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In Reply to: Re: Regarding Boyk and Sussman, Cheever, etc. posted by john curl on March 14, 2007 at 19:50:02:
Okay, I'll scan the relevant pages and email them to you. Device physics is not my thing, so I couldn't follow it in detail.There may be multiple emails, as I can't zip them together because of the Mac/PC thing.
Follow Ups:
Andy, I appreciate anything that you can do. It is difficult to get real insight on these design problems. Of course, you can 'cover' them up with negative feedback, BUT if Boyk is correct, then we are not doing ourselves any favors.
I was attending engineering classes when Spice circuit analysis was first introduced. I was there to hear the active circuit models explained. However, my classmates and I knew that it was only an approximation of the real thing.
When I first worked with Mark Levinson we made complementary differential fet input stages. However, we found a different distortion spectrum when we used National Semi fets instead of Siliconix fets, yet they had the same part number. I know that the amount of higher order harmonics is very important, so I try almost anything to get lower. I don't worry too much about 2'nd or 3rd harmonic. It is just too much part of the music to make me concerned with .01% or less, BUT a little 5th, or worse: 7th or 9th harmonic, at listening levels, and I know that I have failed to make a truly successful product. This is why I measure individual harmonics down to .0001% with noise averaging to resolve xover artifacts that MUST reside in a typical IC op amp's distortion spectrum.
Based on the specifics of the manufacturing, such as layer thickness and uniformity, I could imagine the actual behavior to vary from fet to fet. That being the case, I would guess then that it would be necessary to obtain an actual transfer function for the exact transistor you are using in order to model it adequately. That would be a big PITA I guess.
We usually just build and test a prototype. Excessive reliance on modeling can give only partial results.
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