Electrical – Open loop gain – understanding the simulations

bode plotltspiceoperational-amplifierspice

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:

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

enter image description here

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?


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

enter image description here

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

enter image description here


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

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And this is the simulated circuit:

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Best Answer

In your second edit, you have supply V5 shorted. With the net label 'Vee' tied to ground your opamps are still biased at the negative rail.

Sample loop-gain using the LT6016 for comparison. enter image description here enter image description here