Differential Amplifiers Chapter 17

A differential amplifier +VCC RC v1 > v2 v2 > v1 v1 = v2 vout = 0 RC vc1 vout = vc2 - vc1 vout vc2 v1 v2 RE -VEE

differential output value) A differential amplifier with single-ended output +VCC Block symbol v1 v2 vout A RC vout vout = A(v1 - v2) v1 v2 RE (A is one-half the differential output value) -VEE

Single-ended output and single-ended input. +VCC v1 A vout RC vout = Av1 vout v1 RE -VEE

Inverting configuration +VCC vout A RC vout v2 vout = -Av2 v2 RE -VEE

dc analysis +VCC IT, the tail current, flows in RE RC RC IT = VEE - VBE RE IT IE = 2 RE VC = VCC - IERC -VEE

ac analysis +VCC A = RC 2re’ Differential output case: A = RC re’ RC vout zin = 2bre’ v1 RE -VEE

The input transistors are not exactly the same. IB1 = IB2 Iin(bias) = IB1 + IB2 2 IB1 Iin(off) = IB1 - IB2 IB2 IB1 = Iin(bias) + Iin(off) 2 IB2 = Iin(bias) - Iin(off) 2

Base current can produce a false signal. +VCC RC RC Verror Vin = IB1RB RB RE -VEE

Vin and Verror are minimized with equal base resistors. +VCC Iin(off) < Iin(bias) Vin = Iin(off) RB RC RC Verror RB RB RE -VEE

Input offset voltage The RC values may be different. VBE for each transistor may be different. Other parameters may differ slightly on each side of the diff amp. The input offset voltage would produce the same error in an ideal amplifier. Vin(off) = Verror/A Both bases are grounded when Verror is measured.

Combined dc error V1err = (RB1 - RB2)Iin(bias) V2err = (RB1 + RB2)Iin(off)/2 V3err = Vin(off) Verror = A(V1err + V2err +V3err) V1err eliminated with equal base resistors V2err reduced with smaller base resistors V3err reduced with premium op amps

Common-mode gain +VCC ACM = RC 2RE RC vout Vin(CM) Vin(CM) RE -VEE

Common-mode rejection ratio CMRR = A/ACM CMRRdB = 20log CMRR The higher the CMRR, the better A typical op amp has CMRRdb = 90 dB Much interference is common-mode and a high CMRR means an amplifier will be effective in rejecting interference

The current mirror +VCC IR = VCC - VBE R R IC = IR Compensating diode

In ICs, a current mirror sources the tail current +VCC RC vout R V2 V1 IC = VCC + VEE - VBE R -VEE

The impedance of a current source is very high. +VCC RC vout R V2 V1 CMRR = Zcurrent source re’ -VEE

Active loading produces a very high voltage gain. +VCC R vout V2 V1 -VEE

Loaded diff amps vout 2RC RL vL Thevenin equivalent circuit for differential output RC vL Thevenin equivalent circuit for single-ended output RL vout