Re: MOSFET output not stable

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.

On Mar 19, 3:51 pm, "Phil Allison" <philalli...@xxxxxxxxxx> wrote:
"Ben Vaughan"

** Groper Alert





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,

** Well, you think it is fixed.

But how do you know it has NO variation or noise ?

fixed supply voltage, fixed resistance, constant temperature and yet
my output is fluctuating by quite a lot.

** Huh - it is varying by a TINY amount !

By 10s of mVs only !!

Can someone enlighten me as
to what's happening?

** The FET is amplifying tiny gate voltage variations that are too small for
your test gear to see.

...... Phil


.

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