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# OPA131: Phase of loop gain issue in TI Precision Labs - Op Amps: Stability 2

Part Number: OPA131

Hi,

Actually I have a question mainly foucus on TI training, TI Precision Labs - Op Amps: Stability 2 Slides 8

"To measure the phase margin, we need to know the loop gain phase, or phase of Aol times beta, over frequency. Using the same log properties as before, we can simply subtract the phase of 1 over beta from the phase of Aol to get the phase of loop gain."

The phase is not expressed in log or dB, so how could we get phase(Aol)-phase(1/beta)=phase(Aol*beta)

• A loop-gain, LG, is a ratio of the open-loop gain, AOL, to close-loop gain (1/beta), LG=AOL/(1/beta)=AOL*beta, so to find the magnitide of LG you divide their magnitude in a linear scale but subtract it ONLY in dB scale. When it comes to phase, you just subtract phase directly in degrees; thus in the body plot below, the phase of AOL*beta would be equal ~5 degree: the phase margin of AOL (90 degrees) minus the phase shift of 1/beta (~85 degrees).

• Hello User,

We hope your issue is resolved. I am going to close this thread now. Feel free to reply to to re-open if you have any questions moving forward.

-Tamara