Q7Analog Circuits

The Differential Amplifier is the foundational building block of operational amplifiers. Its primary function is to amplify the voltage difference between two input signals while actively rejecting any signal that is common to both inputs.

Principle of Operation:

The basic circuit consists of two symmetrical emitter-coupled BJTs (or source-coupled FETs) sharing a common constant current source at their emitters.

- Differential Mode: If a voltage difference is applied ($V_1\ne V_2$), one transistor conducts more while the other conducts correspondingly less. The output taken between their collectors is a highly amplified version of the difference $(V_1-V_2)$.

- Common Mode: If identical voltages are applied to both inputs ($V_1=V_2$), such as electrical noise picked up by long cables, both transistors try to conduct equally. However, the shared constant current source prevents both currents from rising, resulting in zero change at the outputs. Thus, common-mode noise is heavily rejected.