1. BJT Differential Pair Transistors Q1, Q2 are matched
Responds to differential mode signals
(a) The differential pair with a common-mode input signal vCM. (b) The differential pair with a “large” differential input signal.
6-1

2. BJT Differential Pair (d) (c)
(c) The differential pair with a large input signal of polarity opposite to that in (b). (d) The differential pair with a small differential input signal vi.
6-2

3. BJT Differential Pair
6-3

4. Small-Signal Operation
BJT Differential Pair
Small-Signal Operation
6-4

5. Differential Voltage Gain
Vd less DC DC
Differential output
Single-ended output
Differential output with emitter
Resistance RE
6-5

6. Differential Half-Circuit
vC2 = -vC1
voltage gain = gain of ½ cct
(output taken differentially)
Fig Equivalence of the differential amplifier (a) to the two common-emitter amplifiers in (b). This equivalence applies only for differential input signals (unless R >> re). Either of the two common-emitter amplifiers in (b) can be used to evaluate the differential gain, input differential resistance, frequency response, and so on, of the differential amplifier (e.g. using hybrid pi model).
6-6

7. Common-Mode Gain Analyzing the equiv. Circuit (b)
For output taken differentially = 0 and common-mode gain = 0
For output taken single-ended
6-7

8. Common-Mode Gain (Cont’d)
Consider the case of a collector resistor mismatch
Q1 has load
Q2 has load
(differential component)
(common-mode component)
6-8
(P)

9. Input Common-Mode Resistance
Input CM resistance is large so we must take into account
CM gain is small, so assume 0V at collector then r0 appears in
parallel with 2R in the emitter and ru appears between input
and ground.
“emitter
reflectance
rule”
6-9
(P)

10. Example6.1 pg. 502
15V
10k
10k
- vo + vs
150
150
200k
1mA
6-10
(P)

11. (c) The worst case common-mode gain if the two collector
Example6.1 pg. 502 (Cont’d)
(c) The worst case common-mode gain if the two collector
resistances are accurate to within 1%
where DRC = 0.02 RC
(worst case)
(d) The CMRR in dB
Using Ad = 40 from (b) and ACM = 5E-4 from ( c)
(e) The input common mode resistance
(assuming VA = 100 and rm = 10 b r0 )
Eqn p 502