I've built a discrete transistor OpAmp, and when the input was off, I saw it oscillating. So, I got interested in OpAmp oscillation and compensation. The main criterion is that a Phase shift of 180 degrees occur when the loop gain is higher than 1.
I tried to demonstrate that in LTSpice but couldn't. I'm not sure if it's an LTSpice limitation, or something I'm doing.
I opened up the included exampled circuit for the LM741, and removed the compensation capacitor. I ran a frequency response and it showed that around 18 MHz, the phase shift is -180 and the open loop gain is greater than unity, so connecting it as a follower, the loop gain should be still greater than unity.
I turned off the AC source and ran simulation expecting it will oscillate (after all LTSpice can do other types of oscillators). The circuit failed to oscillate however, no matter what I did. I tried different things, but in vain, I couldn't get it to oscillate. In fact at 18 MHz, even the basic follower functionality wasn't working.
I wonder if somebody can comment on this cheesy experiment, and let me know what's going on? is it me? or LTSpice? or things are just more complicated than explained in AoE?
Regards,