Taming MOSFET nasties: Hafler - diyAudio
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Old 21st January 2007, 11:45 PM   #1
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Default Taming MOSFET nasties: Hafler

Hafler used these two arrangements to militate against the "nasties" in MOSFET behavior --
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Old 22nd January 2007, 12:13 AM   #2
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Well??????????????
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Old 22nd January 2007, 02:08 AM   #3
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I am asking one of the many grey-beards to opine.
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Old 22nd January 2007, 04:03 AM   #4
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Resistor instead of the cap on the left picture would be better against "nasties". I suppose they need that caps because of some other reasons not shown on your pictures.
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Old 22nd January 2007, 05:27 AM   #5
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well, my beard ain't grey yet, but i'll take a whack at it...... i've noticed that even with complementary pair mosfets, there are differences in gate capacitance between N and P channel devices, with the result that the N channel devices are a bit faster than their P channel counterparts, and so harder to tame. the trick is to tame instability, while maintaining equal phase response and slew rate for both sides of the output pair, and so Hafler does so, possibly finding the proper component values experimentally. the output devices, although complements, are not entirely symmetrical in all of their characteristics.
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Old 22nd January 2007, 10:33 PM   #6
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My beards very gray and alot of snow on the roof also! Caps to stabilize ,ie. prevent oscillation..thats my 2 cents ....do I get any change back! LOL!
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Old 23rd January 2007, 12:04 AM   #7
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The original post sounded like a statement--then it morphed into an implied question, but I'm still not clear as to exactly what the question was.
Are you asking about the Gate stoppers, the caps, the MOSFETs, or the topology as a whole?
What MOSFET nasties were you referring to? Oscillation? You'll find much the same sort of layout in bipolar designs. Tubes, too, for that matter, making allowances for the lack of 'P-ch' devices.

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Old 23rd January 2007, 05:56 AM   #8
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looking at the hafler dh-220 service manual, i see that the driver for the PCH half is heavily compensated, and the NCH half is not compensated at the driver, but at the output device.

one design goal i am familiar with in hafler amps is symmetrical slew rate, and as i was told long ago.... if it works, don't fix it......
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