PURPOSE: To reduce the power consumption and enlarge the ratio of noise to signal of an output signal, by turning on an electron beam of an electronic tube some time before the rise of a burst signal and by turning off the electron beam rapidly at the fall of a burst signal.

CONSTITUTION: The burst signal applied to a signal input terminal A is branched by a signal branching circuit 1, and one is transmitted to a signal output terminal after amplified by a power amplifying tube 2, and the other has the rise and the fall of the burst signal detected by an on/off detecting circuit 3. The output of the on/off detecting circuit 3 has the phase adjusting by a transfer phase adjusting circuit 4 and is given to a power source 5 for control electrode. In the power source 5, the electron beam is turned on at a timing some time before the arrival of the signal of the rise of the burst signal at the power amplifying tube 2, and the electron beam is turned off as soon as possible after the part of the fall of the burst signal is outputted by the power amplifying tube 2.