Fiber Brillouin amplifier provides exceptional gain within an ultra-narrow spectral bandwidth, rendering it particularly suitable for selective amplification of optical frequency combs that have mode spacings indistinguishable from conventional amplifiers. However, the fundamental noise limit in phase-coherence comb line amplification using a Brillouin amplifier has remained unresolved. Here, we demonstrate that amplified-spontaneous Brillouin scattering sets the phase noise limitation of the Brillouin amplifier while exerting negligible impact on comb line coherence. Our Brillouin amplifier achieves 50 dB gain on individual comb lines while preserving its sub-Hz integrated linewidth and frequency stability on the level of 10-16 at 1s averaging time. Furthermore, we demonstrate a tunable microwave synthesizer through bichromatic Brillouin amplification of dual comb lines, achieving a phase noise of -71 dBc/Hz@1Hz for a 7.0 GHz signal. These results establish the fiber Brillouin amplifier as an indispensable tool for preserving ultra-high spectral purity during optical field amplification.