Noise Reduction Apparatus for Hybrid Image Sensor

This invention relates generally to a document reader device for use in facsimile data transmission or the like. In particular, it relates to a system for reducing noise in a large-size document reader device which has been recently developed as a counterpart to an array of MOS photodiodes or a CCD image sensor.

Large-size document reader devices (hereinafter referred to as a "large-size image sensors") are defined as document readers which are physically as large as the document or script to be read. Large-size image sensors "read" (or sense) a document image by employing an optical system, such as an array of either optical fibers or lenses, to focus the document image. This system is advantageous in that since it does not utilize a magnified image of the document, it has a reduced optical path for image focusing, thus making the system in general smaller in size than those systems in the prior art utilizing magnified images.

FIG. 1(a) shows an equivalent circuit of a conventional large-size image sensor, and FIG. 1(b) shows the structure of a photosensitive element 1. As shown in FIG. 1(b), the photosensitive element 1 comprises a photoconductive semiconductor thin film 21 composed of an amorphous material such as Se-As-Te, a -Si, or a polycrystal material such as CdS or CdSe. Thin film 21 is "sandwiched" between a thin film electrode 22 consisting of Al, Cr or Au, and a continuous transparent thin film electrode 23 consisting of either SnO.sub.2 or ITO. Referring to FIG. 1(a), the photosensitive element 1 is schematically represented as a photodiode 1a and a capacitor 1b, the capacitor 1b representing the inherent capacitance of the photosensitive element 1. A plurality of such photosensitive elements are arranged in an array of a density required to resolve a document or script, such as 8 elements/mm.

A charge stored in the capacitor 1b is discharged by the photodiodes 1a as a function of the amount of light which has condensed upon the photosensitive element 1. MOSFETs 2 are successively turned on at certain intervals by a shift register 3, to recharge the capacitors 1b. A current generated uon such recharging is detected by an output terminal 6 to read optical information.

With the large-size image sensor of the above construction, the photosensitive element 1 can be fabricated by forming the conductive thin film 22, the photoconductive thin film 21 and the transparent conductive film 23 on one substrate 24 composed of such materials as glass or ceramics, through typical processes such as evaporation, sputtering, or CVD. Since the MOSFETs 2 and the shift register 3 are independent devices, they cannot be fabricated integral to the photosensitive element 1. It is therefore necessary to mount the MOSFETs 2 and the shift register 3 on the substrate 24 or another substrate, and to connect them to the photosensitive element 1 by way of such methods as wire bonding. To effect such a connection, a wire 7 extends from the photosensitive element 1 to the corresponding MOSFET 2. The length of the wire 7 is in reality larger in proportion to the circuit elements than it appears to be in the equivalent circuit. In addition, signal line 8 has a length of several tens of centimeters in the large-size image sensor. Thus, capacitances between the wires or signal lines are quite large, and cause an amount of non-negligible noise to be introduced into the signal line 8, resulting in a lowered signal-to-noise ratio.

In an attempt to eliminate the foregoing difficulty, the present application has proposed a system which utilizes dummy MOSFETs to effect differential amplification, as described in an application entitled "Document Reader Device" filed on Nov. 10, 1981, in Japan and laid open on May 16, 1983 as No. 80865/83. That Japanese application is commonly assigned. As shown in FIG. 2 herein, the system comprises switching MOSFETs 2, dummy MOSFETs 10, and photosensitive elements 1. The MOSFETs 2 and 10 produce outputs which are differentially amplified such that the composite signal has a good signal-to-noise ratio. However, with two independent MOSFETs connected for each photosensitive element, the area needed to package all of the MOSFETs is twice as large as that required by the typical image sensor circuitry shown in FIG. 1(a). When the MOSFETs of the proposed system are fabricated on an IC chip, such an IC chip will therefore have a large packaging area. The increased packaging area presents an obstacle to the reduction in size of large-size document reader devices.

It is thus an object of the present invention to eliminate the decreased signal-to-noise ratios of large-size image sensors without appreciably increasing the size of the photodetector IC package.

