Saturday, 28 July 2012

Greatly Improved JFET Matcher

So moving on from RG's JFET matcher which measures VGS (or more precisely, the voltage at which RDS is 10K) , this is another version by Sebastian from runoffgroove which allows you to measure VP and IDSS depending on the position of the switch.

The centre off position should be used to insert and remove the JFETs, and as long as you keep the orientation of the switch the same as shown in the layout, up measures IDSS and down measures VP.

A combination of these two layouts will allow you to match JFETs in a number of different ways, so use the appropriate one depending on what characteristic you are required to match, but if in doubt which to use, having VP and IDSS close means the JFETs are more effectively matched and so this method is probably preferable.




And an updated version which is even more simple and will allegedly give more accurate matches:



24 comments:

  1. Replies
    1. It needs an IC with a FET input for the Vp measurement to work properly, so the TL072 should be ok, but don't use something like a JRC4558 or NE5532.

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  2. Added the no IC version to the bottom of my breadboard. Seems to work...never measured em before, so can't say for sure...but I definitely get readings :)

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    1. That's good enough for me! :o)

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  3. Can I test regular FETs with this? like 2N5952 or 2N5457?

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    1. They're JFETs as well, and you test any JFET with this.

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    2. Oh, I must have read something incorrect over the years then that suggested devices like the J201 were JFETs and the 2N5457 and 2N5952 were FETs.

      Thanks for clearing that up and everything you do for the community!

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  4. hey guys, I'm REALLY new to this aspect of building. I'll be making the one on the bottom here. What exactly am I looking to get on the DMM, and how do I read it? I'll be matching 2 pairs of 2n5952s for a phase 90 if that helps at all.

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    1. Build it up as shown and then connect your meter to the DMM+ and DMM- points shown on the board (something like these are great for adding easy measuringpoints to veroboards, particularly if your meter has crocodile clips: http://www.ebay.co.uk/itm/290663751268).

      Then just put your meter in DC voltage mode, take the readings and do the little sums to come up with the Idss and Vp values. You don't even need to do the convert for these values if you don't want to, just match the JFETs by choosing the closest pair of the direct DC voltage meter reading in both switch positions.

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    2. Awesome, thank you! I had no idea what Idss and Vp meant, so i peed a little bit and went fetal

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  5. Hello, I have a newbie question. I have looked on fre stomp box and other DIY sites but I cannot find the reason we want to match jfets in a circuit. I understand biasing and the use in something like the sucker punch, but, why does it , as some suggest, make a plexi drive sound better???

    This article was the closest I could get to an answer...

    http://www.diystompboxes.com/smfforum/index.php?PHPSESSID=356375dc0f8e5516ac29499d5ac3df21&topic=50828.20

    However, the "best link on the web for understanding jfets" could not be found by my browser

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    1. I just found some good info from mark on the 65 thread. From what I understand now matching is the way to avoid different bias resistance for every jfet. Therefore it will make a pedal like the plexidrive sound better because it does not include a bias trimmer... Right? Now my question is you have matched your jfets, now what values should you use for the resister or vv what values should you be looking for in the jets?

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  6. I feel a bit stupid asking this question with such a simple circuit, but just built the most simple version (no opamp) and on switch center position y get the same reading that on down position (switch 1-2)

    Measurements seem to be correct, but getting a measure in switch center postion gets me crazy!

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    1. Are you sure the contact in the switch is making?

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    2. I think that the problem should be there, in the switch, sadly this was my only left spdt on-off-on.

      Anyway, checkin it with de dmm, seems to work ok, no continuity between 1-2 and 2-3 with toggle on center position but can't think of another component failing.

      It seems to get correct measures, just it gets no unconnected on mid position, so i've tested the BF244 using it, and seems to "work" ok.

      BR

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    3. Same here. Vp measurement stays the same when the switch is in center position. Switch is a on/off/on and definitely working

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  7. I've build the no IC version. I get Vp readings in all positions of the switch, but no Idss. I was able to match 4x J201. I just remove the battery when I change a JFET. For my DMM measuring I use two sockets.

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  8. I built the non-IC version, but like the others I get Vp reading in the middle position. If you check the linked schematic from ROG, it is clear why: in the center position the 9V is not off, but the DMM measures the voltage on both resistors in series. Now 100Ohm is like nothing compared to 1MOhm, so basically in the middle position you measure the same voltage as on the 1MOhm resistor only. As a solution you can use a DPDT on-off-on 3-position switch. In this case you would connect the +9V connection of the circuit to the middle lug (lug nr 5) and connect the both outer lugs (4 and 6) both to the +9V supply. If you do this then in the middle position the 9V is not applied to the circuit and you have it switched of.

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    Replies
    1. Can I find an updated layout anywhere for this?

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  9. Could we not design a circuit with a 15 turn pot so that we could dial in a precise ohm per what ever v we are shooting for and then sub fixed resistors in the circuit were building so that instead of having to match jfets we can simply bias each jfet that's going in the circuit and use the "r" the values to keep them biased at the same v?

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  10. Hey, could someone revisit the no-IC layout, to fix the issue of not having an off-position? Vp comes through on the middle position.

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  11. I just built the no IC version myself and I don't seem to be able to make it do much of anything.I went with an off switch and a DPDT switch to go between Idss and Vp. I figured the idea was to read DC mv on your meter, but my readings are 00 to .1 at best.

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  12. Okay here's a stupid question... when are measurements close enough to "match" transistors? I understand from the ROG post that I don't need exact to-the-decimal matches, but what's the acceptable margin?

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  13. Hi All, I have built the non ic version, and have the same issue as you guys. Is there a fix for this or another good way to match the 2N5952's, I am not up to speed with Idss, Vp,Vgs and so on, Can some one give me an actual example..........at my age, I need all the help I can get...
    Regards Peter

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