We present a practical model for optical amplifier noise amplifier noise analysis and its application in conventional intensity modulation- direct detection (IMDD) optical transmission systems. In particular, a general sensitivity calculation formulae for optically pre-amplified receiver is derived and the impact of different system parameters on BER performance is discussed. Measurements on a experimental system with a 2. 5 Gb/s optically pre-amplified receiver show the good coincidence between the theoretical calculation and the experimental results. By using an optical pre-amplifier, the receiver sensitivity of the experimental system improved from -26.5 dBm to -39.5 dBm.