Differential amplifier emitter negative feedback gain control circuit

The controllable gain amplifier is the execution unit of the automatic gain control circuit . There are two main ways to control the gain of the amplifier: one method is to
change certain parameters of the amplifier itself. Such as emitter current, load, current distribution ratio, constant current source current, negative feedback size, etc. to control
its gain; another method is to insert a controllable attenuator to change the gain of the entire amplifier.
    The differential amplifier emitter negative feedback gain control circuit diagram is shown in the figure.
    The reference values ​​of component parameters are as follows:
    1) The transistor is 3DG6×2. The diode is 2CP41.
    2) Resistor: R. l-R spit-5.lkfl,, R. 1-ReZ-Ra=lkfl.
    VT1 and VT2 form a differential amplifier. The signal is input from the two bases of VT1 and VT2 at
both ends and output from the two collectors at both ends. The control signal
is injected . Two diodes VD1, VD2 and resistors R., ruler. constitute the emitter negative feedback, and
there is R. l -Rcz -Rr, Rcl =Rtz =R.. Whether the diodes VD1 and VD2 conduct or not depends
on the voltage drop on Rel and Rsweet.
    When the control voltage When u. is very small, IcJ is very small, and the average current flowing through R and Re2 is
Ie3/2. If kRt/2 is less than the diode conduction voltage, the diodes VD1 and VD2 are cut off, and the differential amplifier gain is minimum
at this time. , when the control voltage I gradually increases, I increases, making Iea Re/2 greater than the diode conduction voltage. Then VD1 and VD2 are turned on, and the conduction resistance rd will decrease with the increase of the conduction current ID.     It can be seen that using When this circuit performs gain control , the control voltage H should decrease as the input signal increases.