Some limitations of the power output capability of VHF transistors

The power output capability of a VHF transistor is often limited by the RF saturation resistance. The resistance is set up by the constriction of the emitter current to the geometric edges of the device. The contributions of large current densities and high frequencies to current crowding are discussed in this paper. An extension of this discussion leads to an interpretation of the first-order effects for large-signal high-frequency operation. The RF saturation resistance was measured under large-signal conditions and was found to increase by a factor of 2 over a frequency range of 40-200 MHz. This variation was compared to results obtained by a large-dc small-signal analysis. The resulting deviation was less than 20 percent.