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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