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