Newnes Circuits Manual Series Audio IC Circuits Manual R.M. Marston CMOS Circuits Manual R.M. Marston Diode, Transistor & FET Circuits Manual R.M. Marston Electronic Alarm Circuits Manual R.M. Marston Instrumentation & Test Gear Circuits Manual R.M. Marston Op-amp Circuits Manual R.M. Marston Optoelectronics Circuits Manual R.M. Marston Power Control Circuits Manual R.M. Marston Timer/Generator Circuits Manual R.M. Marston Op-amp Circuits Manual Including OTA circuits R. M. Marston H N E W N ES To Alex, with best wishes Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford 0X2 8DP A member of the Reed Elsevier group OXFORD MUNICH BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON First published 1989 Reprinted 1993 ©R. M. Marston 1989 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data Marston, R. M. (Raymond Michael) 1937- Op-amps circuits manuals 1. Electronic equipment - Integrated circuits. Operational amplifiers I. Title 621.381735 ISBN 0 7506 0998 2 Printed and bound in Great Britain by Redwood Books, Trowbridge, Wiltshire To Alex, with best wishes Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford 0X2 8DP A member of the Reed Elsevier group OXFORD MUNICH BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON First published 1989 Reprinted 1993 ©R. M. Marston 1989 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data Marston, R. M. (Raymond Michael) 1937- Op-amps circuits manuals 1. Electronic equipment - Integrated circuits. Operational amplifiers I. Title 621.381735 ISBN 0 7506 0998 2 Printed and bound in Great Britain by Redwood Books, Trowbridge, Wiltshire Preface The operational amplifier (op-amp) is a direct-coupled high-gain differential amplifier that can readily be used as the basis of a variety of ac or dc amplifiers, instrumentation circuits, oscillators, tone generators and sensing circuits. It is one of the most popular and versatile 'building blocks' used in modern electronic circuit design and is available in three basic forms - the standard (741, etc.) type, the Norton (LM3900, etc.) type, and the operational transductance amplifier (OTA) (CA3080 and LM13600, etc.) types. This book explains how these devices work and shows how to use them in practical applications. The book is divided into ten chapters and presents a total of over 300 practical circuits, diagrams and tables. The first six chapters deal with the operating principles and applications of the standard op-amp of the 741 (etc.) type. The remaining four chapters deal with special types of op-amp, such as the Norton amplifier (Chapter 7), the OTA (Chapters 8 and 9), and the LM 10 op-amp/reference IC (Chapter 10). The book is specifically aimed at the practical design engineer, technician and experimenter, but will be of equal interest to the electronics student and the amateur. It deals with its subject in an easy-to-read, down-to-earth, non- mathematical but very comprehensive manner. Each chapter starts off by explaining the basic principles of its subject and then goes on to present the reader with a wide range of practical circuit designs. Throughout the book great emphasis is placed on practical 'user' information and circuitry, and the book abounds with useful circuits and data. All of the ICs and other devices used in the practical circuits are modestly priced and readily available types, with universally recognized type numbers. R. M. Marston 1 Basic principles and configurations An operational amplifier (op-amp) can be simply described as a high-gain direct-coupled voltage amplifier 'block' with a single output terminal but with both inverting and non-inverting input terminals, enabling the device to function as either an inverting, non-inverting, or differential amplifier. Op- amps are very versatile devices. When coupled to suitable feedback networks they can be used to make precision ac and dc amplifiers and filters, oscillators, and level switches and comparators, etc. Circuits of all these types are described in this volume. Three basic types of operational amplifier are currently available. The most important and best known of these is the conventional voltage-in voltage-out op-amp (typified by the 741 and 3140, etc.), and the first six chapters of this volume are devoted to looking at the operating principles and applications of this type of device. The other two basic types of op-amp are the current-differencing or Norton op-amp (typified by the LM3900 quad amplifier integrated circuit), which is particularly useful in applications using single-ended power supplies, and the voltage-in current-out variable-gain operational transconductance amplifier (OTA) (typified by the CA3080 and LM13600 ICs), which is particularly useful in voltage-controlled amplifier (VCA), voltage-controlled filter (VCF), and voltage-controlled oscillator (VCO) applications. Chapters 7 to 9 give detailed descriptions of these two families of op-amps. Finally, one unique member of the op-amp family is an integrated circuit (IC) known as the LM10, which houses a special op-amp and a precision voltage reference in an 8-pin T05 package; this device is fully described, complete with sixty-five 'application' circuits, in Chapter 10. 