Simulation of Crossover Distortions in Class AB 0.00002% THD

MATLAB to LTspice Interface in action

The following is an attempt to apply the theory of robust feedback control to the design of Class AB audio amplifiers. The potential designs are analyzed by simulations using LTspice invoked from MATLAB in a batch mode. It is repeatedly demonstrated that application of a standard -20dB/dec feedback loop is broadly equivalent to taking a derivative of the pre-existing non-linear distortions which may lead to the lower distortions - or may not. The proposed analysis and visualization framework aids in better understanding of Class AB amplifiers nuts n’ bolts and serves as an illustration of the non-trivial underlying complexity of feedback control over time-invariant static plants with hard non-linearities. Armed by this better understanding, we can apply nested feedback loops and 50kHz low-pass filtering to lower the total harmonic distortions from 0.5% for 3-stage Emitter Follower down to 0.00002% for frequencies ≤20kHz and power ≤100W for Vcc-Vee=67.2V, RL=4Ohm, according to simulations.