• Book Name: Design with Operational Amplifiers and Analog Integrated Circuits
• Author: Sergio Franco
• Pages: 733
• Size: 5 MB

Design with Operational Amplifiers and Analog Integrated Circuits

During the last decades much has been prophesized that there will be little need for analog circuitry in the future because digital electronics is taking over. Far from having proven true, this contention has provoked controversial rebuttals, as epitomized by statements such as “If you cannot do it in digital, it’s got to be done in analog.” Add to this the common misconception that analog design, compared to digital design, seems to be more of a whimsical art than a systematic science, and what is the confused student to make of this controversy? Is it worth pursuing some coursework in analog electronics, or is it better to focus just on digital?

There is no doubt that many functions that were traditionally the domain of analog electronics are nowadays implemented in digital form, a popular example being offered by digital audio. Here, the analog signals produced by microphones and other acoustic transducers are suitably conditioned by means of amplifiers and filters, and are then converted to digital form for further processing, such as mixing, editing, and the creation of special effects, as well as for the more mundane but no less important tasks of transmission, storage, and retrieval. Finally, digital information is converted back to analog signals for playing through loudspeakers. One of the main reasons why it is desirable to perform as many functions as possible digitally is the generally superior reliability and flexibility of digital circuitry. However, the physical world is inherently analog, indicating that there will always be a need for analog circuitry to condition physical signals such as those associated with transducers, as well as to convert information from analog to digital for processing, and from digital back to analog for reuse in the physical world. Moreover, new applications continue to emerge, where considerations of speed and power make it more advantageous to use analog front ends; wireless communications provide a good example.

Indeed many applications today are best addressed by mixed-mode integrated circuits (mixed-mode ICs) and systems, which rely on analog circuitry to interface with the physicalworld, and digital circuitry for processing and control. Even though the analog circuitry may constitute only a small portion of the total chip area, it is often the most challenging part to design as well as the limiting factor on the performance of the entire system. In this respect, it is usually the analog designer who is called to devise ingenious solutions to the task of realizing analog functions in decidedly digital technologies; switched-capacitor techniques in filtering and sigma-delta techniques in data conversion are popular examples. In light of the above, the need for competent analog designers will continue to remain very strong. Even purely digital circuits, when pushed to their operational limits, exhibit analog behavior. Consequently, a solid grasp of analog design principles and techniques is a valuable asset in the design of any IC, not just purely digital or purely analog ICs.

THE BOOK

The goal of this book is the illustration of general analog principles and design methodologies using practical devices and applications. The book is intended as a textbook for undergraduate and graduate courses in design and applications with analog integrated circuits (analog ICs), as well as a reference book for practicing engineers. The reader is expected to have had an introductory course in electronics, to be conversant in frequency-domain analysis techniques, and to possess basic skills in the use of SPICE. Though the book contains enough material for a two-semester course, it can also serve as the basis for a one-semester course after suitable selection of topics. The selection process is facilitated by the fact that the book as well as its individual chapters have generally been designed to proceed from the elementary to the complex.

At San Francisco State University we have been using the book for a sequence of two one-semester courses, one at the senior and the other at the graduate level. In the senior course we cover Chapters 1–3, Chapters 5 and 6, and most of Chapters 9 and 10; in the graduate course we cover all the rest. The senior course is taken concurrently with a course in analog IC fabrication and design. For an effective utilization of analog ICs, it is important that the user be cognizant of their internal workings, at least qualitatively. To serve this need, the book provides intuitive explanations of the technological and circuital factors intervening in a design decision.

NEW TO THE FOURTH EDITION

The key features of the new edition are: (a) a complete revision of negative feedback, (b) much enhanced treatment of op amp dynamics and frequency compensation, (c) expanded coverage of switching regulators, (d) a more balanced presentation of bipolar and CMOS technologies, (e) a substantial increase of in-text PSpice usage, and (f) redesigned examples and about 25% new end-of-chapter problems to reflect the revisions.

While previous editions addressed negative feedback from the specialized viewpoint of the op amp user, the fourth edition offers a much broader perspective that will prove useful also in other areas like switching regulators and phase-locked loops. The new edition presents both two-port analysis and return-ratio analysis, emphasizing similarities but also differences, in an attempt at dispelling the persisting confusion between the two (to keep the distinction, the loop gain and the feedback factor are denoted as L and b in two-port analysis, and as T and β in return-ratio analysis). Of necessity, the feedback revision is accompanied by an extensive rewriting of op amp dynamics and frequency compensation. In this connection, the fourth edition makes generous use of the voltage/current injection techniques pioneered by R. D. Middlebrook for loop-gain measurements.

In view of the importance of portable-power management in today’s analog electronics, this edition offers an expanded coverage of switching regulators. Much greater attention is devoted to current control and slope compensation, along with stability issues such as the effect of the right-half plane zero and error-amplifier design.

The book makes abundant use of SPICE (schematic capture instead of the netlists of the previous editions), both to verify calculations and to investigate higher-order effects that would be too complex for paper and pencil analysis. SPICE is nowadays available in a variety of versions undergoing constant revision, so rather than committing to a particular version, I have decided to keep the examples simple enough for students to quickly redraw them and run them in the SPICE version of their choice.

