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MALVINO

SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. Differential Amplifiers. Chapter 17. v 1 = v 2. v 2 > v 1. v out = 0. v 1 > v 2. A differential amplifier. +V CC. R C. R C. v out = v c2 - v c1. v c1. v c2. v out. v 1. v 2. R E. -V EE. v 1. v out. A. v 2.

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MALVINO

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  1. SIXTH EDITION MALVINO Electronic PRINCIPLES

  2. Differential Amplifiers Chapter 17

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

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

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

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

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

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

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

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

  11. 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

  12. 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.

  13. 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

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

  15. 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

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

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

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

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

  20. 2RC vout RL vL vL Loaded diff amps Thevenin equivalent circuit for differential output RC Thevenin equivalent circuit for single-ended output RL vout

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