# Open loop gain - understanding the simulations

I tried to simulate a non inverting amplifier, to see the differences from the amplification up to the unity gain, just to learn to be more confident with the LT Spice tool.

Then I saw this video describing ho to analyze the loop: http://www.linear.com/solutions/4449

And is doing the analysis by checking the bode plot, where it become 0dB you can of course find the phase margin and so on. I did the simulation with the two resistor set to 1000 as in the example (DC gain = 2).

The problem is that with their amplifier LT6016 is fine, or reasonable:

When I change with the LM324 (a model found in internet), it never starts from gains greater than unity:

My thoughts and questions:

1. May the LM324 model be wrong? Seems to work with a simulation with a conventional signal.
2. With other opamp, the gain may still be greater than 1 at DC so you can determine the phase margin, but with a different magnitude: what is telling to me this magnitude? I know that according to this value, the phase margin and bandwidth can vary, but is it proportional to the open gain loop, so is it the GPB?
3. When is lower or equal than 1 at DC, if it is correct as in the example with my LM324, how can I proceed for the stability analysis?

EDIT:

This is the test circuit, for which I tried negative VEE (like in the video) and as shown, the result does not change:

This is the famous result of all of three op-amps:

EDIT2:

These are instead the plots with the negative supply CORRECTED. Now is meaningful.

And this is the simulated circuit:

• Both plots don't appear correct typ open-loop gain is 80 db and above for an opamp look at loopgain example in the edu folder for ltspice Commented Jun 17, 2017 at 12:29
• In the app video they called open, because somehow you broke the loop to simulate the closed loop. I was referring to the close loop then, my mistake. Of course these are not the Avol curves. Commented Jun 17, 2017 at 23:48
• The ratio you've plotted would suggest your trying to plot open-loop gain but the results don't match. This can be because you have more than one AC source or the output is railed. However there isn't too much to say without a schematic of your sim. Commented Jun 18, 2017 at 0:01
• Added the missing information. Commented Jun 18, 2017 at 22:35
• Yes, you can see all 3 opamps are biased to operate at the negative Vee rail (ground in this case). If you power Vee as say -6V and Vcc at +6V, you should obtain more realistic results. Commented Jun 18, 2017 at 22:43