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Matching rig for MOSFETs in the sweet region

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  • Matching rig for MOSFETs in the sweet region

    Hi! I have around hundred IRFP140/IRFP9140 (for Ampeg BA115HP/BA115HPT etc) and IRFP240/IRFP9240 (Ampeg SVT-3 Pro etc).
    So I figured I would make a simple matching rig, and measure them to matched pairs/quarts. I've had good success with this one:

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    I used R1=150ohms and Vin=15V. With IRFP140/IRFP9140 it draws 76mA and IRFP240/IRFP9240 it draws 87mA. I measure VGS between the green points in the schematic.
    I just chose 150ohms to have the same a reference for all my matchings.

    Would matching them in the more sweet region, at the levels bass players on average play (increase or decrease R1) make any difference? No need to match them in a region a bass player would never play (e.g. master/volume 1).
    Or will the difference between each MOSFET just be proportionally changed with R1? So my MOSFET list from lowest to highest VGS, remain in the same order?
    Transfer characteristics would differ between even same batches of MOSFETs, so I would suspect matching in the sweet region would be beneficial?
    Which R1 value would in this case be best for the 140/9140 and for the 240/9240?

    And the guy who created the above schematics chose 15V without any explanation, but that was for matching MOSFETs for another application. Not sure if it makes any difference increasing or decreasing this voltage?

    Third question is, how long should I apply power, before noting down each VGS? I see some are dropping quicker than others, but it looks like groups of similar VGS levels drops with the same speed.

    Fourth question is, if I have "perfectly" matched pairs of both high VGS (e.g. 4V) and low VGS (e.g. 2V) of both N and P channel MOSFETs (so 4 pairs). In the amp, would it be best to choose e.g. the low value pairs for both N and P MOSFETs? Or doesn't it matter between P and N (since they are not shared)?
    Would it affect sound if I choose the lowest or highest VGS MOSFETs? Or would it just make my amp run hotter or colder?

  • #2
    Like a valve in pairs, it is good to balance them but only for the point at which they start to conduct, not in full conduction.
    If you measure the voltage across R1 and adjust the gate voltage to just conduct, matching is performed in that fashion for use in amplifiers, exactly the same as valves.
    The purpose is to eliminate crossover distortion without dissipating too much heat.
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    • #3
      I've attached some of my files and Test Fixtures, procedures I've used in Selecting bulk-purchased IRFP 240's and IRFP9240 MosFET's for use in Ampeg SVT3-Pro, SVT4-Pro, etc. Along with an article from Nelss Pass that looks much like the test jig you show. There's also a link to an MEF post I had added the details and results found on one of the SVT4-Pro amps I was working on at the time:

      Ampeg SVT 4 Pro 10V dc offset - Music Electronics Forum (music-electronics-forum.com)

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      Fahey MosFet Matcher.pdf

      How To- matching devices Nelson Pass.pdf

      Matching MosFET's-Nelson Pass.pdf

      Over the years since buying these MosFET's in bulk, and taking the time to select and then match them, noting the numbers on the back of the devices and stacking them back into their tubes as received, I've had a great deal of success with keeping the Ampeg SVTPro Amps working in our Rental Fleet of backline gear here at CenterStaging, LLC in Burbank, CA.
      Attached Files
      Logic is an organized way of going wrong with confidence

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