Little Dumb question on RG's Jfet matching

[barret77]

Hello all

I've built the RG's Jfet matcher from GEO - and I'd just like to confirm if I'm making things right:

When I power-up the circuit with no JFET inserted, the DMM reads -1.69 V;

Then I test a batch of 2N5458 jfets;

the DMM then varies between -0.50V and -0.63V; many match at -0.55V

I understand very little of the theory that's going on here - so my question: Are these values correct and expected? Does it seem that I've made something wrong?

thanks a lot and happy thanksgiving! (I'm having a DIY holiday!)

[bioroids]

According to the datasheet, the 2N5458 should be in the range -1v / -7v . Yours measure a little lower than the minimum, that's suspicious to me, but maybe is posible as the Vds is probably different than in the datasheet.

I've used the fet matcher with no problems, so if you have everything hooked right (triple check the transistor pinout) it should work ok.

Luck!

Miguel

[Processaurus]

I was just using RG's circuit to match some FETs for a phase 45,  it actually took me three tries to get it to work (the shame).  First I didn't have ground hooked up to the opamp, then I had the wrong pinout for the FETs I was using, then the meter ground probe was hooked up to the wrong place, augh!
Maybe check your circuit again, even though its dead simple. 

Some #s I got for Vgsoff with my matcher after I got it working right are

2n5292:  -1.4v to -1.8v
2n5485: -1.8 to -2.4v

2n5458s have a wider tolerance range than the ones I tested, though.

heres some more on RGs matcher:
http://www.diystompboxes.com/smfforum/index.php?topic=4923.0

[R.G.]

Re-check your circuit. Proc's recommendations are on target.