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Old 16th November 2006, 12:57 PM   #1
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Default bjt/mos (class A) CFP

Hallo good people!

1. When designing class a bjt/mos CFP out stage does the bjt see Cgs or Cgd?
2. In aksa`s "Glass-Harmony" amp, he said that he used 2N5551 bjt with 1K collector load and that`s about 4-5mA bias current. Is that enough for driving IRFP9140 in CFP stage... ??
3.What is the most important to know when optimising bjt/mos CFP.
4. Does it need bjt base-collector cap and how to calculate the value?


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Old 16th November 2006, 03:10 PM   #2
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ANyone knows?
I didn`t found "exact" data in previous threads...
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Old 16th November 2006, 04:12 PM   #3
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Quote:
Originally posted by bogdan_borko
ANyone knows?
I didn`t found "exact" data in previous threads...
It is an interesting question.
How to make a Bipolar + HEXFET foldback device.
I am sure it would have some parameters with EXTRAORDINARY PERFORMANCE.



You don't see this often.
Because the HEXFET people don't use bipolar much
and
the Bipolar builders do not use HEXFET much, either.


Anyway,
this is a perfect issue for 'Professor' Pass to answer.
He would be the one to know ....
Maybe this topic would have better luck in Pass Labs forum


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Old 16th November 2006, 04:39 PM   #4
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thanks for reply lineup!


Cfp (bjt/mos) is more stable when operating in class a (that`s what I heard) and high local feedback of cfp is good for distortion minimizing, so I think that this combination could be bull`s eye...

This kind of CFP IMO is an opposite of IGBT.
< base - drain- source>
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Old 16th November 2006, 05:05 PM   #5
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Hugh (AKSA) recommends CFP in class-a because it has good distortion and because of class-a you avoid all the turn-off/on problems.

I have built a class-b amp with TIP29C/30C transistors driving IRF540/9540 devices, and it seemed to work well. I didn't test much with loads or waveshapes though.

I also had a lot of compensation caps all over the place due to an initial oscillation problem, and I never removed the 'excess' caps to see how it went without them.

You cannot calculate the cap values, you have to determine them with the actual board layout and real parts.

IMO 2N5551 and 1k is not man enough to drive MOSFETs, maybe Hugh has done some other trick?
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Old 16th November 2006, 05:56 PM   #6
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Quote:
IMO 2N5551 and 1k is not man enough to drive MOSFETs, maybe Hugh has done some other trick?

so Hugh...? Could you explain this if you have time...?
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