Gain-frequency characteristics of transistors based on flux flow in hysteretic long Josephson junctions (LJJ)

Transistors based on one dimensional flux flow in hysteretic long Josephson junctions (LJJ) and deriving output from the spatial average over the length can be modeled as a section of transmission line. Time domain analyses made on these transmission line sections with both the ends terminated with typical resistance values reveal a fundamental gain-frequency relationship unique to all devices whose output voltage is equal to the spatial average over the length of the junction. The maximum frequency of a transistor depends on the transit time of carriers and the parasitic elements. The analysis shows that the frequency response of a flux flow transistor is related but not equal to the inverse of the transit time of fluxons. Secondly the step response of these transmission line sections varies linearly with time, which indicates a slew rate limitation. The slew rate that affects the high frequency response of a flux flow device is shown to be a function of the critical current density of the junction and material parameters. Analysis made on niobium-lead junctions indicates that the slew rate peaks at an optimum current density level. The results of these analyses and performance comparisons are presented.