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The op amp IC has become the universal analog IC because it can perform all analog tasks. OP AMPS FOR EVERYONE provides the theoretical tools and practical know-how to get the most from these versatile devices. This new edition substantially updates coverage for low-speed and high-speed applications, and provides step by step walkthroughs for design and selection of op amps and circuits.
* Modular organization allows readers, based on their own background and level of experience, to start at any chapter
* written by experts at Texas Instruments and based on real op amps and circuit designs from TI
* NEW: large number of new cases for single supply op amp design techniques, including use of web-based design tool
* NEW: complete design walk-through for low-speed precision op amp selection and circuit design
* NEW: updates, including new techniques, for design for high-speed, low distortion applications.
* NEW: extensive new material on filters and filter design, including high-speed filtering for video and data
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About the Author
Bruce Carter holds dual degrees, Engineering Physics from Texas Tech University, and Electrical Engineering at the University of Texas. He has over 30 years of experience in analog design for military and oil field service companies. He was an applications engineer at Texas Instruments for 9 years, where he authored application notes, contributed articles, and knowledge base FAQ's.
Read an Excerpt
Op Amps for Everyone
By Bruce Carter
ElsevierCopyright © 2013 Elsevier Inc.
All rights reserved.
The Op Amp's Place in the World
1.1 An Unbounded Gain Problem
In 1934 Harry Black commuted from his home in New York City to work at Bell Labs in New Jersey by way of a railroad and ferry. The ferry ride relaxed Harry, enabling him to do some conceptual thinking. Harry had a tough problem to solve; when phone lines were extended long distances, they needed amplifiers, and undependable amplifiers limited phone service. Initial tolerances on the gain were poor, but that problem was solved with an adjustment. Unfortunately, even when an amplifier was adjusted correctly at the factory, the gain drifted so much during field operation that the volume was too low or the incoming speech was distorted.
Many attempts had been made to make a stable amplifier, but temperature changes and power supply voltage extremes experienced on phone lines caused uncontrollable gain drift. Harry knew that passive components had much better drift characteristics than active components; thus if an amplifier's gain could be made dependent on passive components, the problem would be solved. During one of his ferry trips, Harry's fertile brain conceived a novel solution for the amplifier problem, and he documented the solution while riding on the ferry.
1.2 The Solution
The solution was first to build an amplifier that had much more gain and bandwidth than the application required. Then some of the amplifier output signal could be fed back to the input in a manner that made the circuit gain (circuit is the amplifier and feedback components) dependent on the feedback circuit rather than the amplifier gain. The circuit gain was then dependent on the stable passive feedback components rather than the active amplifier. This is called negative feedback, and it is the underlying operating principle for all modern day operational amplifier circuits. Harry had documented the first intentional feedback circuit during his ferry ride. I am sure unintentional feedback circuits had been built prior to that time, but the designers ignored the effect!
I can hear the squeals of anguish coming from the managers and amplifier designers at Bell Labs. I imagine that they said something like this, "It is hard enough to achieve 30 kHz gain—bandwidth (GBW), and now this fool wants me to design an amplifier with 3 MHz GBW. But, all he is making is an audio amplifier". Nevertheless, they accomplished it, the problem was solved, and the benefits of negative feedback were realized. The operational amplifier was born — even though it was not recognized as a component in its own right, or called an operational amplifier until years later.
There is a minor problem that Harry did not discuss in detail, and that is the oscillation problem. It seems that circuits designed with large open-loop gains sometimes oscillate when the loop is closed. A lot of people investigated the instability effect, and it was pretty well understood in the 1940s, but solving stability problems involved long, tedious, and intricate calculations. Years passed without anybody making the solution simpler or more understandable.
In 1945 H.W. Bode presented a system for analyzing the stability of feedback systems using graphical methods. Until this time, feedback analysis was done by multiplication and division, and calculation of transfer functions was a time-consuming and laborious task. Bode presented a logarithmic technique that transformed the intensely mathematical process of calculating a feedback system's stability into graphical analysis that was simple and perceptive. Feedback system design was no longer an art dominated by a few electrical engineers who were also accomplished mathematicians. Any electrical engineer could use Bode's methods to find the stability of a feedback circuit, so the application of feedback circuits began to grow. Operational amplifiers had become easier to use and understand. There was not much call for operational amplifiers, however, until analog computers and transducers came of age.
