A function generator is constructed to generate voltage as any given function of input voltage. The generator approximates any function including a nonmonotonic one by quantizing it into 11 steps. It has fast response (less than 100 nsec), good frequency characteristic (up to 10 MHz), and is easy to construct. It is to be used with aV‐to‐fconverter in a feedback loop of rf confinement of a plasma, where the most favorable frequency is given as a nonlinear function of plasma density.