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MOSFET output not stable

Discussion in 'Electronic Design' started by Ben Vaughan, Mar 19, 2007.

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  1. Ben Vaughan

    Ben Vaughan Guest


    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

    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?

  2. Guest

    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

    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.
  3. Ben Vaughan

    Ben Vaughan Guest

    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.

  4. Robert Baer

    Robert Baer Guest

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