FET inputs or bipolar inputs on a power amp?

[john curl]

For audio, it can be significant.

[lumanauw]

Mr. Curl, what is actually happening? When I make bipolar differential input, it seems without RE sounds better than using RE

[john curl]

I can't tell you what to like. I have not built a bipolar transistor input for many decades. I sometimes use them in my test equipment, when fet inputs (IC based) are too noisy.

[traderbam]

FM distortion is generated by the non-linear S curve that exists WHEN you want BOTH low noise and a bipolar transistor input.

I don't understand. This S curve...what are its axes?

Degeneration resistor free bipolar input stages have a significant amount of FM distortion, unless the open loop bandwidth is very, very high, or the gain bandwidth is very, very high.

Could you be more specific? Are you talking about phase modulation due to the non-linear transfer characteristic of the bipolar transistors in conjunction with their junction capacitances, or something else? Why is this different for FETs which also have non-linear gm, albeit a lower order distortion, and junction capacitance?

[john curl]

Please contact the Walt Jung website for further information. Specifically, the link to Barrie Gilbert's articles on op amp design.
If an individual does not understand that the distortion characteristics of a differential bipolar transistor pair maps like an S curve, then I recommend some beginner courses on transistor theory, perhaps from an internet course before going any further on this topic.

[lt cdr data]

one of the bottom lines is that both types have been used sucessfully in many designs.

Audio subjectivism seems to defy the theory that frequently something is objectively better, yet good things and good sounds can result from something measuring less well.

Another is my pet theory that the more complex an amp. gets, the more 'clinical' the sound.

ie, start mirroring and cascoding and lots of elaborations, and the sound suffers, even tho the measured performance may improve, as elicited by amplifiers in the late 60s/70s, some of these are the finest soudning amps I have ever heard, we have not really moved forward one iota, yet modern ones measure near perfect?

why do they not sound better then?

[PMA]

Nuts. Under direct comparison with good up to date amps the oldies from sixties and seventies cannot compete. Experience, no guessing.

[john curl]

I agree with you on this, PMA. The only exceptions that I know of personally are quality tube amps and the early Electrocompaniet power amp. The Electro-Research power amp would probably qualify as well. Many early power amps had TIM (SID) limitations DUE to the undegenerated bipolar transistor input stages then used, because WE did not know any better. Today, we usually either resistively degenerate the input transconductance or use a fet input which does essentially the same thing, as fets don't have as high a transconductance as bipolar transistors at normal operating currents.
Remember folks, transconductance (Gm) is the enemy of high slew rate, especially in bandwidth limited F(t) power amps.

[forr]

J. CURL
---Electro-Research power amp ---

Its author is Jon Iverson, I think. Has anybody the schematics ? I am looking for it since twenty years.

[jcx]

http://www.diyaudio.com/forums/showt...789#post501789

please point to the spectral components that support the claim that degenerated bjt has higher order distortion than the fet diff input

also note that the the loop gain is integrating for both examples so we expect that (and can see by) the spray of multiple sidebands evidence that the primary distortion is in fact "FM distortion" and is present with both fet and bjt inputs


it sure looks to me that degeneration has reduced "FM distortion" very effectively in the bjt example