because u know positive feedback is used to construct oscillators .... and the condition for that is that the loop gain =BA = 1 ... so total gain =A/(1-BA) = infinity so for zero input there will be output
but why should we worry about oscillation for negative feedback amplifiers :
this is because for any negative FB amplifier ... total gain = A/(1+BA)
so if the phase response ( phase of loop gain (BA) ) =-180 when the magnitude of BA is >1 .... the negative FB will turn into positive FB and oscillation takes place ....
i.e amplifier becomes unstable
that's why we must make sure that for mag(BA)>1
angle(BA) should be >-180 .... and (angle(BA)-(-180)) is called the phase margin should be >0