  Electrons performing Bloch oscillations in an energy band of a dc-biased
superlattice in the presence of weak dissipation can potentially generate THz
fields at room temperature. The realization of such Bloch oscillator is a
long-standing problem due to the instability of a homogeneous electric field in
conditions of negative differential conductivity. We establish the theoretical
feasibility of stable THz gain in a long superlattice device in which the bias
is quasistatically modulated by microwave fields. The modulation waveforms must
have at least two harmonics in their spectra.
