A new analytical model for semiconductor laser amplifiers is presented. It has the virtue of avoiding excessive numerical computation and yet retaining sufficient accuracy for most cases of interest. The essentially new feature is the use of an appropriate mean photon density obtained by averaging the axial field distribution along the cavity length. Calculated results are presented for gain, saturation power and tuning characteristics, and their sensitivity to current density, facet reflectivities, and the spontaneous emission coefficient is explored.