Jfet Vgs off Circuit

[percyhornickel]

Hi, first I want to be clear that I am not an electronic expert, just a self-learning guy (Petroleum Engineer) trying to expand my knowledge in this particular section.
After some study about jfets I would like to hear some opinions about this circuit to measure Vgs Off.

The main idea is to have Vs  (virtual ground) with a voltage slightly higher than max value of the Vgs off  showed in a particular datasheet, so Vcc would be at least  2 x Vs.

Vcc values like 5V and 10V works good in simulation for commun Jfets  (J201, 2N5457, 2N5458, etc  etc).  according their particular Vgs off.

The image shows the Vgs value (green line) when Id (red line) reach 0mA. To get that value we just have to adjust the potentiometer until Id is cero, then Vg - Vs.

I am using 10 ohms at the voltage divider (virtual ground) so the current would not affect the Vs value when Vg decreases.
Until now simulation works very well and the values are according the datasheet and the Spice Vto (Vgs off) for each Jfet.

I know there are some circuits over there but after trying to understand them I was just wondering if this could be used..  ..and of course, I don't have a transistor tester.

Gracias y saludos desde Venezuela.

B.T.W. THIS IS NOT A SPICE SUMULATION TOPIC.

[Vivek]

I had found this article useful

https://stompville.co.uk/?p=112

[Vivek]

[percyhornickel]

I have seem this article before but it was not that clear for me then. I have done the simulación for the Jfet and 20M resistor at source and the Vgs off is pretty close to the spice value for the Jfets I have tried. As Big the resistor - More accurate the Vgs off value as I can see.
10M resistor is used according the DMM internal value.

Vivek the way I see it, both circuits could be used.

[percyhornickel]

I have matched some jfets with the R.G. opamp circuit too, even there is a great vídeo from Erik Vincent (DIYGUITARPEDALS) explaining the correct way to do it too.

I just used the protoboard and three 10K resistor, very simple to understand and usefull.!!

[PRR]

> Vcc values like 5V and 10V works good in simulation for commun Jfets  (J201, 2N5457, 2N5458, etc  etc).

J201 spec sheet says Vgs(off) is measured at 20V. J211(?) at 15V. As I read your plan, you apply 5V. While Id varies very little with Vds above a few volts, 5V is enough on the low side to give numbers different from the datasheet.

> Id (red line) reach 0mA.

What is "zero"? 0.01mA? 1nA? 1fA? There is a very long toe on a JFET cut-off. Tenths of a volt. And it never reaches absolute zero. For production testing, the factory picks a value which is low but not too low to measure reliably on high-speed machinery.

[percyhornickel]

PRR with "Zero" I meant a very low current flowing through Id (mA /uA / pA). You are right about the 20V specified in the datasheet. I was taking a low values for Vcc not considering the datasheet test conditions.
I am working this week (even I'm using my phone to write this in my trailer / oil rig facilities).
My real plan when I get home next week is using a 18V power supply for Vcc and adjust the potentiometer until Id reach near to zero mA, then I could measure Vgs.
The potentiometer with this circuit would be very sensitive using 18V Vcc too, so I will have to be careful.
Thank you for pointing out the Vcc datasheet value!.