Reading through op-amp stability calculations in Op Amps for Everyone, and I am not sure if a minus sign is missing. I'm going through section 6.3 Noninverting Op Amps. If I work through the equations, I also get eq 6-13, matching the text, with no minus sign:
But if you look at the figure:
Output should be a*(-Vtest), with a minus sign, and so the total Aβ should also have a minus sign, no? But that doesn't match 6-13. What am I missing? The amplifier oscillates when Aβ = -1, so sign matters.
Hello Jonathan,
Because the equation for closed loop gain is Acl = Aol / (1+Aol*B), a condition where Aol*B is equal to -1 results in an unbounded gain resulting in stability issues. Aol*B, known as loop gain, is equal to -1 when the magnitude of Aol*B is equal to 1V/V (0dB) and the phase has shifted by 180 degrees. Phase margin is a measure of the remaining phase shift before the phase has shifted by 180 degrees. So a phase margin of 45 degrees indicates that the phase has shifted 135 degrees.
Take a look at the TI Precision Labs videos on Op Amp stability. It contains the same information presented in an easier to understand way and describes a few compensation techniques. We will continue to add additional compensation techniques in the future. You can view the videos at the link below:
