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opamp somehow exceeds open loop gain!

Discussion in 'Electronic Basics' started by Asa Cannell, Apr 28, 2004.

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  1. Asa Cannell

    Asa Cannell Guest

    I have an opamp (OPA348 to be exact) in a non inverting configuration.
    Take a standard noninverting configuration (two resistors in series
    from output to ground, resitor junction connected to inverting input),
    but instead of connecting the resistor network to ground, connect it
    to one side of a capacitor, and the other side of the capacitor to
    ground. Now we frequency variable gain, (high pass).

    In the simulator, if I change the large resistor to 1000k and the
    small resistor to 3.16k, and the capacitor to .001125uF, I am getting
    a gain of 316 at 12KHz. This exceeds the opamps open loop gain
    specified in the datasheet, which is well under 40db (100) at 12KHz.
    I have a 100k resistor on the opamps output as a load, per the
    datasheet.

    How is this possible? Is the opamps phase shifts somehow acting as an
    inductor, and along with the capacitor, somehow a series resonant LC
    circuit is formed and I am getting voltage gain? (please post
    responses in the forum)

    Asa
     
  2. Soeren

    Soeren Guest

    Hi Asa,

    Simulators are nice toys, but they cannot be better than the models used
    (and the programming).

    If you "simulate" with your brain and some real life components...
    WYSIWYG.
     
  3. Roy McCammon

    Roy McCammon Guest

    you are probably getting enough phase shift ( almost 90 for
    the op amp and almost 90 for your feedback) to have almost
    positive feed back. Its also called regenerative gain.
    Bottom line yes you can get extra gain that way
    but the actual gain is very sensitive. In effect
    you have an almost oscillator.
     
  4. Fred Stevens

    Fred Stevens Guest


    The closed loop gain equation Av = 1 + Zf/Zg (Zg the impedance to
    ground) is accurate under the assumption that the open loop op amp
    gain is infinity which is clerly not the case with your circuit!
    Obviously the simulator you are using is making certain "assumptions"

    Fred.
     
  5. Asa Cannell

    Asa Cannell Guest


    By extra gain do you mean gain above open loop gain? (seemingly
    violating the datasheet spec). I notice that the inverting terminal is
    approximately equal to the noninverting terminal over most of the
    freq. band (as expected), but where I have the "extra gain" peak, it
    also peaks, and is no longer equal to the noninverting input at that
    frequency.

    Asa
     
  6. Well, the "simulator" doesn't make those kinds of assumptions. It does
    what it is told to do, usually. It solves equations very accurately. The
    *model* may well have approximations and assumptions that result in
    these accurate solutions but have incorrect results.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  7. Fred Stevens

    Fred Stevens Guest

    What I meant is that the simulator uses a model in which certain
    assumptions (and approximations) are made.

    Fred.
     
  8. Roy McCammon

    Roy McCammon Guest

    so look at the ratio of the output to difference between
    those two pins to see if the gain of the opamp is
    what you expect.
     
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