  Josephson junction systems with a negative differential resistance play an
essential role for applications. As a well known example, long Josephson
junctions of the BSCCO type have been considered as a source of THz radiation
in recent experiments. Numerical results for the dynamics of the fluxon system
have demonstrated that a cavity induced negative differential resistance plays
a crucial role for the emission of electromagnetic radiation. We consider the
case of a negative differential resistance region in the McCumber curve itself
of a single junction and found that it has an effect on the emission of
electromagnetic radiation. Two different shapes of negative differential
resistance region are considered and we found it is essential to distinguish
between current bias and voltage bias.
