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Noise equivalent circuits, which account for thermal noise and Poissonian shot noise in nonlinear resistors (diodes, FETs), are usually derived with the help of device specific, non thermodynamic knowledge. On the other hand, Nyquist’s theorem, which is a thermodynamic, device independent result, successfully describes the equilibrium fluctuations of all kinds of different linear and linearized nonlinear resistors, no matter if the fluctuations are of thermal origin or due to shot noise. Based on Stratonovich’s concept of nonlinear nonequilibrium thermodynamics, we suggest an approximate linear-quadratic generalization of Nyquist’s formula: For biased devices, the bias must not exceed the linear-quadratic approximation, taken at a characteristics’ origin. The model is successfully applied to the thermal channel noise of FETs and to the (Poissonian) shot noise of diodes. The approach is restricted to macroscopic resistors at a close neighborhood of thermodynamic equilibrium, and to sufficiently low frequencies. Phenomena like hot electron noise or shot noise in mesoscopic conductors are not covered. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]