My simple JFET characterization is oscillationg like crazy.. why?

Question

I'm simply trying to characterize a MPF102 JFET. I attached my HP6625A Power supply as follows:

CH1 : 0 - 12VDC with the + on Drain and the - on source CH2 : 0 to 1V (actually 0 to -1 because I place the + on the source and the - on the gate).

While I step up the Drain to Source voltage (CH1) while holding the Gate to Source voltage at 0 (ch2).... I start to get some pretty serious oscillations on both channels (3.62 MHZ).

Any idea why? Shouldn't the HP power supply be able to hold these values steady?

I know that my two probes (for my Tek 3032 scope) have 8pf of capacitance. I am also using about 4 ft 16 gauge twisted pair leads coming from the power supply.

I just purchased the HP power supply off of Ebay and everything up to this appeared fine. Could this be an equipment issue?

Thanks to anyone who might suggest a reason.

Stephen

Tim Wescott · Accepted Answer

Take the schematic for a Hartley oscillator:

.-----------------------------. | | | | | || | | .-------o------||-----o | | | || | | |-+ C| C| '---->| C| C| |-+ C| C| | | | | | | === === === GND GND GND created by Andy´s ASCII-Circuit v1.24.140803 Beta

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Note that the coils don't have to be coupled -- that's convenient but not necessary in a Hartley. Now call the inductors "twisted pair" and the capacitor "gate-drain capacitance". Does this sound at all familiar?

------------------------------------------- Tim Wescott Wescott Design Services

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Stephen · Answer

Thanks Dave... I was just adding the bypass caps and they did indeedwork.

DaveM · Answer

That long TP line from the power supply is introducing a sizable amount of inductance into the setup.  And the MPF102 has a fair amount of transconductance, so it's not suprising to hear that it oscillates quite well with that setup.I'd suggest that you bypass all the power supply lines very close to the transistor.  That should tame that oscillating beast-- Dave MMasonDG44 at comcast dot net  (Just subsitute the appropriate characters in the address)Never take a laxative and a sleeping pill at the same time!!

My simple JFET characterization is oscillationg like crazy.. why?

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