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A theoretical analysis is carried out On characteristics of a large-scale linear image sensor using amorphous silicon (a-Si) photoconductive diodes. This sensor has long wiring patterns between a photodiode array and scanning circuits, giving rise to large parasitic capacitances. Because of these parasitic capacitances, the sensitivity and the modulation transfer function (MTF) are lowered and are forced to have spatial nonuniformity. In this sensor a fixed pattern noise (FPN) is the most serious obstruction to the attainment of a high signal-to-noise ratio. The major causes of the FPN are found to be the error voltage induced by the turning-off of a reset switch, and the offset voltage of a unity-gain amplifier. For further improvements the reduction of the parasitic capacitances and the elimination of their distribution will be required. Appropriate choice of the thickness of a-Si film will also contribute to improving the image quality.