| 摘要 |
PROBLEM TO BE SOLVED: To make the input impedance of signal takeout circuit high while eliminating the feedback capacitor with a circuit of small area and also make accommodation to explosion-proof container unnecessary. SOLUTION: A constant current circuit CT1 is provided. In the constant current circuit CT1, a resistor R5 is made to flow the constant current, and made to generate the constant voltage between both the sides of the resistor R5, the constant voltage is superposed on the output signal of an operation amplifier OP1 to be fed back to a drain D of a field effect transistor Q1, and between the output terminal of the operation amplifier OP1 and the drain D of the field-effect transistor Q1 is held with same AC potential. Thereby, the voltage between the gate G and drain D of the field-effect transistor Q1 becomes the same AC potential, therefore no current is generated through the stray capacitance, and the input impedance of the field-effect transistor Q1 can be made high in the same way as the case that the feedback capacitor is used. COPYRIGHT: (C)2008,JPO&INPIT
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