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:
- May the LM324 model be wrong? Seems to work with a simulation with a conventional signal.
- 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?
- 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?
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:
These are instead the plots with the negative supply CORRECTED. Now is meaningful.
And this is the simulated circuit: