I am trying to implement a cross coupled amplifier with OP-AMP feedback. I am not able to get it stable for a square wave however. The circuit looks as follows: The OP AMP is ideal from Cadence, all parameters can be tuned. The circuit is unloaded. The open loop gain, i.e without feedback, Vout/Vin looks as follows:
As it can be seen the gain is negative, which is correct.
Now the closed loop gain from AC analysis with the feedback opamp i.e Vout/Vin* looks as follows: Looking at the closed loop gain there appears to be a zero at ~10 MHz. I have no idea why, because the open loop gain does not introduce any zeroes.
And the loop gain which is calculated using 'stb analysis' where the loop is broken at 'iprobe' and the loop gain is calculated between the terminals of iprobe. The loop gain also shows that the phase has been shifted to negative, compared to the phase at DC. Hence the entire amplifier is not stable.
Now my questions based on the observations above:
- Where does the zero come from?
- If the closed loop gain and loop gain shows unstable behavior at 10 MHz then the amplifier should oscillate unconditionally, because of negative phase margin. But it does not for a sinusoidal wave and does so for a square wave. Why?
- 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?
Any help will be appreciated.