  In this paper, describing function method is used to analyze the
characteristics and parameters selection of differentiators. Nonlinear
differentiator is an effective compensation to linear differentiator, and
hybrid differentiator consisting of linear and nonlinear parts is the
combination of both advantages of linear and nonlinear differentiators. The
merits of the hybrid differentiator include its simplicity, rapid convergence
at all times, and restraining noises effectively. The methods are confirmed by
some examples.
