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Differential Amplifiers
ES 330 Electronics II
Chapter 8 Sedra & SmithSection 9.2 on BJT Diff Amps
Single-Ended Signals versus Double-Ended (i.e., Differential) Signals
2
The current source is called the tail current.
Differential Pairs – BJT and FET
3
Q1 Q2
Differential Pair Characteristic
4
5
Differential Pair Characteristic
CCY
EECCCX
C
EEC
VV
IRVV
I
II
02
1
Differential Response of a Differential Amplifier (Case 1)
ON
OFF
6
CCX
EECCCY
C
EEC
VV
IRVV
I
II
01
2
Differential Response of a Differential Amplifier
OFF
ON
7
Common Mode Response of a Differential Amplifier
8
2
221
21
EECCCYX
EECC
BEBE
IRVVV
III
VV
9
Small-Signal Analysis of a Differential Amplifier
II
I
II
I
EEC
EEC
2
2
2
1
10
For small changes at inputs, the gm’s are the same, and the respective increase and decrease of IC1 and IC2 are the same, node “P” must stay constant to accommodate these changes. Thus, node “P” is an AC ground, but not a DC ground.
1
2
0limPx
C m
C m
V
I g V
I g V
Virtual Ground in a Differential Amplifier
11
Small-Signal Differential Gain
Since the output changes by -2gmVRC and input by 2V, the small signal gain is –gmRC, similar to that of the CE stage. However, for the same gain as in CE stage, DC power dissipation is doubled.
CmCm
v RgV
VRgA
2
2
Vout = VX – VY = -2gmΔVRC
Vin = 2ΔV
12
Small Signals and Large Signals
Linear Signal
Clipped Signal
(a) (b)
(c) (d)
Input
Output
TransferCurve
13
Small-Signal Model for Differential Amplifier
r0 not included
vout1 vout2
Since VP is grounded, we can treat the differential pair as two CE “half circuits,” with its gain equal to one half circuit’s single-ended gain.
14
From Symmetry We Need Only Analyze a Half Circuit
Cm
inin
outout Rgvv
vv
21
21
vout1 vout2
vout1 vout2
1 2
1 2
out outm O
in in
v vg r
v v
15
Intrinsic Differential Gain of a Differential Amplifier
Note: Resistance associated with these current sources is infinite.Thus, we include output resistors r0.
16
Questions?
An Operational Amplifier Example Using Differential Pair at Input
17
DifferentialInputs
Output