Distribution of Idss in a bunch of random 2SJ74? - Page 4 - diyAudio
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Old 22nd November 2012, 01:54 PM   #31
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Hannes, see post #17... that is the difference in the parts I acquired between them and the original. Idss measured fine.
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Old 23rd November 2012, 07:03 AM   #32
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@bear:

Quote:
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.

Last edited by h_a; 23rd November 2012 at 07:27 AM.
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Old 23rd November 2012, 09:44 AM   #33
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ok, thats easy to do.

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

Thanks!
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