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Chapter 7
Operational-Amplifier
and its Applications
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Outline
Introduction
The 741 Op-Amp Circuit
The ideal Op Amp
The inverting configuration
The noninverting configuration
Integrator and differentiator
The antoniou Inductance-simulation Circuit
The Op Amp-RC Resonator Bistable Circuit
Application of the bistable circuit as a comparator
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Introduction
Analog ICs include operational amplifiers, analogmultipliers, A/D converters, D/A converters, PLL,etc.
A complete op amp is realized by combininganalog circuit building blocks.
The bipolar op-amp has the general purposevariety and is designed to fit a wide range of
specifications. The terminal characteristics is nearly ideal.
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The 741 Op-Amp Circuit
General description
The input stage
The intermediate stage
The output stage
The biasing circuits Device parameters
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General Description
24 transistors, few resistors and only one
capacitor
Two power supplies
Short-circuit protection
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The Input Stage
The input stage consists of transistors Q1 throughQ7.
Q1-Q4 is the differential version of CC and CB
configuration. High input resistance.
Current source (Q5-Q7) is the active load of inputstage. It not only provides a high-resistance load
but also converts the signal from differential tosingle-ended form with no loss in gain orcommon-mode rejection.
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The Intermediate Stage
The intermediate stage is composed ofQ16, Q17and Q13B.
Common-collector configuration forQ16gives this
stage a high input resistance as well as reduces theload effect on the input stage.
Common-emitter configuration forQ17provideshigh voltage gain because of the active load Q13B.
Capacitor Cc introduces the miller compensationto insure that the op amp has a very high unit-gainfrequency.
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The Output Stage
The output stage is the efficient circuit called class ABoutput stage.
Voltage source composed ofQ18 and Q19 supplies the DCvoltage forQ14 and Q20 in order to reduce the cross-over
distortion.
Q23 is the CC configuration to reduce the load effect onintermediate stage.
Short-circuit protection circuitry
Forward protection is implemented by R6and Q15.Reverse protection is implemented by R7, Q21, current
source(Q24, Q22) and intermediate stage.
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The Output Stage
(a) The emitter follower is a class A output stage. (b) Class B output stage.
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The Output Stage
Wave of a class B output stage fed
with an input sinusoid.
Positive and negative cycles are
unable to connect perfectly due to the
turn-on voltage of the transistors.
This wave form has the nonlinear
distortion called crossover distortion.
To reduce the crossover distortion can
be implemented by supplying theconstant DC voltage at the base
terminals.
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The Biasing Circuits
Reference current is generated by Q12, Q11 andR5.
Wilder current provides biasing current in theorder ofA.
Double-collector transistor is similar to the two-output current mirror. Q13Bprovides biasingcurrent for intermediate stage, Q13A for outputstage.
Q5, Q6and Q7is composed of the current source tobe an active load for input stage.
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Device Parameters
Fornpn transistors:
Forpnp transistors:
Nonstandard devices:
Q14 and Q20 each has an area three times that of a standarddevice.
VVAI As 125,200,1014 !!! F
VVAI As 50,50,1014 !!! F
AISA141025.0 v! AISA
141075.0
v!
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The Ideal Op Amplifier
symbol for the op amp
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The Ideal Op Amplifier
The op amp shown connected to dc power supplies.
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Characteristics of the Ideal Op
Amplifier
Differential input resistance is infinite.
Differential voltage gain is infinite.
CMRR is infinite.
Bandwidth is infinite. Output resistance is zero.
Offset voltage and current is zero.
a) No difference voltage between inverting
andnoninvertingterminals.b) No inputcurrents.
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Equivalent Circuit of the Ideal Op
Amp
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The Inverting Configuration
The inverting closed-loop configuration.
Virtual ground.
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The Inverting Configuration
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The Inverting Configuration
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The Inverting Configuration
Shunt-shunt negative feedback
Closed-loop gain depends entirely on passive
components and is independent of the op
amplifier.
Engineer can make the closed-loop gain as
accurate as he wants as long as the passive
components are accurate.
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The Noninverting Configuration
The noninverting configuration.
Series-shunt negative feedback.
