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Old 10th June 2005, 03:07 AM   #11
tlf9999 is offline tlf9999  United States
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Let me ask the question in a slightly different way:

two otherwise identical MOSFETs: one's capacitance goes from 900p to 100p, in a perfectly linear fashion. another's capacitance fluctuates between 400p and 600p randomly (thus, perfectly non-linear).

Which of the two devices would you prefer?
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Old 10th June 2005, 03:58 AM   #12
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Quote:
Originally posted by lumanauw

. . . (Vgs I assume). . .
I assume this way.
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Old 10th June 2005, 04:03 AM   #13
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Quote:
Originally posted by tlf9999
Let me ask the question in a slightly different way:

two otherwise identical MOSFETs: one's capacitance goes from 900p to 100p, in a perfectly linear fashion. another's capacitance fluctuates between 400p and 600p randomly (thus, perfectly non-linear).

Which of the two devices would you prefer?
Tough call. I would try both out, and see what happens. We
have to be clear in understanding that both create distortion,
and its a choice between magnitude and character. Usually
I prefer character, but it depends on how "perfectly nonlinear"
your example might be. If it's really random, then the harmonics
will have high order, and I really dislike that.
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Old 10th June 2005, 04:26 AM   #14
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Good answer! Couldn't have said it better.

Except, I'd probably go back to search!
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Old 10th June 2005, 05:25 AM   #15
quasi is offline quasi  Australia
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Default Relevance ?

With a well designed and low impedance output driver stage the effects of gate capacitance can be minimised to a negligible (non-audible) level. A low impedance drive will swamp out any variations in capacitance.

Visually this can be observed with a 10Khz full rail swinging square wave where with a well designed driver stage the effects observed are quite small. (IMO).

Cheers
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Old 10th June 2005, 05:30 AM   #16
EUVL is offline EUVL  Europe
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Nelson,

Is it your experience that vertical FETs have in general a more gentle variation in capacitance with frequency than lateral FETs ?

Would you be kind enough to show some measurements ?


Patrick
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Old 10th June 2005, 05:39 AM   #17
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Indeed Quasi,

See my 5ohm drive impedance MOSFET 150W power amp,
with the BC546 30mW driver!

It's a goer.

Greg
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Old 10th June 2005, 05:55 AM   #18
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Quote:
Originally posted by amplifierguru
Indeed Quasi,

See my 5ohm drive impedance MOSFET 150W power amp,
with the BC546 30mW driver!

It's a goer.

Greg

Whats you damping factor.....using BC546 as a driver WOW! Great
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Old 10th June 2005, 09:52 AM   #19
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WOW !!!
thnx god !!! so i can use irfp250 and irfp2940 ... !!!
lol thank u ...
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Old 10th June 2005, 10:27 AM   #20
quasi is offline quasi  Australia
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Default Yes you can, but .....

Hi Ahmad_tbp,

Yes you can but you must include thermal tracking compensation in the voltage multiplier stage or elswhere that will reduce the bias current as things warm up and things WILL warm up. Let me explain;

In order to achieve an output signal free of crossover (switching)distortion you must bias the output stage so that output devices are always on at low signal levels. This produces heat.

Unfortunately with the IRF fets you are talking about, as they get warm they will conduct more and then get warmer and conduct more and get hot and conduct heaps and then KABOOM!. This tragic result is guaranteed with the circuit you posted if you do not insert themal compensation. Some methods have already been suggested here and elswhere in this forum.

Very large heatsinks may not help that much because the heating is too localised. This is why most amp layouts have the "thermal detector" very close to the FETs.

As far as circuits go you should look at the amp I posted in "power amp under development" and also Lars' ZETA.

The amp I posted has been built and is currently out on loan to an audiophile friend of mine who wanted to give it a run. Repeated phone calls have failed to get the amp back....so I think he likes it.

Good luck & cheers
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