Hi everyone! Semi-noob here: I'm just following the schematics for a DIY audio project as closely as I can, with very little actual insight, currently just hoping for the best (and hoping to learn by way of osmosis as I go).

I've bought a batch of 2N3819 JFETs (replacement for PF5102 in the schematics, which was hard get a hold of. The ones I got were useless and possibly fake). I need to match a pair of them as closely as possible by Idss value, so I tried measuring it using procedures I googled.

I THINK I got it right, looks simple enough, but the thing is: in a batch of 10, every single one gave an Idss reading of exactly 4.2 mA. From what I understand, the norm is that two 2N3819's are rarely identical, even within the same batch. The chance of this happening for all 10 seems minuscule, so is this result an indication that I did something wrong and shouldn't trust these readings?

For ref, here's a summary:

I understood that I need >15V DC Vds to get an accurate measurement, and all I had was a breadboard power source of 3.5V or 5V. So instead I connected two 9V batteries in series, and verified the output of 18 V up front.

Using the breadboard, I then shorted the Gate to the Source of the JFET, connected them both to ground (negative power supply), connected one pole of a switch to the Drain, and the other pole of the switch to my multimeter (which in turn was connected further to the positive power supply pole), reading the current directly out as an Idss value when I closed the switch for each JFET. Sounds right?

Thanks a lot in advance!

 

your test should have told you tge IDss of the parts.
If you vary the power supply voltage you should see very little change in current.

You might try adding a resistor in series with your test lead to find out whether there is a problem with the test setup.

 

It would be indeed a huge luck to have 10 consecutive JFETs with identical IDSS.

Your circuit description is just fine, so there must be an error or malfunction somewhere.

 

I tested some 2N3819 the Idss for this variety measured typically 4mA to 12mA. None were the same on DCA75
Within the same batch there is less variance, the difference is clearly significant in Idss for both national and fairchild.
I only had a few Motorola MPF102 they measured 8.6mA and 9.2mA the test was limited by the number of test samples.
Possibly the Fairchild matchers were sorted on a friday or what was swept off the floor that could'nt meet specs.
The question remains, I received a package of 1% resistors that are 0.001% there must be some mistake.

 

You might try adding a resistor in series with your test lead to find out whether there is a problem with the test setup.

Hmm, what would be a result then that would tell me about a problem (or vice versa, what should happen if everything is correct) ?

To help me reason about it: can I think of a JFET in practice as a variable resistor? So in this example, if the maximum current at Vds = 18V is indeed 4.2 mA (at Vgs = 0V), it kind of is a 4 kΩ - ∞ resistor, or is there more to it?

 

With a resistor in series with the drain you could try passing a small current through the base-emitter junction and measuring the drain-source voltage as a function of base current. That should at least confirm the JFET is functioning and isn't just a 4k resistor! If all samples then behave the same you're in luck and have a well-matched set.

 

Try the measurement again at 9V. You should get near the same current. That would verify that it is acting like a jfet rather than a resistor.

 

Not an answer to your question but my experience.

The few times I did it to get matched pairs I tested a whole lot of maybe 40 or 50 transistors, 10 of them at a time, ensuring a steady temperature with no fan running nor windows open, and separated them, right there on the bench, classified as per the measurements obtained .

I then rechecked the several small lots to confirm measurements. Weeks or months later, repeating the procedure yielded quite consistent results.

Measuring the transistors immediately after the insertion in the breadboard showed the effect of the temperature due to the manipulation. That's why I left them alone around a minute for their temperature to settle. Working in a cold room (usually my case because I do not enjoy heating much) makes this effect quite evident. So, be patient and wait.

 

Testing them at 9V (with no resistor, just as described above and only using one 9V battery instead of 2): 1.8 mA for 9 of them, 1.7 for the last one, so basically the "same" result but with an unexpected value?

 

They were tested at steady room temperature, no draft. If anything, a little warm.

 

Then they are not likely not what they are supposed to be.

The darasheet shows about 8.5mA at either 9 or 18V with Vgs=0. But the range is 2 to 20mA at 15V. Any jfet should be near constant current over that voltage range.

 

Thanks everyone! Disappointing, but good to clarify!

 

The next step will be to check them for DC gain, which is possibly a better indication of a useful parameter.