Table of Contents
1 Introduction to Analog Design2 Basic MOS Device Physics 3 Single-Stage Amplifiers 4 Differential Amplifiers 5 Passive and Active Current Mirrors 6 Frequency Response of Amplifiers 7 Noise 8 Feedback 9 Operational Amplifiers 10 Stability and Frequency Compensation 11 Bandgap References 12 Introduction to Switched-Capacitor Circuits 13 Nonlinearity and Mismatch 14 Oscillators 15 Phase-Locked Loops 16 Short-Channel Effects and Device Models 17 CMOS Processing Technology 18 Layout and Packaging
Forewords & Introductions
In the past two decades, CMOS technology has rapidly embraced the field of analog integrated circuits, providing low-cost, high-performance solutions and rising to dominate the market. While silicon bipolar and III-V devices still find niche applications, only CMOS processes have emerged as a viable choice for the integration of today's complex mixedsignal systems. With channel lengths projected to scale down to 0.03 um, CMOS technology will continue to serve circuit design for probably another two decades.
Analog circuit design itself has evolved with the technology as well. High-voltage, highpower analog circuits containing a few tens of transistors and processing small, continuoustime signals have gradually been replaced by low-voltage, low-power systems comprising thousands of devices and processing large, mostly discrete-time signals. For example, many analog techniques used only ten years ago have been abandoned because they do not lend themselves to low-voltage operation.
This book deals with the analysis and design of analog CMOS integrated circuits, emphasizing fundamentals as well as new paradigms that students and practicing engineers need to master in today's industry. Since analog design requires both intuition and rigor, each concept is first introduced from an intuitive perspective and subsequently treated by careful analysis. The objective is to develop both a solid foundation and methods of analyzing circuits by inspection so that the reader learns what approximations can be made in which circuits and how much error to expect in each approximation. This approach also enables the reader to apply the concepts to bipolar circuits with little additionaleffort.
I have taught most of the material in this book both at UCLA and in industry, polishing the order, the format, and the content with every offering. As the reader will see throughout the book, I follow four "golden rules" in writing (and teaching): (1) I explain why the reader needs to know the concept that is to be studied; (2) I put myself in the reader's position and predict the questions that he/she may have while reading the material for the first time; (3) With Rule 2 in mind, I pretend to know only as much as the (first-time) reader and try to "grow" with him/her, thereby experiencing the same through process; (4) I begin with the "core" concept in a simple (even imprecise) language and gradually add necessary modifications to arrive at the final (precise) idea. The last rule is particularly important in teaching circuits because it allows the reader to observe the evolution of a topology and hence learn both analysis and synthesis.
The text comprises 18 chapters whose contents and order are carefully chosen to provide a natural flow for both self-study and classroom adoption in quarter or semester systems.
Unlike some other books on analog design, we cover only a bare minimum of MOS device physics at the beginning, leaving more advanced properties and fabrication details for later chapters. To an expert, the elementary device physics treatment may appear oversimplified, but my experience suggests that (a) first-time readers simply do not absorb the high-order device effects and fabrication technology before they study circuits because they do not see the relevance; (b) if properly presented, even the simple treatment proves adequate for a substantial coverage of basic circuits; (c) readers learn advanced device phenomena and processing steps much more readily after they have been exposed to a significant amount of circuit analysis and design.
Chapter 1 provides the reader with motivation for learning the material in this book.
Chapter 2 describes basic physics and operation of MOS devices.
Chapters 3 through 5 deal with single-stage and differential amplifiers and current mirrors, respectively, developing efficient analytical tools for quantifying the behavior of basic circuits by inspection.
Chapters 6 and 7 introduce two imperfections of circuits, namely, frequency response and noise. Noise is treated at an early stage so that it "sinks in" as the reader accounts for its effects in subsequent circuit developments.
Chapters 8 through 10 describe feedback, operational amplifiers, and stability in feedback systems, respectively. With the useful properties of feedback analyzed, the reader is motivated to design high-performance, stable op amps and understand the trade-offs between speed, precision, and power dissipation.
Chapters 11 through 13 deal with more advanced topics: bandgap references, elementary switched-capacitor circuits, and the effect of nonlinearity and mismatch. These three subjects are included here because they prove essential in most analog and mixed-signal systems today.
Chapters 14 and 15 concentrate on the design of oscillators and phase-locked loops, respectively. In view of the wide usage of these circuits, a detailed study of their behavior and many examples of their operation are provided.
Chapter 16 is concerned with high-order MOS device effects and models, emphasizing the circuit design implications. If preferred, this chapter can directly follow Chapter 2 as well. Chapter 17 describes CMOS fabrication technology with a brief overview of layout design rules.
Chapter 18 presents the layout and packaging of analog and mixed-signal circuits. Many practical issues that directly impact the performance of the circuit are described and various techniques are introduced.
The reader is assumed to have a basic knowledge of electronic circuits and devices, e.g., pn junctions, the concept of small-signal operation, equivalent circuits, and simple biasing. For a senior-level elective course, Chapters 1 through 8 can be covered in a quarter and Chapters 1 through 10 in a semester. For a first-year graduate course, Chapters 1 through 11 plus one of Chapters 12 through 15 can be taught in one quarter, and the first 16 chapters in one semester.
