Third edition
`
`A practical
`MOC GtOia®
`electronic circuits
`
`MARTIN HARTLEY JONES
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`Published by the Press Syndicate of the University of Cambridge
`The Pitt Building, Trumpington Street, Cambridge CB2 1RP
`40 West 20th Street, New York, NY 10011-4211, USA
`10 Stamford Road, Oakleigh, Melbourne 3166, Australia
`
`© Cambridge University Press 1977, 1985, 1995
`
`First published 1977
`Reprinted 1978, 1979, 1980, 1981 (with corrections), 1982, 1983
`Second edition 1985
`Reprinted 1987, 1988, 1990, 1992, 1993
`Third edition 1995
`
`Printed in Great Britain at the University Press, Cambridge
`
`A Catalogue record for this book is available from the British Library
`
`Library of Congress cataloguing in publication data
`
`Jones, Martin Hartley, 1942-
`A practical introduction to electronic circuits / Martin Hartley Jones. —3rd ed.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN 0-521-47286-5. — ISBN 0-521-47879-0 (pbk.)
`1. Electronic circuits.
`I. Title.
`TK7867.J62
`1995
`621.3815—dc20
`95-4035 CIP
`
`ISBN 0 521 47286 5 hardback
`ISBN 0 521 47879 0 paperback
`
`Coverillustration from radar processing circuit.
`Courtesy Kelvin Hughes Ltd.
`
`wy
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`
`
`Il
`Integrated circuit analogue building
`bricks
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`11.1 Introduction
`
`The integrated circuit (IC) is clearly the building brick of electronic circuits.
`We have already had a flavour of IC applications when looking at power
`supply regulators. Now we turn towards the full range of IC capabilities. A
`glance through a componentdistributor’s catalogue reveals a seemingly limit-
`less range of ICs for virtually every conceivable function. An IC may contain
`from a dozen transistors to a million depending on the application, together
`with all necessary resistors, diodes etc. The intimate thermal connection
`achieved by fabricating all the components on one chip ofsilicon generally
`leads to excellent stability and predictability in use.
`_ The understanding ofdiscrete components gained from earlier chapters will
`be found essential to the proper interfacing of ICs: in fact even today very few
`analogue circuit applications dispense totally with discrete semiconductors.
`However, the IC designer has today relieved the circuit designer of much of
`the ‘donkey work’. In addition, the small size and low power consumption
`of ICs has made possible products like the Camcorder and hand-held GPS
`position-fixing satellite receiver.
`This chapter deals with applications of linear ICs. They are designed to
`handle analogue signals, which carry their information in terms of amplitude
`and waveshape. Most audio and radio signals come into this category; they
`are distinct from the standard binary pulses of digital circuits which are dis-
`cussed in chapter 13. To give an idea of the scope of this chapter, fig. 11.1
`includes many of the basic building bricks which will be discussed and pro-
`vides a quick reference to the outline circuits.
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`The operational amplifier
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`273
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`11.2 The operational amplifier
`
`11.2.1 Simplifying assumptions
`All the circuits of fig. 11.1 make use of an operational amplifier (op amp).
`The term ‘operational’ is generally used nowadays to describe a high-gain
`voltage amplifier, particularly one in IC form; the name is derived from the
`original use of such amplifiers in analogue computing operations. The charac-
`teristics of an op ampare such that the following simplifying assumptions can
`be made in most practical circuits:
`infinite open-loop voltage gain, Ayo, (typically 2 x 10°)
`infinite input impedance (typically 2 MQ)
`zero output impedance (typically 75 2)
`
`The parameter values quoted aboverefer to the popular 741 type IC ampli-
`fier which is used in many of the practical circuits in this book.
`
`11.2.2 Input bias current and offset voltage
`Theinputterminals of an op amp connect to internal transistor bases or gates
`which must be given some d.c. reference and be able to draw a small bias
`current if the amplifier is to function (there are no coupling capacitors on the
`chip). Input bias current in the 741 amplifier is about 100 nA. Thefirst design
`consideration, therefore, is that each input of any IC amplifier must have. some
`sort of d.c. path to earth, even if it is through a high-value resistor.
