Working principle of two matched transistor pairs

fangfang120

Working principle of two matched transistor pairs [Copy link]

In this picture, what are the changes in the collector voltages of the two transistors? What are the functions of the operational amplifier at the front end and VR10?

maychang

"The role of the operational amplifier at the front end and VR10"

The input of the operational amplifier is the output of transistors Q1Q2, so it is not appropriate to call it the "front end". In terms of signal flow direction, the operational amplifier is after Q1Q2, so it is more appropriate to call it the "back end".

maychang

"The role of the operational amplifier at the front end and VR10"

The function of VR10 is to manually change the gain of the entire amplifier circuit. VR10 is connected in series with R64 to divide the voltage, and the voltage across R64 is the input signal of the entire amplifier circuit.

maychang

"What are the changes in the collector voltages of the two transistors in this picture?"

The collector voltages of the two triode pipes always change in opposite directions, when one rises, the other falls.

wenyangzeng

Differential amplifier only

阿卡时间段

There should be this picture in the analog electronics book

maychang

In fact, this differential amplifier circuit is widely used in bipolar op amps. The input stage of bipolar op amps is of this structure.

maychang

From the application point of view, the first circuit has no value, Q1Q2 and its auxiliary components can be omitted, and the signal can be directly input to the op amp's non-inverting input. Of course, the gain of the op amp needs to be changed to suit the needs.

maychang

Therefore, this circuit is just an exercise, as an assignment for learning analog circuits. This discrete triode differential amplifier circuit will not be used in actual design work.