Why do some amplifiers oscillate with capacitive loads?
Oscillations caused by capacitive loads are a result of interactions between the op-amp”s output impedance and the capacitive load. The output impedance and the capacitive load form an R-C pole in the output stage, causing additional phase lag in the feedback signal. CMOS amplifiers have a higher output impedance that causes the pole to come near, or below, the op-amp”s unity-gain frequency. The additional phase lag of the pole will erode the phase margin of the op-amp, causing the total amplifier phase lag to increase to more than 180 degrees at the unity-gain frequency, resulting in oscillation due to a total feedback phase shift of greater than 180 deg at unity gain. CMOS parts can have an output impedance between 100 and 500, causing the pole frequencies to be relatively low. Similar speed bipolar op-amps have output impedances in the 1 to 100 range, resulting in pole frequencies much higher than the CMOS op-amp, keeping the pole away from the unity-gain frequency of the part. Th
