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

[Stephen]

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

 

[DaveM]

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

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 M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[Stephen]

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

 

[Tim Wescott]

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

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 www.tech-chat.de

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?