low-biasing or high biasing in Class-AB amps

[mauropenasa]

I have read Your interventions, don't understand of thing it you is speaking, but me amuses your heat.
You teach me that there are about tens of configurations of the output stage, and diverge devices by use, and that every device has a his exact characteristic of transfer.
D.Self has analysed a part of this (MOSFET and BJT) in any EF and CFP configurations ( Complementary feedback pair ) and has noticed the polarizations with minor crossover THD ( 114mA EF and 11mA @Re=0.22 CFP class B stage).
They exist a lot of techniques sophisticated to improve the polarization, as the " non linear common mode loop", "square law rules" etc.
How to be has to polarize?: simple, with the near current studied for that circuit, from it more from it less...

Ciao

Mauro

[sam9]

My comment regarding more is better in the case of L-MOSFETs was intended to convey a certain wry humor regarding some practices and recomendations. Just because you *can* do something dopesn't mean it is a good idea.

Regardinf BJT's I think Self is valid. However, I don't think the specific optimal bias voltages cited are applicable to all topologies. For instance when dealing with a mirror image VAS, a EF biased at Self's 2.89V ran unreasonably hot and wanted to run away even with a very large heatsink. He alludes to other topologies requiring a different bias in his comments regarding output tripples.

[mauropenasa]

Hi, Sam9.
Be saying the same thing. I have done reference to Self to show that in every circuit is has to analyse the point of work "ideal" ( and he has done it ).
My irony is tied up one's own to the tendency to generalize the concept. Sense of humour to depart, I concern me to scientific and demonstrable concepts when intervene in this forum.
Creed that my affirmation:"-How to be has to polarize?: simple, with the near current studied for that circuit, from it more from it less..." non-being from it extremist from it questionable!

Ciao

Mauro

[Rudy]

Originally posted by janneman

I think with competent design nowadays thermal stability is no longer an issue. As to thermal stressing, I would love to have some info on that. I have never seen an amp failing because of thermal stressing/cycling, so I wonder if that would be a reason to select a certain bias.

Don't get me wrong here Jan, i do not mean that you should selekt your bias settings for this, not at all.

I just mean that if you got less difference in the amount off power that needs to be dissipated than there will be less thermical stress, and will also result in less mechanical stress, witch is a good thing for livespam.

Rudy

[sam9]

I think with competent design nowadays thermal stability is no longer an issue.

If you mean avoiding runaway, I agree. If you mean maintaining optimal bias under all conditions, I think EF configurations still offer "opportunities for improvement". Mitigating it, if my experience is valid, is that there is a useful window where sub-optimal bias has little discernable effect. The preliminary announcement by Onsemi of BJTs with and integral diode for thermal tracking is very interesting.

[amplifierguru]

Sorry about yesterday's comments - I had some local stress issues that spilled over.

On the AB bias issue, in 1974 I produced a range of commercial PA amplifiers using the then new MJ15003/4. The output stage of all models used CFT's - complementary feedback triples! Each output BJT was biassed at 20-25mA. They also used twin slope SOA protection and supply fuses. THD was typically 0.005%.

My point is it's really interesting reading of the virtues of CFP's etc 25-30 years later by someone who cannot even acknowledge the importance of output stage/power supply commutation when even his own experiments showed a reduction in THD from 0.04% to 0.006% just by regulating the supply eliminating it - but dwells on the nuances of crossover optimisation. Please.

The other point was that one should not need to assiduously tweak for optimum bias for distortion to achieve performance goals - as variations occur with spreads,ageing,etc.. My experience is that once there is not a discontinuity causing loss of gain in the forward path then the global NFB does it's job of defining the THD performance.

[djk]

"I must remember the more published guff the more cred..."

Gee, my IM analyzer can't read, and my tests were done around 1985 (Self had not published this by then).

The emitter resistors on most of the amps I checked were about 0R47, so the bias currents were more on the order of 20mA~40mA.

I would drive at full power for a few minutes to heat things up, and then re-check the bias to make sure it wasn't overcompensated.

One amp (which will remain nameless) drew about 8A at full power. When I turned the generator down it still drew 8A! The bias seemed to be a little undercompensated on this model

[tlf9999]

As to the 20-30mv figure for "optimal" biasing, is it possible that it has something to do with Re' (Re prime) for bipolars?

Just curious.

[Workhorse]

OK guys the spark has already turned into flames... and the Thread is going hot and hot and that what i like most........

Thanks to all of you the experts.....

[janneman]

Originally posted by sam9

[snip] The preliminary announcement by Onsemi of BJTs with and integral diode for thermal tracking is very interesting.

Interesting, yes. But I see no info on the relative temp drift of the diodes versus the B-E junctions. My gut feeling is that a lot of work needs to be done to come up with a circuit that balances the two, to avoid either over- or undercompensation. Integrating the diodes on the power chip is NOT enough by a long shot. Again, Doug Self has studied this in depth and the result was that is it very hard to get it just right.

BTW, I can't find that Self article on the biasing of power stages mentioned before; anybody else has that info?

Jan Didden