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As of now most of the negative feedback systems I have encountered have a feedback factor that is less than 1 but positive. However in some situations it seems the system can behave as positive feedback if the feedback factor is not negative.. I just don't seem to comprehend how this negative attenuation is accomplished just using resistive networks.. Ones I have used till now in my studies.

nn08
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  • example, please – analogsystemsrf Jul 27 '19 at 10:03
  • Three common source amplifiers cascaded produce a negative low frequency gain.. If put inside a negative feedback loop for which IF the feedback factor is positive makes the negative feedback itself positive doesn't it..? – nn08 Jul 27 '19 at 10:07
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    also examine how high_gain amplifiers can be stable, with lots of phase shift at low frequencies, but still be stable overall because the phaseshift near UnityGain is approximately zero degrees. Examine G/(1 + GH) for those conditions. – analogsystemsrf Jul 28 '19 at 04:02

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I just don't seem to comprehend how this negative attenuation is accomplished just using resistive networks

There are some reasons why this can happen, and in fact happen all the time:

  1. The output signal polarity may be reversed (or have enough phase shift) due to the plant transfer function. When you take a sample of this signal you can feed it back to the input directly because it already is negative feedback.
  2. If the plant transfer function doesn't reverse polarity, then the injection point of the feedback at the input may reverse it instead, thus having the same effect. That's the reason why we use the negative input of opamps for feeding back the output, for example.

These situations are effectively equivalent to what you call "negative attenuation" and are used extensively in amplifier design, etc.

Enric Blanco
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