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MOSFET output not stable

B

Ben Vaughan

Jan 1, 1970
0
Hi,

I have a very simple MOSFET circuit that is not performing how I
thought it would, I'm hoping someone might be able to tell me what's
happening.

The circuit is as follows: I have a drain resistor of 750 ohms, the
source goes to ground and the gate has a voltage applied directly from
a power supply. I'm using a 2N7000 FET and the supply is as 20V. The
gate voltage I can vary. If I set the gate voltage so that the FET
should be within it's Linear range, say around 2.5V for example, the
output starts at 6.7V and drops at an exponential rate to around 6.22V
and then seems to be fluctuating quite a lot, with 10's of millivolts
change. Also the room temperature is a constant 19 degrees celsius. I
have tried 5 different FET's all with similar results.

I don't understand why this is happening? I have a fixed gate voltage,
fixed supply voltage, fixed resistance, constant temperature and yet
my output is fluctuating by quite a lot. Can someone enlighten me as
to what's happening?

Cheers,
Ben.
 
Well, that power supply driving the gate is not all that stable or
quiet, and the power that the FET dissipates does warm it up - which you
see as a dropping drain voltage.
So....trash those incorrect assumptions and re-start

One problem we occasionally had with MOSFETs at Cambridge Instrument
was their tendency to oscillate at a few hundred MHz. This wasn't
always visible on regular oscilliscopes, and could lead to very odd DC
measurements.

We ended up putting 10R resistors close to and in series with the gate
of pretty much every MOSFET we used. The seemed to be enough to kill
the tendency to high frequency oscillations in most cases, and you
could always change it to 100R or something larger if it didn't
provide enough damping in a particular situation.
 
B

Ben Vaughan

Jan 1, 1970
0
One problem we occasionally had with MOSFETs at Cambridge Instrument
was their tendency to oscillate at a few hundred MHz. This wasn't
always visible on regular oscilliscopes, and could lead to very odd DC
measurements.

We ended up putting 10R resistors close to and in series with the gate
of pretty much every MOSFET we used. The seemed to be enough to kill
the tendency to high frequency oscillations in most cases, and you
could always change it to 100R or something larger if it didn't
provide enough damping in a particular situation.

Thanks guys for the responses.

Paul your certainly right about switching circuits being easier, this
is really my first time dealing with FET's in a linear sense and
there's a lot more involved than I had first imagined! Thanks for your
explanation of why the feedback kills the overshoot, I understand a
little more now :)

Bill I'll try adding a low series resistor to the gate tomorrow and
see if it further improves things.

Cheers,
Ben.
 
R

Robert Baer

Jan 1, 1970
0
Ben said:
Hi guys,

Thanks for your responses. I agree that there is noise in my supply
and that is part of what I am seeing. However at the beginning when I
switch the circuit on there is a significant time (30 seconds +)
before the output voltage of the FET has settled. It seems to drop
around 2 volts in this time. Once it has settled then yes I am just
looking at noise in the supply, which I suppose I should be using some
bypass caps to alleviate.

Is this settling time normal?

Cheers,
Ben.
It is not "settling"; it is the FET warming up as i mentioned.
 
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