Jfet Matcher question.

[compuwade]

Hey there! I was thinking of building the Phase 90 so I decided to breadboard the Jfet Matcher from geofex. I do not have the specified fets, but I do have a whole bunch of j201's. When testing these fets I get a reading of 100-300 mV But according to the spec the minimum should be 300mV or more. I don't think I understand what I'm actually checking for so I was hoping that someone could explain to me what these readings mean and whether or not I should even consider using any of these fets.

Thanks,
Wade

[R.G.]

The J201 is something of an anomaly for JFETs. It has an extremely low Vgsoff, the reverse bias needed to make it turn off completely.

The Vgsoff is what is specified on the datasheet.

However, Vgsoff is NOT what you use in a phaser. You want some voltage less negative than Vgsoff, because you want the JFET to act like a variable resistor, which means it is not totally off.

The  JFET matcher at geofex measures the voltage needed to make the JFET have the same DC drop as a 10K resistor. So it's result for a JFET will be between Vgs=0V (totally on) and Vgsoff (totally off).

Your results may be fine for the J201. I say "may be"just because I have never personally measured J201s in that FET matcher.

I personally would not pick the J201 for a phaser, as my opinion is that there are much better devices. But I also realize that any JFET one has *may* work, and that's better than the perfect JFET which is at the other end of the world.    And some people have reported good results with the J201 in a phaser, so it probably *can* work.

If it were me, I would recheck the pinouts of the devices I was testing, then test some and get a matched set. I would try to get them on the high side, towards 300mV, for compatiblity with the LFO in the phaser. And then I'd try it. Worst thing that can happen is that you have to order JFETs.

[compuwade]

Very cool! Thank you very much for your explaination as I have a better understanding of what I'm looking at now. I guess I'll match some fets and build the circuit and try them. If they work, cool, if not, at least I learned something!

Thanks again!

-Wade