It is difficult to analyze the harmonic distortion of a self-oscillating power amplifier (SOPA)

because the SOPA is a hard nonlinear system without an external clock. The single or multiple sinusoidal inputs describing function (DF) method is commonly used to linearize a nonlinear element

but this method considers only the components at the same frequencies as the input signals (i.e.

fundamental components) at the nonlinear element’s output. In this paper

besides the fundamental components

the third harmonic components are also calculated at the output of a comparator with three sinusoidal inputs

to create a linearized model of the comparator

and thus of the SOPA. The third harmonic distortion of the SOPA is calculated. The models of the zeroth and the first order SOPA are verified by behavioral simulation using MATLAB.