Preliminary tests on 2SJ109. Fake?

[Bikeman]

Hi All.

So I managed to source a few toshiba 2sj109 BL.
I realize that there's probably a lot of fakes around out there, but I bought from what seems to be a decent dealer so I have some small hope...
I won't name them at this stage, not until I'm sure one way or the other.

Now, I am a complete n00b when it comes to jfets, so I grabbed the data sheet:
http://ezphysics.nchu.edu.tw/prophys/ael/File/Datasheet/2sj109.pdf
and did a quick measurement of IDSS.
The part came up as 10ma, which looked promising, but I have a couple of concerns, both about my results, and methods

So is this right?:
I have the Gate and Source connected (Vgs=0V) and I am putting 10 V from Source to Drain (Vds = -10V).
Then I measure the current Ids.

Is this right?

The other concern I have is that if I am reading the datasheet properly, Ids shouldn't vary much as I change Vds from say -2V to -10V.
This is where things get interesting, as I am measuring around 6ma at -4V which steadily increases to 10ma at -10V.
Should it vary this much?
I'm trying to avoid plotting curves...

Any thoughts?

Mark

 

[davidsrsb]

No. The graph shows Idss only falling at Vds <1V, so suspicious.
Check Vgs(off) by connecting drain to -9V, gate to 0V and a resistor of about 100k from source to 0V. Measure across the resistor for the pinch off voltage, which should be <2V

 

[AndrewT]

2sj109 is a pchannel jFET.
Vgs is +ve when measured from G to S.

Id varies a lot as Vds is varied.
i.e. expect >50% variation in Id as Vds is swept from -2V to -10V

This is why we put a low gm jFET around the j109 as a cascode to nearly eliminate the variation in Id as the supply voltage is varied.