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OperationalAmplifier
Part3
Dr. M. J. AkhtarDept. of Electrical Engineering
IIT Kanpur
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Twoimportantpropertyforanalyzingidealopampcircuits
+12V 1. v vi1
v1
v2
vo
-12V 1 2. i2
e npu s e opamp appears o e e a s or an an opencircuit
simultaneously !
This
property
does
not
hold
under
certain
conditions
such
as
openloop,
positivefeedback
2
or eopamp ssa ura e .
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Integrator/v R
C
+VCC
s
VOR
S
-VCC
( )S O O SC V t v dt R dt RC
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Differentiator SdvC
R
Sdv
+VCCCdt
VO
VS
-VCC
( )O S SOv v
C V t RC R dt dt
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Log Amplifier V/v R
D ( 1)T
SI I e
+VCC
s
( 1)
O
T
V
VS
vI e
VOR
VS
-VCC
O
T
V
VSv
SI R
ln 1 lnS Sv v
V V V
5S SRI RI16/03/2015
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Temperature Sensor ?
+ CC
VOR
VS ln( ) ;S B
O T T
S
V V V
RI q
-VCC
But IS is a function of temperature as well.
D
CC
VO
VSR2
2
ln( )SO T
S
V VR I
-VCC616/03/2015
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D
+VCC
VS
1
1ln( )O T
S
V V R I
-VCC
1O1
f
R
D
VO
R
+VCC
VSf f f
R R R
-VCC
O2
V
01 02O
fR R R R
2
2
nO T
SR I
02 01f
OV V VR 2 1ln( ) ln( )f S S
O T
S SV VR R I R I 716/03/2015
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D 2lnf S S
O T
R V R IV V
+VCC
VS
1
1
ln( )SO T
S
VV V
R I
1 S S
-VCC
1O1
f
R
D
VO
R
+VCC
VS
-VCC
O2
2 ln( )S
O T
VV V
2
1
ln( )f
O TV V
R R
2 S
BT
k TV
Output voltage is directly proportional to
temperature816/03/2015
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AntiLog AmplifierS
V
R1
S
TV
I I e ( 1)TVsI I e
+D
VS
O
S
T
V
VOV
- CC SR
S S
T T
V V
V V 9
O S S
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Multiplier1 (ln( ) ln( ))
X YO T
S S
V VV V
RI RI
+VCC
D
ln( )XOX TS
VV V
RI
1 ln( )X Y
O T
V VV V
-VCC
RX
VOXR
R
S
D
R VO1
+VCC
VO
+VCC
RVY
VOY
-VCC
1O OX OY V V V
-VCC
ln( )YOY T
S
VV V
RI
1O
T
V
V X YV V
SRI 1016/03/2015
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Difference Amplifier3R
Rf
RIN = R1
23 1
1(1 )
o sv v R R
R
1
1( )f
s
R
vVS1
V
R2
3
32 1
Choose R R 2 11
( )o s sv v vR
A drawback is that input resistance is relatively Lower
Need to be checked input resistance
To change gain, we have to change two resistors and a slight
m sma c can ras ca y re uce common mo e re ec on ra o
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Instrumentation Amplifier
R4
A1
VS1 1 1 2o sv v i R 1sv
R3
R2
i
R3
31
R2
VO1 2 1s s
v v
A2
V
4 2 13
( )o o ov v vR 2Sv
2 2 2o sv v i R
4R 4R 3R 3R
R R
122 1 23
( 2 )o s s
v v v iRR
2 13 1o s s
v v vR R
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Instrumentation Amplifier
R4
A1
VS1 1 1 2o sv v i R 1sv
R3
R2
i
R3
31
R2
VO1 2 1s s
v v
A2
V
4
2Sv
2 2 2o sv v i R
4 22R R 2 13 1
o s SR R
Very high input Resistance
Can change one resistor R1 and change gain 1316/03/2015
