An apparatus and method include a transmission line carrying a propagating signal between an inlet port and an outlet port. The propagating signal can include a forward traveling wave and optionally a backward traveling wave. A feedback stage samples a the propagating signal at the outlet port, generates a feedback signal the includes a time translation and a gain translation in the feedback energy, and routes the feedback signal to the inlet port such that the gain translation constructively interferes with the forward traveling wave and thereby increases the amplitude of the forward traveling wave.