JPS62114313A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To secure a frequency characteristic for a high gain and attain a wide-band frequency characteristic and a wide variable gain width by providing a peaking circuit consisting of a diode and a capacitor in a diode circuit of a gain control circuit connected to a differential amplifier. CONSTITUTION:A series circuit of resistances R1 and R2 of an attenuating circuit 1b of a gain control circuit 1 is connected between emitters of differential pair of transistors TRs Q3 and Q4 of a differential amplifier 2 of a variable gain amplifier. Anodes of diodes D1 and D2 of a diode circuit 1c of the circuit 1 are connected to emitters of TRs Q3 and Q4, and diodes D3 and D4 are connected in series to diodes D1 and D2, and a capacitor C is connected between the connection point of diodes D1 and D2 and that of diodes D3 and D4. TRs Q1 and Q2 of a differential control circuit 1a of the circuit 1 are connected to the connection point of diodes D3 and D4 and that of resistances R1 and R2. The circuit 1c is operated as a peaking circuit in case of a high gain to provide the amplifier with a wide-band frequency characteristic and a wide variable gain width.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a variable gain amplifier.

[Technical background of the invention and its problems]

Conventionally, a variable gain amplifier shown in FIG. 2 has been known (see Japanese Patent Laid-Open No. 57-211814).

The illustrated amplifier includes a differential control circuit 1a, an attenuator Mlb
.

It has a gain control circuit 1 consisting of a diode circuit IC.

R, for the attenuator of the attenuation circuit 1b receives the supply of the input signal Sl, and receives the output signal 8. It is connected in series to the emitter electrodes of the differential transistors Q, , Q, of the differential amplifier 2 which outputs . When the control signal S3 is applied to the control terminal 3 of the gain control circuit 1, the connection point 4 of the diodes D, , D of the diode circuit 1a and the attenuation circuit 1
b attenuator) 1. , , R, changes the current at the connection intermediate point 5, the input current of the differential amplifier 2 increases or decreases, and the gain is controlled.

The maximum gain 0 and minimum gain G of this variable gain amplifier are expressed by the following equations.

R.L. G cod=□ T 4・□ I.

C RL: Collector resistance (R8, Gourd) E: Emitter resistance (R1 + FL2) As is clear from the above formula, by setting the resistance large and the resistance RB small, the variable gain amplifier described above can be widened. Although it has a variable width, it also has the drawback that its frequency characteristics deteriorate due to the influence of the pace capacitance and collector capacitance of the differential transistors Q, , Q.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to obtain a variable gain amplifier having wide frequency characteristics and a wide variable gain width.

[Summary of the invention]

The variable gain amplifier according to the present invention includes diodes D, D! in a diode circuit lc on a gain control circuit, for example, as shown in FIG. Add series diodes D, , D, to
Furthermore, by arranging a capacitor C in parallel, a peaking circuit that operates at high gain is constructed.

This makes it possible to obtain both broadband frequency characteristics and a wide variable gain width.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention. The difference from the conventional variable gain amplifier shown in FIG. 2 lies in the configuration of the diode circuit IC. That is, diodes D, , D, are added in series with each of the diodes D, , D.

Further, a capacitor C is connected in parallel with the diodes D, ,D.
A diode circuit 1c is constructed by adding .

According to the variable gain amplifier configured as described above, when current flows to the diode circuit IC side (at high gain) in accordance with the on/off of the control transistors Q, , Q* of the six differential control circuits 1a, the capacitor C Due to this action, the amount of feedback in high waves is reduced, compensation (peaking) of band characteristics is performed, and wide band characteristics of the same wave number can be obtained. Further, when current flows to the attenuation circuit Ib side (at low gain), the band characteristic compensation described above is not performed, so that a wide variable gain width can be maintained as in the conventional circuit.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to provide a variable gain amplifier that has both broadband frequency characteristics and a wide variable gain width.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a variable gain amplifier according to an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional variable gain amplifier. 1... Gain control circuit, 2... Differential amplifier. D, ~D,... evening ion, C... capacitor. R,,,R,...Resistance, Q,,Q,...
・Control transistor. Q, , Q,...Differential transistor. Figure 1

Claims (1)

[Claims] two series resistors connected between one emitter electrode and the other emitter electrode of a differential pair transistor constituting a differential amplifier; a first diode whose anode end is connected to the one emitter electrode; , a second diode whose anode end is connected to the cathode end of this first diode, a third diode whose cathode end is connected to the cathode end of this second diode, and an anode of this third diode. a fourth diode whose cathode end is connected to the other end and whose anode end is connected to the other emitter electrode; and a connection point between the first and second diodes and a connection point between the third and fourth diodes. and a current changing means for differentially changing the current at the intermediate connection point of the series resistor and the connection point of the second and third diodes according to the control signal, A variable gain amplifier characterized in that the gain is changed by increasing or decreasing the gain.