Analog Design for CMOS VLSI Systems

382 Pages·2003·19.705 MB·English

by  Franco Maloberti (auth.)

ANALOG DESIGN FOR CMOS VLSI SYSTEMS
ANALOG DESIGN
FOR CMOS
VLSI SYSTEMS
by
Franco Maloberti
Texas A & M University, U.S.A. and
University of Pavia, Italy
KLUWER ACADEMIC PUBLISHERS
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eBookISBN: 0-306-47952-4
Print ISBN: 0-7923-7550-5
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C
ONTENTS
Preface xi
Chapter 1
The MOS Transistor 1
1.1 Electrical Conduction in Solids 1
1.2 Fermi-Dirac Statistic 3
1.3 Properties of Materials 10
1.3.1 Silicon 10
1.3.2 Silicon dioxide 14
1.3.3 Polysilicon 16
1.3.4 Silicon Nitride 17
1.4 CMOS Technology 18
1.5 MOS Threshold Voltage 19
1.6 I-V Characteristics 26
1.6.1 Weak Inversion Region 27
1.6.2 Linear (or Triode) Region 28
1.6.3 Saturation Region 30
1.7 Equivalent Circuits 32
vi Contents
1.7.1 Large Signal Equivalent Circuit 32
1.7.2 Small Signal Equivalent Circuit 35
1.8 More Sophisticated Models 39
1.9 Noise 45
1.10 Layout of Transistors 49
1.11 Design Rules 53
1.12 References 55
1.13 Problems 55
Chapter 2
Resistors, Capacitors, Switches 59
2.1 Integrated Resistors 59
2.1.1 Accuracy of Integrated Resistors 63
2.1.2 Layout of Integrated Resistors 69
2.2 Integrated Capacitors 72
2.2.1 Accuracy of Integrated Capacitors 75
2.2.2 Layout of Integrated Capacitors 78
2.3 Analog Switches 81
2.3.1 ChargeInjection 86
2.3.2 Charge Injection Compensation 89
2.4 Layout of Switches 94
2.5 References 96
2.6 Problems 96
Chapter 3
Basic Building Blocks 99
3.1 Inverter with Active Load 99
3.1.1 Small Signal Analysis 101
3.1.2 Noise Analysis 107
3.1.3 Design of Inverters with Active Load 109
3.2 Cascode 114
3.2.1. Small Signal Analysis 115
3.3 Cascode with Cascode Load 121
3.3.1 Small Signal Analysis of Cascode Gain Stages 127
Contents vii
3.3.2 Gain EnhancementTechniques131
3.4 Differential Stage 133
3.5 Source Follower136
3.6 ThresholdIndependent Level-shift141
3.7 ImprovedOutput Stages142
3.7.1 Source Follower with Local Feedback 143
3.7.2 Push-PullOutputStage 146
3.8 References 151
3.9 Problems 151
Chapter 4
Current and Voltage Sources 155
4.1 Current Mirrors 155
4.1.1 Simple Current Mirror 156
4.1.2 Wilson Current Mirror 160
4.1.3 Improved Wilson Current Mirror 163
4.1.4 Cascode Current Mirror 165
4.1.5 Layout of Modified Wilson and Cascode Current Mirrors 167
4.1.6 Modified Cascode Current Mirror 168
4.1.7 High Compliance Current Mirror 171
4.1.8 Enhanced Output-Impedance Current Mirror 173
4.1.9 Current Mirrors with Adjustable MirrorFactor 176
4.2 Current References 178
4.2.1 Simple Current Reference 178
4.2.2 Self Biased Current Reference 180
4.2.3 Self Biased Micro-Current Generator 184
4.2.4 Start-up Circuits 188
4.2.5 Use of Parasitic BJT for Current Reference 190
4.2.6 Based Current Reference 190
4.2.7 Bases Current Reference 192
4.3 Voltage biasing 196
4.3.1 Voltage Divider 197
4.3.2 Diode-Connected Voltage Bias 201
4.4 Voltage References 201
4.4.1 Multiplier 202
4.4.2 Multiplier 203
4.4.3 Voltage Reference Based on Threshold Difference 204
4.4.4 Band-Gap Reference Voltage 205
4.4.5 Curvature Error 212
viii Contents
4.5 References 213
4.6 Problems 214
Chapter 5
CMOS Operational Amplifiers 217
5.1 General Issues 217
5.2 Performance Characteristics 221
5.3 Basic Architecture 228
5.4 Two Stages Amplifier 229
5.4.1 Differential Gain 230
5.4.2 Common Mode dc Gain 230
5.4.3 Offset 231
5.4.4 Power Supply Rejection 235
5.4.5 Effect of External Components on the PSRR 240
5.5 Frequency Response and Compensation 242
5.6 Slew Rate 255
5.7 Design of a two stage OTA: Guidelines 258
5.8 Single Stage Schemes 259
5.8.1 Telescopic Cascode 259
5.8.2 Mirrored Cascode 265
5.8.3 Folded Cascode 269
5.8.4 Single Stages with Enhanced dc Gain 273
5.9 Class AB Amplifiers 277
5.9.1 Two Stages Scheme 278
5.9.2 Unfolded Differential Pair 280
5.9.3 Single Stage AB-class OTA 282
5.10 Fully Differential Op-Amps 286
5.10.1 Circuit Schematics 286
5.10.2 Common Mode Feedback 289
5.10.3 Continuous-time Common-mode Feedback 291
5.10.4 Sampled-data Common-mode Feedback 295
5.11 Micro-Power OTA’s 297
5.11.1Dynamic-biasing of the Tail Current 298
5.11.2Dynamic Voltage Biasing in Push-pull Stages 299
5.12 Noise Analysis 301
5.13 Layout 308
5.13.1 Parasitic Effects 308
Contents ix
5.13.2Stacked Layout 313
5.14 References 319
5.15 Problems 320
Chapter 6
CMOS COMPARATORS 325
6.1 Introduction 325
6.2 Performance Characteristics 326
6.3 General Design Issues 330
6.3.1 Architecture of the Gain Stage 331
6.4 Offset Compensation 333
6.4.1 Implementation of the Auto-zero Technique 335
6.4.2 Auto-zero in Multi-stages Architectures 339
6.4.3 Fully Differential Implementation 341
6.4.4 Use of an Auxiliary Stage 346
6.5 Latches 349
