Below is a schematic of a op-amp designed by Jake Baker for a power line spread spectrum communications chip designed by ITRAN communications. The process used is Tower Semiconductors 0.35 um process. This op-amp is used to drive powerline and filters with an effective input resistance of 30 ohms. It has the feature that it can be put into a disabled mode so that it doesn't draw any significant amount of current. Some interesting features of this design include the use of small current sources and sinks to lower the quiescent current in the output stage. This (small bias current in the output stages relative to the diff-amp bias current and device sizes) causes the stability of the op-amp to become marginal under certain load conditions. The pole associated with the output stage is comparable, and under some situations has a value less, than the pole associated with the output of the first stage. It was decided that this was OK to minimize quiescent current draw while at the same time having a circuit that can drive 30 ohms within 150 mV of the ground and 3.3V supplies (drive 100 mA into the load). Another problem with this topology is that since current source loads are used on both the top and bottom of the diff-pair the output stage is not guaranteed to operate Class AB. This had to be done in order to have fast high and low drive (this op-amp is used on the output of a DAC). In order to balance the switching currents in the output p-channel and n-channel stages the PMOS and NMOS devices were sized the same (so that they had the same input capacitance.)

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