| 摘要 |
PURPOSE:To improve the response speed or to apply the titled switch even under an environment where a strong AC magnetic field is applied by discriminating an output of a current feedback type oscillating circuit at a prescribed level and providing a comparison means inactivating a switching element at the reduction of an oscillation output, increasing a feedback current at the reduction of oscillated amplitude so as to keep the oscillation. CONSTITUTION:When an object approaches, the oscillation amplitude is decreased and lower than a reference level Vref2 or below of the comparator circuit 25, a comparison output from the comparator circuit 25 is stopped to turn off a switching transistor (TR) 16 of the oscillation circuit 1. Then a current mirror circuit CM3 is activated, one collector current Ib of a TR 11 flows to the current mirror circuit CM3 and the other collector current is circulated in an LC resonance circuit. When the emitter area of the TRs 13, 14 is equal, the collector current Ic of the TR 15 is nearly equal to the Ib and the current value flowing to the LC resonance circuit is the sum of the three collector currents Ia and Ic of the TR 11. Thus, a feedback current If is increased to improve the gain of the oscillation circuit 1.
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