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Review of Jim Williams: AN47 High Speed Amplifier Techniques

Download AN47 High Speed Amplifier Techniques (1991, Linear Technology, 132 pages, 5,09 MB pdf file)

Of all the excellent application notes that were written by Jim Williams, this is perhaps the most interesting one. It covers the problems that you may encounter when you design high speed circuits and how you can avoid them. It also contains a distillation of the booklet ABCs of Probes by Tektronix (Appendix A, page 69) which is very good (assuming that you already have a basic knowledge about scopes, otherwise XYZs of Oscilloscopes, also by Tektronix, will be more useful).

The most important sections of this app note are "Mr. Murphy's Gallery of High Speed Amplifier Problems" (pages 7 - 14) and the "Tutorial" (pages 15 - 31). The "Problems" section shows 25 scope screen shots. Each of them is typical for a certain type of problem, so if you ever see this waveform on your scope this might give you an idea what the cause might be. Each of these problems is described in detail. Additionaly, each screen shot has a telephone symbol with a number attached to it. This tells you how often the application guys at Linear Technology get a call for help from a customer that be traced back to this problem. So this will give you a good idea about the relative importance of each problem. The "Tutorial" section complements the "Problems" section. It shows how you can avoid to run into the problems. I would advise you to read both sections completely, if you have to design circuits.

The "Applications" sections (pages 32 - 66) are also very interesting, but because the discussed circuits are independent of each other, it is not absolutely necessary to study all of them. Just look at page 3 of the Table of Content and select the circuits that are most interesting for you. The most important appendices are "ABCs of Probes" (page 69 - 81), and "The Oscillation Problem" (page 86 - 92).

When you read this app note, you should keep in mind that it was written in 1991, so some details are outdated. For example, today you may find faster amplifiers than those that are listed in the table on page 6, or those that are used as examples of the state-of-the-art in the applications sections. Also the statements in the preface about the lack of speed of most available monolithic amplifiers are not true anymore. Today affordable high-speed op amps are a reality. However, the list of problems that you may encouter when you design high speed amplifiers, and the tutorial section that tells you how to avoid them, are as relevant today as they were 20 years ago. And you will find many inspirations for your own circuits in the applications sections, even if you should choose to use more modern op amps to get the highest performance.

You may also be interested in some Recommendations for Introductions to Oscilloscopes. And if you found the app note useful and liked the authors writing style, you should also have a look at some other Application Notes by Jim Williams.

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