JPH0284804A - Gain control circuit - Google Patents

Abstract

PURPOSE:To facilitate direct connection to the next stage keeping an output DC level constant by providing a circuit which inputs a difference between a reference current to a variable gain control circuit. CONSTITUTION:An input signal is supplied to the base of the first transistor TR Q1 of a differential amplifier in a gain control circuit, and a first variable current source IA is connected to the emitter of the TR Q1 connected to a second TR Q2, and a load resistor R1 is connected to the collector of the TR Q1. Also, the collector of a third TR Q4 is connected to that of the TR Q1, and a current mirror is comprised of the TR Q4 and a fourth TR Q5, and a second variable current source IB is provided at the base and collector of the TR Q4. When the gain of the amplifier of operating format is varied by varying the current source IA of the gain control circuit, the output DC level can be kept constant by the TRs Q4 and Q5 and the second current source IB. Then, a discharge circuit which supplies the difference between the reference current generated by varying the current source IA can be formed.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a variable gain control circuit.

[Conventional technology]

Figure 3 is a circuit diagram of an example of a conventional variable gain amplifier circuit.The input signal is input to the base of transistor Q10, and a variable gain amplifier circuit is constructed by Ql and Q, and variable current source IA>load resistor R8. The gain of this amplifier is varied by changing the current value of the variable current source. Calculating the gain is expressed as, if the current value of the current source is the current value.

From this equation, the gain can be varied by varying the value of .

[Problem to be solved by the invention]

The above-mentioned conventional variable gain amplifier circuit has the disadvantage that by varying the gain, the output DC level fluctuates, and it is not easy to connect it directly to the next stage.

[Means to solve the problem]

The variable gain amplifier circuit of the present invention includes a first transistor to which an input signal is supplied to the base, a second transistor forming a differential amplifier with the tenth transistor, and a first transistor connected to the first and second emitters. a current source, a first load resistor connected to the collector of the first transistor, and a third resistor, the collector of which is connected to the collector of the first transistor.
a fourth transistor configured as a current mirror with the third transistor; and a second current source connected to the base and collector of the fourth transistor, the first current source being variable; A circuit is provided for keeping the output DC level constant using the third and fourth transistors and the second current source when varying the gain of the differential amplifier made up of the first and second transistors.

In this way, the output DC level can be kept constant by providing the output circuit that supplies the difference in the reference current generated by varying the variable current source.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of the principle of the present invention. That is, in contrast to the conventional example shown in FIG. 3, transistors Q, ,Q,.

A current mirror circuit including resistors R4 and R5 and a current source I3 corresponding to a gain control signal are provided. In other words, a differential amplifier circuit (Ql, QI
R1), a variable current source (IA) that changes the gain of this differential amplifier circuit, and an output circuit (IB) that keeps the output DC level constant.

Q4. Ql, R4, R8).

FIG. 2 shows a specific example. transistor Ql,
Q6 each constitutes a current source.

Now, if the base potential of Q is VA, then the collector current of Qs is %.The gain is calculated here.

L= ・・・・・・(3) Next, calculate the output DC voltage in this circuit from equations (1), (2), and (3). Vo=V
cc-Rt ・I y ..." (1) Here, the work is expressed by the following formula. (In the case of 1, ≧ work.) I?=II L=L-Ii=L-Is+I.
...(2) Also, to set R2 = 2 Rs, A I s ” = I +
...(3) From equations (3) and (2), L=I.・・・・・・(4)
Therefore, R3 is always machined. flows, so the output DC voltage v0 is kept constant.

〔Effect of the invention〕

As explained above, the present invention is a standard electrician. By having a circuit that inputs the difference between the two, the output DC level can be kept constant, and there is an effect that it can be easily directly connected to the next stage.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of the principle of the present invention, FIG. 2 is a circuit diagram of a specific embodiment, and FIG. 3 is a circuit diagram of a conventional example. a...Input terminal, b...Output terminal, C
...Power terminal, d...Ground terminal, Q1
~Q6...Transistor% R+~R6...
...Resistance, ■...Volume, ■.・・・・・・
・・Constant current source, work・・・・・variable current source, V
A...Base potential of Q and Q6, ■. ...
...Output DC potential. Agent Patent Attorney Susumu Uchihara

Claims (1)

[Claims] first and second transistors connected in a differential manner and having an input signal supplied between their bases;
a first current source that is connected to the emitter coupling point of the transistor and generates a current according to the gain control signal; a second current source that generates a current according to the gain control signal; and the second current source. A gain control circuit comprising: a current mirror circuit having an input end connected to the collector of the first transistor and an output end connected to the collector of the first transistor, the gain control circuit maintaining a constant output DC level.