Simulating the open loop gain

Question

My questions regarding the op amp simulation are as follow :

1. Why do we need closed loop for DC and open loop for AC ? Why this separation?

2. The inductor will behave like short at DC so the op amp will not saturate but capacitor will behave like open at DC at the same time, if so how do we inject our signal ?

3. As far as I know we use an inductor to have proper DC bias, if I need a proper DC bias why do we need open loop for AC ?

Answer 1

1. Well, the picture actually tells you: DC closed loop is for the SPICE simulation to have a feasible DC operating point preventing the op amp from driving its output to one of the rails. For AC you need an open loop, because open loop AC analysis is just one way to perform stability analysis. In your example you can directly measure the phase margin (a measure for stability) at Vo: The phase margin is the phase of the loop gain at the frequency where the loop gain is 0 dB. Be aware that the term "loop gain" usually refers to the open loop containing the feedback network and the amplifier, while the term "open loop gain" usually refers to the op amp only.
2. Any signal of interest, but DC, will pass the capacitor.
3. See answer 1.