PURPOSE: To eliminate the influence of the charge voltage of a bias capacitance means on the bias voltage of a driving signal in a switching means by providing the bias voltage circuits of polarity and reverse polarity so as to change capacitance by means of the switching means. ;CONSTITUTION: When a transistor(TR) 3 is turned on, a DC power source 1 charges a capacitance 8 with diodes 4, 5 and TR3 and, at the same time, TR2 (with the diode 6) and TR3 are reversely biased. When TR3 is turned off after that, the capacitor 8 charges the capacitor 9 in a reverse direction as against a last time with resistors 10 and 11 so as to normally bias TR2. Therefore, it is regarded that the capacitor 8 is operated as a power source capacitor and the DC power source 1, TR3, the diodes 4, 5 and the capacitor 8 constitute the other DC power source. Then, TR2 can be normally biased or reversely biased even if only one DC power source is used. Since the capacitors 8 and 9 are connected to the DC power source 1 in parallel when TR3 is turned on, the charge voltage of the capacitor 8 is not influenced by the charge voltage of the capacitor 9.;COPYRIGHT: (C)1993,JPO&Japio
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