A method is provided for forming a power semiconductor device. According to the invention, a driving transistor also serves as an erasing transistor and the driving transistor is operated in a saturation region. The linear regulator is designed so as to control the operating voltage of the power amplifier with sufficient bandwidth to faithfully reproduce the desired amplitude modulation waveform. When voltage is developed between the electrodes 24, 25, the polarized active portions 40 of the piezoelectric sheets 54-56 that are sandwiched between the electrodes 24, 25 contract in the planar direction, so that the second portion S arches downward. The remaining fourth card becomes the \u201ckicker. Thus, the stroke control valve can be provided in any position in the associated hydraulic circuit.