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Old 16th May 2010, 11:20 AM   #1
Nrik is offline Nrik  Denmark
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Default Power J-fet + PNP buffer

...trying to figure a way to use my Lovoltech JFETs other than The Pass F3 and circlotron, and always wanting to test new small amps for my 110dB compression drivers, I came up with this selfbiased sweetie.

I was thinking of a supply voltage of +-9V, and an idle current of say 100mA-200mA. I plan to include it into a feedback loop of a gainstage, but it could work on its own as a buffer.

The only question is - and this is were YOU clever guys can have a go - how to deal with thermal compensation of the PNP?

NB: the Lovoltech JFET will ask for something like -1.4V gate-source for 100-150mA.
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Old 16th May 2010, 01:08 PM   #2
Nrik is offline Nrik  Denmark
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Simplest thing would be something like this I guess.
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Old 16th May 2010, 02:39 PM   #3
Bigun is offline Bigun  Canada
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Nice devices JFETs. I've never used any but they appeal to me because of there unique transfer characteristic (and they aren't static sensitive).

This looks like a variation of the Technics implementation of the Julius Futterman toplogy.

6C33C OTL
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Old 16th May 2010, 04:11 PM   #4
Nrik is offline Nrik  Denmark
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Bigun - yes Power Jfets are nice - they looove the Lovoltechs over at the Pass forum.
But Futterman is a circlotron - and tubes can only be 'n-channel'.

I want complementary topology, and both circlotron and 'all N-channel' (or quasi-complementary) is something else.

But since there are no P-channel Power JFets around, one have to do something else.
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Old 16th May 2010, 04:35 PM   #5
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Quote:
Originally Posted by Nrik View Post
The only question is - and this is were YOU clever guys can have a go - how to deal with thermal compensation of the PNP?

NB: the Lovoltech JFET will ask for something like -1.4V gate-source for 100-150mA.
Hi Nrik, i have never used a Jfet but i might have a simple idea for you

Why not simply connect the Jfet gate to the bottom of the resistor & select the resistor to allow the bias current you are looking for? Ok so you'll drop a bit more voltage accross the resistor, but the bias will no longer be subject to the Vbe of the PNP transistor.

Failing that put a 1N4148 between the gate & transitor base & thermally connect it to the PNP transistor. I realise both methods involve a higher voltage drop accross the resistor.

You wanted simple though
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Old 16th May 2010, 05:03 PM   #6
Nrik is offline Nrik  Denmark
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Originally Posted by event horizon View Post
Hi Nrik, i have never used a Jfet but i might have a simple idea for you

Why not simply connect the Jfet gate to the bottom of the resistor & select the resistor to allow the bias current you are looking for? Ok so you'll drop a bit more voltage accross the resistor, but the bias will no longer be subject to the Vbe of the PNP transistor.

Failing that put a 1N4148 between the gate & transitor base & thermally connect it to the PNP transistor. I realise both methods involve a higher voltage drop accross the resistor.

You wanted simple though
Thanks for your feedback
Your first suggestion (turning the JFET into a constant current source) will still leave the PNP to do its thermal runaway. Besides - the CCS would then transform the circuit into a single-end class A.
Your second suggestion is more or less what I am doing in my second post - I am trying it out right now so Ill get back on that one.
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Old 16th May 2010, 05:22 PM   #7
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You shouldn't get any thermal runaway as long as the PNP transistor has some kind of heatsink. If the Jfet was turned into a CCS then the only way the transistor could turn on more current is if the load accepts it. Yes, the base emitter voltage may drop due to heating but it wouldn't be able to turn on more current.

Having said that i'm not sure about the behaviour of the power Jfet under heating, no idea if this would be positive of negative temperature coefficient so you might be right

As i said i have never used a jfet, let alone one designed for higher power usage.

Hope you get it sorted, looks like you are trying for some kind of SRPP action
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Old 16th May 2010, 05:34 PM   #8
Nrik is offline Nrik  Denmark
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hmmm okay you are right about the CCS.
The Fet will probably behave nice this way ( thats how Nelson uses them)

SRPP? No, it is supposed to be a classic source/emitter follower - would make a nice headphone amp.
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Old 16th May 2010, 05:48 PM   #9
Nrik is offline Nrik  Denmark
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...yep - now its working. Now all I need is to build a stereo set and listen.
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Old 16th May 2010, 09:31 PM   #10
Bigun is offline Bigun  Canada
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Just my way of thinking, call me narrow minded but I think it would be a waste to use the JFET as a CCS; it is the centrepiece here.
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