Need Help with JFETS

[Mark F]

Hi All, I have quite a few JFETS that I got from GodBout Elec. (bring back some memories from you old timers?)they are house numbered & I can't find any data on them. Some are dual and some are single. I've had them for quite a few yrs. & I just recently figured out that they are FETs as they were sent to me unrequested as they were getting rid of all their stock. Anyway, to get to the point, I built RG's circuit for gate to source cutoff voltage & it works quite well. Is there a test circuit I can build to find the aprox. gain(transconductance?) I built RG's FET Doubler & it looks good on the scope but I had to keep trying different FETs as the gains were quite variable. I hope this makes sense. Thanks in advance.

[Jered]

There is a fet tester also at RG's site that I believe tests Vgs.
Jered

[Boofhead]

To measure Idss you start with say a 10V rail,  put a 220ohm resistor in series with the + rail - the resistor protects the JFET in case you connect it incorrectly.  If you have a variable supply use that.  Connect the source and gate together and connect that point to 0V.  Connect  a current meter to the 220R then connect that to the drain.  The measured current is Idss.

RG's Vgs tester doesn't actually give you the pinch-off voltage Vp - the magnitude of the measured Vgs is always less than Vp.  The reason for this is because there is a finite amount of current going through the JFET in the test ckt - Vp is defined at Id = 0.   If you have already measure Idss then you need to measure/calculate the drain current in RG's test ckt. The from the JFET equation you can find the Vp required to make the equation satisfy the measure Vgs, Id of the test circuit.   You can also modify RG's  test ckt to run the JFET at very low current so the measured Vgs is closer to Vp.

The (maximum) transconductance Yfs0 can be subsequently calculated from 2 Idss/Vp.

Jfet pinouts and P/N channel type can be worked out with a diode tester on a multimeter.

[Mark F]

Thanks, Boofhead I do have a variable supply and a good DMM so I'll try your suggestion. Thanks again.