| 摘要 |
PURPOSE:To accurately amplify only an AC component in a signal made of superimposing an unidirectional AC component with a DC component by adding a clamp circuit to a signal amplifier circuit of a conventional AC coupling type. CONSTITUTION:A DC component of an emitter voltage of a photo transistor 2 changes depending on the intensity of an external disturbing light as shown in solid and broken lines and the DC component is superimposed on the AC component depending on the difference from the reflected luminous quantity by a bar code. The signal eliminated with the DC component through a capacitor 5 is fed to an inverting input of an operational amplifier (OP) 8 whose operating voltage is clamped to the reference voltage at a clamp circuit 20 comprising an OP 9 and a diode 10. Part of the output signal of the OP 8 is converted as shown in broken lines by an automatic threshold value setting circuit comprising diodes 14, 15 and a capacitor 16, a threshold value of an OP 13 is set and the result is fed to a noninverting input of the OP 13, where an output signal of the OP 8 is binarized.
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