Feedback Circuits and Op. Amps

196 Pages·1990·5.11 MB·English

by  D. H. Horrocks (auth.)

Feedback Circuits
. and Op. Amps
TUTORIAL GUIDES IN ELECTRONIC ENGINEERING
Series editors
Professor G.G. Bloodworth, University of York
Professor A.P. Dorey, University of Lancaster
Professor J.K. Fidler, University of York
This series is aimed at first and second year undergraduate courses. Each text is
complete in itself, although linked with others in the series. Where possible, the
trend towards a 'systems' approach is acknowledged, but classical fundamental
areas of study have not been excluded. Worked examples feature prominently and
indicate, where appropriate, a number of approaches to the same problem.
A format providing marginal notes has been adopted to allow the authors to
include ideas and material to support the main text. These notes include references
to standard mainstream texts and commentary on the applicability of solution
methods, aimed particularly at covering points normally found difficult. Graded
problems are provided at the end of each chapter, with answers at the end of the
book.
1. Transistor Circuit Techniques: discrete and integrated (2nd edition)
G.J. Ritchie
2. Feedback Circuits and Op. Amps (2nd edition) - D.H. Horrocks
3. Pascal for Electronic Engineers (2nd edition) - J. Attikiouzel
4. Computers and Microprocessors: components and systems (2nd edition) -
A.C. Downton
5. Telecommunication Principles (2nd edition) - J.J. O'Reilly
6. Digital Logic Techniques: principles and practice (2nd edition)
T.J. Stonham
7. Transducers and Interfacing: principles and techniques - B.R. Bannister and
D. G. Whitehead
8. Signals and Systems: models and behaviour - M.L. Meade and C.R. Dillon
9. Basic Electromagnetism and its Applications - A.J. Compton
10. Electromagnetism for Electronic Engineers - R.G. Carter
11. Power Electronics - D.A. Bradley
12. Semiconductor Devices: how they work - J.J. Sparkes
13. Electronic Components and Technology: engineering applications -
S.J. Sangwine
14. Optoelectronics - J. Watson
15. Control Engineering - C. Bissell
16. Basic Mathematics for Electronic Engineers: models and applications -
J .E. Szymanski
17. Software Engineering - D. Ince
Feedback Circuits
and Op. Amps
Second edition
D.H. Horrocks
Senior Lecturer, School of Electrical,
Electronic and Systems Engineering,
University of Wales College of Cardiff
CHAPMAN AND HALL
University and Professional Division
LONDON. NEW YORK • TOKYO. MELBOURNE. MADRAS
UK Chapman and Hall, 2-6 Boundary Row, London SEI 8HN
JAPAN Chapman and Hall Japan , Thomson PublishingJapan,
Hirakawacho Nemoto Building, 7F, 1-7'11 Hirakawa-cho,
Chiyoda-ku, Tokyo 102
AUSTRALIA Chapman and Hall Australia, Thomas Nelson Australia,
102 Dodds Street, South Melbourne 3205
INDIA Chapman and Hall India, R. Seshadri, 32 Second Main
Road, CIT East, Madras 600 035
First edition 1983
Reprinted 1984, 1985
Second edition 1990
Reprinted 1991
© 1983,1990 D.H. Horrocks
Typeset in Singapore by Coiset Pte Ltd
ISBN-13: 978-0-412-34270-7 e-ISBN-13: 978-94-011-7609-5
DOl: 10.1007/978-94-011-7609-5
ISSN 0 266 2620
All rights reserved. No part ofthis publication may
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means, electronic, mechanical, photocopying, recording
or otherwise, or stored in any retrieval system of any
nature, without the written permission of the copyright
holder and the publisher, application for which shall
be made to the publisher.
The publisher makes no representation, express or
implied, with regard to the accuracy of the information
contained in this book and cannot accept any legal
responsibility or liability for any errors or omissions
that may be made_
British Library Cataloguing in Publication Data
Horrocks, D_H. (David H.)
