Now I have time to go somewhat into details:
When speaking about feedback it is very important to know the terms.
* negative feedback means: Feedback to the inverting input terminal. This sounds simple, however, you should note that each negative feedback at higher frequencies turns into positive feedback due to phase shifting of real amplifiers. But this positive feedback will not harm you when for these signals the loop gain amplitudes are below unity.
* Positive feedback also for low frequencies is achieved by feeding back to the non-inverting input. For amplifiers this is done always in conjunction with appropriate negative feedback (which must dominate because of stability reasons).
* An amplifier with positive feeedback only cannot work as an amplifier - it is a kind of Schmitt-Trigger (switching capabilities between output extreme values).
Exception: The example given by FLATULENT with very small amount of positive feedback (pos. feedback can narrow the bandwidth, as negative feedback will enlarge it).
*Oscillator condition: If the positive feedback leads to a loop gain of unity
at one frequency only and if the loop gain drops down for lower and for higher frequencies, than you have a harmonic oscillator. This is only a simplified description as oscillating properties of amplifiers with feedback are a rather complicated area.
*As an example: In all textbooks dealing with the subject a necessary oscillation criterion (Barkhausen rule) is mentioned. However, no book contains a formulation for a
sufficient oscillation criterion! That means, up to now, I couldn´t find such a criterion. I would be happy if somebody could show me that I am wrong!!!
* From the above: To answer the question - in principle, there is no difference in loop gain measurement (simulation) between negative and positive feedback. But it becomes complicated if you cannot open both loops at the same time (that means: at the same node).
The problem is: For two different feedbacks (negative resp. positive), then you have three different loops (positive open, negativ open, both open) with three different loop gains. And it is not easy to decide and to know which loop of the three dominates and sets the margin. This depends on the specific topology.
* Finally: Feedback is one of the most involved and challenging phenomenon within the field of analog electronics.
Added after 19 minutes:
biff44 said:
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Another example is a simple conrol loop. You will find that classical feedback control loops can have a unity gain MUCH GREATER than unity, and be perfectly stable. A phase locked loop might have 80 dB of low frequency open loop gain, and yet be stable, for example. The key is that the round trip phase shift must stay withing certain limits.
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More than that - normally one is interested in a very high loop gains becaus of many wanted properties associated with it. However, at a phase shift of 360 deg it should be less than unity (I suppose, that was the meaning of the contribution from DEDALUS).
But - as another example for the complicated subject of loop gain - there is something called "conditional stability" which allows a loop gain even larger than unity at a phase shift of 360 deg. !