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Chapter 7
Operational-Amplifier
and its Applications
SJTU Zhou Lingling
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Outline
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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, analog
multipliers, A/D converters, D/A converters, PLL,
etc.
• A complete op amp is realized by combining
analog circuit building blocks.
• The bipolar op-amp has the general purpose
variety 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
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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 through
Q7.
• Q1-Q4 is the differential version of CC and CB
configuration.
• High input resistance.
• Current source (Q5-Q7) is the active load of input
stage. It not only provides a high-resistance load
but also converts the signal from differential to
single-ended form with no loss in gain or
common-mode rejection.
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The Intermediate Stage
• The intermediate stage is composed of Q16, Q17
and Q13B.
• Common-collector configuration for Q16 gives this
stage a high input resistance as well as reduces the
load effect on the input stage.
• Common-emitter configuration for Q17 provides
high voltage gain because of the active load Q13B.
• Capacitor Cc introduces the miller compensation
to insure that the op amp has a very high unit-gain
frequency.
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The Output Stage
• The output stage is the efficient circuit called class AB
output stage.
• Voltage source composed of Q18 and Q19 supplies the DC
voltage for Q14 and Q20 in order to reduce the cross-over
distortion.
• Q23 is the CC configuration to reduce the load effect on
intermediate stage.
• Short-circuit protection circuitry
 Forward protection is implemented by R6 and 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 the
constant DC voltage at the base
terminals.
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The Output Stage
QN and QP provides the voltage
drop which equals to the summer
of turn-on voltages of QN and QP.
This circuit is call Class AB
output stage.
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The Biasing Circuits
• Reference current is generated by Q12, Q11 and R5.
• Wilder current provides biasing current in the
order of μA.
• Double-collector transistor is similar to the twooutput current mirror. Q13B provides biasing
current for intermediate stage, Q13A for output
stage.
• Q5, Q6 and Q7 is composed of the current source to
be an active load for input stage.
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Device Parameters
• For npn transistors:
I s  10 14 A,   200,VA  125V
• For pnp transistors:
I s  10 14 A,   50,VA  50V
• Nonstandard devices:
I SA  0.25 10
14
A
I SA  0.75 10 14 A
Q14 and Q20 each has an area three times that of a standard
device.
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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
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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
and noninverting terminals.
b) No input currents.
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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
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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 Noninverting Configuration
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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
Ra Rc
Ra Rc
Rc
Rc
vo  v1 ( )( )  v2 ( )( )  v3 ( )  v4 ( )
R1 Rb
R2 Rb
R3
R4
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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
R2
vo1   vI 1
R1
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A Single Op-Amp Difference
Amplifier
Application of superposition.
Noninverting configuration.
vo 2  (1 
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R4
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）vI 2
R1 R4  R3
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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 amp–RC resonator obtained by replacing the inductor L 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 amplifier K.
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The Op amp-RC Resonator
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Pole frequency
0  1
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LC6  1 C4C6 R1R3 R5 R2
Pole Q factor
Q  0C6 R6  R6
C6 R2
C4 R1R3 R5
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Bistable Circuit
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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 when
appropriate 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 op
amp connected to an input signal vI.
R1
v  vo
 vo 
R1  R2
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Bistable Circuit
The transfer characteristic of
the circuit in (a) for increasing vI.
Positive saturation L+ and
negative saturation L-
VTH  L 
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Bistable Circuit
The transfer characteristic
for decreasing vI.
VTL  L 
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Bistable Circuit
The complete transfer characteristics.
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A Bistable Circuit with Noninverting
Transfer Characteristics
R2
R1
v  v I
 vo
R1  R2
R1  R2
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A Bistable Circuit with Noninverting
Transfer Characteristics
The transfer characteristic is
noninverting.
VTH   L（
 R1 R2）
VTL   L（
 R1 R2）
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Application of Bistable Circuit as a
Comparator
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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 a
comparator 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 the
comparator
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
1  （ L L )
T1  RC ln
1 
1  （L L )
T2  RC ln
1 
1 
T  2RC ln
1 
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Generation of Triangle Waveforms
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Generation of Triangle Waveforms
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