Jfet matching problems

[oldrocker]

I've searched for past messages on this subject but I still don't understand this setup.  What opamp should I use here?  I tried a 741.  When I measure voltage at (pins + & -)  I get 4.47v.  It doesn't matter if there's a transitor plugged in it or not. At this point I don't think anything in the circuit does anything to this voltage.  What size cap should I use here?  I'm seeing a range from 10uF to 47uF.  I used a 47uF.  I'm testing 2N5952's for a P-45.  I have 12 of them.  Looks like a simple circuit (Improved JFET Matching) but I'm not doing something right here.

[Seljer]

I used a TL071 and a 22uF since they were the first thigns I found, following the article on Geofex. Worked fine for all the 2n5952s, 2n5485s and 2n5457s I had. Out of the 14 2n5952s I had I managed to get 2 precisely matched ones for a Phase 45.

[oldrocker]

Thanks Seljer,  I realized after your response that I needed to use a different IC.  I took out the 741 (ooops) and installed a TL082 and it seems to be working now thanks.

[stm]

JFET characterization for matching can be easily done as described here (one 9V battery, one digital voltmeter and two resistors is all you need):

http://www.runoffgroove.com/fetzervalve.html#11