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P-channel version of the BF862 JFET

Why there are some nice n-channel JFETs with low input capacitance, Idss, gate threshold voltage etc but p-channel types are rare?

There seem to be the J177 types, and possibly a device from Central Semiconductor ("contact factory" - looks expensive for just messing around) and that's about it. Am I missing some obvious device, are they extraordinarily difficult to make, or is there a way of using n-channel types in some sneaky way so p-channel types aren't often needed?

(I'm looking at bootstrapping a photodiode with positive bias instead of the more usual negative bias, based on the classic circuits in Linear Tech app notes like DN399 and their LTC6244 datasheet.)

Reply to
Nemo
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I'm not a fet connoisseur, but have you tried Interfet?

Reply to
Simon S Aysdie

Yes, I flailed around on their website for a while, but it's like National Semiconductor's - they assume you know what you want before you go, and divide things into their own categories which aren't necessarily how I think, and the part numbers are meaningless without some kind of key 8) So it's not searchable if you don't really know what you're after. I did burrow around for about 40 minutes before deciding they probably didn't have anything better than a J177, though I grant there may be something in a corner of their site I didn't visit.

Reply to
Nemo

Try

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They have some real nice parts and a cross reference search engine.

Reply to
JW

The fundamental properties of "holes" (e.g. mobility) are different than those for electrons. There may also be some differences due to the particular elements used as dopants. These differences occur in bipolar transistors (where the interesting part is the minority carriers) as well as FETs. (majority carriers). This means that the tradeoffs for gain, breakdown voltage, and the like cannot be the same for N and P type devices.

Sometimes there can be good reasons for using a particular biasing scheme, though it can be inconvenient.

HTH..

Reply to
cassiope

Well explained, I see your point. Alas I have to use this polarity of bias due to other constraints, but I'll see if I can get an N-channel JFET to work in some configuration.

In reply to JW, yes, I checked out Linear Systems, but thanks for checking I knew about them 8)

Nemo

Reply to
Nemo

I don't think you need a p-channel fet. It's a follower, and its output is ac-coupled into the pd supply, so the BF862 should work fine.

Just flip the pd and its bias supply.

John

Reply to
John Larkin

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