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Old 18th July 2002, 09:23 PM   #1
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Default Driving a MOSFET power follower

I am trying to drive 1400pF worth of gate capacitance from +10v to -10v.

IRFP140N biased at 3A 12v. CCS underneath at 3A -12v.

For low distortion at high frequencies.. (10khz-20khz) I need a pre with low (<200ohms) output impedance.

Can I make a small JFET follower as a buffer before the MOSFET? i.e connect it to a set of +24 to -24 batteries... and run it at 50mA or so?

Thoughts? ideas? part #'s?

Thanks!!!!
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Old 18th July 2002, 10:07 PM   #2
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Default See AKSA advice

at thread "Linearize..."
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Old 18th July 2002, 10:18 PM   #3
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Would a JFET give you that low output resistance? .. I don't know .. and are you shure aboput the 1400pf.. that looks a bit high ..

I once build a buffer for headphones with power MOSFET's and the bandwidth obtained with a 10KOhm seies resistance of the input pot was way more than expected.... i think the datasheet input capacitance is not that important

...maybe because since there's not very much voltage to be charged (it is a folower right? so all voltage relations are quite constant..)

gr,
Thijs
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Old 18th July 2002, 10:39 PM   #4
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I say hook it up to your source through 200 ohms or
so and see what you get. Probably it will work
fine. My experience with followers is that around
1000 ohms you start seeing some real degradation.

Remember that the high Gate capacitance is mostly to
the Source pin, which is following, and you are not
really charging that capacitance nearly as much as you
might imagine.
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Old 18th July 2002, 11:04 PM   #5
mlloyd1 is offline mlloyd1  United States
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This is an excellent point I remember being told when I first started building MOSFET amps and was worrying about the capacitance!
Quote:
Originally posted by Nelson Pass
...Remember that the high Gate capacitance is mostly to
the Source pin, which is following, and you are not
really charging that capacitance nearly as much as you
might imagine.
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Old 19th July 2002, 12:19 AM   #6
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Default Todays MOS/HEXFET

Has much lower input capacitance,
then yesterdays.
And who would say he knew better than Nelson??
Only a fool....

gromanswe
the fool on the hill
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Old 19th July 2002, 08:35 AM   #7
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Default Miller effect

As Nelson said, it is not so much the capacitance but the change in voltage across that capacitance. So if your gate goes up by 10v and your source follows it by 9v then G/S capacitance really only sees 1v change.

Now if you have a common source cct (not to be confused with no-name tomato sauce) as well as the driver seeing the full capacitance of G/S because the source stays put, the gate/drain capacitance is *multiplied* approximately by the voltage gain of the cct. If the gate goes up by 1v and it causes the drain to come down by 10v then the gate/drain came 11v closer for 1v drain voltage change, making the G/D capacitance look 11 times larger. Yoicks!

GP.
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Old 19th July 2002, 02:13 PM   #8
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Default low Ciss MOSFETS

Then this begs the question:

Any low Ciss MOSFETS that can handle a bias of 3A at 12v with high S and low Rds ?

The IRFP140N is 1400pF...

Does it help by putting in a low value (200mOhms) resistor in the source of the MOSFET? sort of local negative feedback?

Thanks for this insight... I just assumed that this capacitance is for any difference between gate and source... but if the source follows the gate then the capacitance is not really an issue.

Richard
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Old 19th July 2002, 06:36 PM   #9
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I prefer
IRF530 HEX 600pf N-channel 0.18 ohms
IRF540 have 1450pF 0.06 ohms
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Old 19th July 2002, 08:15 PM   #10
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Default Mosfet power follower

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