1. The problem statement, all variables and given/known data Consider a feedback amplifier for which the open loop gain A(s) is given by: A(s) = X/ [(1+s/Y)*(1+s/Z)2] |s=jω Where X = 7500, Y = 90000, and Z = 800000 a) If the feedback factor is independent of frequency, find the frequency at which the phase shift is 180°. b) Find the critical value of at which oscillation will commence. 2. Relevant equations Af(s) = A(s)/ 1+A(s)B(s) (gain with feedback) 3. The attempt at a solution I'm not sure how to start this problem. If is constant value then I get -tan-1(ω/90,000) - 2*tan-1(ω/800,000) = tan(180) or (ω/90,000) + (ω/800,000) = tan(60) and when I try to solve I get ω=140,000 rad/s, but maybe i'm doing it wrong.