Powering fets..

[mnordbye]

What is your favourite way of powering a JFET transistor? I've been using trimpots, but find them unreliable (at least the ones i have). Is there any other method you guys prefer?

Thanks

[alanlan]

I presume you mean biasing.

Search the forum for "FET biasing" and maybe study this:
http://www.datasheetcatalog.org/datasheet/vishay/70595.pdf

[mnordbye]

Yeah, guess i mean biasing. A bit late here over here in Norway.

Thinking of trying out a two resistors in series from 9V to Ground, then the point between those resistors will be 4,5V. Is my theory correct?

Other suggestions welcome.

[aziltz]

Yeah, guess i mean biasing. A bit late here over here in Norway.

Thinking of trying out a two resistors in series from 9V to Ground, then the point between those resistors will be 4,5V. Is my theory correct?

Other suggestions welcome.

"hard" biasing like that will work for certain BJT and MOSFET circuits, but the reason we have to use a trimpot to get 4.5 is because the FET's all vary in some way, and the resistance needed to get 4.5 at the output is always different.

there ARE other ways to bias, but the trimpot is the simplest one, IMO

[Earthscum]

I've been screwing around a bit with the fets as well. I tried the trick of getting your bias set with the trim, then pull the trimmer and measure the resistance, then use a resistor network to replace the trimmer. After I put the resistors in, my bias was off again. I tried this a couple times. I didn't just grab a 220R resistor and assume that it was 220R, I actually handpicked the closest resistance and had the exact resistance as the trimmer.

Is this just because of the 5% resistors? Does this method actually work better with 1% res?

BTW, I personally like the 15-turn trimmers. They are kind of a pain to fit to a PCB, but on a breadboard they work great. Alot easier to get an acurate biasing.

[mnordbye]

Yeah, guess i mean biasing. A bit late here over here in Norway.

Thinking of trying out a two resistors in series from 9V to Ground, then the point between those resistors will be 4,5V. Is my theory correct?

Other suggestions welcome.

"hard" biasing like that will work for certain BJT and MOSFET circuits, but the reason we have to use a trimpot to get 4.5 is because the FET's all vary in some way, and the resistance needed to get 4.5 at the output is always different.

there ARE other ways to bias, but the trimpot is the simplest one, IMO

I see trimpots is the way to go then. I will have to buy some quality ones, i tend to always go for the cheapest stuff around.

[aziltz]

i really like the idea of the trimpot in the final build because it makes it easier to test out different FETs without re-soldering.  Make sure to use sockets for the FETs though