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A study of amplitude modulation (AM) and phase modulation (PM) noise has been carried out in an SiGe bipolar amplifier, operating in small- and large-signal conditions. Experimental and simulation results show that the evolution of flicker noise and white noise contributions versus carrier power is different in the AM and PM noise spectra. This indicates that specific mechanisms are involved in the conversion processes, thus producing AM and PM noise. The degradation of white noise, introduced by the amplifier as a function or carrier power, has been studied and characterized through the effective noise figure (NF) obtained from the white noise portion of the AM and PM spectra. As expected, the results show that white noise only produces identical contributions to AM and PM noises if the amplifier is operating in a small-signal regime. The effective NF in large-signal conditions needs to be determined from both the AM and PM noise spectra. An expression compatible with the standard IEEE definition of NF is proposed to extract the effective NF from the AM and PM noise spectra.