Dear Dennis...
EN-Genius Network's Dennis Feucht answers your design
queries in his new Circuit Design Clinic!
December, 2008
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EN-Genius Network presents a new, interactive
analog design service to readers! Send us your design questions (with
relevant data; schematics in JPEG or GIF, please) for some free
engineering advice from EN-Genius Network's circuit
consultant, Dennis Feucht, on how you might solve a design problem or
improve circuit performance. Submissions may be edited for clarity or
brevity, and submitters and their email addresses will remain anonymous
(unless otherwise indicated). Please send your questions to Dennis here.
Analog Circuits: World Class Designs
by Dennis L Feucht
Edited by Robert A Pease
Newnes, May 2008; 432 pages; paperback; ISBN: 978-0-7506-8627-3; $39.95
This book is a compilation of chapters of different authors. Some chapters are republished or reworked from previously published material. About half the book appears to be new material. It is similar in style and depth/width of material as Jim Williams’ two compilations in Butterworth-Heinemann books bearing the words Analog Circuit Design in the titles. Pease has included not only a republished article of Williams from one of those two books, but also articles from EN-Genius Network/EDN regular contributor, Bonnie Baker.
As for content, there are several chapters on analog filters by Marc Thompson. Also by him is the lead chapter, a review of feedback analysis. The following chapter, by Phil Perkins of LTX Corp., presents his approach to feedback analysis. The overlapping subject-matter in this kind of book tends to strengthen it through the perspectives of different authors. Both authors approach feedback with an emphasis on Bode plots and the complex-frequency domain.
Thompson also contributes a chapter which is a case study of the venerable 741 op-amp, a good walk-through for newer engineers wanting a better understanding of what is “under the hood” of such ICs. The 741 makes a good study, but it has been analyzed rather extensively in the past. I would have preferred a newer, rail-to-rail op amp topology with complementary emitter-coupled BJT input stage, but the 741 has some unusual circuitry worth perusing. Thompson also covers bipolar, complementary-BJT output stages and their crossover dead-band. (For a more detailed analysis and simplification of this rather complicated but familiar output stage, see my book, High-Performance Amplifiers in the Analog Circuit Design book-set.) Baker contributes an op amp chapter oriented to their use, and presents the most common circuits and where to use them.
Thompson’s chapter on passive components and board layout might be the most valuable to developing engineers who already have the basic textbook circuits in their heads.
Richard Burwen, one of the three founders of Analog Devices, Inc., wrote the chapter titled How to Design Analog Circuits Without a Computer or a Lot of Paper. Each chapter is prefaced with a short comment by Pease; for this one, he expresses his agreement that the author “has the right attitude for designing analog systems and circuits.” And what is this attitude? It is one of keeping the design calculations so simple as to “generally do the calculations in my head.” Although any good engineer would want to follow Einstein’s dictum, once printed on a Tektronix calendar, that “Everything should be made as simple as possible but not simpler,” even when extraordinary complications require pages of derivations, the inclination of this chapter and the book generally is to not lose sight of the physical meaning of the mathematics. Even more so, the desire is to circumvent much math through higher-level concepts that avoid having to apply Kirchhoff’s Laws to do circuit analysis – a symptom that one is empty of ideas on how to apply the more powerful, “intuitive” circuit theorems that allow writing down the design formulas by mere circuit inspection.
What I liked about Burwen’s chapter was not its illustration of this minimalist approach (though it is used) but that he exercises the patience to go step-by-step through a circuit design, explaining in detail what he is doing. There is too little of this kind of literature for newer engineers and this offers a tutorial on a design as a case study. Such detailed plodding through a design reveals insights not found in textbooks about what and how the designer is actually thinking, and provides an example to follow for the apprentice engineer. Burwen, though he was involved in the founding of ADI, prefers to remain an independent engineer. I think I should get to know Mr. Burwen; we seem to have something in common.
A revision of Sergio Franco’s old classic article from EDN (5 January 1989) on current-feedback op amps (or what Barrie Gilbert of ADI prefers to call transconductance op amps) is a good tutorial on this subject and covers somewhat more ground than my (previously cited) book does. (Why, you might ask, prefer “transconductance” over “current-feedback” to name this kind of amplifier? For a big hint, see my TechNote on feedback amplifier misconceptions.)
The book then turns quasi-analog (or is that “mixed-signal?”), with two chapters on ADCs by Bonnie Baker. This basic tutorial material is reminiscent of what is in her book, which I also reviewed earlier. The book ends with two chapters by Pease. The first, on error budgets, reminds the reader that for a one-op-amp differential amplifier, 1% resistors not only result in gain error but also common-mode error. The last chapter finds Bob beating away again at BJT junction VBE characteristics – another good topic for newer engineers to master. Bob’s graphic approach is intuitive, though some of us still like equations and simple rules of thumb that result when they are worked out, such as Is having a fractional TC of 16%/°C at room temperature.
At the front of the book it is noted by the publisher that Pease, “self-declared Czar of Proofreading,” contributed to the accuracy of the book with his sharp editing eye. I would like to also contribute a little in noting that on page 387, in eqns (17-2) and (17-3), that the numeric subscripts have wandered considerably from their correct positions (and font sizes). Also, though not an error as such, the BJT curve illustrated on page 390 really could use some axes to illustrate quantitatively the quantitative point Bob is making about low VCE in using it. And on page 380, the text refers to R1, R2, R3 but the corresponding resistors are not designated in the illustration. Perhaps a little additional “currying” of the text might be applied before the next printing. The book also includes “free Newnes online membership.”
Now we come to that time to put on my black robe and appraise the book. Like previous (hardback!) Newnes compilations and Baker’s book, this one belongs on the bookshelf of engineers who are no longer green but who have much yet to learn. The level of material is introductory for most chapters, and moves from green to yellow in a few. For red hot, read some chapters from Williams’ books such as the one by John Addis on how to design fast amplifiers for oscilloscopes. For the guy next bench over out of school, I recommend that engineering managers buy a stack of these books (this one included) and distribute them to new recruits in your business unit.
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