DESIGNOFLOW-VOLTAGE,LOW-POWER OPERATIONALAMPLIFIERCELLS THE KLUWER INTERNATIONAL SERIES IN ENGINEERING AND COMPUTER SCIENCE ANALOG CIRCUITS AND SIGNAL PROCESSING ConsultingEditor Mohammed Ismail Ohio State University RelatedTitles: CHARACTERIZATIONMETHODSFORSUBMICRONMOSFETs,editedbyHishamHaddara ISBN:0-7923-9695-2 LOW-VOLTAGELOW-POWERANALOGINTEGRATEDCIRCUITS,editedbyWouterSerdijn ISBN: 0-7923-9608-1 INTEGRATED VIDEO-FREQUENCYCONTINUOUS-TIME FILTERS: High-Performance RealizationsinBiCMOS,ScottD.Willingham,KenMartin ISBN: 0-7923-9595-6 FEED-FORWARDNEURALNETWORKS:VectorDecompositionAnalysis,ModellingandAnalog Implementation, Anne-JohanAnnema ISBN: 0-7923-9567-0 FREQUENCY COMPENSATION TECHNIQUES LOW-POWER OPERATIONAL AMPLIFIERS,RuudEaschauzier,JohanHuijsing ISBN: 0-7923-9565-4 ANALOG SIGNAL GENERATION FOR BIST OF MIXED-SIGNAL INTEGRATED CIRCUITS,GordonW.Roberts,AlbertK.Lu ISBN: 0-7923-9564-6 INTEGRATEDFIBER-OPTICRECEIVERS,AaronBuchwald,KennethW.Martin ISBN: 0-7923-9549-2 MODELINGWITHANANALOGHARDWAREDESCRIPTIONLANGUAGE, H.AlanMantooth,MikeFiegenbaum ISBN: 0-7923-9516-6 LOW-VOLTAGECMOSOPERATIONALAMPLIFIERS: Theory, DesignandImplementation, SatoshiSakurai,MohammedIsmail ISBN: 0-7923-9507-7 ANALYSISANDSYNTHESISOFMOSTRANSLINEARCIRCUITS,RemcoJ.Wiegerink ISBN: 0-7923-9390-2 COMPUTER-AIDEDDESIGNOFANALOGCIRCUITSANDSYSTEMS,1.Richard Carley, RonaldS.Gyurcsik ISBN: 0-7923-9351-1 HIGH-PERFORMANCECMOSCONTINUOUS-TIMEFILTERS,JoseSilva-Martinez,Michiel Steyaert,WillySansen ISBN: 0-7923-9339-2 SYMBOLICANALYSISOFANALOGCIRCUITS:TechniquesandApplications,LawrenceP. 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Catalogue record for this book is available from the Library of Congress ISBN 978-1-4419-5165-6 ISBN 978-1-4757-2489-9 (eBook) DOI 10.1007/978-1-4757-2489-9 Printed on acid-free paper All Rights Reserved © 1996 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1996. Softcover reprint of the hardcover 1st edition 1996 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. Third Printing 2000. This printing is a digital duplication of the original edition. Preface ix Listofsymbols xi 1 Introduction..................................................................................................1 1.1 Low-voltage low-power CMOScircuits 2 1.2 Designissues 2 1.3 Organization ofthework 3 1.4 References 4 2 Low-VoltageAnalogDesign Considerations 5 2.1 Introduction 5 2.2 ClassificationofCMOS low-voltagecircuits 6 2.3 Electrical properties ofMOS transistors 7 2.3.1 Stronginversion 7 2.3.2 Weakinversion ll 2.3.3 Moderate inversion 12 2.4 Rail-to-railsignals 13 2.5 Rail-to-rail stages 15 2.6 Conclusions 17 2.7 References 17 3 InputStages 19 3.1 Introduction 19 3.2 Singledifferential inputstage 20 3.3 Rail-to-rail inputstage 27 3:4 Constant-g.,rail~to-railinputstages 35 3.4.1 Rail-to-rail inputstages withcurrent-based gmcontrol 36 3.4.2 Rail-to-rail inputstages withvoltage-based gmcontrol., 51 3.4.3 Rail-to-rail inputstage withWoverLbasedgmcontrol 57 3.5 Conclusions 60 3.6 References 63 Low-VoltageLow-PowerCMOSOperationalAmplifierCells v 4 OutputStages 65 4.1 Introduction 65 4.2 Common-sourceoutputstage 66 4.3 Class-AB outputstages 68 4.3.1 Feedforwardclass-AB output stages 72 4.3.2 Feedbackclass-AB output stage 79 4.4 Conclusions 85 4.5 References 86 5 OverallTopologies 89 5.1 Introduction 89 5.2 Single-stageamplifier 90 5.3 Two-stage amplifiers 95 5.3.1 Millercompensation 96 5.3.2 Millerzerocancellation 108 5.3.3 CascodedMillercompensation Ito 5.3.4 Nested cascodedMillercompensation 115 5.3.5 Comparisonoffrequencycompensation methods 119 5.4 Three-stageamplifiers 120 5.4.1 Nested Millercompensation 120 5.4.2 Multipath nested Millercompensation 127 5.5 Four-stageoperational amplifiers 131 5.5.1 Hybrid nested Millercompensation 132 5.5.2 Multipath hybrid nested Millercompensation 138 5.6 Conclusions 141 5.7 References 143 6 Realizations 147 6.1 Introduction 147 6.2 3-Vcompactoperational amplifiers 149 6.2.1 Topology of thecompactopamp 149 6.2.2 Input stage with gmcontrol bythree-times currentmirrors .154 6.2.3 Inputstage with gmcontrol by anelectronic zener 165 6.2.4 Inputstage with gmcontrol byaone-timecurrentmirror 172 vi Low-VoltageLow-PowerCMOS OperationalAmplifierCells 6.2.5 Input stage with gmcontrol by multiple inputpairs 175 6.2.6 Conclusions 181 6.3 1.5-Voperational amplifiers 