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MALVINO & BATES
SEVENTH EDITION
Electronic
PRINCIPLES
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Nonlinear Op-Amp Circuits
ChapterChapter2222
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Topics Covered in Chapter 22
• Comparators with zero reference
• Comparators with non-zero references
• Comparators with hysteresis
• Window comparator
• The integrator
• Waveform conversion
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Topics Covered in Chapter 22
(Continued)
• Waveform generation
• Another triangular generator
• Active diode circuits
• The differentiator
• Class D amplifier
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Comparators with zero reference
• Called zero-crossing detectors
• Diode clamps used to protect
comparators against large inputs
• Usually interface their outputs with
digital circuits
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Inverting comparator
vout
vin
+VCC
-VEE
Diode clamps
for protection
AVOL ≅ ≅ ≅ ≅ 100,000
vin
vout
+Vsat
-Vsat
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Inverting comparator with input bias compensation
vout
vin
+VCC
-VEE
RB
RB
The trip point of a zero-crossing
detector should be near zero volts.
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Bounded output zero-crossing detector
vout
vin
+VCC
-VEE
vin
vout
+VZ + 0.7
-VZ - 0.7
..
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Comparators with
non-zero references
• Sometimes called limit detectors
• Op amps may be used
• IC comparators are optimized by
removing the internal compensating
capacitor which increases switching
speed
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Non-inverting threshold detector
vout
vin
+VCC
-VEE
vin
vout
+Vsat
-Vsat
R2
R1
R2
R1+R2
Vref = (−VEE)
Vref
.
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IC comparators
• Op amps have speed limitations due to
the internal compensation capacitor.
• Comparator ICs are optimized for non-
linear operation so the capacitor is not
needed.
• Comparator ICs often have open-
collector outputs to make interfacing
more flexible.
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vout
+V
Using a pullup resistor with an open-collector output stage
IC comparator
pullup
resistor
When the
transistor
is off, the
resistor pulls
vout up to +V.
.
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Comparators with hysteresis
• Noise is any unwanted signal not related
to the input
• Noise can cause false triggering
• Positive feedback creates hysteresis
• Hysteresis prevents false triggers and
speeds up switching
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The Schmitt trigger uses positive feedback
voutvin
+VCC
-VEE
vin
vout
+Vsat
-Vsat
R1
R2
LTP
UTP
R1
R1+R2
B =
UTP = BVsat
LTP = -BVsat
H = 2BVsat
.
.
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vin
vout
+Vsat
-Vsat
vin
vout
+Vsat
-Vsat
LTP
UTP
Comparing a zero-crossing detector to a
Schmitt trigger when both have noisy input signals.
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Window comparator
• Called a double-ended limit detector
• Detects when the input voltage is between
two limits
• Uses two comparators with two different
trip points
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Window comparator operation
voutvin
+VCC
-VEE
+VCC
-VEE
RL
LTP
HTP
D1
D2
vin
vout
+Vsat
LTP HTP
Output is low
when input is
in window
. . .
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Integrator
• Useful for converting rectangular pulses
into linear ramps
• Earliest part of exponential charge is
used
• Output ramps are almost perfect
• Used to create time bases for
oscilloscopes
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Integrator circuit
vout
vinC
R
≥≥≥≥10R
vout
V = vin
-T
RC
V
T
vin
0
0
..
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Waveform conversion
• A Schmitt trigger can be used to convert
a sine wave to a rectangular wave
• An integrator can convert a square wave
to a triangular wave
• The duty cycle of a limit detector can be
controlled with an adjuster resistor
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Rectangular to
triangular converter
vout
vinC
R
≥≥≥≥10R
vout
+VP
-VP
vin
vout(pp) = 2fRC
VP
..
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Triangle to pulse converter
vout
vin
+VCC
vin
vout
R2
R1
R2
R1+R2
Vref = VCC
Vref
.
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Waveform generation
• Oscillators require no external input and
still generate an output signal
• Oscillators work because of positive
feedback
• A relaxation oscillator uses a charging
capacitor
• A triangular waveform is produced by
cascading a relaxation oscillator and an
integrator
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Relaxation
oscillator vout
R2
R
vout
vC
R1
C
UTP
LTP
T = 2RCln1+B
1-B
.
.
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R2
R3
R1
C1vout
C2
R4
R5
Triangular waveform generator
.
..
..
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R2
R1
vout
CR3
R4
Another triangular waveform generator
R1
R2
VsatUTP =
H = 2UTP Vout(pp) = HR2
4R1 R3 Cf =
Schmitt trigger and cascaded integrator
.. .
.
..
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Active half-wave rectifier
vin
RL
An op amp with negative feedback allows the rectification
of signals with amplitudes less than the diode knee voltage.
VK(CL) =VK
AVOL
.
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Active peak detector with reset
vin
RL
Reset
RLC > 10T
C
. . .
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Active positive clamper
vin
RL
An op amp with negative feedback allows the clamping
of signals with amplitudes less than the diode knee voltage.
0
0..
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Differentiator
• When a square wave drives an RC
differentiator, the output is a series of
narrow positive and negative voltage
spikes
• Using an op amp will improve the
differentiator and provide a low output
impedance
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0.01R to 0.1R
R
C
Differentiator circuit and operation
vin
vout
..
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Class D amplifier
• Uses output transistors as switches
• These transistors are alternately driven
into saturation and cutoff by the output
of a comparator
• Provides very high circuit efficiencies
• Popular in portable equipment