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opamp phase shift, how to think about it

Discussion in 'Electronic Basics' started by Asa Cannell, May 6, 2004.

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  1. Asa Cannell

    Asa Cannell Guest

    I am having trouble understanding opamp phase shift. For instance, its
    common to have 90 degrees open loop phase shift for a wide frequency
    range, but if you were to connect the opamp in a non-inverting
    configuration, there would not be any phase shift in the output vs.
    the input in that 90degree range. Where is the phase shift? Or if you
    were trying to make a phase shift oscillator, you would only need to
    add the extra 90 degrees phase shift at one frequency to get it to
    oscillate, so clearly the opamp is supplying the other 90 (plus 180
    for the inverting input), but in the non-oscillating band, the phase
    shift is zero, how does the phase shift suddenly appear?

    Asa
     
  2. It is still there exactly as it was before you closed the feedback.
    That gain and phase shift data you see on the data sheet is the output
    relative to the differential voltage on the two inputs. As long as
    the opamp is not in saturation or slew limited output swing, that data
    applies. That is a completely different thing than the gain and phase
    between some other two points, like the + input and the connected
    output and - input (which is really the voltage between the inputs).
    When you connect up the feedback path, the output becomes two voltages
    in series, with respect to the input. A very tiny one that is the
    difference between the + and - input and a much larger one that is the
    output. there is still a 90 degree phase shift between these two
    parts of the total, but the differential input voltage is so small
    compared to the output voltage (by the open loop gain of the
    amplifier at that frequency) that you just don't notice it and only
    see the output that is almost perfectly in phase with the + input (and
    very nearly as large in amplitude). If the output did anything else,
    there would not still be a tiny difference between the inputs so the
    output would be driven toward the above described situation.
    Since the phase shift oscillator does not usually use the opamp as a
    strictly differential input open loop amplifier, but has some
    additional feedback across the opamp, the opamp is not providing a 90
    degree phase shift to the oscillator loop. It can be connected to
    have very near zero phase shift or very nearly 180 degrees phase shift
    with a fixed gain. The feedback network that connects the output of
    this gain block back to the input must provide 180 degrees of
    additional phase shift to form a stable oscillator.

    If you have a specific schematic in mind that would make this easier
    to talk about.
     
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