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In this paper, the best linear approximation (BLA) is extended to include second-order nonlinearities. This extension is particularly useful for the analysis of low-frequency (LF) distortion due to self-mixing. The self-mixing of a modulated signal due to even-order nonlinear distortion creates a spectrum around dc, as well as around the high order even harmonics. The frequency response at dc can be used to determine long-term memory effects such as trapping and self-heating. The extended BLA is extracted for a GaN-based HEMT to analyze the LF distortion and demonstrate the possibilities with the proposed method.