LTspice IV:Stability of Opamp Circuits

[@GiGaSzS]

I have tried your method for some of my analog circuits that I failed to analyse. I cannot thank you enough for showing this easy method for finding system transfer function. You also enlightened my knowledge about stability which so many professors, articles and books failed to do.

[@fredthechamp3475]

THIS IS JUST WHAT I NEEDED! THANK YOU

[@Sn3innovationsLLC1]

Fantastic video! We just started coming out with a series on Op Amp stability. This simulation method is one of our favorites. Of course a general idea can be made looking at the step response of the circuit in the time domain. We introduced the idea with that...while using LTSpice of course :)

[@skottedont]

Hello, isn't this "Loop Gain", and not "Open Loop Gain"? "Loop Gain" being the product of the "Open Loop Gain" and the "Feedback Factor", which is evaluated at 0dB to determine phase margin? "Open Loop Gain" should be the response of the amplifier with no feedback, correct? Thank you and great video!

[@fredthechamp3475]

That is just semantics. Some books call the product of all transfer functions in the loop for open loop gain. Some books call it loop gain.

[@vergildvb7702]

What should i do for non-inverting opamp? how to add ac source?

[@udohuhn-rohrbacher1406]

I tried your method and compared the result with the datasheet of a LT1001. Unfortunately the max. Gain between my LTspice simulation and the datasheet differs by about 10dB roughly. Can you verify your method by applying the LT1001 in your circuit arrangement. Thank you for your response.

[@paulg.3067]

4:25 How does the right scale at 0dB (at -40°) correspond to the phase margin shown in the box window (56°) ? Whats the math behind that?

[@KeiJazz]

That does not involve math or magic. The phase at the 0dB gain frequency is +56 deg, not -40. The gain in dB is shown as a continous line, and the dB's are on the left hand y axis. The phase (in degrees) is the dotted line, and can be read on the right hand axis.

[@hhao0011]

Has anyone tried this method with Vee set at 0V ? It does not seem to work. Thanks

[@amentothatt]

Very useful. But have you used Tian's method, calculating the PM?

[@MclarenF1rocket]

Where can I find more information about the reasoning and effect of inserting that capacitor Cff ?

[@LinearTechnology]

Hello, please visit the EngineerZone, our online technical community, at ez.analog.com/power/

[@MclarenF1rocket]

@@LinearTechnology Thanks!

[@paulg.3067]

4:28 And maybe some time in the future the developer of LTSpice will decide to make the fields in the cursor window editable and I know it will not be in 2021... :D

[@yanwenlai]

Could you please give a more specific explanation for this method? I mean for my understanding it's not pretty straight forward expressing the open-loop gain with the ratio Vfb/Vinm.

[@yanwenlai]

And does it work for every op amps? It'll be much better if a derivation is illustrated. Thanks!

[@vodashark]

Very useful!!

[@NorbertNipken]

I didn't get the same plots in my simulation, even using the demo files provided.

[@LinearTechnology]

Hello, for an answer by one of our engineers, please visit the Power by Linear technical community at ez.analog.com/community/power. Thanks!

[@IchrisiI]

great - thank you. analog device rules

[@amirb715]

I think it should have been V(out)/V(inm)

[@yveslesage8525]

I do agree with you and I point out that I would have added a minus sign : -V(out)/V(inm).

[@GiGaSzS]

V(out)/V(inm) is not whole transfer function, you will neglect what happens across R1. You can also see if you check V(inm)/V(Vout) you get pure opamp transfer function. Try it on any more complex circuit and you will be surprised :) (Vout being opamp output) (Vout/Vinm VS Vinm/Vout is only a matter of sign/voltage source position)