There are two cases, one where Vs is 0 and the other non zero.
G = AB
Case one Vs = 0. So lets start by using Vs to inject a signal into the loop, then
turn it off. So signal, if loop G < 1, slowly keeps adding to Vs = 0, so its irrelevant.
and so it makes it first pass around the loop itself as it transverses the loop.
Now assume G is .9, so it passes thru the G elements (AB) but now is smaller, and that
process iteratively each pass keeps reducing signal until its 0. Corollary if G = 1 the loop
is sustained with the signal when Vs was reduced to 0. If G > 1 then it keeps growing
until physical circuit limits saturate.
Case two Vs is non zero. So Vs flows into summer and is is added to the fdbk
portion. If G >=1 then each transversal of loop the output of the summer grows
until physical limitsof the circuits saturate, typical the supply V on the circuits as
an approximation. Corollary, G < 1 then each time thru loop less and less is added
to Vs coming out of the summer, and ultimately the loop simple settles to Vs flowing
around loop, eg. stable.
Of course the above is not discussing the phase impact, but you can walk the loop and
see the impact of how phase affects loop. Keep in mind phase affects at the summer,
where you can experience less or more added to loop depending on phase, can even
experience sign reversal where summer actually becomes subtractor.
Regards, Dana.