JFET matching information
- Nelson Pass
Some information on your JFET requirements
- The gold standards for JFET matching are the Toshiba 2SK389 and Linear Systems LSK389 duals, whose Idss is matched ±1mA
- The JFET matching tolerance specified for any of Nelson's projects is the same, at ±1mA
- At ±1mA, the effect of tolerances in other components of your amplifier will have more effect and be more important than your JFET match. Adjusting for those variations is the purpose of the trim pots in your amplifier, and you can make those adjustments at operating temperature.
- There is no such thing as a perfect component. What matters is that you feel you have spent your money wisely and you sleep well at night.
About our current batch of matched JFETs
Our current range of matched JFETs were tested at 12V and 22'C. Current was read from a 100Ω resistor (dropping the voltage to 11V), with the source shorted to the gate. They were then sorted into 0.1mA bins. This results in a ±0.1mA match, which is an order of magnitude tighter than required.
- N and P channel JFETs use different materials to achieve their opposite effect (pentavalent vs trivalent elements). They are not exact mirror images of each other and do not exhibit a perfectly linear mirrored response with relation to input. Different test conditions (voltage, temperature, test duration) will alter the match between N and P.
To measure a JFET:
- Get a 5 to 13 volt supply and a DC voltmeter (we used 12V)
- Connect the +V to the Drain
- Connect the Gate and Source together to a 100Ω resistor
- Connect the other lead of the resistor to ground
- Measure the voltage across the resistor, which gives the Idss (1 volt = 10 mA of current)
- Try to measure all devices under the same conditions, temperature, and duration of test
Note: The DCA Peak Atlas Pro semiconductor tester will give inaccurate results for testing these JFETs as it cannot provide adequate current during the test. Please refer to the user manual for more information on its specifications. This is a great tool but should not be used for testing high current / high voltage devices.