Matching Power Transistors

Going down the line, testing all the MOSFETs on one amp section of the dual amp Ampeg SVT4Pro.

http://power-amplifier.blogspot.com/2007/10/zeus-mosfet-matching.html

http://www.firstwatt.com/pdf/art_matching.pdf

http://sound.westhost.com/transistor-matching.htm

http://music-electronics-forum.com/t24947/

Matching transistors step 1 is to buy a large quantity from a trusted retailer like Mouser. Check the "factory package" quantity, or at least buy at the first quantity discount break.

A package from the factory is likely to contain devices all from the same "batch" which should be pretty well matched already.

Take your pick of transistor matching methods. All of them have the same purpose: to test the Vgs or Vbe under the same conditions. You want devices that hit the same numbers... but what is the tolerance?? That is the biggest question for me from an academic view point, but in practice it isn't an issue (as long as you followed "step 1" above). I like to shoot for all of the Vgs/Vbe reading to be within 100mV, which I think is overkill and that's why I do it (overkill beats underkill). The Pass article gives 200mV for the limit. Note that his article seems to be for Hi-Fi where power amps should never be pushed to their limit. Bass guitar power amps are regularly pushed to their limits, and unmatched devices will fail in those operating conditions. Exactly where "unmatched" becomes "matched" is a big gray area.

I just test all the transistors in the tube, line them up with their test numbers, then match up the devices in groups that are all within 100mV or better. Often I get 50mV groups.