Approximate theoretical expressions for gain, bandwidth, and root gain-bandwidth are derived by introducing negative absorption into equations for a Fabry-Perot interference filter. Resuits of gain vs frequency calculations are given for a 10-cm maser with high single-pass gain and a 100-cm maser with low single-pass gain. Root gain-bandwidth is shown to be a constant for a given cavity as long as the cavity bandwidth is much smaller than the Doppler or fluorescent linewidth of the maser transition. Frequency drift due to temperature variations and the necessity for precise control of single-pass gain are two practical problems. These must be solved in order to make a stable amplifier with high gain.