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MALVINO & BATES
SEVENTH EDITION
Electronic
PRINCIPLES
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Linear Op-Amp Circuits
ChapterChapter2020
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Topics Covered in Chapter 20
• Inverting amplifier circuits
• Noninverting amplifier circuits
• Inverter/noninverter circuits
• Differential amplifiers
• Instrumentation amplifiers
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Topics Covered in Chapter 20
(Continued)
• Summing amplifier circuits
• Current boosters
• Voltage-controlled current sources
• Automatic gain control
• Single-supply operation
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Inverting amplifier circuits
• One of the most basic circuits
• Voltage gain equals the ratio of the
feedback resistance to the input
resistance
• Example circuits include: ac coupled and
adjustable bandwidth
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ac coupled inverting amplifier
vout
vin
R1
Rf
RL
C1
C2
AV =-Rf
R1 Rf
R1
+1
funityf2 =
fc1 = 2ππππR1C1
1fc2 = 2ππππRLC2
1
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Rf
Adjustable bandwidth
voutvin
R1
RAV
f
-Rf
R1
R1 R + Rf
R1 RB =
f2 = Bfunity
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Noninverting amplifier circuits
• One of the most basic circuits
• Advantages include: stable voltage gain,
high input impedance, and low output
impedance
• An example circuit is an ac coupled
noninverting amplifier
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R1
Rf
ac coupled noninverting amplifier
C3
C2
C1
vout
R2
RL
vin
fc3 = 2ππππR1C3
1
Bdc = 1
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Inverter / noninverter circuit
• When an input signal drives both inputs,
inverting and noninverting amplification
result at the same time
• Total voltage gain equals voltage gain of
the inverting channel plus voltage gain of
the noninverting channel
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R
JFET controlled reversible gain
voutvinR
R
0 V
VGS(off)
Low gate: AV = 1
High gate: AV = −1
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R
Adjustable gain of ± 1
voutvin
R
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R’
Phase shifter
vout
vin
R
R’
C
φφφφ
φ φ φ φ = -2 arctan f/fc
t
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R2
Differential amplifier
voutvin
R1’
R1
R2’
vin(CM)
vin(CM)
CMRR limiting factors
are the op amp itself and
the tolerance of the resistors.
AV =-R2
R1
±2 < AV(CM) < ±4∆∆∆∆R
R∆∆∆∆R
R
Ideally:
R1 = R1’
R2 = R2’
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R2
Wheatstone bridge circuit
vout
R1
R4
R3
Transducer
+V
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Wheatstone bridge
• The differential output signal is small.
• The common-mode output signal is large.
• Differential amplifiers are a good match.
• Input transducers convert nonelectrical
quantities into an electrical quantity.
Examples:
� photoresistor: light to resistance
� thermistor: temperature to resistance
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Instrumentation amplifiers
• Differential amplifiers optimized for dc
performance
• Large differential voltage gain
• High CMRR
• Low input offsets
• Low temperature drift
• High input impedance
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R
vout
vin
R
R
R
vin(CM)
vin(CM)
±2∆∆∆∆R
RAV(CM) =AV =
2R1
RG
+1
R1
R1
RG
Instrumentation amplifier
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Monolithic instrumentation
amplifiers
• Use laser-trimmed resistors for high performance.
• Resistor RG is external and is selected to set the differential gain.
• Resistor RG can be split into two devices for guard driving (bootstrapping the cable shield to the common-mode potential).
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RG
2
RG
2
vout
Guard driving
Guard voltage (common-mode voltage)
Instrumentation
amplifier.
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Summing amplifier circuits
• Examples include: summing on both
inputs and D/A converter
• D/A converter is used in multimeters to
measure voltages, currents, and
resistances
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Summing amp with inverting
and noninverting inputs
vout
R1Rf
R2
R3
R4
v1
v2
v3
v4
R5
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D/A converter
vout
R R
2R
4R
8R
v3
v2
v1
v0
vout = -(v3 + 0.5v2 + 0.25v1 + 0.125v0)
Possible combinations = 2N = 24 = 16
N = 4
vin18
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Current booster
• Used when short-circuit output current
of one amp is too low
• Used on the output side of the circuit
• Typically, a transistor whose base
current is supplied by the op amp
• Output current increased by the β factor
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Unidirectional current booster
R2
R1 RL
vin
+VCC
vout
741
Imax = ββββdcISC = 100(25 mA) = 2.5 A
ββββdc = 100
ISC = 25 mA
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Bidirectional current booster
R2
R1
RL
vin
+VCC
vout
-VEE
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Voltage controlled current sources
• Input voltage controlled current sources can be configured
• Loads can be floating or grounded
• Load currents can be unidirectional or bi-directional
• The Howland current source is a bi-directional voltage controlled current source
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AGC circuit
vout
R1vin
R5
R6
R3
R4
R2
+VAGC
-VEE
As the signal level increases,
VAGC goes more positive.
As the JFET rds drops,
the input signal is attenuated.
Voltage-controlled
resistance
(rds)
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Single-supply operation
• Op amps typically use dual supplies.
However some applications are better
suited to a single supply.
• Some op amps are optimized for single
supply operation.
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Single-supply inverting amplifier
vout
vin
R1
R2
RL
C1
C2
R
+VCC
RC3
+VCC
2