How it works
A differential amplifier has two inputs V1 and V2 and produces output proportional to (V1 − V2). Any signal common to both inputs is the common-mode signal (Vcm); the difference is the differential signal (Vd = V1 − V2). Differential gain Ad = −RC/2re for the basic BJT diff amp. Common-mode gain Acm = −RC/(2REE) where REE is the tail resistance; replacing REE with a current source drives REE → ∞ and Acm → 0. CMRR = |Ad/Acm| = REE/re when using a resistive tail, or approaches infinity with an ideal current source. In practice, CMRR for op-amps is 80–100 dB. Output can be single-ended (one collector) or differential (between two collectors); differential output doubles the gain and preserves symmetry.
Key points to remember
Differential mode gain Ad = RC/2re; common mode gain Acm = −RC/(2REE). CMRR = Ad/Acm = REE/re — higher tail resistance or a current source dramatically improves CMRR. CMRR in dB = 20 log(Ad/Acm); the μA741 specifies CMRR > 90 dB typically. Mismatch between the two transistors or resistors degrades CMRR; matched transistor pairs like the MAT02 achieve CMRR > 100 dB. The tail current source sets total emitter current IE(total), splitting equally between Q1 and Q2 when V1 = V2; differential input shifts this balance, steering current from one side to the other. Input offset voltage (a key datasheet parameter) arises from transistor mismatch and is typically 1–5 mV for general-purpose op-amps.
Exam tip
The examiner always asks you to derive CMRR for a differential amplifier — compute Ad = RC/2re and Acm = RC/(2REE), then CMRR = REE/re, and explain why replacing REE with a current source maximises CMRR.