JFETs or BJTs

[Gus]

http://www.its.caltech.edu/~musiclab/feedback-paper-acrobat.pdf

I posted this link in another thread.

Figure 3, Figure 4,  Figure 7, Figure 8, etc.

[Paul Marossy]

Once I did an A/B comparison between an FET and a BJT opamp in a distortion pedal I built. I could see some differences on the scope, but I sure as heck couldn't really hear a discernable difference. I suppose it depends on what you want to accomplish with your circuit. In something where you want the minimum amount of distortion possible, it might make some difference. But then again, I wonder how many people can actually hear a difference. And if not, what is the point? 

Just my two centavos... 

[Quackzed]

i'd be interested to hear what you may haave gleaned from this paper.
after a quick scan, it seems like the crux is that feedback on all 3 types of devices lowers the overall distortion level, but also raises the relative amount of high order distortion... that is to say, low order harmonics (2ns 3rd....seventh) are less with feedback- cleaner-, but there is an added higher order 'hash' of non-musical low level distortion also, that isnt present with non-feedback setups...
as far as harmonic content of each dvice etc... i dunno... ?!?

[BubbaFet]

i'd be interested to hear what you may have gleaned from this paper...

Yes, some FEEDBACK would be appreciated! 
                                                                         .

[sault]

Well, I can briefly summarize, or at least try to.

This paper measures the effect that different components and circuits have on intermodulation distortion, ie what the end result is when you play two tones at the same time.

Single-ended

FET
single-ended, no feedback - only 2nd harmonics
single-ended, with feedback - lots more harmonics; the harder it gets pushed, the more distortion and trebly harmonics are added

BJT
single-ended, no feedback - lots of harmonics
single-ended, with feedback - more feedback clears up the signal and reduces distortion and harmonics dramatically
... for both ; as with single-ended FET w/feedback, the harder you push it the more distortion/harmonics are added

Triode
single-ended, no feedback - harmonics, but less distortion than either comparable FET or BJT
single-ended, with feedback - ditto, even when driven harder

Push-pull

FET
in class A, no distortion, with or without feedback.
in class B, no even-order distortion, but some high-level harmonics are added. also, less input = more distortion!

BJT
eliminates even-order harmonics, but introduces more higher-level harmonics
the frequencies introduced depends on feedback

Triode
low distortion, all distortion eliminated with feedback present


Differential

For all types, more input = more harmonics (especially higher order), feedback lowers these though.


...

Until a tube starts to distort, its intermodulation harmonics are lower than either FET or BJT.
Perfectly matched push-pull FETs should theoretically have no distortion. Perfectly matched? Good luck with that.
Well, we already basically know that FETs get very trebly when we hit 'em hard, so that's not new.
Less feedback is better for FETs, more feedback is better for BJTs.
Probably don't want to run FETs in class B.

In each case, these attributes were measured with FETs and tubes being driven many times harder than the BJTs!
(in other words, at similar signal levels, you'd need a ton more feedback on the BJT to keep it as clean as the others)

Finally,

Feedback isn't automatically a bad thing, as some in the hi-fi world would claim.