Feedback circuits and op. amps. - 2nd ed.
1. Electronic equipment. Feedback circuits
I. Title
621.381535
ISBN-13: 978-0-412-34270-7
Preface
Feedback circuits in general, and op. amp. applications which embody feedback
principles in particular, playa central role in modern electronic engineering. This
importance is reflected in the undergraduate curriculum where it is common practice
for first-year undergraduates to be taught the principles of these subjects. It is right
therefore that one of the tutorial guides in electronic engineering be devoted to
feedback circuits and op. amps.
Often general feedback circuit principles are taught before passing on to op.
amps., and the order of the chapters reflects this. It is equally valid to teach op.
amps. first. A feature of the guide is that it has been written to allow this approach
to be followed, by deferring the study of Chapters 2, 4 and 5 until the end.
A second feature of the guide is the treatment of loading effects in feedback
circuits contained in Chapter 5. Loading effects are significant in many feedback
circuits and yet they are not dealt with fully in many texts.
Prerequisite knowledge for a successful use of the guide has been kept to a
minimum. A knowledge of elementary circuit theory is assumed, and an under
standing of basic transistor circuits would be useful for some of the feedback circuit
examples.
I am grateful to series editor Professor G.G. Bloodworth for many useful discus
sions and suggestions. I am also grateful for the expert typing of Mrs. B. Richards
and for help in preparing solutions to the probl~ms given by Mr. R. Hor. Finally, I
thank my wife, Chris, and son, Jim, for their forbearance.
With this second edition the opportunity has been taken to enhance the first
edition in several ways. Chapter 9 has been added to extend significantly the range
of op. amp. circuit applications covered by the tutorial guide. Further applications
of feedback and op. amps. can be explored through the bibliography which has been
included. The text makes some use of phasors and the j-notation with which some
readers may not be familiar. To help such readers, an appendix introducing these
topics is provided.
v
Contents
Preface v
1 Introduction
2 General Properties of Feedback Amplifiers 4
Basic definitions and equations 4
Positive and negative feedback 7
Reduced sensitivity to gain variations 11
Reduction of noise and distortion 13
Multiple-loop feedback 15
3 Amplifiers Without Feedback 21
Amplifier models 21
Connection of signal-source, amplifier and load 26
Frequency response effects 29
Differential amplifiers 39
4 Feedback Amplifier Circuits 47
The four feedback circuit configurations 47
Effect of feedback on input and output impedance 49
Shunt-voltage feedback circuit example 56
Series-voltage feedback circuit example 59
Series-current feedback circuit example 61
Shunt-current feedback circuit example 64
5 More About Feedback Amplifiers 69
Loading effects between forward amplifier and feedback block 69
Effect on feedback amplifier of source and load impedances 82
Frequency response of feedback amplifiers 88
Instability 90
6 The Ope Amp. - Basic Ideas and Circuits 97
What is an operational amplifier? 97
Inverting voltage amplifier 100
Circuits based on the inverting voltage amplifier 105
Non-inverting voltage amplifier 108
Power supply connections to the op. amp. 111
7 Ope Amp. Non-idealities 115
The importance of op. amp. non-idealities 115
Offset voltages 115
Bias currents and offset currents 120
Op. amp. frequency response 124
Slew rate and full-power bandwidth 126
vi
8 Selected Ope Amp. Applications 131
Precision difference and instrumentation amplifiers 131
Analogue computation 135
Wien bridge oscillator 139
The inverse function principle 142
Triangle wave/square wave generator 145
9 Further Ope Amp. Applications 149
Circuits derived from the precision difference amplifier 149
A.C. amplifiers 156
Active filters 161
Precision rectifier circuits 170
Appendix: Steady-State Network Analysis using Phasors and
Complex Variables 176
Bibliography 184
Answers to Numerical Problems 185
Index 187
vii
Introduction
Many examples of feedback can be found in everyday life. One example is the
temperature regulation of a heated room, shown schematically in Fig. 1.1. The
heater supplies heat to the room and the temperature of the room, TM, is sensed.