6.6 References 356
6.7 Problems 356
Appendix A 359
Appendix B 361
Appendix C 365
Index 369
P
REFACE
The purpose of this book is to describe the design techniques of analog
integrated circuits and to teach the reader how to properly design CMOS oper-
ational amplifiers and comparators for mixed analog-digital integrated sys-
tems. Analog circuits have become an increasingly critical factor for the
systems design. The huge amount of transistors made available by sub-micron
technologies allows us to integrate entire systems on chip (SoC). Therefore,
analog sections, digital parts, and possibly sensors must use the same technol-
ogy, the same supply voltage, the same silicon substrate and so on. Moreover,
the increased speed of operation and the augmented resolution of digital proc-
essors require performing analog functions: such as speed, dynamic range,
power supply and noise rejection. Without a deep understanding of the opera-
tion and the limits of basic analog circuits, the designer can not properly
design the analog processing functions required by modern systems. Thus,
this book gives a fairly detailed study of CMOS circuit configurations, learn-
ing performances and limits.
This book evolved from a set of lecture notes written in 1986 for an in-
house training short course presented in a semiconductor company. Later the
material formed the basis of a graduate course on analog CMOS integrated
circuit offered since 1998 at the Pavia University, Italy. The initial set of lec-
ture notes progressed in time following the technology evolution. The refer-
ence technology migrated from a CMOS to the CMOS used in
this book. The circuit techniques moved from design targets like 5 V and tens
of MHz bandwidth to the present 1.8 V and hundreds of MHz bandwidth.
However, such an amazing change did not modify much of the basic philoso-
xii Preface
phy: to follow a bottom-up approach. The teaching starts from basic physics
elements, discusses the features of the MOS transistor, studies the passive
components, and considers circuit design of basic blocks, current and voltage
reference. In this way the reader acquires all the elements necessary to prop-
erly design op-amps and comparators.
The book contains six chapters. Chapter 1 provides those physical, techno-
logical and device modelling issues necessary to properly comprehend the
behaviour of MOS transistors and their modelling. Chapter 1 starts with a
resume of the basic principles of solid state physics and discusses the proper-
ties of the basic materials used in microelectronics. This enables the model-
ling of MOS transistors, both at a simple level, and at a more complex level
for computer simulation. Finally, the chapter studies noise performances and
discusseslayout techniques.
Chapter 2 examines the basic properties of integrated resistors, capacitors
and analog switches. The features of integrated components are quite different
from the ones of discrete elements. Therefore, it is essential that the reader
knows limits and performances well.
Chapter 3 studies simple gain stages, differential pairs, differential to single
ended convertors, output stages, etc. The approach conforms the hierarchical
view of the teaching pattern: the readermust become familiar with and under-
stand the features and performances of simple cells before studying more
complex cells.
The basic building blocks studied in Chapter 3 and other analog functions
require, as essential elements for their operation, current generators and volt-
age biases. Chapter 4 covers the basic architecture of current and voltage
sources. This enables the reader to know how to design these “auxiliary”
blocks appropriately, to recognize their functional limits, and to estimate costs
and benefits for the best design decision.
Chapter 5 deals with operation amplifiers (usually referred to as op-amps).
The function and operation of op-amps should be well known to the reader.
For this reason, the chapter deals with those circuit implementations that are
specifically used in CMOS integrated VLSI systems. Namely, the chapter
studies a special category of op-amp: the operational transconductance ampli-
fiers (OTAs). An OTA achieves a large gain exploiting its large output resist-
ance. The reader will learn that when used inside an integrated architecture an
op-amp drives capacitive loads. This makes the request of having a low output
impedance of little importance.
Chapter 6 studies CMOS comparators. A comparator is together with the
op-amp the basic block used in analog signal processors. Ideally, it generates
an output logic signal as responseto an analog input. Since a real circuit does
not achieve the ideal function it is essential to know how the limitations affect