  We consider a periodic array of graphene nanoribbons under the action of a
strong dc electric field $E_0$ and an external electromagnetic excitation with
the frequency $\omega$ and the lateral wave vector $q$. Solving the
quasi-classical Boltzmann kinetic equation and calculating the surface dynamic
conductivity $\sigma_{2D}(q,\omega,E_0)$ and the absorption coefficient of such
a system we show that the real part of the conductivity and the absorption
coefficient may become negative under certain conditions. Physically this
corresponds to the amplification of the electromagnetic waves at the expense of
the energy of the direct current source. The results are discussed in
connection with experiments on the surface acoustic waves and on the
Smith-Purcell-type graphene-based terahertz emitter.
