A RF amplifier includes a RF input section for receiving a RF input signal. At least one single-sided slow-wave structure is associated with the RF interaction section. An electron ribbon beam that interacts with the RF input supported by the at least one single-sided slow-wave structure so that the kinetic energy of the electron beam is transferred to the RF fields of the RF input signal, thus amplifying the RF input signal. A RF output section outputs the amplified RF input signal.