A new user just asked this some minutes ago:
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.
I happened to be online and answered the following:
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.
Two other users commented on his question post, too.
Then he briefly accepted the answer, then unaccepted it, then deleted the question. I don't know why, maybe he felt embarrassed... who knows. I browsed back to the deleted post and then raised a flag requiring moderator intervention. This is what I've said to them:
This new user has asked an interesting question that might be useful for future users, but once he's got an answer then he has deleted the question. I found this to be disrespectful both for future users that might have a similar question, but also for those who took the time to read, comment and answer to the question. Should the question be undeleted and protected from user vandalisation?
My question is: Was I right to raise the moderation flag, or it's just a waste of time? Do we, as a community, try to recover questions from their vandalising OPs because they belong to the site? Or it's just a fact of life that OPs can do whatever they want with their question, ignoring future users and efforts from those who answered?
I want to know just in case this happens to me again, not to bother moderators without a valid reason.
Update: It looks like the question has been undeleted, upvoted and accepted. Go figure. :)