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OPA333: Phase margin equation

Aivaras Sakauskas
Intellectual 1440 points
Part Number: OPA333

Hi,

I was hoping you could help me with some data that keeps coming up in TI resources for the analysis of Phase Margin using % overshoot and AC gain peaking.

See: https://training.ti.com/sites/default/files/understanding-opamp-stability-intuitively-and-with-spice-simulations.pdf

Slide 23 and 24 have “indirect phase margin measurements”. Could you tell me what equation was used to generate this data so I can use it more readily than trying to eye-ball a graph?

Thanks.

  • 0
    Guru 48395 points

    Well actually, I worked those out quite some time ago now, These are basically going back and forth from phase margin to Q or zeta - once you have those (or the angle the poles make to the negative s axis) you can get all those other things. 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Incidentally, the missing piece in all of this is what is the Fo vs LG=0 crossover - it is definitely not that crossover F (unless it is a single pole system, but then the phase margin is always 90deg). 

    That was a much more recent breakthrough for me with the resulting plot for F-3dB/Fxover in this article - so you could imagine backing into the Fo from that F-3dB using the equation above. the curve in this article is exact 2nd order loop gain analysis, not an estimate - of course no loop is just 2nd order, so it still ends up being approximately ok for predicting closed loop shape from LG analysis. 

    https://www.planetanalog.com/why-is-amplifier-gbp-so-confusing-insight-12/#