  Explicit analytical expressions for conductivity of a superconducting film
above and below critical temperature in an arbitrary electric field are derived
in the frameworks of the time dependent Ginzburg-Landau theory. It is confirmed
that slightly below critical temperature the differential conductivity of
superconducting film can become negative for small enough values of electric
field. This fact may cause generation of electromagnetic oscillations if the
superconducting film is appropriately coupled of with a resonator. Their
maximal frequency is proportional to the value of critical temperature of
superconducting transition. The obtained results can stimulate the development
of Terahertz generators on the basis of high temperature superconducting films.