According to the present invention, noise reduction is achieved, and hence signal-to-noise ratio is improved, without increasing the number of MOSFETs used. More specifically, two signal lines are connected to the MOSFETs, and adjacent MOSFETs are grouped together and are coupled alternately to the signal lines, such that suitable switching of the MOSFETs for differentially amplifying the signals delivered over the signal lines is achieved. The improved differential amplification results in a reduction in the noise of the composite image signal, which enhances the signal-to-noise ratio of the system.

FIG. 1(a) is an equivalent circuit diagram of a conventional large-size image sensor, and FIGS. 1(b) and 1(c) are plan and side views of a photosensitive element of a conventional large-size image sensor;

FIG. 2 is a circuit diagram of a prior art circuit for reducing noise in a conventional large-size image sensor;

FIG. 3 is an equivalent circuit diagram of a large-size image sensor according to the present invention;

FIG. 4 is a timing chart, along with a diagram showing the waveforms of the corresponding output signals, for the operations of the elements of the large-size image sensor of the present invention.

The present invention will now be described with reference to FIG. 3. In FIG. 3, two signal lines 34, 35 are connected alternately to the sources 41a, 41b . . . 41i of MOSFETs 32a, 32b . . . 32i. Although in the illustrated embodiment the MOSFETs are separated and grouped such that three adjacent MOSFETs are connected to the same signal line, it is to be understood that they may be divided into groups ranging from one to several tens of adjacent MOSFETs.

The operation of the document reader device as shown in FIG. 3 will be described with reference to the timing chart of FIG. 4.

The shift register 33 in FIG. 3 is driven by a clock signal 30. Signals 40a-40i are supplied to the gates of the MOSFETs 32a-32i by the shift register. Each of the signals turns on the corresponding MOSFET when the signal is at the low logic level, and turns off the MOSFET when it goes high. First, the signals 40a, 40b, 40c successively go low to turn on MOSFETs 32a, 32b, 32c by successively introducing the pulses 40a--1, 40b--1, 40c--1, respectively, to the gates of the MOSFETs. Signals 34a, 34b, 34c, which contain both optical signals and noise, are now delivered over signal line 34. Then, the signals 40d, 40e, 40f successively go low to produce pulses 40d--1, 40e--1, 40f--1 which turn on the MOSFETs 32d, 32e, 32f, respectively, in succession. Thus, signals 35d, 35e, 35f, which are a mixture of optical signals and noise, are delivered over the signal line 35. Simultaneously with the above operation, the signals 40a, 40b, 40c are rendered low successively to produce pulses 40a--2, 40b--2 and 40c--2, which cause MOSFETs 32a, 32b, and 32c to deliver signals 34d, 34e, 34f (containing noise only) to the signal line 34. Thus, one set of signals, comprising optical signals and noise, and another set of signals, which comprises of noise only, are simultaneously transmitted over the signal lines 34 and 35. The signals supplied through the signal lines 34 and 35 are differentially amplified by a differential amplifier 38 such that the signal appearing at its output terminal 39 comprises of optical signal 39 only. In FIG. 4, the solid line portions of optical signal 39 are generated when light falls upon photosensitive elements 31, and the dotted line portions are generated when no light falls upon the photosensitive elements 31.

In summary, the document reader device of the present invention is capable of producing an accurate output signal with an enhanced signal-to-noise ratio. As a result of this improved signal-to-noise ratio, a source of light for illuminating a document or script may be reduced in intensity, and the speed at which the document is read can be increased. Further, the advantages of the document reader device of the prsent invention hold true regardless of the length of the signal lines 34, 35. Thus, the present invention is highly advantageous when applied to large-size image sensors in which the signal lines 34, 35 are more than 20 centimeters long, in that it eliminates the capacitive noise produced by such long couplings. The number of MOSFETs used to equal to the number of photosensitive elements, such that the packaging area in which the MOSFETs are disposed may be reduced.

It is apparent that modifications of this invention may be practiced without departing from the essential novelty thereof.