《电路设计技术与技巧(英文第三版)》.pdf

The Circuit DesignersCompanionThird EditionPeter WilsonAMSTERDAM?BOSTON?HEIDELBERG?LONDON?NEW YORK?OXFORDPARIS?SAN DIEGO?SAN FRANCISCO?SINGAPORE?SYDNEY?TOKYONewnes is an imprint of ElsevierNewnes is an imprint of ElsevierThe Boulevard,Langford Lane,Kidlington,Oxford,OX5 1GB,UK225 Wyman Street,Waltham,MA 02451,USAFirst edition 1991Second edition 2005Third edition 2012Copyright?2012,2005,1991 Elsevier Ltd.All rights reserved.No part of this publication may be reproduced or transmitted in any form or by any means,electronic ormechanical,including photocopying,recording,or any information storage and retrieval system,withoutpermission in writing from the publisher.Details on how to seek permission,further information about thePublishers permissions policies and our arrangement with organizations such as the Copyright ClearanceCenter and the Copyright Licensing Agency,can be found at our website: book and the individual contributions contained in it are protected under copyright by the Publisher(other than as may be noted herein).NoticesKnowledge and best practice in this field are constantly changing.As new research and experience broadenour understanding,changes in research methods,professional practices,or medical treatment may becomenecessary.Practitioners and researchers must always rely on their own experience and knowledge in evaluating andusing any information,methods,compounds,or experiments described herein.In using such information ormethods they should be mindful of their own safety and the safety of others,including parties for whom theyhave a professional responsibility.To the fullest extent of the law,neither the Publisher nor the authors,contributors,or editors,assume anyliability for any injury and/or damage to persons or property as a matter of products liability,negligence orotherwise,or from any use or operation of any methods,products,instructions,or ideas contained in thematerial herein.British Library Cataloguing in Publication DataA catalogue record for this book is available from the British LibraryLibrary of Congress Control Number:2011940053ISBN:978-0-08-097138-4For information on all Newnes publications visit ourwebsite at Printed and bound in the United Kingdom12 13 14 1510 9 8 7 6 5 4 3 2 1IntroductionWhenIwasfirstapproachedtoproduceathirdeditionofTheCircuitDesignersCompanion,Iwasatfirstreluctantto“messwith it”.It is rare to have a companion book that is not just a textbook,or a handbook,but isseen in many respects to contain all the essential information that a“real”circuit designerneedstonot only produce a working circuit,but to enable that designer to understand all the related topics thatmake the design robust,tolerant to noise and temperature,and able to operate in the system that it wasdesigned for.This book is a rare example of just that,and there is no other comparable text that providessuch a broad range of design skills to be passed on to the next generation of circuit designers.It is interesting to note that twenty-one years on from the original edition of this book there is nodiminutionofdemandfor analogandmixedsignaldesignskills,however,most universitiesand collegesstillteachasyllabusinelectronicsthatisdominatedbydigitaldesign techniques.ThecommentmadebyTim in the introduction to the first edition that analogelectronics were“hard”and therewas a reluctanceto embark on analog electronics could have been written this year,rather than two decades ago!Duringthe revision of this book,it was also interesting to note that much of the content was still completelyvalid in todays electronic systems,albeit some of the individual technology elements have of coursemoved on,with many of the fundamental concepts being essentially the same.Peter WilsonINTRODUCTION TO THE FIRST EDITION(TIM WILLIAMS,1990)Electronic circuit design can be divided into two areas:the first consists in designing a circuit that willfulfil its specified function,sometimes,under laboratory conditions;the second consists in designing thesame circuit so that every production model of it will fulfil its specified function,and no other undesiredand unspecified function,always,in the field,reliably over its lifetime.When related to circuit designskills,these two areas coincide remarkably well with what engineers are taught at college basic circuittheory,Ohms law,The venin,Kirchhoff,Norton,Maxwell and so on and what they learn on the job that there is no such thing as the ideal component,that printed circuits are more than just a collection oftracks,and that electrons have an unfortunate habit of never doing exactly what theyre told.This book has been written with the intention of bringing together and tying up some of the looseends of analog and digital circuit design,those parts that are never mentioned in the textbooks andrarely admitted elsewhere.In other words,it relates to the second of the above areas.Its genesis camewith thegrowing frustration experienced as a senior design engineer,attempting