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Regarding Boyk and Sussman, Cheever, etc.

One of the things that Boyk and Sussman do is to look at a common-source FET amp with source degeneration, using an idealized FET model. In this model, the device by itself has only second-order distortion. They evaluate three different configurations, corresponding to three different levels of feedback. Yet they only show data for two of the three configurations. The data for amplifier c, with the highest amount of feedback, is not shown. They state, "The spectrum of amplifier c is not shown. Its additional feedback compared to b makes for a distortion spectrum which is similar except that all of the products are pushed down in level". I'll return to this idea in a moment.

Cheever makes his point in a way that's easier to visualize than Boyk and Sussman's approach. He deals with harmonic distortion only. Using the same approach of assuming the open-loop amplifier has only second-order distortion, he shows the relative values of fundamental and various orders of harmonics as a continuous function of the amount of feedback applied. He shows that the relative values of the higher harmonics rise as the amount of feedback is increased from zero, then decrease again as feedback is increased further and further. Of course, the higher-order products were assumed to be zero without feedback, so in this case feedback cannot improve higher-order harmonics by the definition of the problem itself.

But this raises a practical question, and that is, "What practical device has only second-order distortion when no feedback is applied?". The answer to this question is "None". The quadratic equation used by Boyk and Sussman to model the JFET is an empirical curve fit, chosen for ease of hand computation. A more accurate equation relating drain current to gate-source voltage based on the device physics is more complex than a simple quadratic (Ref: Massobrio and Antognetti, equations 3-11 and 3-12). This is also borne out by measurement.

This raises yet another question, and that is, "Has anyone done actual measurements of a JFET amplifier with varying amounts of feedback to determine the harmonic levels as a function of the amount of feedback applied?". The answer to this question is "Yes!". Peter Baxandall, in his series in Wireless World from 1978 to 1979 does exactly this. In fact, if you look at the graph of harmonic levels vs. feedback in the Cheever thesis, this was taken from these very articles by Baxandall. They are not Cheever's own work. But Baxandall overlays the actual measured data from a JFET amplifier on top of these theoretical plots, allowing one to see the effects of feedback on a practical amplifier.

Quoting Baxandall, here are some of his results for the JFET amp. "...referring again to Fig. 7, 16.5 dB of negative feedback is sufficient to reduce the third harmonic to the same level as it has without feedback, whereas about 35 dB is required for reducing the sixth harmonic to its no-feedback level". He also goes on to say, "The magnitude of harmonics of extremely high order will be increased by the application of negative feedback, no matter what practical amount of feedback is employed. But this is of no consequence if, when thus increased, they are, say, 120 dB below the fundamental".

I have a PDF of these documents. The first four articles in the series would be an excellent introduction to feedback for those with a scientific background, but no formal EE introduction to the subject. If anyone is interested, just shoot me an email through the forum system. It's a 3.4 MB file, so keep that in mind.

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