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feedback-paper-acrobat - Small-Signal Distortion in...

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Unformatted text preview: Small-Signal Distortion in Feedback Amplifiers for Audio 1 James Boyk 2 and Gerald Jay Sussman 3 April 22, 2003 1 c 2003 Boyk & Sussman. You may copy and distribute this document so long as the source is appropriately attributed. 2 Pianist in Residence, Lecturer in Music in Electrical Engineering, and Director of the Music Lab, California Institute of Technology 3 Matsushita Professor of Electrical Engineering, Department of Electrical En- gineering and Computer Science, Massachusetts Institute of Technology Abstract We examine how intermodulation distortion of small two-tone signals is affected by adding degenerative feedback to three types of elementary am- plifier circuits (single-ended, push-pull pair, and differential pair), each im- plemented with three types of active device (FET, BJT and vacuum triode). Although high precision numerical methods are employed in our analysis, the active devices are modeled with rather simple models. We have not investigated the consequences of more elaborate models. Though negative feedback usually improves the distortion characteristics of an amplifier, we find that in some cases it makes the distortion messier. For instance, a common-source FET amplifier without feedback has a distor- tion spectrum displaying exactly four spurious spectral lines; adding feedback introduces tier upon tier of high-order intermodulation products spanning the full bandwidth of the amplifier (as suggested by Crowhurst in 1957). In a class-B complementary-pair FET amplifier, feedback mysteriously boosts specific high-order distortion products. The distortions we are dealing with are small, but we speculate that they may be psychoacoustically significant. This work also casts light on the relative virtues of the three types of active devices and the three circuit types. For instance, a FET pair run in class-A produces zero distortion even without feedback. 1 Some experienced listeners report favorably on the sound quality of non- feedback amplifiers. This is surprising, because such amplifiers have much more nonlinear distortion than amplifiers that use negative feedback. Indeed, the appropriate use of negative feedback improves almost all of the theoretical and measurable parameters of an amplifier. Of course, the listeners may be mistaken. Alternatively, some subtle consequent of negative feedback may be responsible for the difference in perception. Here we investigate one such possibility. Of the various proposals attempting to explain how these perceptual dif- ferences might arise, most have suggested bad design errors in the application of feedback. For example, transient intermodulation distortion occurs in amplifiers with inadequate slew rate; 1 but real-world transients have rise times that are easy to accommodate with modern circuits....
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feedback-paper-acrobat - Small-Signal Distortion in...

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