torecruit people for junior engineer positions in companies whose foundations rested on analog designexcellence.Increasingly,it became clear that the people I and my colleagues were interviewing hadonly the sketchiest of training in electronic circuit design,despite offering apparently sound degree-level academic qualifications.Many of them were more than capable of hooking together a micro-processor and a few large-scale functional block peripherals,butwere floored by simple questions suchas the nature of the pn junction or how to go about resistor tolerancing.It seems that this experience isby no means uncommon in other parts of the industry.The colleges and universities can hardly be blamed for putting the emphasis in their courses on theskills needed to cope with digital electronics,which is after all becoming more and more pervasive.Ifxiiithey are failing industry,then surely it is industrys job to tell them and to help put matters right.Unfortunately it is not so easy.A 1989 report from Imperial College,London,found that few studentswere attracted to analog design,citing inadequate teaching and textbooks as well as the subject beingfound“more difficult”.Also,teaching institutions are under continuous pressure to broaden theircurriculum,to produce more“well-rounded”engineers,and this has to be at the expense of greater in-depth coverage of the fundamental disciplines.Nevertheless,the real world is obstinately analog and will remain so.There is a disturbing tendencyto treat analog and digital design as two entirely separate disciplines,which does not result in goodtraining for either.Digital circuits are in reality only over-driven analog ones,and anybody who hasa good understanding of analog principles is well placed to analyze the more obscure behavior of logicdevices.Even apparently simple digital circuits need some grasp of their analog interactions to bedesigned properly,as Chapter 6 of this book shows.But also,any product which interacts with theoutside world via typical transducers must contain at least some analog circuits for signal conditioningand the supply of power.Indeed,some products are still best realized as all-analog circuits.JimWilliams,a well-known American linear circuit designer(who bears no relation to the author of thefirst two editions of this book),put it succinctly when he said“wonderful things are going on in theforgotten land between ONE and ZERO.This is Real Electronics.”Because analog design appears to be getting less popular,those peoplewho do have such skills willbecome more sought-after in the years ahead.This book is meant to be a tool for any aspiring designerwho wishes to develop these skills.It assumes at least a background in electronics design;you will notfind in here more than a minimum of basic circuit theory.Neither will you find recipes for standardcircuits,as there are many other excellent books which cover those areas.Instead,there is a serioustreatment of those topics which are“more difficult”than building-block electronics:grounding,temperature effects,EMC,component sourcing and characteristics,the imperfections of devices,andhow to design so that someone else can make the product.I hope the book will be as useful to the experienced designer who wishes to broaden his or herbackground as it will to the neophyte fresh from college who faces a first job in industry with trepi-dation and excitement.The traditional way of gaining experience is to learn on-the-job through peercontact,and this book is meant to enhance rather than supplant that route.It is offered to those whowant their circuits to stand a greater chance of working first time every time,and a lesser chance ofbeing completely redesigned after six months.It does not claim to be conclusive or complete.Elec-tronic design,analog or digital,remains a personal art,and all designers have their own favorite tricksand their own dislikes.Rather,it aims to stimulate and encourage the quest for excellence in circuitdesign.I must here acknowledge a debt to the many colleagues over the years who have helped me towardsan understanding of circuit design,and who have contributed towards this book,some withoutknowing it:in particular Tim Price,Bruce Piggott and Trevor Forrest.Also to Joyce,who has patientlyendured the many brainstorms that the writing of it produced in her partner.INTRODUCTION TO THE SECOND EDITION(TIM WILLIAMS,2004)The first edition was written in 1990 and eventually,after a good long run,went out of print.But thedemand for it has remained.There followed a period of false starts and much pestering,and finally thexivIntroductionauthor was persuaded to pass through the book once more to produce this second edition.The aimremains the same but technology has progressed in the intervening fourteen years,and so a number ofanachronisms have been corrected and some sections have been expanded.I am grateful to those whohave made suggestions for this updating,especially John Knapp and Martin OHara,and I hope itcontinues