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:



Posted by at 16:40
Tags: , ,

41 comments:

  1. Alex Irae28 July 2012 at 21:15

    Do it works with TL072?

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    1. IvIark28 July 2012 at 21:27

      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.

      Delete
  2. Anonymous30 July 2012 at 04:32

    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. IvIark30 July 2012 at 09:46

      That's good enough for me! :o)

      Delete
    2. j-pee11 August 2019 at 22:06

      hi,

      it is very good!
      such a beautiful tool!
      we only have to use the Voltmeter to get two readings... including a current value :)

      NOTE:
      the switch mentioned in the layout DPDT -- aka 2P2T ...
      per def does NOT have a mid setting...
      only 2, A and B :)

      so I suggest either buying a 2P3T switch :)
      or placing 2 switches on the board...
      which I did...

      one to turn it ON and OFF..
      and one to choose the measurement method...

      I also suggest adding at least one LED... as an indicator of the ON state :) but there could be another one, too, indicating the measurement, although this is totally NOT necessary...
      only decoration :)

      but the ON LED is important, I think...

      I suggest making this not in a box whatsoever...
      but as flat thing, I used a beer pad -- see my post at guitar-fx-layouts:
      http://guitar-fx-layouts.42897.x6.nabble.com/which-jfet-matcher-do-you-use-think-is-most-reliable-td30460.html#a47122

      Delete
  3. Jacob30 July 2012 at 20:27

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

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    1. IvIark30 July 2012 at 20:31

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

      Delete
    2. Jacob30 July 2012 at 20:35

      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!

      Delete
  4. mikejoyce25 August 2012 at 17:24

    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. IvIark25 August 2012 at 17:37

      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.

      Delete
    2. mikejoyce25 August 2012 at 18:04

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

      Delete
  5. Gc662 December 2012 at 14:35

    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

    ReplyDelete
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    1. Gc662 December 2012 at 14:45

      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?

      Delete
  6. JaviCAP14 February 2013 at 00:54

    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. IvIark14 February 2013 at 12:19

      Are you sure the contact in the switch is making?

      Delete
    2. JaviCAP14 February 2013 at 21:33

      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

      Delete
    3. Mr.Baiser12 June 2013 at 08:30

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

      Delete
    4. DC7 May 2020 at 11:35

      I know this is an ancient question, but people still refer here. What you are seeing is residual capacitance from the 47u DC smoothing capacitor. After disconnecting the power, the capacitor starts slowly discharging. If you watch the voltage reading it will slowly start to drop.

      Delete
    5. DC7 May 2020 at 11:37

      Sorry, the answer below is correct. I am talking about the voltage readings that you get after disconnecting power. I built this on a breadboard and don't have a switch, just jumpers that I move around.

      Delete
  7. H.A.M.17 July 2013 at 11:18

    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. drdFX14 August 2013 at 09:00

    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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    1. Gc6630 August 2013 at 15:51

      Can I find an updated layout anywhere for this?

      Delete
  9. Unknown17 September 2013 at 18:05

    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. agitprop20 June 2014 at 02:04

    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. motterpaul7 October 2014 at 07:30

    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. jaredmcohen6 March 2015 at 03:35

    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?

    ReplyDelete
  13. Unknown20 March 2015 at 01:25

    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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  14. Chance21 December 2016 at 12:36

    I built the IC version and matched a quad of 2N5485's within 5mv for a phase 90. Works brilliantly, thank you!

    ReplyDelete
  15. Unknown15 March 2017 at 14:40

    built the IC version and was able to match 4 jfets for a phaser. thank!!

    ReplyDelete
  16. pakkee6621 March 2017 at 15:25

    This comment has been removed by the author.

    ReplyDelete
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    1. pakkee6629 March 2017 at 15:28

      firstly, apologize for my comment about the mpf102 get smoke on the board.. I know whats the matter now, the only things need to pay attention is [china market], the answer is here: http://i1244.photobucket.com/albums/gg569/pakkee66/DIY%20PEDAL/fake%20mpf102_zpsru1opnxh.jpg
      so that's it..and sorry again.

      Delete
  17. Unknown2 June 2017 at 21:18

    I tried testing a J201 with the smaller version and it got hot and smoked. Why?

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    1. Unknown3 June 2017 at 04:16

      You either put the jfet in the wrong way or have an error in your build. These jfet are very sensitive, I have burned up a few just using a meter probe and shorted them to ground. this tester is extremely accurate and useful for lots of jfet builds

      .what are you making Jay? Good luck triple check your build before trying any more jfets. I seem to make more mistakes on the easy builds than the mega ones.If you want to post some pics of your matcher i would be happy to help check it for you.

      Delete
    2. Unknown3 June 2017 at 04:16

      You either put the jfet in the wrong way or have an error in your build. These jfet are very sensitive, I have burned up a few just using a meter probe and shorted them to ground. this tester is extremely accurate and useful for lots of jfet builds

      .what are you making Jay? Good luck triple check your build before trying any more jfets. I seem to make more mistakes on the easy builds than the mega ones.If you want to post some pics of your matcher i would be happy to help check it for you.

      Delete
  18. Unknown4 June 2017 at 06:36

    Shitty imitation JFETs from China is my guess. I tried it with some good ones from guitarpcb and they worked fine.

    ReplyDelete
  19. crustone13 June 2019 at 04:04

    i've just built this little jewel, to run some tests...it gives me strange results: 9 2n5457 tried: 2 of them give 3.5v; 2 of them 0.9v (i think they are faulted); 5 of them are 0.4v...does anyone knows which is the "right" value of this circuit for 2n5457?
    anyway, for anyone who builds this, i suggest this item: LCR-T4 328 mega...you can find for a few bucks on ebay, and it's really usefull and quite precise; it identifies every component you put on, without minding the pinout

    ReplyDelete
  20. Unknown15 November 2019 at 20:36

    How about using 2N3819 as I have 10 approx lying around in spares box

    ReplyDelete
  21. DC7 May 2020 at 12:23

    Built this on a breadboard. Got 3 matched pairs from my 10 pack of J201s :)

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  22. Unknown10 November 2020 at 04:29

    Aloha,

    I've just put together the Greatly Improved JFET Matcher II on a breadbaord and have tested two 2SK170 (for discrete Op-amp build). The figures I get are: 0.67V/9.38mA & 0.62V/7.05mA.

    I'm wondering if someone can confirm that these figures jive with what I should expect from an authentic component. I Bought them NOS from a local electronic store whose sales clerk told me he hadn't sold any since 2011...

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    1. CrackinTheShat17 April 2022 at 03:23

      Those values match what you'd expect from a 2SK170, specifically the BL variant: Vp -0.2-1.5V, Idss 6-12mA

      For future reference, you can find the spec'd values in the datasheet: https://www.mouser.com/datasheet/2/408/6909-57550.pdf

      Delete
  23. Dazz8 August 2022 at 23:42

    Is the opamp acting as a buffer there? Won't it limit the mínimum Vp reading to 1.5V, since that's the lowest the opamp can go at the output?

    ReplyDelete

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