op amp driving mosfet question

Discussion in 'Homework Help' started by bug13, Dec 4, 2012.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    here is a simple constant current load circuit from eevblog.com, it's possible that U2 and Q1 might oscillate, a series resistor need to put between U2 and Q1 to stop it.

    So what cause the oscillation here? and why?

    [​IMG]
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The input of the Mosfet will behave as a capacitor.
    A resistor in series with the gate of the Mosfet will reduce the peak current into the gate.

    Bertus
     
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  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Sorry to dig this old post up, what if I replace the MOSFET with a bipolar transistor, does it still oscillate?
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,023
    3,236
    Some op amps are sensitive to output load capacitance and will oscillate unless there is a resistor added in series with the capacitance. A BJT is much less likely to cause oscillations in that circuit since it has a much lower input capacitance than a MOSFET.
     
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