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Twin Tee Sine Wave Oscillator

Discussion in 'Electronic Basics' started by zero, Mar 17, 2005.

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  1. zero

    zero Guest

    How does a twin-tee sine wave oscillator work?

    The schematic I have shows an op-amp with a twin-tee between the output and
    the - input, and a voltage divider formed by a 10K resistor from the output
    to the + input, and a 1K resistor from the + input to ground.

    I understand that at the frequency w= 1/ RC the twin-tee produces a -180
    degree phase shift. But at w= 1/RC, wouldn't the - input of the op-amp
    become 0V because of the twin-tee's notch effect? Then the loop gain AB =0?
     
  2. Andrew Holme

    Andrew Holme Guest

    AB =0?

    The notch has finite depth, typically 60dB i.e. B = 0.001

    The potential divider on the +ve input provides a little positive
    feedback to make sure the oscillator starts reliably.
     
  3. zero

    zero Guest

    If B = 0.001 would I have to change the voltage divider on the + input of
    the op-amp to produce a gain of A = 1000 to keep AB = -1 at the desired
    frequency?
     
  4. Andrew Holme

    Andrew Holme Guest

    No.

    For one thing: B is not 0.001; that's just a typical "ballpark" figure.
    It will vary with component tolerances, temperature e.t.c.

    Another thing: you can't control gain like that with POSITIVE feedback.

    Sustained oscillation requires zero (or 360 degrees) phase shift and
    unity gain. Due to temperature coefficients and component tolerances,
    you can't get exactly unity gain, so oscillators are designed with
    gains greater than unity. The question then is: what is the amplitude
    limiting mechanism?

    The amplitude may be allowed to grow until limited by the power supply
    rails, the designer may use the non-linear characteristic of an active
    component to limit the amplitude, or some form of automatic gain
    control may be employed - whichever way it is done, the gain is
    effectively unity!

    The amplitude limiting mechanism affects the purity of the sine wave.
     
  5. John  Larkin

    John Larkin Guest


    The positive feedback path wants to make it oscillate; assume it wants
    to oscillate at all frequencies. The negative feedback through the t-t
    overcomes the positive feedback and kills the oscillation. But at the
    tt notch frequency, there is no neg feedback, so that's the only
    frequency it can oscillate at, so it does.

    John
     
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