The noise temperature, band width, and gain modulation characteristics of a transmission maser are obtained as a function of iris coupling for fixed desired gain and other parameters. It is found with the help of a characteristic curve that certain of these characteristics can be optimized at the expense of others. The same functions are compared in the case of a reflection maser. The reflection maser is shown to be superior in most respects, although the same noise temperature may be attained with the transmission maser at a substantial sacrifice in band width and gain modulation. The reflection maser is also limited by the availability of good circulators, while a transmission maser can be made independent of load. A figure of merit for both types of maser amplifiers is obtained. This isQ0Z, whereQ0is the unloaded cavityQ;Zis given byZ=4&pgr;&xgr;&khgr;0−Q0−1; &xgr; is the filling factor; and &khgr;0is the negative rf susceptibility. When the cavity band width is much less than the susceptibility band width, an auxilliary figure of merit isZ.