Measuring VGs off in JFETs

[Toney]

 I rigged up the GEO JFET tester for the first time yesterday and was a bit confused at the low readings I was getting with J201s (I just bought a batch due to the coming possible shortage). Anyway, I sort of just rushed it together on the breadboard. A good lesson in what what assumptions will get me  
 Re-reading the article, slowly, properly it dawned on me that its actually testing the transistors in sort-of mid point of usage, with the 10K resistor emulating the channel "a bit" closed, not for VGs as I was shooting for.
So to test for actual VGs off, or very close to it, is it correct to assume I can replace the 10K with a much higher value to replicate a closed channel, say 1-10M? Or is the current too small for accurate readings?
Perhaps someone knows another method?

[R.G.]

There is a very similar circuit designed to measure Vgsoff. I'll try to look it up. It's quite similar to the GEO matcher.

There is a problem with measuring that, and it's one of objectives. How "off" is off? You have to decide that more than some resistance drain-source is off. That may be 1M, 100M, 1G, 100G, 1T, but you have to decide where off is before you can test for it.

The J201 *is* an anomalous JFET, in that it's Vgsoff may be as little as 0.1V according to the datasheets.

[DiscoVlad]

Vgs(off) is also dependent on the applied voltage Vds (all the data sheets I have for small signal FETs specify 15V), and some value of leakage(?) current; Id = 10nA.
which in this instance means they're using an "off" resistance of 15V ÷ 10^-6A = 15 MΩ

[Toney]

 I am just looking to see if my batch of J201's are in spec - I followed all the "good" advice and bought them from Ebay (alarm 1) and China (alarm2).
Heh heh I know, I know... but I was a bit late stocking up for the great through-hole Jfet hoarding and my regular parts place is already out of stock - none coming.
So in this case I am simply attempting to measure VGs off as the J201 has a particularly low VGs off and that seems like a good starting point. I want to see if it falls within the datasheet spec as I am suspicious of these parts.They are supposedly Fairchild J201's but have the classic sanded and branded look to them. That's to say the faces of the transistors look much like auto paint that's been wet and dry sanded flat and dull, then possibly re-branded. Maybe, maybe not I cant tell but they don't have the look and feel of the ones I bought from Aron or my local parts store.
Perhaps earlier or different fab ones have this look but I have learned that anything is possible. They couldn't be bothered facing and painting individual Jfets yet.... could they? Lol.

Also, I would like to generally know how to measure VGs off  

[Toney]

 Thanks DiscoVlad - so 15M is a "decent" off figure.

OK, well I found  a good simple trick to measure VGsoff using just a 9v battery, the device and the usual 10M input resistor of the average multimeter in the circuit at the bottom of the source. Should be very close.

Like so:

                                         

Original pages with good simple info here:
http://stompville.co.uk/?p=112

[kingswayguitar]

how about p-fet?

[Eb7+9]

I recently devised a simple method for producing good approximations of jFET's Idss and Vgs(off) figures (extracted at a fixed reference Vds voltage) ... it involves using a signal generator with tunable DC offset on the output (HP 3310B in my case) to drive a triangle wave input on a Source grounded jFET transistor having a very small resistive load on the Drain (... done to avoid Drain-Gate modulation based errors)

for an n-channel device the input wave goes from zero to a negative voltage // the linear-time aspect of a triangle wave plays the (visual) role of that of a DC swept voltage and so can be used to plot out the characteristic Id(Vgs) curve for a relatively-fixed (or in this case semi-fixed) Drain voltage directly on a scope ... by keeping the Drain voltage semi-fixed I'm doing two things, (i) satisfying the requirement of defining Idss and Vgs(off) at the same Vds "reference" voltage, and  (ii) producing something that approximates small-signal (zero-limiting) linear measurement ... two stones with one bird

by observing these results on the scope I came upon the observation that varying input bias conditions would make each side of the pseudo-parabolic curve "meet" in either an overly-flat way or in a non-flat way ... having the response to perfectly flat at the meeting point, in the middle of the other two cases, would give the condition whereby Vgs is lying at or very near Vgs(off) ... aiming for a "true" parabolic shape // in this condition Idss and Vgs(off) data pairs can be given with fairly good accuracy (with no data interpolation required) ...

and, from there true jFET device matching can be performed // and things like Rmin, rellevant to Voltage-Controlled Resistor (phasor) applications, predicted reliably ... AS STATED SEVERAL TIMES PREVIOUSLY any other system with NFB acting upon the device will forcibly "lump" both these numbers into an equivalent (OP) operating point "figure" which has absolutely no bearing on parameter matching of any sort (read my prior posts for detailed explanations) ...



a gif movie posted at my blog shows the three curve shape conditions contrasted against each other ...

www.viva-analog.com

[Gurner]

how about p-fet?

Just reverse the test....so, a  high value resistor between 9V & the source, drain goes to ground, gate goes to 9V too. Now measure the voltage at the source leg & subtract that voltage from 9v.....so for example 7.5V measured at the source = 1.5V cutoff.

[kingswayguitar]

measure v-off in n-jfets all the time with good results in the subsequent builds
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/j201off.jpg.html

was having trouble with p-jfets but now all is well
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/j117off.jpg.html

woops...it's actually a j177 not j117

i just measure the voltage across the source resistor with the red lead of the multimeter on the hot side of the resistor