The JFET-matcher

While building the Borbely amp from the previous post I had to measure and match a number of JFETs, both for the input circuit and for the current sources. I have done that before using a test setup based on a solderless bredboard and I used that here as well, but afterwards I started thinking if there was an easier way because I always have to rebuild the setup and it can be a little bit fiddly to use I think.

The result is this little PCB. It has screw clamps for input and output connections which means you can connect the leads from a multimeter directly – no crocodile clips and resistor leads required :D. There are connections for the power source and for volt-, amp- and ohmmeters (if you are measuring current sources with a resistor and not the raw Idss of the JFET). In addition, there are a few other features which I thought might be handy:

  • Silkscreen on the back of the boards to keep board connections to hand.
  • A jumper (marked “Idss”) to bypass the resistor/trimpot and measure Idss.
  • 2×3 pattern for a trimpot to allow connections of pots with both inline and staggered pins (soldered or via socket strips)
  • Connections for a switch to control whether the ohmmeter is connected or not. This means that you can connect the ohmmeter permanently to the board using the screwclamps. Then measuring is as simple as connecting the JFET, adjusting the trimpot to read the right current and then remove the JFET and pressing the switch to get the corresponding resistance reading – should help if measuring a larger batch of FETs.

The board measures app. 50x41mm and if there’s any interest I can easily share the eagle files. I also have a few spare boards (as I don’t really need 10 of these :D) so if  you just want a single board send me a message.

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5 Responses to The JFET-matcher

  1. Jeremy says:

    I could really use one of these !!!
    Could I possible pry one loose from you 🙂

  2. Pingback: Project files: B1 buffer/preamp | theslowdiyer

  3. Wim Groenier says:

    Hey,
    I just discovered this article, is there one left? I live in France, I can send you money to cover the costs 🙂 Let me know price & account.
    Cheers,

    Wim

    • theslowdiyer says:

      Hi Wim,

      I should have spares of most of the designs. If you send me an email through the form on the “contact me” page with what you need I’ll have a look.

      /U.

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