  We consider a high-frequency response of electrons in a single miniband of
superlattice subject to dc and ac electric fields. We show that Bragg
reflections in miniband result in a parametric resonance which is detectable
using ac probe field. We establish theoretical feasibility of phase-sensitive
THz amplification at the resonance. The parametric amplification does not
require operation in conditions of negative differential conductance. This
prevents a formation of destructive high-field domains inside the superlattice.
