Download - Lecture20(20-3-11)
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7/27/2019 Lecture20(20-3-11)
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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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