Stability analysis of cross-coupled amplifier in Cadence
Where does the zero come from?


Do a &quot;&quot;small signal analysis&quot; on the circuit to find the poles and zeros. So start drawing the small signal equivalent model of the circuit and perform some simple network calculations. This is all done by hand on a piece of paper. You could confirm your findings using a simulator. You could even build the small signal equivalent model and simulate with it.


If the closed loop gain ... for a square wave. Why?


Applying a sinewave and/or squarewave as an input signal to do stability analysis is &quot;silly&quot;, you want to see if the circuit generates its own signals (oscillates) so why apply a signal??? The proper way that all experienced EEs use is 1) the small signal stability analysis and to confirm and double check 2) do a transient (time) simulation but  with a pulse or a step as the input signal. Then look at the step response of the circuit. Much has been written about stability of systems and their step response (the two are strongly related).


I know some compensation caps need to be added, but where should I add them? Basically how do I go about to make the circuit stable?


When you have the small signal equivalent model it becomes more evident where the poles and zeros are and what can be done to make the system stable. Many books have been written about making unstable systems stable, there is no need to repeat all that here.