  The investigation of the ultimate limits imposed by quantum mechanics on
amplification represents an important topic both on a fundamental level and
from the perspective of potential applications. We propose here a novel setup
for an optomechanical amplifier, constituted by a mechanical resonator
dispersively coupled to an optomechanical cavity asymmetrically driven around
both mechanical sidebands. We show that, on general grounds, the present
amplifier operates in a novel regime-- which we here call phase-mixing
amplification. At the same time, for a suitable choice of parameters, the
amplifier proposed here operates as a phase-sensitive amplifier. Furthermore,
we show that both configurations allow amplification below the standard quantum
limit in a parameter range compatible with current experiments in microwave
circuit optomechanics.
