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Consider a feedback amplifier with a nonlinear output device and an open-loop amplifier A having the same gain and the same output device . If the gains of the forward amplifier and feedback network are independent of frequency, it is well known that under the usual conditions the nonlinear distortion of due to is related to the nonlinear distortion of by . In this paper conditions are determined under which a similar formula, namely , applies when and are strictly stable linear time-invariant amplifiers. The interpretation of the formula is the following: is the function of time which is output of an amplifier whose gain is when is its input, and is the nonlinear distortion of the open-loop amplifier A.