to give the same level of help that the first edition evidently achieved.INTRODUCTION TO THE THIRD EDITION(PETER WILSON,2012)The third edition of the book has really been an exercise of revision rather than revolution,and I havetried to keep the philosophy the same as the original author intended.As with the second edition,theaim has been to update the technological aspects in the book,expand some sections and offer a slightlydifferent personal perspective to hopefully further enhance the book.I am very grateful toTim Williams for allowing me to make these revisions,and for his discussions about the booksprevious editions.I also acknowledge the advice,teaching and knowledge of many friends andcolleagues over the past three decades which have provided much insight into the art of analogelectronics,including my father,Tom Wilson,Frank Fisher while at Ferranti,Professor Alan Mantoothat the University of Arkansas,and Dr Neil Ross and Dr Reuben Wilcock,at the University ofSouthampton.I must also thank my wife,Caroline,who has tolerated my fascination with electronicsfor many years.I hope that further generations of electronic designers will find this edition useful andthat the book will continue to provide the assistance and help to circuit designers that the previouseditions have done over the last two decades.IntroductionxvGrounding and wiring1CHAPTER OUTLINE1.1 Grounding.2When to consider grounding.31.1.1 Grounding within one unit.41.1.2 Chassis ground.41.1.3 The conductivity of aluminum.5Other materials.61.1.4 Ground loops.61.1.5 Power supply returns.8Varying loads.9Power rail feed.10Conductor impedance.111.1.6 Input signal ground.11Connection to 0V elsewhere on the PCB.11Connection to 0V within the unit.11External ground connection.111.1.7 Output signal ground.13Avoiding the common impedance.131.1.8 Inter-board interface signals.14Partitioning the signal return.151.1.9 Star-point grounding.161.1.10 Ground connections between units.17Breaking the ground link.181.1.11 Shielding.19Which end to ground for LF shielding.19Electrostatic screening.20Surface transfer impedance.211.1.12 The safety earth.211.2 Wiring and cables.221.2.1 Wire types.22Wire inductance.22Equipment wire.23Wire-wrap wire.231.2.2 Cable types.241.2.3 Power cables.24CHAPTERThe Circuit Designers Companion.DOI:10.1016/B978-0-08-097138-4.00001-XCopyright?2012,2005,1991 Elsevier Ltd.All rights reserved.11.2.4 Data and multicore cables.25Data communication cables.26Structured data cable.26Shielding and microphony.271.2.5 RF cables.281.2.6 Twisted pair.291.2.7 Crosstalk.30Digital crosstalk.321.3 Transmission lines.33Transmission line effects.34Critical lengths for pulses.341.3.1 Characteristic impedance.341.3.2 Time domain.35Forward and reflected waves.35Ringing.36The Bergeron diagram.36The uses of mismatching.381.3.3 Frequency domain.38Standing wave distribution versus frequency.38Impedance transformation.39Lossy lines.40Understanding the transmission line impedance graphically.421.1 GROUNDINGA fundamental property of any electronic or electrical circuit is that the voltages present within it arereferenced to a common point,conventionally called the ground.This term is derived from electricalengineering practice,when the reference point is often taken to a copper spike literally driven into theground.This point may also be a connection point for the power to the circuit,and it is then called the0V(nought-volt)rail,and ground and 0Vare frequently(and confusingly)synonymous.Then,whenwe talk about a five-volt supply or a minus-twelve-volt supply or a two-and-a-half-volt reference,eachof these are referred to the 0 V rail.At the same time,ground is not the same as 0 V.A ground wire connects equipment to earth forsafety reasons,and does not carry a current in normal operation.However,in this chapter the word“grounding”will be used in its usual sense,to include both safety earths and signal and power returnpaths.Perhaps the greatest single cause of problems in electronic circuits is that 0 Vand ground are takenfor granted.The fact is that in a working circuit there can only ever be one point which is truly at 0 V;the concept of a“0 V rail”is in fact a contradiction in terms.This is because any practical conductorhas a finite non-zero resistance and inductance,and Ohms law tells us that a current flowing throughanything other than a zero impedance will develop a voltage across it.A working circuit will havecurrent flowing through those conductors that are designated as the 0 V rail and therefore,if any one2CHAPTER 1 Grounding and wiringpoint of the rail is actually at 0 V(say,the power supply connection)the rest of the rail will not be at0V.This can be illustrated with the example in Figure 1.1.Now,after such a trenchant introduction,you might be tempted to say well,there are millions ofelectronic circuits in existence,they must all have 0 V rails,they seem to work well enough,so whatsthe problem?Most of the time there is no problem.The impedance of the 0 V co