FRF Measurement of Nonlinear Systems Operating in Closed Loop

To prevent unstable behavior or saturation, a frequency response function (FRF) measurement is often performed under closed-loop conditions (e.g., open-loop gain measurements of an operational amplifier). The difficulty of such FRF measurements is that the nonlinear (NL) distortions also perturb the input via the feedback loop. The latter introduces a bias in the estimate of the best linear approximation (BLA) and jeopardizes the interpretation of the output NL distortions. In this paper, we solve these problems via a generalized definition of the BLA that is valid for NL systems operating in feedback. The classical definition for open-loop systems follows as a special case.