182 6.3.1 Overall designs 187 6.3.2 Measurementresults 187 6.3.3 Conclusions 194 6.4 Fully differential operational amplifiers 195 6.4.1 3-Vone-stageoperational amplifier 196 6.4.2 3-Vtwo-stageoperational amplifier 197 6.4.3 2-Vfour-stage operational amplifier 199 6.5 Conclusions 202 6.6 References 203 INDEX 205 Low-VoltageLow-PowerCMOSOperationalAmplifierCells vii Preface This book addresses the design and realization of low-voltage low-power CMOS operational amplifier cells. It is the result of a Ph.D. project performed at the Delft University of Technology, The Netherlands. The book discusses all the circuit parts necessary to realize low-voltage low powerCMOS operational amplifiers,such asconstant-g.,rail-to-rail input stages, c1ass-AB rail-to-rail output stages and frequency compensation methods.In addition, several silicon realizations are treated.The contents of this book will be of particular interest to professional designers and graduate students. The reader is presumed to have basic knowledge of analog integrated circuit design. In thefirstchapterof this book, the motivation behind this book is given. The question is answered why low-voltage low-power CMOS operational amplifiers areneeded. Chapter 2 gives a classification of low-voltage circuits in terms of gate-source voltages and saturation voltages. Two groups of low-voltage circuits aredistinguished: low-voltageandextremelylow-voltagecircuits. The first group requires a minimum supply voltage of two gate-source voltages andtwosaturation voltages.Typicalvaluesliebetween 2and 3V. The second group requires a minimum supply voltage of one gate-source voltage and one saturation voltage.Typical values are between 1and 2V. Further some design considerations that are encountered in low-voltage design aredescribed inthischapter. Chapter3addresses the design of input stages.The properties of a conventional input pair as well as the properties of acomplementary rail to-rail input stage are discussed. To obtain a power-optimal frequency compensation a rail-to-rail input stage should have a constant transconductance. Toachieve this several rail-to-rail input stages having a constant transconductance are designed.Constant transconductance input stages which areable tooperate inweakorstrong inversion aredescribed. Chapter 4treatsthedesign oflow-voltagepower-efficientc1ass-AB output stages.Two classes of c1ass-ABcontrol circuits are distinguished: feedforward and feedback control. The feedforward c1ass-AB control Low-Voltage Low-PowerCMOSOperationalAmplifierCells ix circuits are able to operate on low supply voltages while the feedback control circuits are able torun under extremelylowsupply voltages. Chapter5discussesoverall topologies andfrequency compensation techniques of operational amplifiers. The first section of this chapter is dedicated tothe high-frequencyperformanceofaone-stageamplifier.The next sections discuss several frequency compensation techniques for two, three, and four-stage operational amplifiers. Chapter 6 discusses the design and realization of several complete operational amplifiers. The amplifiers are constructed using the circuit parts describedinthepreviouschapters. Sixdifferent implementationsofa 3-V compact rail-to-rail two-stage operational amplifier are discussed.In addition two versions of a 1.5-V four-stage operational amplifier are discussed.At the end of this chapterthree different operational amplifiers with adifferential input and output are discussed. Ron Hogervorst Johan H.Huijsing Delft, July 12, 1996 x Low-VoltageLow-PowerCMOS OperationalAmplifierCells Listofsymbols Symbol Quantity Unit p transconductancefactor AJV2 y bulk-thresholdparameter INI/2 0 gate electricfieldparameter IN A,R bandwidthreductionfactor 2Ns Il chargecarriermobility cm ~ source-drainelectricalfield parameter urn/V Ij>f flat-band voltage V 0- real partofthe complex frequency rad/s co polefrequency radls COu unity-gainfrequency rad/s Ao DC open-loopgain dB AT closed-loopvoltagegain cgs gate-sourcecapacitance F CMRR common-moderejection ratio dB C capacitor F C Millercapacitor F M C load capacitor F L Cox normalizedoxidecapacitance F/m2 f frequency Hz .a-I gm transconductance .a-I go outputconductance ld draincurrent A lpull pull current A lpush push current A Low-VoltageLow-PowerCMOSOperationalAmplifierCells xi