The problem sets at the end of each chapter are designed to extend the reader's understanding of the material and complement it with additional practical considerations. A solutions manual is available for instructors.
Behzad Razavi
July 2000
Read an Excerpt
Chapter 1: Introduction to Analog Design
1.2 Why Integrated?The idea of placing multiple electronic devices on the same substrate was conceived in the late 1950s. In 40 years, the technology has evolved from producing simple chips containing a handful of components to fabricating memories accommodating more than one billion transistors as well as microprocessors comprising more than 10 million devices. As Gordon Moore (one of the founders of Intel) predicted in the early 1970s, the number of transistors per chip has continued to double approximately every one and a half years. At the same time, the minimum dimension of transistors has dropped from about 25 um in 1960 to about 0.18 F, m in the year 2000, resulting in a tremendous improvement in the speed of integrated circuits.
Driven by primarily the memory and microprocessor market, integrated-circuit technologies have also embraced analog design extensively, affording a complexity, speed, and precision that would be impossible to achieve using discrete implementations. Analog and mixed analog/digital integrated circuits containing tens of thousands of devices now routinely appear in consumer products. We can no longer build a discrete prototype to predict the behavior and performance of modern analog circuits.
1.3 Why CMOS?
The idea of metal-oxide-silicon field-effect transistors (MOSFETs) was patented by J. E. Lilienfeld in the early 1930s-well before the invention of the bipolar transistor. Owing to fabrication limitations, however, MOS technologies became practical much later, in the early 1960s, with the first several generations producing only n-type transistors. It was in the mid-1960s thatcomplementary MOS (CMOS) devices (i.e., both n-type and p-type transistors) were introduced, initiating a revolution in the semiconductor industry.
CMOS technologies rapidly captured the digital market: CMOS gates dissipated power only during switching and required very few devices, two attributes in sharp contrast to their bipolar or GaAs counterparts. It was also soon discovered that the dimensions of MOS devices could be scaled down more easily than those of other types of transistors. Furthermore, CMOS circuits proved to have a lower fabrication cost.
The next obvious step was to apply CMOS technology to analog design. The low cost of fabrication and the possibility of placing both analog and digital circuits on the same chip so as to improve the overall performance and/or reduce the cost of packaging made CMOS technology attractive. However, MOSFETs were quite slower and noisier than bipolar transistors, finding limited application.
How did CMOS technology come to dominate the analog market as well? The principal force was device scaling because it continued to improve the speed of MOSFETs. The intrinsic speed of MOS transistors has increased by more than three orders of magnitude in the past 30 years, becoming comparable with that of bipolar devices even though the latter have also been scaled (but not as fast). Multi-gigahertz analog CMOS circuits are now in production.
1.4 Why This Book?
The design of analog circuits itself has evolved together with the technology and the performance requirements. As the device dimensions shrink, the supply voltage of integrated circuits drops, and analog and digital circuits are fabricated on one chip, many design issues arise that were unimportant only a decade ago. Such trends demand that the analysis and design of circuits be accompanied by an in-depth understanding of their advantages and disadvantages with respect to new technology-imposed limitations.
Good analog design requires intuition, rigor, and creativity. As analog designers, we must wear our engineer's hat for a quick and intuitive understanding of a large circuit, our mathematician's hat for quantifying subtle, yet important effects in a circuit, and our artist's hat for inventing new circuit topologies.
This book describes modern analog design from both intuitive and rigorous angles. It also fosters the reader's creativity by carefully guiding him/her through the evolution of each circuit and presenting the thought process that occurs during the development of new circuit techniques.
1.5 General Concepts
1.5.1 Levels of Abstraction
Analysis and design of integrated circuits often require thinking at various levels of abstraction. Depending on the effect or quantity of interest, we may study a complex circuit at device physics level, transistor level, architecture level, or system level. In other words, we may consider the behavior of individual devices in terms of their internal electric fields and charge transport [Fig. 1.8(a)], the interaction of a group of devices according to their electrical characteristics [Fig. 1.8(b)], the function of several building blocks operating as a unit [Fig. 1.8(c)], or the performance of the system in terms of that of its constituent subsystems [Fig. 1.8(d)]. Switching between levels of abstraction becomes necessary in both understanding the details of the operation and optimizing the overall performance. In fact, in today's IC industry, the interaction between all groups, from device physicists to system designers, is essential to achieving a high performance and a low cost. In this book, we begin with device physics and develop increasingly more complex circuit topologies.
1.5.2 Robust Analog Design
Many device and circuit parameters vary with the fabrication process, supply voltage, and ambient temperature. We denote these effects by PVT and design circuits such that their performance remains in an acceptable range for a specified range of PVT variations. For example, the supply voltage may vary from 2.7 V to 3.3 V and the temperature from 0° to 70°. Robust analog design in CMOS technology is a challenging task because device parameters vary significantly from wafer to wafer...
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