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The effect of input and output amplifier impedance on the amplifier amplitude-modulation (AM) and phase-modulation (PM) noise, as well as on noise figure (NF) is studied in linear and nonlinear regimes. Simulations and measurements performed on different implemented amplifier prototypes corroborate that the behavior of the amplifier AM, PM, and effective NF levels in large-signal conditions depends on input and output matching conditions. Moreover, correlation between noise degradation in compression and the shape of the derivatives of the AM-AM and AM-PM conversion curves is analyzed and discussed. Conclusion of the study can be applied at the design stage to optimize the amplifier noise performances in the nonlinear regime.