This is fed back and compared with the desired temperature, To. The difference, TE
= To - TM (called the error signal), is passed to the regulator. The regulator uses
the error signal to control the heater to maintain the room temperature close to the
desired temperature. This automatic control technique is an example of negative
feedback, so called because, in generating the error signal, the function of the
comparator is to subtract the measured temperature from the desired temperature.
The subject of this tutorial guide is the application of feedback principles to
electronic amplifying circuits containing active devices such as transistors. The use
of integrated circuit transistor amplifiers, called op. amps. (an abbreviation of
operational amplifiers), is particularly important and is considered in some detail.
The elements of a negative feedback amplifier system are shown in Fig. 1.2. The
amplifying circuit uses active devices, normally transistors, to increase the
magnitude of the electronic signal applied to its input. The input signal to the
amplifier comes from a comparator which subtracts from the external input signal a
fraction {3 of the amplifier output signal. If the amplifier has high gain it requires
only a small signal at its input. Therefore the external input signal is approximately
equal to the fraction {3 of the output signal. It follows that the output signal must be
approximately equal to the input signal divided by {3. This technique gives a system
with a stable overall gain if the components in the feedback circuit have stable
values. This is relatively easy to achieve using passive components such as resistors. A very significant result.
The amplifying circuit itself does not need to have stable gain (but the gain must be
high). This is of importance because the stable and predictable gain of systems is
highly desirable for most electronic systems, but active devices so far invented do
(T'-
Error Signal
TD- TM Temperature
Desired sensor
Temperature Regulator Heater Room
To
Measured
temperature T M
------------- ----------- --------
Feedback path
Fig. 1.1 Temperature regulation of a heated room.
Forward path
Mixer
Input Amplifying -'" Output
signal---·' circuit signal
Proportion {3 of output signal
(
Feedback
circuit
•
Feedback path
Fig. 1.2 Operating principle of a negative feedback amplifier system.
not have stable characteristics. Fortunately the requirement for the feedback system
that the amplifier should have high gain is readily and cheaply achieved nowadays
with transistors, particularly in integrated circuit form.
It should be noted that the stability of gain provided by negative feedback is not
merely stability with the passage of time. Negative feedback also reduces the
variations of g~n with environmental conditions such as temperature, with
manufacturing component spreads, and with the frequency and amplitude of the
signals.
From Fig. 1.2 it is apparent that if a fraction {3 of the output signal is added to the
input signal the overall gain is increased. This is called positive feedback.
During the early development of thermionic valve amplifiers positive feedback
was used to increase the gain because valves were expensive and had low
performance.
Non-linearities are also present A serious problem of valve amplifiers was distortion of signals caused by non
in modern transistors. linearities, that is to say variation of gain with signal amplitude in the valves. In 1927
H.S. Black proposed the use of negative feedback for the first time, to reduce
Black conceived the idea 'in a
flash' while commuting to work distortion in valve amplifiers. This discovery is seen in retrospect to have been one of
on a ferry across the Hudson the most significant ideas in the 20th century and has led to important advances in
river, IEEE Spectrum, December
engineering. From it has growjl whole disciplines such as control engineering, and
1977, pp. 56-60.
the idea has been used by biologists, economists and others, to understand and
model the operation of systems in their disciplines. However, it is particularly in
electronic engineering that the idea is all-pervasive as transistors can provide high
gain very cheaply, but cannot provide stable gain. This book shows that negative
feedback not only stabilizes gain but also has other advantages such as enabling
input and output impedances to be controlled, and bandwidth to be extended.
In Chapter 2 the general properties of feedback amplifiers and some of the main
advantages of negative feedback are analysed. An introduction to the analysis of
multi-loop systems is also included.
2