By making use of the matching characteristics of a rising‐sun type dual photodiode, an optically modulated photo‐IC with an improved external light illumination allowance was developed. In this photo‐IC, the frequency response of the preamplifier of one of the photodiodes is kept low. Only the dc component of the incident photo‐current is current‐voltage transformed and its output is applied by the current feedback to the preamplifier of another photodiode so that the external photocurrent is compensated automatically.
In this method, the size ratio of the two photodiodes is compensated by the ratio of the gain resistance and the feedback resistance of the preamplifier. Hence, by the amount of the diode size ratio, the saturation characteristic of the preamplifier has a margin. The external light illumination allowance is improved by the corresponding amount. The minimum receiving sensitivity of the fabricated optically modulated photo‐IC was as small as 50 1x and its external light illumination allowance was as high as 20,000 1x. This circuit technique can be applied to various circuit systems which must be operated under bright environment described in the foregoing. Hence, its application to a system operated from outside a room is expected.
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