More on FET Matching

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Well as promised here is a quick run down on my journey through the FET matching process. Pictured here is my latest mixer and above it are three different little test jigs that I built to help match and measure FET characteristics. From left to right we have a FET Voltage at Pinchoff (VP) test tool, a FET matching bridge and at far right a very simple jig for measuring FET IDSS. It simply shorts the Gate and Source together.

All of the information for building these devices started at the web site of QRP Homebuilder. Sadly in recent days this outstanding treasure trove of electronic goodness has been pulled from the Internet. Thankfully Todd Gale, VE7BPO the owner and creator of QRP Homebuilder has archived the entire site and made it available as a single (large) PDF document. See here for details.

I found measuring IDSS is a pain because as the current flows heat is caused and circuit conditions change. This in turn causes the meter readings to change before your very eyes as you watch it. I’ve read that manufacturers actually pulse DC current into the FET to stop this from happening when they test for IDSS. I have mentioned in a previous post that I settled for simply switching the current flow on. Waiting a set number of seconds (10 in my case, the same number for each test) for things to settle down a bit and then simply recorded the number displayed on the volt meter at that point in time.

The FET matching bridge is very precise. It tells you when two FET’s are exactly the same. Which is pretty much what you want for a FET mixer of any type, be it Passive, Active or whatever. But this does not tell you what the pinch-off voltage is (VP). Which could be very important if your trying to exceede VP deliberately for the purpose of creating a switching or “chopper” mixer. The Minima mixer is supposed to be a mixer of this type. Equally, if you wanted to build a mixer that remains in the non-linear square-law region. Then the reverse would become important. You would want to make sure your mixer was NOT exceeding VP.

So that’s what the most complicated test jig pictured above is for. The FET Voltage at Pinchoff (VP) test tool. This tool allows you to adjust the FET bias such that you just hit pinch-off (VP) when your digital volt meter reads zero volts across a resistor. Once this is done you then connect your volt meter across another set of test points in the circuit to read off the actual pinch off voltage (VP).

Again all of the knowledge I gathered together to build and use these tools came straight from the QRP Homebuilder web site. Which is now archived into a PDF linked above.

One final comment.

What I found (like Professor Vasily Ivanenko before me) was that if two FET’s have matching IDSS values then the VP of those same two FET’s would be very close indeed. So I think for all practical purposes that Amateurs can match FET’s for use in mixers by simply using the most simplest of the test jigs above. The IDSS test tool. So simple in fact, it does not really need a jig at all. It is only a time saving convienience if you plan on testing multiple FET’s in the one session.

So this then will get you a “matched” set of FET’s. Having got that far then all you the need to make sure that your mixer circuit is well and truely exceeding VP, or not. Depending on the mixer type your trying to build. For the Minima it is supposed to be a switching or “chopper” mixer. As such VP needs to be exceeded. I checked mine and it was. But there is some general concern about that the original Minima mixer design may not be able to deliver sufficient voltage swing to drive the J310 FET’s into cut-off. This does not mean that the mixer would not function. In fact it may even mix quite well. But it would not be operating as a high performance commutating (switching) mixer. Which has implications for the rest of the circuit as a whole.

Next up, some crystal filters…

73, Steve

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