Hello.
First I'm new to this forum. Great pleasure to meet all of you

My question is:
I have an opamp (one stage with differential input and current mirror as a active load - the basic configuration). I wanted to measure the DC Open-Loop Gain, Closed-Loop Gain, PM & GM.

First I have made STB sim(with no C as a load at output): I put the opamp in the voltage follower configuration and in the loop i put a DC voltage source with 0 value as a probe. I have made the sim and the result for PM, for example, was PM=41.2deg.

Second I have made AC sim(with no C as a load at output): I put the opamp in the voltage follower configuration and in the loop i put an inductor L=1G H and a capacitor to ground C=1G F. I have made the sim and the result for PM, for example, was PM=-85.8deg.

Strange. So I applied a step at input and in the voltage follower configuration(same, with no C as a load at output) I looked at output to see the wave form, and the wave form looks like for a PM of around 40-45.

https://i.ibb.co/RpVf3Bj/STB-vs-AC-loop.png


I put the image below.
*in the image there are 3 simulation for both STB in the right and AC in the left, with 3 different values of output C as load.
All of them are different, hence offering different results.
* also in the image it can be seen the step response, looking almost like a PM around 45.

What is the explanation ?
STB and AC should show the same result or not ?

Ok, I understand your point of view.

But:
1. Which one says the correct result for PM,GM,Open-Loop or Close-Loop Gian, 3db BW, unity gain freq ? AC or STB ?
 
1.1. Because in a corner simulation, or monte-carlo, both will show different values.

1.2. You can not make the calculations for all the designs you are doing, just to check which one is giving you something near the right answer.
      And in this way we return to the question 1 and 1.1.



2. Why L-C method is showing different answer ? Although all the books are using it.

2.2. AC method (with L-C), basic tell you the "Gain and Phase" of the signal, that means in other words, the gain form the output all the way to the input !!!!
     On the other hand, STB give you the "Loop Gain", that means the gain inside the loop or the gain of the loop. So just from theoretical point of view, STB is not giving you any information about the gain, !!! attention !!!, from the output all the way to the input.

You can find a lot of information on loop gain simulation (probably more than you ever wanted to know) on my webpage at https://sites.google.com/site/frankwiedmann/loopgain

The location of the iprobe for the stb analysis does not matter if there is only one loop in the circuit and all signals are referenced to the global ground. If there are several loops in the circuit, you must insert the iprobe into a wire that is common to all loops (see https://kenkundert.com/docs/cd2001-01.pdf for details). If the signals connected to the iprobe are referenced to a node that is different from the global ground, you must enter the node name in the field "Local Ground Name" of the setup form for the stb analysis.

The stb analysis can use either the loop-based or the device-based algorithm, depending on what you specify as the probe instance (see https://support.cadence.com/apex/techpubDocViewerPage?path=spectreuser/spectreus...). I (and probably most other people as well) only use the loop-based algorithm, however.