The Little FET that Oscillated

[wrenchone]

Last night. I was trying to sort some Supertex DN2530N3 depletion mode mosfets (TO-92 package, 300V) for use as current sources. I tried one with a 100 ohm resistor from gate to source, a regulated DC supply connected from drain to gate, and a meter across the resistor to measure the voltage drop. To my surprise, the fet oscillated so violently that it totally confused the power supply and caused a big blast of noise from the stereo playing on the floor above (I was toiling away in the basement). A 1k resistor in series with the gate lead totally quiets down the little b**%$@@@, but I was surprised how much havoc a little TO-92 device could cause if not handled properly. The fatal factors were no doubt the lack of any stopper on the gate and the long leads I was using to hook up up the device under test. Just a word of caution to anyone else trying to use these devices.

[lineup]

this is one reason we use Gate Stopper resistors, not only in JFET

And what we make with this R, is one low pass filter
( RC filter, where C is the capacitance of the JFET )
Even if this lowpass filter can have a very high filter frequency
of several tenths of MHz ( >= 10 MHz )

[Rikard Nilsson]

Try using a Zobel filter from the gate to ground. (after the stopper resistor)

Something like 100 ohms in series with 47 pF will probably do the trick.

[wrenchone]

Nothing a baroque as a zobel was needed - a 1k gate resistor was sufficient. I posted mainly because this device seems to be extra slippery. I use stopper resistors as a matter of course in my finished circuits, but haven't included them in sorting jigs. I've never had any big problems until this FET. This includes all sorts of JFETs and enhancement mode devices from ON semi and Supertex. My newest sorting jig includes the stopper resistors, but it's still under construction.

This particular Supertex depletion mode part gave squirreley results with both my original sorting jig and the rough setup described in my original post. Sorting using a 10 mA ring of two current sink and a 1k gate stopper resistor was successful. Funny enough, the TO-220 version of this part in 400V flavor (DN2540N5) didn't give me any trouble. Those were a set of older parts purchased on Ebay. I have a set of much newer devices recently purchased from Mouser. I think I'll be extra careful when it comes to sorting those.