`Ideally, both the inverting and non-inverting inputs should ‘see’ the same
`resistance to earth; otherwise, as fig. 11.2 shows, an effective input offset
`voltage will appear. We can assumethat the two input bias currents are equal,
`i.e.
`
`I,=1,.
`
`Hence, if R,=R,, V, and V, will be equal and there will be zero effective
`differential offset voltage (V.—V,) at the amplifier inputs. In most circuits,
`the inverting input will normally have a feedback resistor R; connected through
`to the output as in fig. 11.3 and therefore a proportion of the bias current will
`be drawn from the output through R, Now,if our circuit is designed correctly
`so that the offset voltage is zero, then the output will be at 0 V level under
`quiescent conditions. This means that, as far as the input bias current is
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`274
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`Integrated circuit analogue building bricks
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`(a)
`
`(6)
`
`
`; F
`
`ig. 11.1. (a) Non-inverting amplifier. (b) Voltage follower. (c) Inverting amplifier.
`(d) Adder. (e) Integrator. (f) Differentiator.
`
`9(R,+Rj)
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`2
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`The operational amplifier 275
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`
`
`R
` —k logoat)
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`Fig. 11.1. (g) Differential amplifier (subtractor). (4) Precision rectifier. (() Current
`to voltage converter. (j) Multiplier. (k) Divider. (2) Logarithmic amplifier.
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`The operational amplifier
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`277
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`The difference between the two input bias currents is known as the input
`offset current and typically amounts to 20 nA in the 741. It is therefore not
`worth bothering to match the input resistance to better than 20% orso.
`Yetanother factor to be considered is that, irrespective of external voltages
`at the inputs, the IC itself has a small inherent input offset voltage; in the
`741, for example, it is about 1 mV. Bias and offset currents of the order of
`tens of nanoamps (10° A) sound very small until it is realized that the resist-
`ance seen by each inputin a typical circuit may be some hundreds of thousands
`of ohms. A current of 100 nA flowing in 100 kQ produces a potential differ-
`ence of 10 mV, which may be comparable to the input signal.
`Moreserious than the actual value of the resultant offset is the fact that the
`bias and offset currents vary with temperature. The offset current drift in a
`741 is typically 1 nA/deg C which can result in output voltage drift.
`For these reasons it is advisable to set an upper limit of 500 kQ on the
`resistor value used from each input to earth on a 741. Good industrial design
`practice would restrict the maximum value to 50 kQ for better d.c. stability.
`If higher value input resistors are required, op amps with much smaller bias
`currents are available, though at higher cost than the 741. The LM308 IC uses
`very high gain (‘super-8’) input transistors and achieves a typical input bias
`current of 7 nA and an input offset current of only 1.5 nA.
`For much lowerinput currents, circuits with FET inputs are needed. In this
`category is the Texas TLO71 IC which has an input bias current of 30 pA and
`an input offset current of 5 pA, enabling input resistances as high as 100 MQ
`to be used. The use of FET inputs involves somesacrifice in inherent input
`offset voltage, this being typically 3 mV for the TLO71 compared with the
`1 mV of the 741. This is primarily due to the difficulty of producing two
`identically matched FETs; this fact also leads to the input offset voltage being
`more sensitive to temperature than in a bipolar circuit.
`
`11.2.3 Offset null circuit
`
`From the discussion above, it should be clear that it is virtually impossible
`to build an amplifier without an offset voltage at the input. In other words,
`even if there is no external input signal on the amplifier, it will be sitting with
`a few millivolts d.c. input of its own making. In a high-gain amplifier, this
`can be serious: if the overall voltage gain is 1000, one millivolt input offset
`appears as one volt at the output.
`If the amplifier is intended for a.c. operation only and uses a coupling
`capacitor at the output, this will cut out any d.c. offset and all will be well,
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