1.3 The Birth of the Op Amp as a Component
The first computers were analog computers! Programming consisted of configuring wiring and passive components to a series of circuits that performed mathematical operations on voltages. The heart of the analog computer was Harry's invention: a device called an operational amplifier because it could be configured to perform many mathematical operations such as multiplication, addition, subtraction, division, integration, and differentiation on the input signals. The name was shortened to the familiar op amp, as it is now known.
General-purpose analog computers were found in universities and large company laboratories because they were critical to the research work done there. Although early op amps were designed for analog computers, it was soon discovered that op amps had other uses and were very handy to have around the physics lab. There was a parallel requirement for transducer signal conditioning in lab experiments, and op amps found their way into signal conditioning applications. As the signal conditioning applications expanded, the demand for op amps grew beyond the analog computer requirements.
The hard wiring limitation eventually caused the declining popularity of the analog computer. Even when analog computers lost favor to digital computers, the op amp survived because of its importance in universal analog applications. Eventually digital computers completely replaced analog computers (a sad day for real-time measurements), but the demand for op amps increased as measurement applications increased.
1.3.1 The Vacuum Tube Era
The first signal conditioning op amps were constructed with vacuum tubes before the introduction of transistors, so they were large and bulky. During the 1950s, miniature vacuum tubes that worked from lower voltage power supplies enabled the manufacture of op amps shrunk to the size of a brick used in house construction, so the op amp modules were nicknamed bricks. Vacuum tube size and component size decreased until an op amp was shrunk to the size of a single octal vacuum tube.
One of the first commercially available op amps was the model K2-W, sold by George A. Philbrick Research. It consisted of two vacuum tubes, and operated from a ± 300 V power supply! If that is not enough to make you cringe, then its fully differential nature is sure to. A fully differential op amp, as opposed to the more familiar single-ended op amp, has two outputs: a non-inverting output and an inverting output. It requires you to close two feedback paths, not just one. Before panic sets in, the two feedback pathways only require duplication of components, not an entirely new design methodology. Fully differential op amps are currently enjoying a resurgence because they are ideal components for driving the inputs of fully differential analog-to-digital converters (ADCs). They also find use in driving differential signal pairs such as digital subscriber line (DSL) and balanced 600[ohm] audio. Suffice it to say, op amps have come full circle since their original days.
1.3.2 The Transistor Era
Transistors were commercially developed in the 1960s, and they further reduced op amp size to several cubic inches, but the nickname brick still held on. Now this nickname is attached to any electronic module that uses potting compound or non-integrated circuit (IC) packaging methods. Most of these early op amps were made for specific applications, so they were not necessarily general purpose. The early op amps served a specific purpose, but each manufacturer had different specifications and packages; hence, there was little second sourcing among the early op amps.
1.3.3 The Integrated Circuit Era
Although ICs were developed during the late 1950s and early 1960s, it was not until the mid-1960s that Fairchild released the µA709. This was the first commercially successful IC op amp, and Robert J. Widler designed it. The µA709 had its share of problems, but any competent analog engineer could use it, and it served in many different analog applications. The major drawback of the µA709 was stability; it required external compensation. In addition, the µA709 was quite sensitive because it had a habit of self-destructing under any adverse condition. The legacy of its instability remains — few uncompensated amplifiers are sold today owing to the problem of misapplication. Stability remains one of the least understood aspects of op amp design, and one of the easiest ways to misapply an op amp. Even engineers with years of analog design experience have differing opinions on the topic. The wise engineer, however, will look carefully at the op amp data sheet, and not attempt a gain less than its specification. It may be counterintuitive, but the op amp is least stable at its lowest specified gain. Future chapters will delve more deeply into this phenomenon.
