High voltage (>60V) P-channel JFET? - diyAudio
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Old 10th December 2003, 10:05 AM   #1
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Default High voltage (>60V) P-channel JFET?

Hi, does anyone know of any good high-voltage P-channel JFET`s
(ca 0.5W)?
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Old 10th December 2003, 05:51 PM   #2
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No. Why so high of a voltage? Ever consider a cascode stage?

Or a good MOSFET?

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Old 10th December 2003, 06:13 PM   #3
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Heck, Jocko, I'd be delighted to find such an animal, too- the crosscoupled circuit I like to use is often limited in swing by the amount of voltage the p-jfet will handle.
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Old 10th December 2003, 06:28 PM   #4
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Default Get your Digi-key catalog out, SY....

I've used some Zetex parts.......I believe that they are good for that voltage, maybe more.

No, I don't remember the number. Memory is not as sharp these days. (I bet Cap'n P. knows the number. Unless his memory is over the hill too.)

But as Phred would probably say:

"Search engine! Do I have to research all this stuff for you guys?" [joke]

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Old 10th December 2003, 06:54 PM   #5
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I doubt if there is anything available. Usually, P channel is lower voltage than N channel, all else being equal. Cascode with 1/2 A Hitachi mos fet like the 2SJ79 gets you up to 200V. Works great, been using it for more than a decade. Other, low current, mosfets will also work.
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Old 10th December 2003, 07:19 PM   #6
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I am curious about this. What's so foundamental about those devices that they don't seem to be able to withstand higher voltage?
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Old 10th December 2003, 07:30 PM   #7
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Well, even high voltage n-jfets are pretty rare birds- Siliconix makes some, so did Teledyne some years back, but that's about it. My guess, as a materials guy, is that it's more a matter of demand than technology. How much of a mass market is there for a 200V jfet? Not much. Mosfets won't work for my application, but audio is pretty niche-y, and high end audio is microminiscule. For the high volume stuff, Mosfets work just fine and have better perfomance in areas like gm and on-resistance that are important in mass applications. I can't think of anyone outside of audio who would need a 200V jfet with a 1nV/rt-Hz noise density.
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Old 10th December 2003, 07:36 PM   #8
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OK.......stupid question.......why wont a MOSFET work? The capacitance?

In which case, what is wrong with the cascode?

Jocko
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Old 10th December 2003, 08:10 PM   #9
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No, it's that I need depletion mode to make the biasing work out without huge issues. And noise, too, but that's a bit more secondary.
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Old 10th December 2003, 09:36 PM   #10
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I`m not using the JFET in an amplifier stage.
I found a solution to the problem...

Quote:
In which case, what is wrong with the cascode?
It has high output resistance, making it difficult to f.ex. drive a darlington pair...
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