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Oscillation amplitude of a opamp sine wave oscillator

Discussion in 'General Electronics' started by Bill yg, Nov 17, 2003.

  1. Bill yg

    Bill yg Guest

    Dear All,

    Perhaps this is a dumb question but I really find confused.

    I am building a simple RC-phase shift oscillator using an opamp, with
    +/- 15V power supplies. I used a trimmer in the feedback resistor
    across the opamp. However, I found something interesting.

    The output amplitude is normally +/- 15V. The waveform is somewhat a
    fine-looking sine wave.
    If I set a larger opamp gain, the output waveform is distorted and
    looked like a square wave.
    If I set a smaller opamp gain just enough to start oscillation, it
    will result in a +/- 12V or smaller sine wave.

    The question is:
    Why is this so? Why won't the +/- 12V sine wave grow up to +/- 15V?
    What I mean is, in theory, the oscillation should grow up to
    infinity... and so why now is bounded by +/- VCC? and even with a
    smaller gain, bounded by +/- 12V?

    Thank you very much.

    Billy yg
     
  2. Robert Baer

    Robert Baer Guest

    Op amps at best, cannot go beyond the supply voltages; that should be
    obvious.
    The non-rail-to-rail opamps normally have problems going within about
    3V of the positive supply and within 1V of the negative supply.
    If there is too much gain, then the opamp simply clips the peaks
    because it cannot go beyond the supplies.
    If you do not have enough gain, it *cannot* oscillate!
    If you get a +/- 12V sine wave at some gain settings, that is an
    indication you have some nonlinear amplitude limiter in the circuit.
     
  3. Bill yg

    Bill yg Guest

    Thanks for your reply. So, is this circuit-related? I mean, there is
    no mathematics that governs why at a particular gain, the output is
    +/- 12V? Or even there IS mathematics, the equations themselves vary
    from one opamp to the other?
     
  4. Robert Baer

    Robert Baer Guest

    The math behind the required gain and phase for oscillation has
    nothing to do with the op-amps.
    The op-amps are the means of realizing gain and phase reversal (if
    needed).
    So, start with a phase-retard oscillator circuit: "source" to R (TP1),
    R from TP1 to TP2, R from TP2 to TP3; C from TP1 to ground, C from TP2
    to ground, C from TP3 to ground; gain and phase reversal stage from TP3
    to "source". I think the minimum gain is about 39 (too lazy to
    calculate); the waveform at TP3 is close to a sine wave (3% distortion)
    if "source" is a square wave.
    Amplitude is limited by the gain and phase reversal stage limiting as
    i mentioned about op-amps.
    Your 12V output level comes from some kind of non-linear device in
    your circuit: zeners, incandescent lamp, FET as either gain adjust or
    amplitude adjust, an op-amp limiting.
    Cannot say what, as i know nothing about your circuit and what parts
    you are using.
     
  5. Bill yg

    Bill yg Guest

    Thank you very much !
     
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