Next generation rf amplifiers, in particular the inductive output amplifiers (IOAs), will require electron sources characterized by high current density, high brightness, low emittance, and the ability to be emission gated. The strong interaction between the beam and the resonant or synchronous electromagnetic field may enable compact, highly efficient, and moderate gain X-band power booster amplifiers. An analysis of amplifier demands on generic emitter candidates is provided. Of the emitter candidates available, two (namely, field emitter arrays and wide-band-gap semiconductors) are amenable to an analysis predicated on a simple model of field emission from hyperbolas and ellipsoids. The simple model is used to investigate two problems of critical concern : for field emitter arrays (FEAs), we will investigate the conditions under which important space charge effects exist, and from the model predict optimum FEA performance characteristics for rf IOAs; for wide-band-gap materials, the simple model identifies parameters which limit the maximum electron transport across a metal-semiconductor interface, thereby limiting current flow.