Abstract: In a high efficiency F-class amplifier, output current may be reduced to cause distortion of the current, or deterioration of high efficiency operation of the amplifier, when an instantaneous output voltage of an amplifying transistor becomes lower than a saturation voltage. To prevent this, the peak value of the high freqency instantaneous output voltage is restricted by a peak value restricting circuit provided in parallel to a load. Thus, an ideal high efficiency amplifier can be provided by avoiding distortion of the output current.

Abstract: Disclosed is a magnetron for a microwave oven which has a drain electrode (60) near a cathode (25) and in a position where an electron beam path is not interfered with. The cathode and the drain electrode are formed in spiral shapes. Turns of the cathode are alternately arranged with those of the drain electrode. When the magnetron is operated, the drain electrode is kept at the same potential as the cathode or is kept at a negative potential relative to that of the cathode. Therefore, positive ions in the vicinity of the cathode are collected at the drain electrode, thus decreasing high frequency noise.