[sdiy] FET for VCO cap discharge

[JH.]

Hi Ian,

> The BF256C is denoted as a VHF/UHF amplifier, whereas the 2N4391 is a
> switching device -- therefore they are spec'ed differently.  I have only
> ever tried switching devices, although other people have reported using
> small signal ones.
>
> According to the data sheet I have, the 256 has a zero-bias drain current
> of under 20 mA.  They don't spec the maximum drain current, but let's
> suppose it's 20 mA.

I don't even have a data sheet of the 256 at hand. It was just a desperate
move to try what I had in storage, and I was surprised that it worked
(slightly) better than the 4391.

> This calculation assumes,
> of course, that the FET can actually carry the peak current, which it may
> not.

Good point. I didn't think much about max. current rating - just about
the maximum current _capability_ . Isn't a small signal JFET more or less
a current limiting device (as long as the gate isn't forward biased), which
would take care of it's max current all by itself? Leaving maximum _power_
considerations, of course, but here the small pulse width would come in.

> So I'm obviously missing something important here.  Any FET experts who
can
> straighten me out on this?

Me too. I'm not a FET expert either. When I looked for a JFET with higher
specified current rating than the 4391 at one of my local suppliers, I got
the impression
it was pretty much what there is. So I'd either go MOSFET or bipolar, or
parallel JFETs.

The most ridiculous reason (but that's what it was, as un-scientific it may
sound!) to try the 256 wasn't even the lower price, but that with this flat
plastic package, I could stick 3 into the solderless breadboard where the
metal can 4391 would have required 3 more wire connections
to a second set of contacts. (;->)

Now that contrary to my expectations, that 256 seems to work quite well,
I just had to blurt it out.

JH.