A semi-classical model is proposed theoretically and demonstrated experimentally on the optical receiver sensitivity enhancement by single-band (signal or idler) and dual-band (signal and idler) fiber optical parametric amplifier (FOPA). The sensitivity enhancement by single-band is determined by the gain of FOPA and the transmission loss of signal and idler, and it can be further improved by up to 3-dB using amplified signal and phase-conjugated idler together at dual-band configuration. The theoretical results are experimentally verified in both fiber communication and biomedical imaging applications. This detection sensitivity enhancement scheme can be potentially applied in the scenarios where ultrafast broadband signal at low-power level is being handled.