Distribution of Idss in a bunch of random 2SJ74?

[bear]

Hannes, see post #17... that is the difference in the parts I acquired between them and the original. Idss measured fine.

[h_a]

@bear:

do you have a tip for us how to determine the transconductance "on the kitchen table"?

Transconductance is change in drain current for a given change in gate-source voltage; means measure IDSS (VGS=0) first, then insert a source resistor (VGS!=0) and measure ID. Divide the differences and you have an approximative transconductance value.

It's not so straight forward as transconductance depends a lot on the drain current, but this way should be sufficient to distinguish a genuine high transconductance jfet from a low transconductance part with both having the same IDSS.

Alternatively (but less practical), one could measure pinch-off voltage (when the jfet stops conducting), because that is very different also; for K170 it is about -1-2V VGS or so, for long channel low transconductance parts about -3-4 VGS or so.

[nr12]

ok, thats easy to do.

As I have some genuine K170 to compare this is ok for me.

Thanks!