| 摘要 |
A vital checking circuit for assuring the reception of only one input signal including a differential amplifier having a pair of PNP transistors. A voltage divider having a first and a second biasing resistor connected to the input of one of the pair of transistors. An a.c. signal transformer coupled to the input of the one of the pair of transistors. A third biasing resistor connected to the input of the other of the pair of transistors. A plurality of current-drawing circuits connected to the third biasing resistor, and an a.c. output signal derived from the other of the pair of transistors when and only when one of the plurality of current-drawing circuits is drawing current through the third biasing resistor.
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