The document addresses the trend towards comprehensive circuit simulation using SPICE, noting that existing transistor-level models are too slow for complex circuits containing multiple op amps. It argues that while some macro-models exist, they often lack sufficient accuracy or fail to represent real-world behaviors like output current splitting between supply rails.
The text details a new macro-model design developed by Mark Alexander and Derek F. Bowers at Analog Devices. This improved model uses a modular approach combining simplification and build-up techniques to accurately mimic the frequency response, input characteristics (including support for bipolar, JFET, and MOSFET inputs), and power supply interactions of real operational amplifiers without excessive computational cost.
Macro-models reduce simulation time significantly by representing the op amp with fewer nonlinear devices while maintaining accuracy for DC and AC characteristics.
The minimum requirement is a differential-input stage, a gain stage, and an output stage.