  We analyze the scalar radiation emitted by a source in a circular geodesic
orbit around a spherically symmetric black hole. The black hole spacetime
considered is quite general, in the sense that it encompasses the solutions of
Schwarzschild and Reissner-Nordstr\"{o}m, and also the Bardeen solution of a
regular black hole. We use the framework of quantum field theory in curved
spaces to compute the one-particle emission amplitude of scalar particles and
related quantities.
