Q.What is a differential amplifier?

A differential amplifier is the circuit that gives an op-amp its defining behaviour: it amplifies the difference between two input signals and rejects whatever is common to both. It is built from two closely matched transistors that share a common emitter current, with one input driving each transistor. Because the two halves are identical, a voltage that appears equally on both inputs — a common-mode signal such as hum, noise or drift — is cancelled, while a voltage that differs between the inputs — the differential signal — is amplified.

A differential amplifier can be connected in four modes according to how many inputs are driven and how the output is taken:

• Dual-input, balanced-output — both bases driven, output taken between the two collectors.
• Dual-input, unbalanced-output — both bases driven, output taken from one collector.
• Single-input, balanced-output — one base driven, output between the two collectors.
• Single-input, unbalanced-output — one base driven, output from one collector.

In the important dual-input balanced-output mode, each input contributes to both outputs, and by superposition the output responds to $(V_1 - V_2)$. Its ability to amplify the difference while suppressing common-mode interference is exactly why it is placed at the front of an op-amp, letting the device detect small wanted signals buried in noise.