Matching of BS170 FETs
Yet another method of matching the BS170s.
This matching configuration allows the comparison between a Reference FET (TR1) and the group of test transistors TR2 etc etc.
The circuit is capable of comparing the difference in Pinch Off voltages as well as the difference in Transconductance over a part of the operating dynamic range of the FETs. Operation:
1) VR1 is originally set with its wiper at Zero volts wrt ground.
2) The first FET (TR1) is the Reference transistor, all others placed in the TR2 position
shall be compared against this reference transistor.
3) To test for a good match, the potentiometer VR1 is adjusted from its Zero volts position
up to the supply voltage. During this adjustment, the operator observes the variation
of the voltage between the Drain of TR1 and TR2.
4) Each transistor tested in the TR2 position which exhibits only small variations on the
voltmeter shall be selected as a reasonable match against the reference transistor. 5) Typical variations are 200 mV Drain voltage differential in the Pinch Off region. This
equates to approximately a differential Drain current of 3 mA.
6) As VR1 is advanced toward the supply voltage, the Drain differential voltages shall
typically indicate lower than 20mV, which equates to a difference of Drain currents of
0.3 mA.
7) Continue to insert several transistors in the TR2 position and make a selection of the
best matches.
8) If you find that there is a very poor match on nearly every one then it is likely to be the
Reference transistor chosen that is the atypical device and it should be replaced and
the complete test sequence repeated and a new selection of “best matches” made. 9) If you find that there is NO variation in voltage then check your circuit !
Measure the voltage from the Drain of TR2 w.r.t earth while adjusting VR1 over its total range. The Drain voltage shall vary from the supply voltage down to approximately 0.4 Volts.
10) Each measurement taken should last no greater than 10 seconds to remain within the maximum ratings of the transistors under these operating conditions.
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I don't know who the author is, making the jig was pretty easy.
