1. Differential Amplifiers
Chapter 17

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

3. 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

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

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

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

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

8. 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

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

10. 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

11. 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.

12. 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

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

14. 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

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

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

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

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

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