector of the first transistor 8;
and an output terminal 11 connected to the collector of the transistor 9, and further has an emitter connected to the input terminal 32, a collector connected to ground, and a control polarity opposite to that of the first transistor 8, which has a base as a control input terminal. transistor 1
2.

Therefore, if the current amplification factor is K and the input current is IN, then the output current is ■.

UT becomes IOUT=KIIN "-"-= (1), and the emitter current ■E of the control transistor 12 is IB-(1-K)IIN ・・・・・・・・・・・・
... (2) becomes.

The amplification factor K changes depending on the control input voltage applied to the base of the control transistor 12, as shown in FIG.

Vc shown in FIG. 5 is a control input voltage applied to the base of the control transistor 12, with the horizontal axis representing the input current IIN and the vertical axis representing the output current IOUT.

In FIG. 3, it is necessary for a sufficient current to flow through the control transistor 12, and therefore the first, second and third transistors 8, 9 and 10 and the control transistor 12 are transistors with opposite polarities, and the input impedance is is lowered.

Furthermore, in the circuit of FIG. 1, the base current of the transistor 1 cannot be ignored, and therefore the input current and the output current do not match by that much. However, by providing the third transistor 10 as shown in FIG. The base current is 1/β (where β indicates the current amplification factor.

), so it can be completely ignored and the accuracy of the circuit can be improved.

The present invention aims to provide a variable gain amplifier with even better characteristics by improving the principle circuit as shown in FIG. 3, and in FIG. A first current inverting circuit 17 is formed by the second transistors 14 and 15 and a third transistor 16 whose base is connected to the collector of the first transistor 14 and whose emitter is connected to the base, respectively. fourth and fifth transistors 18 and 19;
and a sixth transistor 20 whose base is connected to the collector of the transistor 18 and whose emitter is connected to the base.
A current inversion circuit 21 is a seventh transistor 22 whose base is connected to the collector of the second transistor 15 and whose collector is connected to the collector of the fifth transistor 19.
and a diode 23; 24 is a first control transistor 25 that controls the amplification factor of the first current inversion circuit 13; and a second control transistor 26 that controls the amplification factor of the second current inversion circuit 17. and a third transistor 27 that controls the base currents of both control transistors 25 and 26.

One of the pair of signal input terminals 28 and 29 is connected to the collector of the fourth transistor 18, and the other 29 is connected to the collector of the first transistor 14.
The output terminal 30 is connected to the collector of the seventh transistor 22, and the control signal input terminal 31 is connected to the base of the third control transistor 27.

Now the signal input terminals 28 and 29 are connected to ■＋△i,■, respectively.
Assuming that an input signal of △i is supplied and the current amplification factor is K, the first transistor 14 and the second transistor 1
The collector current of the transistor No. 5 becomes K(I-Δi), and the collector current of the fourth transistor 1B and the fifth transistor 19 becomes K(I+Δi).

Also, the emitter current of the first control transistor 25 is (1-K
)(I-△i), the emitter current of the second control transistor 26 is (1-K)(I+△i), and the collector current of the third control transistor 27 is 2I (1-K)/(
1+β).

That is, the collector current of the third control transistor 27 is controlled by a control input signal applied to the base of the third control transistor 27, and the amplification factor K is changed.

At this time, the alternating current components of the base currents of the first and second control transistors 25 and 26 cancel each other out, so that no adverse effects are exerted on the circuit.

Furthermore, the output current of the second transistor 15 is inverted by the third current inversion circuit 21 and added to the collector current of the fifth transistor 19 at the collector of the seventh transistor 22 .

That is, at the summing point A, the collector current K(I-△i) of the seventh transistor 22 flows in the inflow direction, and the collector current of the fifth transistor 19 flows in the outflow direction, so the output current IOUT is IOUT=K(I-△ i)-K(I+△i)=-2Ka
i(3), and an output current flowing in the direction of flowing into point A is obtained at the output terminal 30.

That is, since only the alternating current component appears at the output terminal 30, there is no fear of adverse effects caused by fluctuations in the direct current level caused by the controlled variable.

When this variable gain amplifier is used as a muting circuit, a muting signal may be applied to the control input terminal 31.

And if the magnitude of the muting signal is dog, then
Since the collector current of the third control transistor 2T increases, and therefore the emitter currents of the first and second control transistors 25 and 26 increase, the first and fourth transistors 14
and 18 decrease, and the collector currents of the second and fifth transistors 15 and 19 also decrease.

That is, the output current obtained at the output terminal 30 becomes zero or extremely small, and the signal is not transmitted to the subsequent stage, resulting in a muted state.

As described above, the variable gain amplifier according to the present invention can completely compensate for alternating current component changes caused by the addition of a control transistor, and can completely prevent changes in the direct current level caused by changes in the control amount. It is particularly suitable for use as a muting circuit and for integration into an integrated circuit.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams of a conventional variable gain amplifier, FIG. 3 is a circuit diagram showing the basic principle of the present invention, FIG. 4 is a circuit diagram showing an embodiment of the present invention, and FIG. 5 is a circuit diagram showing the basic principle of the present invention. 3 is a characteristic diagram showing the input/output characteristics of the circuit of FIG. 3. FIG. Explanation of main drawing numbers, 13...First current inversion circuit,
17...Second current inversion circuit, 21...
Third current inversion circuit, 24... control circuit.

Claims (1)

[Claims] a first current inverting circuit having a single output terminal; a second current inverting circuit having an input terminal to which an input signal having a phase opposite to the input signal applied to the input terminal of the first current inverting circuit is applied; a third current inverting circuit connected to the output terminal of the first current inverting circuit and combining the output signal of the first current inverting circuit and the output signal of the second current inverting circuit; and the input of the first current inverting circuit. a first control transistor having a collector-emitter path inserted between the terminal and ground;
a second control transistor with a collector-emitter path inserted between the input terminal of the current inversion circuit and ground, and gain control by controlling the first and second control transistors simultaneously with a single control signal. A variable gain amplifier characterized in that it is configured to perform.