  We analyze the current dynamics of a superlattice Bloch oscillator under
conditions where, in addition to the dc bias field causing a negative
differential ac conductivity, a similarly strong ac field is present, at a
frequency somewhat below the Bloch frequency associated with the dc field. The
differential conductivity at the ac drive frequency is then negative, but,
because of the strong non-linearities of the system, the dc differential
conductivity is modified by the ac field. For a sufficiently strong ac field,
there will be a dc bias range inside which the dc differential conductivity has
turned positive, while the ac conductivity remains negative. The phenomenon
should lend itself to the suppression of the domain-like space charge
instabilities that are normally associated with a negative dc differential
conductivity.
