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EE 332

DEVICES AND CIRCUITS II

Lecture 18

DEFERENTIAL AMPLIFIERS ANDOPERATIONAL AMPLIFIER DESIGN (1)

DIFFERENTIALAMPLIFIERS/INTRODUCTION

BasicTopology:Emittercoupledpair

Differentialamplifiersare

deignedtoamplifythedifference

betweentwoinputs(vi1vi2)AND

simultaneouslyignore(not

amplify)theaverageofthetwo

inputs.

vi1 vi2

VCC

RC2vo2vo1

RC1

RE

DIFFERENTIALAMPLIFIERS

/4DIFFERENTMODES

vi1 vi2

VCC

RC2vo2vo1

RC1

R

Fourdifferentdifferentialamplifiermodes

1. Signalended:

1signal:

v(t),

input/outputrespecttoGND

2. Balanced:[ v(t), v(t)]2

signals:equalmagnitudes,

oppositesigns

3. Differential:differentsignal(vi1

vi2)or(vo1vo2)

4. Common:theaveragesignalE

[(vi1+vi2)]or[(vo1+vo2)]

Lecture Goals

Understanding of concepts related to:

Differential Amplifiers/Modes Differential Amplifiers/Small signal Model Differential Amplifiers /Purely Differential InputSignal Differential Amplifiers /Purely Common-mode

Input Signal

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DIFFERENTIALAMPLIFIERS

/DIFFERENTIALSIGNALANDCOMMONMODESIGNAL

vi1

vi2

VCC

RC2

vo2vo1

RC1

RE

DifferentialSignal:

1 2d i iv v v

CommonmodeSignal:

1 2

2

i i

c

v vv

+=

and

1

2

1

21

2

i c d

i c d

v v v

v v v

= +

=

vd andvc (differentialmodeandcommon

mode)signalsareeasiertoanalyze,because

circuitsymmetrycanbeexploited.

DIFFERENTIALAMPLIFIERS/PURELYDIFFERENTIALSIGNALS

Applyapurelydifferentialmodesignal vi2=vi1

IF , and

2 1 2

1 2 1 2

( )

0

X i i

i i i i

X

V R i i

v v v v

then

V V

= += =

=

Sameanalysis,VE isgrounded.

vi1

vi2VX

REcanbeignored.

ii2

ii1

Ii1

+ii2

DIFFERENTIALAMPLIFIERS/PURELYDIFFERENTIALSIGNALS

PurelyDifferentialinputsignal: 1 2i iv v

Evaluatehalfcircuit,thenobtaintheresponseofwhole

RE=short

vi1 vi2

RC1 RC2

r2r1 ro1

ro2

vo2

vo1

circuitsincebothsidesaresymmetrical.

DIFFERENTIALAMPLIFIERS/SMALL

SIGNAL

MODEL

vi1 vi2

VCC

RC2

vo2vo1

RC1

RE

vi1 vi2

RC1 RC2

r2r1ro1

ro2

RE

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DIFFERENTIALAMPLIFIERS

/

PURELY

DIFFERENTIAL

SIGNALSPurelyDifferentialinputsignal:

1 2i iv v

vi1

RC1

r1

ro1

1 1 1 1 1

1 1

1 1 1

( / / )2

2

( / / )

od

o m be C o

vd

id i be

vd m C o m C

v

v g v R r A

v v v

A g R r g R

= = =

=

11

1 2

22

2 2 2

; sinceid iid id i

id id

id

v vR v v

i i

R r r r

= = =

= = =

; since11

1 1

22

2( / / ) 2( / / )

od o

od od o

od od

od C o C o

v vR v v

i i

R R r R r

= = =

= =

vo1

Rid

iid

Rod

DIFFERENTIALAMPLIFIERS

/INPUTRESISTANCE

; where and

; recall

2

2 ( 1)2

2 ( 1)

icic

ic

b ic

b b ic b

b

b

ic b E

E

vRi

v ir v v i

i

rR r r R

r R

=

= = =

= = + +

+ +

Q

2ic

R =

vi1

RC1

r1 ro1

vo1

1 2i iv v

2RE

=Purelycommonmodeinputsignal:

DIFFERENTIALAMPLIFIERS

/OUTPUTRESISTANCE

; where and

; recall1 1

1

/ /2

/ / 2(1 )

2 2

2/ / (1 )

2

2

oc

oc

oc

c oc

C c c oc c

c

C c E

oc c o

E B

E

C o

E

oc

v

R i

v iR r v v i

i

R r RR r r

R R r

RR r

R rR

=

= = =

= = ++ +

++ =

Q

vi1

RC1

r1 ro1

2RE

vo1

1 2i iv vPurelycommonmodeinputsignal:

DIFFERENTIALAMPLIFIERS

/PURELY

COMMON

MODE

INPUT

1 2i iv vPurelycommonmodeinputsignal:

Nocurrentflowsbetweenthetwosidesandcananalyzeusing

onlyonehalf.

i=0

vi1 vi2

RC1 RC2

r2r1 ro1 ro2

2RE 2RE

ReplaceRE with2RE

vo1vo2

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DIFFERENTIALAMPLIFIERS

/GAIN

Recall:

where:

1

1

1

1

1

1 1 1

1

1

( 1)

( / / / / )

2 ( 1)

( / / )

11 2 ( )

2

oc o

vc

ic i

CE

v m L E Q

E

L E Q C o L

vc m L E Q

E

m C o

vc

E m

C

vc

E

v vA

v v

rA g R

r R

R R r R

rA g R

r R

g R rA

R gr

RA

R

= =

=+ +

= =

+ + =+ +

vi1

RC1

r1 ro1

2RE

vo1

1 2i iv vPurelycommonmodeinputsignal:

End of Lecture 18

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