Third Edition CMOS Analog Circuit Design Phillip E. Allen Professor Emeritus, Georgia Institute of Technology Douglas R. Holberg Consultant NewYork Oxford OXFORD UNIVERSITY PRESS Oxford University Press,Inc.,publishes works that further Oxford University’s objective of excellence in research,scholarship,and education. Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Copyright ©2012,2002 by Oxford University Press,Inc. For titles covered by Section 112 of the U.S. Higher Education Opportunity Act,please visit www.oup.com/us/he for the latest information about pricing and alternate formats. Published by Oxford University Press,Inc. 198 Madison Avenue,New York,NY 10016 www.oup.com Oxford is a registered trademark of Oxford University Press All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted,in any form or by any means, electronic,mechanical,photocopying,recording,or otherwise, without the prior permission of Oxford University Press. ISBN 978-0-19-976507-2 Printing number:9 8 7 6 5 4 3 2 1 Printed in the United States of America on acid-free paper PREFACE The objective of the third edition of this book continues to be to teach the design of CMOS analog circuits. The teaching of design reaches far beyond giving examples of circuits and showing analysis methods. It includes knowing the necessary fundamentals and background and applying them in a hierarchical manner that the novice can understand. Probably of most importance is to teach the concepts of designing analog integrated circuits in the context of CMOS technology. These concepts enable the reader to understand the oper- ation of an analog CMOS circuit and to know how to change its performance. In today's com- puter-oriented thinking, it is crucial to maintain personal control of a design, to know what to expect, and to discern when simulation results may be misleading. As integrated circuits become more complex, it is crucial to know “how the circuit works.” Simulating a circuit without the understanding of how it works can lead to disastrous results. How does the reader acquire the knowledge of how a circuit works? The answer to this question has been the driving motivation of this text beginning with the first edition. There are several important steps in this process. The first is to learn to analyze the circuit. This analysis should produce simple results that can be understood and reapplied in different cir- cumstances. The second is to view analog integrated-circuit design from a hierarchical view- point. This means that the designer is able to visualize how subcircuits are used to form circuits,how simple circuits are used to build complex circuits,and so forth. The third step is to set forth procedures that will help the new designer come up with working designs. This has resulted in the inclusion of many “design recipes,”which became popular with the first and second editions and have been enlarged in the third edition. It is important that the designer realize that there are simply three outputs of the electrical design of CMOS analog circuits. They are (1) a schematic of the circuit, (2) dc currents, and (3) W/L ratios and component values. Most design flows or “recipes” can be organized around this viewpoint very easily. Previous Editions The first edition of CMOS Analog Circuit Design published in 1987 was the first to present a hierarchical approach to the design of CMOS analog circuits. Since its introduction,it has xi xii PREFACE found extensive use in industry and classrooms worldwide. Of course, technology advances and methodologies mature,making it clear that the first edition needed revision. The second edition resulted from a unique blending of industry and academia. Between the period of the first and second editions (15 years),over 50 short courses were taught from the first edition to over 1500 engineers worldwide. In these short courses, the engineers demanded to understand the concepts and insights to designing analog CMOS circuits, and many of the responses to those demands were included in the second edition. In addition to the industrial input to the second edition, the authors have taught this material at Georgia Institute of Technology and the University of Texas at Austin. This experience provided insight that was included in the second edition from the viewpoint of students and their ques- tions. Moreover,the academic application of this material has resulted in a large body of new problems that were given as tests and included in the second edition. Third Edition The third edition has focused on cleaning up the material and removing that which is not used. Homework problems that were not effective have been removed and replaced by better prob- lems. This edition has introduced the idea of design problems. These problems give the desired specifications and a score for grading the problem. The reader is to do the design by hand and then use the computer to simulate the performance and extract the score. These are great vehicles for teaching the trade-off of optimizing the score versus the time spent. Also in this edition,answers to selected problems are found at the back of the book. Key changes to the third edition are as follows. • The technology in Chapter 2 has been updated and a new appendix created to give details on layout (Appendix B). • In Chapter 3 the large-signal MOS model has been extended to include velocity saturation. • In Chapter 4,the bandgap section has been updated and completely rewritten. • The cascode op amps in Chapter 6 have been updated and the enhanced-gain technique used to create op amps with ultra large voltage gains. • In Chapter 7, the differential-in, differential-out op amps have been updated and the material on output common-mode feedback expanded. • Chapter 9,on switched capacitor circuits,was removed and condensed into Appendix E. • A design illustration was presented in Section 10.5 to show the steps in designing an open-loop,buffered,sample-and-hold circuit. • Also included in Chapter 9 at the conclusion is the website to an Excel spreadsheet that has all