Re: JFET on resistance?

On Sun, 12 Apr 2009 14:01:31 -0400, "tempus fugit"
<toccata@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:tbj1u41pm09rjphl1odd4230qvmb1vl2om@xxxxxxxxxx
On Sun, 12 Apr 2009 10:50:21 -0400, "tempus fugit"
<toccata@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Wow thats awful..

Yup, he did make a truly bad guess, off by roughly 50:1.



I'm looking for a JFET to switch a guitar signal in and out of effects
pedals. I'll need 8 in total, but the signal will only ever be passing
through 4 JFETS at any 1 time. I am going to bias the drain and source at
1/2 V+ (4.5v in this case) and would like to switch the JFETs on and off
using a PIC. Since the PIC runs at 5v, I'm assuming it's control outputs
will also be 5v, so I'll need a fairly low Vgs off to switch the JFETs
completely off to isolate the effects from the signal chain when I want
them
bypassed. I realize the Ron doesn't need to be extremely low (although
the
lower the better, I suppose) but I also need the lower Vgs off. I'm
trying
to find a nice balance between those parameters.

Thanks



Why not use a cmos analog switch like an HC4066?

Why not bottom post?

John


I'm trying to get the switching to be click free, and I've tried a MAX4622
CMOS switch, with no success. I tried using an MPF 102 and the switching was
much quieter (only a tiny thump in one direction), and I've read that it's
easier to ramp up the control voltage on a JFET than it is on CMOS.
Which has led me to try ordering up a bunch of suitable JFETs to see if I
can get them to behave.

Thanks





OK, you can slow down the gate drive and de-click the switching, but
you can't de-thump it using a simple jfet switch. Go slow enough, and
you get lots of distortion during the transition.

But that jfet is a real wimp, intended as an electret mike amp. You'll
need something beefier if you want to do jfet switching.

Don't people already make thumpless analog switch chips?

Led-photoresistor things could be interesting.

John


.

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