The µA741 followed the µA709, and it is an internally compensated op amp that does not require external compensation if operated under data sheet conditions. It is also much more forgiving than the µA709. The legacy of the µA741 is much more positive than its predecessor's. In fact, the part number "741" is etched into the memory of practically every engineer in the world, much like the "2N2222" transistor and the "1N4148" diode. It is usually the first part number that comes to mind whenever an engineer thinks of an op amp. Unlike the µA709, the µA741 will work unless grossly misapplied — a fact that has endeared it to generations of engineers. Its power supply requirements of ± 15 V have given rise to hundreds of power supply components that generate these levels, much as +15 V has been driven by transistor—transistor logic (TTL) and ± 12 V has been driven by RS232 serial interfaces. For many years, every op amp introduced used the same ± 15 V power supplies as did the µA741. Even today, the µA741 is an excellent choice where wide dynamic range and ruggedness are required.
There has been a never-ending series of new op amps released each year since the introduction of the µA741, and their performance and reliability have improved to the point where present-day op amps can be used for analog applications by anybody who can understand a data sheet. The latest generations of op amps cover the frequency spectrum from 5 kHz GBW for extremely low power devices to beyond 3 GHz GBW. The supply voltage ranges from guaranteed operation at 0.9 V to absolute maximum voltage ratings of 1000 V. The input current and input offset voltage has fallen so low that customers have problems verifying the specifications during incoming inspection. The op amp has truly become the universal analog IC because it performs all analog tasks. It can function as a line-driver, amplifier, level shifter, oscillator, filter, signal conditioner, actuator driver, current source, and voltage source, and in many other applications.
It should be noted that there is no op amp that is universally applicable. An op amp that is ideal for transducer interfaces will not work at all for radio frequency (RF) applications. An op amp with good RF performance might have miserable DC specifications. The hundreds of op amp models offered by manufacturers are all optimized in slightly different ways, so your task is to weed through those hundreds of devices and find the ones that are appropriate for their application. This edition includes a design methodology for doing so, at least in the case of signal chains.
This book deals with op amp circuits, not with the innards of op amps. It does not get bogged down in detailed calculations. Engineers should not have to be mathematicians to perform routine designs. Math should be required only for advanced applications, not reinvented over and over again for routine ones. Using this book, you can start at the level you understand, and quickly move on to advanced topics as needed.
The op amp will continue to be a vital component of analog design because it is such a fundamental component. Each generation of electronics equipment integrates more functions on silicon and takes more of the analog circuitry inside the IC. Don't fear; as digital applications increase, analog applications also increase because the predominant supply of data and interface applications is in the real world, and the real world is an analog world. Thus, each new generation of electronics equipment creates requirements for new analog circuits; hence, new generations of op amps are required to fulfill these requirements. Analog design and op amp design are fundamental skills that will be required far into the future.
Excerpted from Op Amps for Everyone by Bruce Carter. Copyright © 2013 Elsevier Inc.. Excerpted by permission of Elsevier.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents1: The Op Amp's Place in the World
2: Review of Circuit Theory
3: Development of the Ideal Op Amp Equations
4: Single Supply Op Amp Design Techniques
5: Feedback and Stability Theory
6: Development of the Non-Ideal Op Amp Equations
7: Voltage Feedback Op Amp Compensation
8: Current Feedback Op Amp Analysis
9: Voltage and Current Feedback Op Amp Comparison
10: Op Amp Noise Theory and Applications
11: Understanding Op Amp Parameters
12: Instrumentation I: Sensors to A/D Conversion
13: Instrumentation II
14: Instrumentation III
15: Circuit Board Layout Techniques
16: Wireless Communication: Signal Processing for IF Sampling
17: Interfacing D/A Converters to Loads
18: Sine Wave Oscillators
19: Active Filter Design Techniques I
20: Active Filter Design Techniques II
21: Active Filter Design Techniques III
22: Active Filter Design Techniques IV
23: Active Filter Design Techniques V
24: Active Filter Design Techniques VI
25: Designing for Low Voltage Op Amp Circuits
A: Single Supply Circuit Collection
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