Which THD Components arises if different Ube Voltages by parallel Power BjT's

[tiefbassuebertr]

...present?

This question is obviously still not investigated. Also not in relation to different Hfe values.
Both at low and higher idle currents.
Both at single ended and push-pull buffer versions.
Who knows more about this?
What spread range would still be just acceptable?
Please note, the emitter resistors in most cases lower than 1 Ohm, sometimes even not exist.

Some guys tell me, the only way to avoid unwanted THD/IM effects through the spread of Ube voltage values and the AC current gain factor "hfe" never use paralleling output power devices (no matter whether BjT's or MOSFET's; only paralleling complete amplifiers include the front end is possible without unwanted selecting of transistors and unwanted THD enhancing at the same time. This is often to observe by IC amps like TDA7293 or LM3886/LM4780. But by discrete amps this I have never see.

An other way it could be to use a large amount of paralleling (small/medium signal) transistors with emitter resistor of 10 - 30 ohms. But I am not shure. By simulation unfortunately I am not able to make a variation of Ube voltage and hfe.

[tiefbassuebertr]

No comments to this frequently used parallel connecting of output devices?

[Telstar]

I would also wish to know

[h_a]

I doubt there is any significant THD-difference.

For most of the time VGS of mosfets is matched to within 100mV to guarantee that all mosfets turn on. Source degeneration resistors take care of current sharing. For bjts this is pretty pointless due to tiny variations in VBE.

Matching is not always done to decrease THD. It's mostly done for the peace of mind - and in some distortion cancellation circuits (differentials...)

But that's only the technical side of things, so the stuff you can actually measure. Others may give complementary input.

Have fun, Hannes

EDIT:

This question is obviously still not investigated.

I dare to disagree. Even if there was data posted here, most would plainly ignore it. Matching is poor man's circuit optimization and I don't have a problem with that.

[Telstar]

Originally Posted by h_a

For bjts this is pretty pointless due to tiny variations in VBE.

AFAIK, BJTs should be matched by hfe.

PS: matching supply to the intrinsec very poor linearity of active devices, esp. transistors (instead of using feedback and other compensations).

[h_a]

Well, honestly I don't know how to react on that.

If you want to believe in that, be it so. I don't want to take the fun from the guys that even match 1% resistors.

Really, no problem with that.

A THD analyzer sees things differently.

Have fun, Hannes

[tiefbassuebertr]

I am still looking arround for papers/articles (like e. g. AES) to this matter.

[WuYit]

You won`t find any. There is no unequivocal answer to that essentially immaterial question.

[DF96]

If parallel BJTs have slightly different Vbe or current gain then the main effect is that one will do more work than others. Give the typical local feedback around the output stage there will be little effect on distortion.

If the opposite sides of a P-P output have different gains then the main effect will be a small rise in even-order distortion. Local feedback will deal with most of this in a well-designed amp, then global feedback will finish the job.

The reason nobody seems to have done any work on this is that it is really a non-problem.

[WuYit]

It`s like conducting differently skilled musicians, they don´t play in unison.