# 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 – sstobbe Jun 17 '17 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. – thexeno Jun 17 '17 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. – sstobbe Jun 18 '17 at 0:01
• Added the missing information. – thexeno Jun 18 '17 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. – sstobbe Jun 18 '17 at 22:43

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.  • Fixed! I didn't know at all how the negative rails would be so important for this analysis. I suppose it is due to a lack of a DC bias in the AC generator, that leads to a need of a negative rail. Moreover, I think is not always possible to know the DC bias with precision, in order to derive the simulation in the single supply rail setup. – thexeno Jun 22 '17 at 8:41

Here is openloop versus closedloop gain, and gain_accuracy, for OPA211 with Avcl goal of 2x. Input signal is 1vpp. SNR is 111dB (set by the 2 resistors of 1Kohm)  