Controllable gain amplifier is an automatic gain control circuit of the execution unit, the main method of controlling the gain of the amplifier in two ways: One way is through
changing some parameters of the amplifier itself. As the emitter current, the load current distribution ratio, constant current source, and so to control the size of the negative feedback gain; Another method is to insert a controllable attenuator change the gain of the amplifier. Emitter negative feedback differential amplifier gain control circuit diagram shown. Component Parameters Reference values are as follows: 1) transistors 3DG6 2. 2CP41 diode. 2) Resistance: R. l a R spit a 5.lkfl, R. 1 a ReZ a Ra lkfl. VT1 and VT2 composed of the differential amplifier, the signal from VT1, VT2 base of two double-ended input, a collector from two double-ended output control signal. Note from VT3 base into the two diodes VD1, VD2 and resistor R., ruler. composing emitter degeneration, and there R.l -Rcz -Rr, Rcl Rtz R. diodes VD1, VD2 conduction or not depends on Rel and R sweet drop. when the control voltage u. is small, IcJ small, and the average current flowing through the R. Re2 on each of Ie3/2. as kRt/2 is smaller than the diode turn-on voltage of the diode VD1, VD2 cut stop, then the minimum gain of the differential amplifier when the control voltage it increases, I increase, so Iea Re/2 is greater than the diode conduction voltage is VD1, VD2 conduction, conduction resistance rd will conduct current ID as Yun increases. visible, use when this gain control circuit, the control voltage H. should decreases as the input signal.