For whatever reason, I seem to create a lot of oscillators in the course of developing circuits involving op-amps.
I'm wondering if the frequency of oscillation, which normally appears to be quite stable, can be used for diagnostic purposes?
For example, could I assume that the open loop gain is 0dB at the oscillation frequency and then back-plot a 40dB/decade line from that point to identify the frequency of a pole I need to compensate out by where it crosses the open loop gain plot from the datasheet?
Answer
The terminology is a bit confusing - but you must distinguish between "open-loop gain Aol" (gain of the opamp without feedback factor B) and "loop gain (Aol*B)" (gain of the complete loop). Thus, if an opamp with feedback oscillates, you can assume that at the oscillating frequency the loop gain (AolB) is unity. More than that, it is correct that at this frequency the loop gain magnitude roll-off will be app. -40dB/dec (phase shift -180 deg). However, at which point of such an asymptotic line you will find the pole frequency? This is possible only if you know the first (lowest) pole frequency
Therefore, the best method for identifying the second pole is to record the magnitude function vs. frequency.
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