1 2 Basic principles and configurations Op-amp basics In its simplest form, a conventional op-amp consists of a differential amplifier (bipolar or FET) followed by offset compensation and output stages (as shown in Figure 7.7), all integrated on to a single chip and housed in an IC package. The differential amplifier has a high-impedance (constant-current) tail, to give a high-input impedance and good common-mode signal rejection to its two input terminals, and has a high-impedance collector (or drain) load, to give high signal-voltage gain (typically about 100 dB). +ve supply rail Offset compensation stage Inverting input • Out Non-inverting input (0 V)« -ve supply rail Figure 1.1 Simplified op-amp equivalent circuit The output of the differential amplifier is fed to the circuit's output stage via an offset compensation network which, when the op-amp is suitably powered, causes the output to centre on zero volts when both input terminals are tied to zero volts. The output stage takes the form of a complementary emitter follower, and gives a low-impedance output. Op-amps are represented by the standard symbol shown in Figure 1.2(a). They are normally powered from split supplies, as shown in Figure 1.2(b), providing positive, negative and common (zero volt) supply rails, enabling the op-amp output to swing either side of the zero volts value and to be set at zero when the differential input voltage is zero. They can, however, also be powered from single-ended supplies if required. Basic principles and configurations 3 +ve supply Inverting « _ input Non-inverting · 1 + input (a) -ve supply (b) Figure 1.2 (a) Basic symbol and (b) supply connections of an op-amp Basic configurations The output signal voltage of an op-amp is proportional to the differential signal voltage between its two input terminals and, at low audio frequencies, is given by: tout = A(e-e) 0 l 2 where A is the low frequency open-loop voltage gain of the op-amp (typically 0 100 dB, or χ 100,000, e is the signal voltage at the non-inverting input x terminal, and e is the signal voltage at the inverting input terminal. 2 Thus, an op-amp can be used as a high-gain inverting ac amplifier by grounding its non-inverting terminal and feeding the input signal to its inverting terminal via C\ and R as shown in Figure 1.3(a), ox it can be used l9 as a non-inverting ac amplifier by reversing the two input connections as shown in Figure 13(b), or as a differential amplifier by feeding the two input signals to the op-amp as shown in Figure 1.3(c). Note in the latter case that if identical signals are fed to both input terminals the op-amp should, ideally, give zero signal output. The voltage gains of the Figure 1.3 circuits depend on the individual op- amp open-loop voltage gains and on the actual frequencies of the input signals. Figure 1.4 shows the typical frequency response graph of the well known type 741 op-amp. Note that the device gives a low-frequency (below 10 Hz) voltage gain of 106 dB, but that the gain falls off at a 6 dB/octave (20 dB/decade) rate at frequencies above 10 Hz, and eventually falls to unity (0 dB) at anfy 'unity gain transition' frequency of 1 MHz. This graph is typical of most modern op-amps, although individual op-amp types may offer different value of low-frequency gain and fy. One special application of the 'open-loop' op-amp is as a differential voltage comparator, one version of which is shown in Figure 1.5(a). Here, a 4 Basic principles and configurations +ve (c) Differential ac amplifier Figure 1.3 Methods of using the op-amp as a high-gain open-loop ac amplifier +120 Frequency, Hz Figure 1.4 Typical frequency response curve of the 741 op-amp fixed reference voltage is applied to the inverting terminal and a variable test or sample voltage is fed to the non-inverting terminal. Because of the very high open-loop voltage gain of the op-amp, the output is driven to positive saturation (close to the positive rail value) when the sample voltage is more than a few hundred microvolts above the reference voltage, and to negative Basic principles and configurations 5 Supply +ve t +ve saturation Differential {e\ - <?2) input Reference 0.1 0.2 0.3 X voltage Sample voltage -ve saturation 1 Supply -ve 0 V (a) (b) Figure 1.5 (a) Circuit and (b) transfer characteristics of a simple differential voltage comparator saturation (close to the negative supply rail value) when the sample is more than a few hundred microvolts below the reference value. Figure 1.5(b) shows the voltage transfer characteristics of the above circuit. Note that the output voltage magnitude is determined by the magnitude of the differential input voltage; thus, if a 2 V reference is used and a differential voltage of only 200 μν is needed to swing the output from a negative to a positive saturation level, this change can be caused by a shift of only 0.01% on a 2 V signal applied to the sample input. This circuit thus functions as a precision voltage comparator or balance detector. Closed-loop amplifiers The best way of using an op-amp as an ac amplifier is to connect it in the closed-loop mode, with negative feedback applied from output to input as shown in the circuits of Figure 1.6, so that the overall gain of each circuit is precisely determined by the values of the external feedback components almost irrespective of the individual op-amp characteristics (provided that the open-loop gain, A , is large relative to the closed-loop gain, A). Note from 0 the graph of Figure 1.4 that the signal bandwidth of such circuits equals the IC'sfy value divided by the circuit's closed-loop voltage gain value. Thus, the 741 gives a bandwidth of 100 kHz when the gain is set at χ 10 (20 dB), or 1 kHz when the gain is set at χ 1000 (60 dB). Figure 1.6(a) shows the op-amp wired in the closed-loop mode as a fixed- gain inverting ac amplifier. The voltage gain (A) is determined by the ratios of /?! and R, and equals R/Ri, and the circuit's input impedance equals the 2 2 Ri value. This circuit can thus easily be designed to give any desired gain and input impedance values.