On 6/28/2013 12:34 PM, John Larkin wrote:
On Fri, 28 Jun 2013 08:44:57 -0700, John Larkin
wrote:

On Fri, 28 Jun 2013 10:33:35 -0400, JW wrote:

On Fri, 28 Jun 2013 06:51:11 -0700 John Larkin
wrote in Message id:
:

On Fri, 28 Jun 2013 08:45:19 -0400, JW wrote:

Looking for a suitable cross-reference for a National PF5301 TO-92 N
channel JFET as I can't find the original part anywhere. Hell, I can't
even find an original datasheet. The critical parameter for the sub is
Igss which for the National part is 1pA. The only thing I've been able to
find so far is a Linear Systems LS5301 which has a Ig of 1pA, but at $40 a
pop (!), I'd like to find an alternate.

Does anyone know of any possible substitutes?

Thanks.

What's the application? Do you really need a jfet?

I'm pretty sure I do, as it looks to be a fairly sensitive circuit. It's
for a repair on a Keithley 220 programmable current source.
http://www.google.com/url?sa=t&rct=j...eEEYYk4sluspuA

The schematic is on page 6-31/6-32 (4 pages from the end) location Q302
near the top of the page. It's part of circuit which uses a AD515K
FET-Input Electrometer Op-amp.

Thanks.

Yikes, it's a jfet darlington! Really a weird circuit, overall.

I bet a 2N7000 would work. It's just an AC amplifier. Gate currents on a 2N7000
are femtoamps. You can turn one halfway on and leave the gate open, and it will
stay halfway on for hours. Fun to play with.

https://dl.dropboxusercontent.com/u/...rts/2N7000.jpg


Hmmm, biasing might be tricky. It might be better to replace both jfets with one
2N7000.

Q302 has a lot of drain-source voltage. I'm wondering why it doesn't have a lot
of gate current.


Maybe it's a cascode internally?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net