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Oscillation conditions: Feedback phase shift

  1. Apr 17, 2010 #1
    One of the conditions for oscillation is that the (regenerative) feedback loop must provide a 180 degree phase shift. This is due to the fact that, for a regenerative effect, the signal must undergo n*360 degrees phase shift: 180 from the amplifier and another 180 from the feedback network.

    My question is, doesn't this depend on the type of configuration used with the amplifier?

    For instance, a common-source amplifier is an inverting configuration, so this condition would certainly apply. For a common-drain, or source follower, configuration though, the output is in phase with the input, isn't it?

    That also brings up another doubt I have. I have never been able to "pin point" exactly what it is in an inverting configuration that makes...inverting. Some brief explanation would be greatly appreciated.
  2. jcsd
  3. Apr 17, 2010 #2
    Yes the feedback oscillator requires a negative feedback amplifier.

    Negative feedback is another way of saying inverting amplifier which is another way of saying 180 phase shift.

    Another way of looking at the 180 phase shift required of the feedback network is that is simple cancels out the phase shift introduced by the amplifier, putting you back where you started.

    Note the phase shift is only to be applied at the desired frequency of oscillation, at all other frequencies it is not 180.

    Nor should you think the reason we use an inverting amplifier is that a non inverting one won't work.
    It will oscillate all to well and we would likely loose control of it as the feedback is self reinforcing.

    However neither type of amplifier will oscillate unless the gain is sufficient to compensate the losses in the feedback network. A phase shift network requires a gain of at least 29. Common drain or common collector amplifiers have a gain of just less than 1.
  4. Apr 18, 2010 #3


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    If you have no option but to use a non-inverting, low voltage-gain amplifier, you can achieve inversion (and voltage magnification) with a transformer or, at higher frequencies, some appropriate transmission line.
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