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EE 332

DEVICES AND CIRCUITS II

Lecture 20

DEFERENTIAL AMPLIFIERS ANDOPERATIONAL AMPLIFIER DESIGN (3)

OPERATIONALAMPLIFIERDESIGN

/STAGETHREE(OUTPUTSTAGE)

Outputstageusuallyrequireshighinputresistance,low

outputresistanceandunitvoltagegain

ClassAamplifier:transistorconductsduringentire

360 ofinput

waveform

Example:common

drain(sourcefollower)vi

RLISS

vo

VDD

VSS

OPERATIONALAMPLIFIERDESIGN/STAGETHREE(CLASSAAMPLIFIER)

vi

RLISS

vo

VDD

G S i n ou t T N

D S G S T N

D D ou t i n ou t T N

i n D D T N

v v v V

v v V

V v v v V

v V V

VSS

= >

+

IfRL=:Operationrangeofamplifier:(saturation)

D

S

G

vDS

vGS

Lecture Goals

Understanding of concepts related to:

Opamp Design Stage Three /Class A Amplifier Opamp Design Stage Three/Class B Amplifier Opamp Design Stage Three/Class AB Amplifier

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OPERATIONALAMPLIFIERDESIGN

/STAGE

THREE

(CLASS

A

AMPLIFIER)

vi

RL

ISS

vo

VDD

VSS

IfRL=:

D

S

G

vDS

vGS

1

2( )

2

2

2

out in GS

n

SS GS TN

SS

GS TN

n

SS

out i n TN

n

K

v v v

KI v V

Iv V

K

Iv v V

K

= =

= + =

Q

1442443

Whatistheoutputrange?

Theoutput

followstheinputwith

anoffset

K1

OPERATIONALAMPLIFIERDESIGN/STAGETHREE(CLASSAAMPLIFIER)

ma x( )ou t D D v V

and,ma x( )i n D D G S v V v

== +

L

out

SSDS

R

vII +=IfRL: KeepsthetransistorON

(vout)min occurswhenthetransistorturns

off,

IDS is

at

minimum

and

mi n

mi n

( )

( )

( )

ou t D S SS L

o u t SS L

G S T N

i n SS L G S SS L T N

v I I R

v I R

v V

v I R v I R V

= =

== + = +

At cu tof f

vi

RL

ISS

vo

VSS

vDS

vGS

IDS

OPERATIONALAMPLIFIERDESIGN

/STAGETHREE(CLASSAAMPLIFIER)

vi=

VDDsint

RLISS

vo

VSS

vDS

vGS

iDS

2

0

22

0

2

0

2 2

( sin )1

1(sin )

1 (1 cos 2 )

2

1

ac

av

TDD

ac

L

TDD

L

TDD

L

P

P

V tP dt

T R

Vt dt

T R

V tdt

T R

V VT

===

=

TheproblemwiththeclassAamplifierisefficiency

Efficiencyistheratiobetweenpowerdeliveredtoload

(maximum)andpowerdeliveredtoamplifier

VDD

2 2D D D D L LT R R= =

OPERATIONALAMPLIFIERDESIGN

/STAGE

THREE

(CLASS

A

AMPLIFIER)

vi

RLISS

vo

VDD

VSS

IfRL=:

D

S

G

vDS

vGS

Theswingofvout isnotrestricted

bythetransistorsinceitstayson

toconduct

min

max

min

max

( )

( )

( )

( )

ou t SS

ou t D D

i n SS GS

i n D D GS

v V

v V

an d

v V v

v V v

= == += +

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OPERATIONALAMPLIFIERDESIGN

/STAGE

THREE

(CLASS

A

AMPLIFIER)

vi

RL

ISS

voVSS

vDS

vGS

iDSSS DD

Current drawnfrom V Current drawn from V

into drain

0

sin1[ ]

0 ( )

ac

av

TDD

av SS SS SS D D D D

L

SS SS SS D D SS SS D D

P

P

V tP I V I V V dt

T R

I V I V I V V

== + +

= + + = +

12314444244443

with

2

2

2 1;

2 4

DD

L

DD SS

DD L

V

RV V

V R = = =

OPERATIONALAMPLIFIERDESIGN

/STAGETHREE(CLASSBAMPLIFIER)

sini n DD

v V t

Consider:

Mostdistortion

increases

as

signalsizeincreases

Largesignal:harmonicdistortion

Smallsignal:crossoverdistortion

Wehavecreatedadistortion

problem,buthowwelldid

weimprovetheefficiency

Crossoverdistortionproblem

V

t

t

vout

vin

vin

OPERATIONALAMPLIFIERDESIGN/STAGETHREE(CLASSBAMPLIFIER)

vin

RL

vo

VSS

VDD

M1

M2

ac

av

P

P =ClassBefficiency

/ 2

0

/ 2

0

2

2

2

1sin

1 2sin

2

278.5%

2 4

TD D

av D D

L

TD D

D D

L

D D

L

D D

L

D D

L

VP V t d t

T R

VV t dt

T R T

V

R

V

R

V

R

===

= = =

(half cycle)

1

OPERATIONALAMPLIFIERDESIGN

/STAGE

THREE

(CLASS

BAMPLIFIER)

ClassBAmplifier:adeviceconductsforonly180 out

of360 possibleduringfullcycle improvedefficiency

Howisworks:

Whenvin ispositive

M1turnson;M2turnsoff

IDS1 chargesvout

Whenvin isnegative

M1turnsoff;M2turnson

IDS2 dischargesvout

M1onwhenM2offandviceversa;pushpullaction

vin

RL

vo

VSS

VDD

M1

M2

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OPERATIONALAMPLIFIERDESIGN

/STAGE

THREE

(CLASS

AB

AMPLIFIER)

vin

RL

vo

VSS

VDD

M1

M2

WecanfixdistortionproblemintheclassBamplifier

whilestillgettingimprovedefficiencybycombiningthe

ClassAandClassBamplifiers

VGG/2cankeepboth

transistorsslightlyonatall

times.

VGG ischosen

to

create

asmallQcurrentinM1,and

M2,butnosomuchQ

currentthattheefficiency

dropsdrastically.

VGG/2

VGG/2

End of Lecture 20

SHORTCIRCUITPROTECTIONIftheoutputofafollowerisaccidentallyshortedtoground,the

transistorcanbedestroyedduetohighcurrentandhighpower

dissipation.

Rsenses

current

supplied

by

Q1.

If

IE1R=VBE2,Q2turnsonanddirectsbase

currentawayfromQ1toprotectitfrom

supplyingtoomuchcurrenttoshort

currentatoutput.

vi

VCC

R

vo

Ri

RL

Q1

Q2IE1

Tomakeopampsasrobust as

possible,circuitryisoftenaddedto

theoutputstagetoprovide

protectionfromshortcircuits.

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