published ADC converters from 1997 through 2010. This information is extremely useful for understanding the trends in converters. • Design problems have been introduced. These problems give the desired specifications and a score for grading the problem. The reader is to do the design by hand and then use the computer to simulate the performance and extract the score. These are great vehicles for teaching the trade-off of optimizing the score versus the time spent. • Readers of the previous editions have requested answers to the problems. In this edi- tion,answers to selected problems are found at the back of the book. Preface xiii Overview of the Chapters Unchanged from the second edition,the hierarchical organization of the third edition is illus- trated in Table 1.1-2. Chapter 1 presents the material necessary to introduce CMOS analog circuit design. This chapter gives an overview of the subject of CMOS analog circuit design, defines notation and convention,makes a brief survey of analog signal processing,and gives an example of analog CMOS design with emphasis on the hierarchical aspect of the design. Chapters 2 and 3 form the basis for analog CMOS design by covering the subjects of CMOS technology and modeling. Chapter 2 reviews CMOS technology as applied to MOS devices, pn junctions,passive components compatible with CMOS technology,and other components such as the lateral and substrate BJT and latch-up. Chapter 3 introduces the key subject of modeling, which is used throughout the remainder of the text to predict the performance of CMOS circuits. The focus of this chapter is to introduce a model that is good enough to pre- dict the performance of a CMOS circuit to within ±10% to ±20% and will allow the design- er insight and understanding. Computer simulation can be used to more exactly model the circuits but will not give any direct insight or understanding of the circuit. The models in this chapter include the MOSFET large-signal and small-signal models, including frequency dependence. In addition,how to model the noise and temperature dependence of MOSFETs and compatible passive elements is shown. This chapter also discusses computer simulation models. This topic is far too complex for the scope of this book,but some of the basic ideas are presented so that the reader can appreciate computer simulation models. Other models for the subthreshold operation are presented along with how to use SPICE for computer simula- tion of MOSFET circuits. Chapters 4 and 5 present the topics of subcircuits and amplifiers that will be used to design more complex analog circuits,such as an op amp. Chapter 4 covers the use of the MOSFET as a switch followed by the MOS diode or active resistor. The key subcircuits of current sinks/sources and current mirrors are presented next. These subcircuits permit the illustration of important design concepts such as negative feedback,design trade-offs,and matching prin- ciples. Finally, this chapter presents independent voltage and current references and the bandgap voltage reference. These references attempt to provide a voltage or current that is independent of power supply and temperature. Chapter 5 develops various types of amplifiers. These amplifiers are characterized from their large-signal and small-signal performance, including noise and bandwidth where appropriate. The categories of amplifiers include the inverter,differential,cascode,current,and output amplifiers. Chapters 6,7,and 8 present examples of complex analog circuits. Chapter 6 introduces the design of a simple two-stage op amp. This op amp is used to develop the principles of compensation necessary for the op amp to be useful. The two-stage op amp is used to for- mally present methods of designing this type of analog circuit. This chapter also examines the design of cascode op amps, particularly the folded-cascode op amp. This chapter concludes with a discussion of techniques to measure and/or simulate op amps. Chapter 7 presents the subject of high-performance op amps. In this chapter various performances of the simple op amp are optimized, quite often at the expense of other performance aspects. The topics include buffered output op amps,high-frequency op amps,differential-output op amps,low- power op amps,low-noise op amps,and low-voltage op amps. Chapter 8 presents the open- loop comparator,which is an op amp without compensation. This is followed by methods of designing this type of comparator for linear or slewing responses. Methods of improving the performance of open-loop comparators,including autozeroing and hysteresis,are presented. xiv PREFACE Finally,this chapter describes regenerative comparators and how they can be combined with low-gain, high-speed amplifiers to achieve comparators with a very short propagation time delay. Chapter 9 covers the topics of CMOS digital–analog and analog–digital converters. Digital–analog converters are presented according to their means of scaling the reference and include voltage, current, and charge digital–analog converters. Next, methods of extending the resolution of digital–analog converters are given. The analog–digital converters are divid- ed into Nyquist and oversampling converters. The Nyquist converters are presented accord- ing to their speed of operation—slow,medium,and fast. Finally,the subject of oversampled analog–digital and digital–analog converters is presented. These converters allow high reso- lution and are very compatible with CMOS technology. Five appendices cover the topics of circuit analysis methods for CMOS analog circuits, integrated circuit layout, CMOS device characterization (this is essentially Chapter 4 of the first edition),and time and frequency domain relationships for second-order systems. In addi- tion,an appendix that covers switched capacitor circuits is included. The material of the third edition is more than sufficient for a 15-week course. Depending on the background of the students, a 3-hour-per-week, 15-week-semester course could include parts of Chapters 2 and 3, Chapters 4 through 6, parts of