We investigated the effect of the phase noise of fiber amplifiers on the linewidth of the amplified signal. We used three different sources as input signal, with linewidth between 5 kHZ and 18 GHz. We did not observe any spectral broadening of the signal linewidth at the output of the fiber amplifier. This is in opposition with the generally admitted description of phase noise in fiber amplifiers, but it confirms some recently published results. We propose a new theoretical description, which is in better agreement with the experimental data.