FET Matching
To make life easy, I have adapted this text from the A75 construction article from the Passlabs site.
The test is simple and requires a power supply, a resistor, and a DC voltmeter. The figure 12 shows the test hookup for N- and P-channel types. The supply source resistance (R1) is nominal, and is found from I = (V -4)/R1. Consistency is the most important thing here. The given voltage is 15 and, adjusting for about a 4V VGS, we will see about 11V across the resistor.
I used a precision 24V/25A regulated supply. Nelson states the input pair are biased at 20mA and the output at "slightly greater than 3 amps". For the 9610s, I = (V-4)/R1 we wanted a R1 = 20/0.02 = 1K. Given there are 3 devices in the output, each must be biased at about 1 amp, so this is where I wanted to match them. R1 = (V-4)/I = 20ohm. Fortunately I had a Dale 50W 20 ohm power resistor lying around which was perfect (this will be 0.5A if you use 6 devices). This was going to generate some heat, so I made a small test heatsink and secured the FET in question with a strong "bull-dog" paper clip on a rubber insulting washer. This is important, because as you power-up the VGS will exponentially decay to an equilibrium value as the FET heats and you need to leave each one idling for a while before taking the measurement (I used VGS at 5 minutes). Continuing on .....
Keep in mind the caveats about electrostatic discharge: touch ground before you touch the parts.
Matching input MOSFETs is critical, because they must share equally the 20mA of bias current from the current source, and they will not do that unless their VGS is matched.
If you are unable to find input devices matched to within 10mV, you must insert resistance in the source to make up the difference. The resistance is calculated by the difference of the two values of VGS divided by 10mA. For example, if the difference in VP1GS is 100mV, then 0.1/0.01 = 10ohm. You would then place 10ohm in series with the MOSFET source having the lower VGS.
We use the same test setup for the MOSFETs in the TO-247 packages ....
..... we like to see the load shared, and recommend that you group the outputs by VGS as closely as possible. Matching within 0.2V will work, and O.1V is even better. Within a population of 150 transistors, you can easily get 12 sets matched to O.1V VGS.
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