| 摘要 |
PURPOSE:To linearly vary from the maximum attenuation to the minimum attenuation of a semiconductor variable resistor by a single positive power supply by using a bias circuit utilizing a division circuit comprising an opera tional amplifier circuit and a resistor so as to bias gates of two FETs and source of one FET properly. CONSTITUTION:A voltage given to an attenuation control terminal 113 is given to a gate terminal of an FET 104 via a resistor 104 and a gate potential of an FET 102 is decided by an operational amplifier circuit. The characteristic of the operation circuit is decided based on a reference voltage fed to a noninverting input terminal of the operational amplifier and the slope is decided by the ratio of resistances of resistors 112 and 109. Thus, in selecting the ratio of the resistances of the resistors 109, 112 and the ratio of resistances of resistors 105, 119, 120 to a proper value, the degree of change with respect to the attenuation control voltage is uniformized. Thus, the voltage changing direction of the gate terminal voltage of the FETs 101, 102 is made opposite and the change from the maximum to the minimum attenuation is attained.
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