JFET on resistance?

[Rich Grise]

On Sun, 12 Apr 2009 08:39:10 -0400, tempus fugit wrote:

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
tempus fugit wrote:

Hi;

Is there a way to figure out the R(on) of a JFET when this parameter
isn't
specified in the data sheet?

Measure one ?

I mean before I place the order.

If it's not fully spec'd, don't buy it.

Good Luck!
Rich

 

[tempus fugit]

"Tim Wescott" <tim@seemywebsite.com> wrote in message
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tempus fugit wrote:
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
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On Sun, 12 Apr 2009 10:50:21 -0400, "tempus fugit"
toccata@quitspammingme.ciaccess.com> 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




So what's the matter with an MPF-102? Not available?

MOSFETs will work, too.

CMOS switches will probably provide soft switching if you ramp the
'enable' input -- the simpler ones are just a pair of MOSFETs with
none-too-spectacular Rds-on figures.

--
Actually I breadboarded my design using an MPF 102, since it was all I had

lying around. Unfortunately, it has a Vgs off of 8V, so it didnt provide
great isolation when it was off. You're suggesting a MOSFET also? I've got
some BS170s lying around so I'll do some work with them.

Thanks