Possible J201 substitution

[Casey73]

I found a Fairchild JFET, PF5102, that looks like a possible substitute for the J201, which is no longer being made by Fairchild. The only specs that don't match up are under small signal characteristics. The J201 data sheet lists yfs 500 minimum. The PF5102 data sheet lists gfs as 11,000, and several other characteristics (goss, ciss and crss) but make no reference to yfs.

I'm not an engineer, so I don't know how significant those numbers are in the application we use JFETs for and would like for someone smarter than me to render an opinion.

[CynicalMan]

This article has good information on JFET characteristics.
http://www.runoffgroove.com/fetzervalve.html

The specs that we usually worry about are VGS(off) and IDSS. Just taking a quick look at the datasheets, the IDSS of the PF5102 seems too high to act as a J201 substitute.

[ubersam]

How about a Vishay/Siliconix J204? http://www.datasheetcatalog.org/datasheet/vishay/70233.pdf. The same minimum V_GS(off) but the Max is 0.5v higher than a J201, the same minimum I_DSS but the Max is 2V higher, and it has a lower V_(BR)GSS (-25V, J201 is -40V). On second thought, maybe it's not a very good sub. Anyway, you can still get J201s from tayda, futurlec, or smallbear.

[R.G.]

"yfs" is the transconductance, in Siemens. That is, how many amperes does the drain current change for each volt of change on the gate-source?

I believe that the "500" is in microsiemens, so that's actually 0.5 ma/V. 11,000 is 11ma/V, so if I read that right, the PF5102 has 22 times as much change in current for a given change in gate voltage, and so has a much higher gain, if everything else is the same.

Everything else is never the same, but we can use that as a rough guide. With a higher transconductance and a higher Idss, the PF5102 is capable of much higher gains.

I think. Haven't read the datasheet yet.