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The Voltage follower
(a) The unity-gain buffer or follower amplifier.
(b) Its equivalent circuit model.
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The Weighted Summer
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The Weighted Summer
)()())(())((4
4
3
3
2
2
1
1R
RvR
RvR
R
R
RvR
R
R
Rvv cc
b
ca
b
cao !
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A Single Op-Amp Difference
Amplifier
Linear amplifier.
Theorem of linear
Superposition.
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A Single Op-Amp Difference
Amplifier
Application of superposition
Inverting configuration
1
1
21 Io v
R
Rv !
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A Single Op-Amp Difference
Amplifier
Application of superposition.
Noninverting configuration.
2
34
4
1
22 )(1( Io v
RR
R
R
Rv
!
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Integrators
The inverting configuration with general impedances in
the feedback and the feed-in paths.
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The Inverting Integrators
The Miller or inverting integrator.
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Frequency Response of the
integrator
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The op-amp Differentiator
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The op-amp Differentiator
Frequency response of a differentiator with a time-constant CR.
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The Antoniou Inductance-
Simulation Circuit
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The Antoniou Inductance-
Simulation Circuit
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The Op amp-RC Resonator
An LCR second order resonator.
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The Op amp-RC Resonator
An op ampRC resonator obtained by replacing the inductorL in the LCR
resonator of a simulated inductance realized by the Antoniou circuit.
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The Op amp-RC Resonator
Implementation of the buffer amplifierK.
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The Op amp-RC Resonator
Pole frequency
Pole Q factor
25316460 11 RRRRCCLC !![
531
2
4
66660
RRRR
C
CRRCQ !![
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Bistable Circuit
The output signal only has two states: positive
saturation(L+) and negative saturation(L-).
The circuit can remain in either state indefinitely
and move to the other state only whenappropriate triggered.
A positive feedback loop capable of bistable
operation.
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Bistable Circuit
The bistable circuit (positive
feedback loop)
The negative input terminal of the opamp connected to an input signal vI.
Foo vRR
Rvv !
!
21
1
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Bistable Circuit
The transfer characteristic of
the circuit in (a) for increasing vI.Positive saturation L+ and
negative saturation L-
F!LVTH
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Bistable Circuit
The transfer characteristic
for decreasing vI.
F
!LVTL
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Bistable Circuit
The complete transfer characteristics.
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A Bistable Circuit with Noninverting
Transfer Characteristics
21
1
21
2
RR
Rv
RR
Rvv oI
!
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A Bistable Circuit with Noninverting
Transfer Characteristics
The transfer characteristic is
noninverting.
21
21
RRLV
RRLV
TL
TH
!
!
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Application of Bistable Circuit as a
Comparator
Comparator is an analog-circuit building block
used in a variety applications.
To detect the level of an input signal relative to a
preset threshold value.
To design A/D converter.
Include single threshold value and two threshold
values. Hysteresis comparator can reject the interference.
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Application of Bistable Circuit as a
Comparator
Block diagram representation and transfer characteristic for acomparator having a reference, or threshold, voltage VR.
Comparator characteristic with hysteresis.
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Application of Bistable Circuit as a
Comparator
Illustrating the use of
hysteresis in thecomparator
characteristics as a
means of rejecting
interference.
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Making the Output Level More
Precise
For this circuit L+ = VZ1+ VD and L= (VZ2
+ VD), where VD is the forward
diode drop.
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Making the Output Level More
Precise
For this circuit L+ = VZ+ VD1+ VD2
and L= (VZ+ VD3+ VD4
).
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Generation of Square Waveforms
Connecting a bistable multivibrator with inverting transfer characteristics in a
feedback loop with an RC circuit results in a square-wave generator.
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Generation of Square Waveforms
The circuit obtained when the bistable multivibrator is
implemented with the positive feedback loop circuit.
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Waveforms at various nodes of
the circuit in (b).
This circuit is called an astable
multivibrator.
Time period T = T1+T2
F
F
!
1
)1ln1
LLRCT
F
F
!
1
)1ln2
LLRCT
F
F
!
1
1ln2RT
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Generation of Triangle Waveforms
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Generation of Triangle Waveforms