Electronic – How to identify the circuit has negative feedback or positive feedback

feedbackintegrated-circuitnegative feedbackoperational-amplifierpositive-feedback

I am having difficulty in identifying whether feedback is negative or positive (i'm not talking about feedback topology)

My teacher told that there is a method in which, after deactivating independent sources, we break the loop and move from that breaking point around the loop, then and return to it after traversing some path. This allows us to determine the loop gain

He says that we can break it at any point that we wish but he always breaks it on Opamp's output. He said that is for convenience without giving reason; in his demonstrations he always moves from output to input to output.

If the calculated loop gain is negative then the circuit has negative feedback. If the calculated loop gain is zero zero then the circuit has zero feedback. If the calculated loop gain is positive then the circuit has positive feedback.

After that he gave some examples (in all examples there was at least one op amp present). But I want to know: does this method only work for op amps, and if it not works for all circuits then why not?

Please give me insight into intuition behind such method. Sorry for the long question

thank you for answering

Best Answer

  1. It will work with nearly all circuits, not just op-amp circuits.
    1. And I'm only saying "nearly all" because there's always exceptions -- just assume that it always works.
    2. It can get tricky if you break it at a point where the resulting circuit's output impedance is close to the resulting circuit's input impedance. Then you have to account for how that "input" loads that "output".
    3. Note, too that for some circuits and for some break points, you may want to consider the feedback to be a current, not a voltage -- there are some transistor amplifiers like this.
  2. Your instructor was probably breaking the circuit at the op-amp output because the output impedance is low, and it "looks into" a relatively high input impedance. As an exercise, you can always take some of his examples and try breaking the circuit at some other point (the next most convenient point in an op-amp circuit would be at the \$V_-\$ pin). Work out the math, and see if you get an answer that's the same, or substantially so.
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