As in the previous editions, the presentation is enhanced by carefully thoughtout examples and end-of-chapter problems emphasizing intuition, physical insight, and problem-solving methodologies of the type engineers exercise daily on the job. The desire to address general and lasting principles in a manner that transcends the latest technological trend has motivated the choice of well-established and widely documented devices as vehicles. However, when necessary, students are made aware of more recent alternatives, which they are encouraged to look up online.

THE CONTENTS AT A GLANCE

Although not explicitly indicated, the book consists of three parts. The first part (Chapters 1–4) introduces fundamental concepts and applications based on the op amp as a predominantly ideal device. It is felt that the student needs to develop sufficient confidence with ideal (or near-ideal) op amp situations before tackling and assessing the consequences of practical device limitations. Limitations are the subject of the second part (Chapters 5–8), which covers the topic in more systematic detail than previous editions. Finally, the third part (Chapters 9–13) exploits the maturity and judgment developed by the reader in the first two parts to address a variety of design-oriented applications. Following is a brief chapter-by-chapter description of the material covered.

Chapter 1 reviews basic amplifier concepts, including negative feedback. Much emphasis is placed on the loop gain as a gauge of circuit performance. The loop gain is treated via both two-port analysis and return-ratio analysis, with due attention to similarities as well as differences between the two approaches. The student is introduced to simple PSpice models, which will become more sophisticated as we progress through the book. Those instructors who find the loop-gain treatment overwhelming this early in the book may skip it and return to it at a more suitable time. Coverage rearrangements of this sort are facilitated by the fact that individual sections and chapters have been designed to be as independent as possible from each other; moreover, the end-of-chapter problems are grouped by section.

Chapter 2 deals with I -V, V-I , and I -I converters, along with various instrumentation and transducer amplifiers. The chapter places much emphasis on feedback topologies and the role of the loop gain T .

Chapter 3 covers first-order filters, audio filters, and popular second-order filters such as the KRC, multiple-feedback, state-variable, and biquad topologies. The chapter emphasizes complex-plane systems concepts and concludes with filter sensitivities.

The reader who wants to go deeper into the subject of filters will find Chapter 4 useful. This chapter covers higher-order filter synthesis using both the cascade and the direct approaches. Moreover, these approaches are presented for both the case of active RC filters and the case of switched-capacitor (SC) filters.

Chapter 5 addresses input-referrable op amp errors such as VOS, IB, IOS,CMRR, PSRR, and drift, along with operating limits. The student is introduced to datasheet interpretation, PSpice macromodels, and also to different technologies and topologies.

Chapter 6 addresses dynamic limitations in both the frequency and time domains, and investigates their effect on the resistive circuits and the filters that were studied in the first part using mainly ideal op amp models. Voltage feedback and current feedback are compared in detail, and PSpice is used extensively to visualize both the frequency and transient responses of representative circuit examples. Having mastered the material of the first four chapters using ideal or nearly ideal op amps, the student is now in a better position to appreciate and evaluate the consequences of practical device limitations.

The subject of ac noise, covered in Chapter 7, follows naturally since it combines the principles learned in both Chapters 5 and 6. Noise calculations and estimation represent another area in which PSpice proves a most useful tool.

The second part concludes with the subject of stability in Chapter 8. The enhanced coverage of negative feedback has required an extensive revision of frequency compensation, both internal and external to the op amp. The fourth edition makes generous use of the voltage/current injection techniques pioneered by R. D. Middlebrook for loop-gain measurements. Again, PSpice is used profusely to visualize the effect of the different frequency-compensation techniques presented.

The third part begins with nonlinear applications, which are discussed in Chapter 9. Here, nonlinear behavior stems from either the lack of feedback (voltage comparators), or the presence of feedback, but of the positive type (Schmitt triggers), or the presence of negative feedback, but using nonlinear elements such as diodes and switches (precision rectifiers, peak detectors, track-and-hold amplifiers).

Chapter 10 covers signal generators, including Wien-bridge and quadrature oscillators, multivibrators, timers, function generators, and V-F and F-V converters.

Chapter 11 addresses regulation. It starts with voltage references, proceeds to linear voltage regulators, and concludes with a much-expanded coverage of switching regulators. Great attention is devoted to current control and slope compensation, along with stability issues such as error-amplifier design and the effect of the righthalf plane zero in boost converters.

Chapter 12 deals with data conversion. Data-converter specifications are treated in systematic fashion, and various applications with multiplyingDACs are presented. The chapter concludes with oversampling-conversion principles and sigma-delta converters. Much has been written about this subject, so this chapter of necessity exposes the student only to the fundamentals.

Chapter 13 concludes the book with a variety of nonlinear circuits, such as log/antilog amplifiers, analog multipliers, and operational transconductance amplifiers with a brief exposure to gm-C filters. The chapter culminates with an introduction to phase-locked loops, a subject that combines important materials addressed at various points in the preceding chapters.

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