JPS62114312A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To secure a band characteristic for a high gain to attain a wide-band frequency characteristic and a wide variable gain width by providing a transistor TR, which can vary the capacity between the base and the emitter of a differential TR, for the differential TR of a differential amplifier connected to a gain control circuit. CONSTITUTION:An input signal S1 is inputted to bases of differential TRs Q3 and Q4 of a differential amplifier 2 of a variable gain amplifier, and an output signal S2 is outputted from collectors. A gain control circuit 1 is connected to respective emitters of TRs Q3 and Q4, and TRs Q7 and Q8 whose collectors and emitters are connected mutually respectively are connected there, and emitters of TRs Q7 and Q8 are connected to the control circuit. Capacities between bases and emitters of TRs Q3 and Q4 are varied by TRs Q7 and Q8, and the band characteristic is peaked for 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 as shown in FIG. 2 has been known (see Japanese Patent Laid-Open No. 57-211814).

The illustrated amplifier includes a differential control circuit 1a and an attenuation circuit 1b.

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

The attenuators R, , R, of the attenuation circuit 1b are connected to the emitter electrodes of the differential transistors Q, , Q, of the differential amplifier 1@unit 2, which receive the input signal S1 and output the output signal S. connected in series. 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 connection midpoint of the attenuators R, , R, of the attenuation circuit 1b. 5 changes, the input current of the differential amplifier 2 increases or decreases, and the gain is controlled.

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

T 4 ・□ O VT 2⇠g+2・− O RL: Collector resistance (R3, R4) RB: Emitter electrode (⇠t, Rt) Ia: Difference a 4 transistor Qs, as is clear from the above formula for the collector current of Q4 By setting the resistor RL large and the resistor RB small, the gain variable amplifier described above has a wide variable range, but the differential transistor Q,...
It also had the disadvantage that its frequency characteristics deteriorated due to the influence of the base capacitance and collector capacitance of Q.

[Objective of the Invention The present invention has been made in view of the above-mentioned conventional problems,
The purpose of this invention is to obtain a variable gain amplifier that has wide frequency characteristics and a wide variable gain width.

[Summary of the invention]

The variable gain amplifier according to the present invention includes differential transistors Q, , Q of a differential amplifier 2, for example, as shown in FIG.

A respective base electrode is connected to each emitter electrode.

Transistors Q, , Q, whose collector electrodes are connected to each other and emitter electrodes to each other are attached, and the capacitance between the base and emitter electrodes of these transistors Q, , Q is varied, and peaking is applied at high gain to achieve a wide band frequency. This makes it possible to obtain both characteristics and a wide variable gain range.

[Embodiments of the invention]

FIG. 1 shows an embodiment of the present invention. The difference from the conventional gain variable amplifier shown in Figure 2 is that the transistors Q.
i, resistance Rg, 6. and the addition of current source (1). That is, the differential transistor Q of the differential amplifier 2
, , Q, are connected to the base electrodes of transistors Q, , Q, respectively.

Corella transistor Q? , Q, are both connected to a power supply via a resistor R,. Furthermore, the transistor Q1. The emitter electrodes of Q, are commonly connected and connected to the collector of transistor Q. The base electrode of this transistor Q is connected to the base electrode of the control transistor Q of the differential control circuit 1a, and the emitter 1tffl is connected to the emitter electrode of the transistor Q6 and to the current source 11. Furthermore, the base electrode of the transistor Q6 is connected to the base electrode of the control transistor Q2, and the collector electrode is connected to the power supply via the resistive island.

According to the variable gain amplifier configured as described above.

The capacitance between the base and emitter electrodes of the transistors Qヮ and Q8 changes depending on the current flowing through the transistors, thereby compensating (peaking) the band characteristics at high gain. That is, depending on whether the control transistors Q, , Q2 of the differential control circuit 1a are turned on or off, the transistors Q7, . The amount of current flowing through Q is controlled. For example, the control transistor Q turns on, and the diode circuit I
When current flows to the C side (at high gain), current also flows to transistors Q, , Q, and the capacitance between the emitter electrodes of the pace board increases.

Also, the control transistor Q2 side is turned on, and - the attenuation circuit l
When 16 current flows to the b side (at low gain), the amount of current flowing through the transistors Q, , Q decreases, and the capacitance between the base and emitter electrodes also decreases.

Therefore, in the variable gain amplifier according to the present invention, due to the capacitance change of the transistor Qγ+Qs as described above, the feedback bond at high frequencies is reduced, and when the gain is high, band characteristic compensation (peaking) is performed, and wide band frequency characteristics can be obtained. I can do it. Furthermore, since peaking is not performed when the gain is low, a wide variable gain width can be maintained as in the conventional circuit.

〔Effect of the invention〕

As explained above, according to the invention, both broadband frequency characteristics and wide variable gain width are provided.

A variable gain amplifier can be provided.

[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. P,, P2...Diode, R,, R,...
・Resistance. Ql, Q,,Q,...control transistor. Q,,Q,...differential transistor, Qy,Qg・
...Transistor.

Claims (1)

[Claims] Two series resistors are connected between one emitter electrode and the other emitter electrode of the differential pair transistors constituting the differential amplifier, and two opposite resistors are connected in parallel to the series resistors.
a diode, first and second transistors whose base electrodes are respectively connected to one emitter electrode and the other emitter electrode of the differential pair transistors, and whose collector electrodes and emitter electrodes are connected to each other; Current changing means that differentially changes the current at the intermediate connection point of the series resistor and the intermediate connection point of the two diodes according to a control signal, and also changes the current flowing through the first and second transistors. A variable gain amplifier comprising: a variable gain amplifier configured such that the gain changes according to an increase/decrease in the control signal.