Chapter 7, Chapter 8, and Chapter 9. At Georgia Tech, this text is used along with the fourth edition of Analysis and Design of Analog Integrated Circuits in a two-semester course that covers both BJT and CMOS analog IC design. Appendix E and Chapter 9 are used for about 70% of a semester course on analog IC systems design. The background necessary for this text is a good understanding of basic electronics. Topics of importance include large-signal models, biasing, small-signal models, frequency response,feedback,and op amps. It would also be helpful to have a good background in semi- conductor devices and how they operate, integrated-circuit processing, simulation using SPICE,and modeling of MOSFETs. With this background,the reader could start at Chapter 4 with little problem. Acknowledgments The authors would like to express their appreciation and gratitude to the many individuals who have contributed to the development of the third edition. These include both under- graduate and graduate students who have used the second edition and offered comments, suggestions, and corrections. It also includes the over 1600 industrial participants who, over the last 8 years, have attended a one-week course on this topic. We thank them for their encouragement,patience,and suggestions. We also appreciate the feedback and cor- rections from many individuals in industry and academia worldwide. In particular, the authors would like to thank the following individuals for providing useful feedback on the new edition. Dr. Ron Pyle,Independent Consultant Ka Y. Leung,Silicon Labs,Inc. Suat Ay,University of Idaho Degang Chen,Iowa State University Preface xv Yun Chui,University of Illinois at Urbana–Champaign Roman Genov,University of Toronto Michael Green,University of California,Irvine Dong S. Ha,Virginia Tech Timothy Horiuchi,University of Maryland Pedro Irazoqui,Purdue University Hongrui Jiang,University of Wisconsin–Madison Youngjoong Joo,University of Texas at San Antonio Aydin Karsilayan,Texas A&M University Bruce Kim,University of Alabama Eun Sok Kim,University of Southern California Ron Kneper,Boston University Boris Murmann,Stanford University Sameer Sonkusale,Tufts University Ashok Srivastava,Louisiana State University Jin Wang,University of South Florida Francis Williams,Norfolk State University The authors gratefully acknowledge the patience and encouragement of Caroline DiTullio, acquisitions editor of Engineering, Science, and Computer Science at Oxford University Press, during the development of the third edition and the firm but gentle shep- herding of the third edition through the production phase by the assistant editor, Claire Sullivan. Lastly,the assistance of Keith Faivre in helping with detail work associated with the third edition is greatly appreciated. Ancillaries There are several ancillaries (supplementary materials) that will help the instructor in using the third edition. They are listed below along with their impact. • Answers to the problems are at the end of the text. The students will now know whether their work is correct or not and will be able to interact with the instructor on homework questions more efficiently. • A complete solutions manual to all problems in PDF format. This will help in provid- ing solutions to the homework problems beyond just the answers at the back of the text. • A set of all figures in Microsoft PowerPoint format is available to help in preparing lectures. • Both authors maintain websites that provide resources that permit the downloading of short course lecture slides,short course schedules and dates,class notes,problems and solutions,etc.,in PDF format. More information can be found at: www.aicdesign.org (P. E. Allen) www.holberg.org (D. R. Holberg) xvi PREFACE These sites are continually updated and the reader or instructor is invited to make use of the information and teaching aids contained on these sites. For example: • At http://www.aicdesign.org/edresources.html, 40 worked problems pertaining to the text can be found. • At http://www.aicdesign.org/scnotes10.html, 40 lectures starting with Chapter 1 and finishing with Chapter 9 can be found in PDF format. These slides would be excellent for instructors as a resource for lecture notes. In addition to the above examples, many other resources are available at the two websites. Fernandina Beach,FL,and Wimberley,TX Phillip E. Allen Douglas R. Holberg CONTENTS Preface xi Chapter 1 Introduction and Background 1 1.1 Analog Integrated-Circuit Design 2 1.2 Notation,Symbology,and Terminology 7 1.3 Analog Signal Processing 9 1.4 Example of Analog VLSI Mixed-Signal Circuit Design 10 1.5 Summary 15 Problems 16 References 17 Chapter 2 CMOS Technology 18 2.1 Basic MOS Semiconductor Fabrication Processes 19 2.2 The pn Junction 33 2.3 The MOS Transistor 40 2.4 Passive Components 46 2.5 Other Considerations of CMOS Technology 53 2.6 Summary 61 Problems 61 References 65 Chapter 3 CMOS Device Modeling 67 3.1 Simple MOS Large-Signal Model (SPICE LEVEL 1) 68 3.2 Other MOS Large-Signal Model Parameters 77 3.3 Small-Signal Model for the MOS Transistor 86 vii viii CONTENTS 3.4 Computer Simulation Models 90 3.5 Subthreshold MOS Model 96 3.6 SPICE Simulation of MOS Circuits 99 3.7 Summary 108 Problems 109 References 112 Chapter 4 Analog CMOS Subcircuits 114 4.1 MOS Switch 115 4.2 MOS Diode/Active Resistor 126 4.3 Current Sinks and Sources 128 4.4 Current Mirrors 138 4.5 Current and Voltage References 147 4.6 Temperature-Independent References 156 4.7 Summary 173 4.8 Design Problems 173 Problems 174 References 183 Chapter 5 CMOS Amplifiers 185 5.1 Inverters 186 5.2 Differential Amplifiers 198 5.3 Cascode Amplifiers 218 5.4 Current Amplifiers 230 5.5 Output Amplifiers 236 5.6 Summary 247 Problems 248 References 260 Chapter 6 CMOS Operational Amplifiers 261 6.1 Design of CMOS Op Amps 262 6.2 Compensation of Op Amps 271 6.3 Design of the Two-Stage Op Amp 286 6.4 Power-Supply Rejection Ratio of Two-Stage Op Amps 303 6.5 Cascode Op Amps 310 6.6 Simulation and Measurement of Op Amps 328 6.7 Summary 342 Problems 342 References 351