  We study two-stream instabilities in a non-equilibrium system in which a
stream of electrons is injected into doped graphene. As with equivalent
non-equilibrium parabolic band systems, we find that the graphene systems can
support unstable charge-density waves whose amplitudes grow with time. We
determine the range of wavevector $\boldsymbol{q}$ that are unstable, and their
growth rates. We find no instability for waves with wavevectors parallel or
perpendicular to the direction of the injected carriers. We find that, within
the small wavevector approximation, the angle between $\boldsymbol{q}$ and the
direction of the injected electrons that maximizes the growth rate increases
with increasing $\boldsymbol{|q|}$. We compare the range and strength of the
instability in graphene to that of two and three dimensional parabolic band
systems.
