JPH0568129A - Image sensor - Google Patents

Abstract

PURPOSE:To obtain a high resolution image sensor having no afterimage phenomenon by holding the potential of a node between a pair of photodiodes in photodetecting elements at an approximately fixed level during the reading period of an image signal. CONSTITUTION:An output level control means 5 controls the output level of a reading pulse impressing means 4 during the reading period of an image signal and holds the potential of a node between the 1st and 2nd photodiodes 2a, 2b in the photodetecting elements 2 at an approximately fixed level. Thereby such a trouble that the potential of the node between the 1st and 2nd photodiodes 2a, 2b is dropped during the reading period of the image signal and the dropped part is superposed to an image signal in the succeeding line can be prevented. Thereby the generation of an afterimage phenomenon due to the drop of potential can be prevented and the resolution of a read image can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image sensor used in an input section of a facsimile, an image scanner or the like, and more particularly to a light receiving element in which a pair of photodiodes are connected in series so that their polarities are opposite to each other. The present invention relates to an improvement of an image sensor in which a plurality of lines are arranged side by side.

[Prior Art]

2. Description of the Related Art Conventionally, as an image sensor used for reading an image in a facsimile or the like, for example, as shown in FIG. 3, a photodiode PD1 and a photodiode PD2 are connected in series so that their polarities are opposite to each other. One light receiving element 21 is formed, and this light receiving element 2
A structure in which a plurality of 1s are arranged in a line has already been proposed.

In this image sensor, one end of the photodiode PD1 of each light receiving element 21 is connected to the common electrode line 23.
One end of the photodiode PD2 is connected to each output terminal 3 of the shift register SR via the individual electrode line 22.
1, 32, 33, ... Connected to the shift register S
The read pulses are sequentially output from the R output terminals 31, 32, 33, ....

The reading of the image signal by such an image sensor is performed as follows. That is, the photodiode PD1 that has already been charged is irradiated with the reflected light from the document surface (not shown), and the charge corresponding to the irradiation light amount of the light is discharged (accumulation period). Then, a pulse voltage is applied to the photodiode PD2 by the shift register SR via the individual electrode line 22, and the photodiode P
D2 is forward biased and diodes PD1 and PD
The battery is charged so that the voltage between two points (point C) becomes a substantially constant value (signal reading period). At this time, charges are supplied from the outside via the common electrode line 23 so as to replenish the charges lost by the discharge within the accumulation period. Therefore, by detecting this amount by the charge reading circuit 24, the image signal output is performed. Obtainable. The above operation is performed for each light receiving element 21 each time a drive pulse is applied from the terminals 31, 32, 33, ... Of the shift register SR at timings such as the waveforms 43, 44, 45 ,. The image signal of one line can be obtained in time series.

[0005]

However, the above-mentioned image sensor has the following technical problems. That is, as shown in FIG. 5, with respect to the drive pulse 51 of each light receiving element, the potential at the point C must be charged to the V _cc potential within the signal reading period, but the pulse response of the photodiode PD2. As shown in the waveform 52, when the signal reading period ends, ΔV
However, there was a phenomenon that an afterimage was generated as an image because this amount was superimposed on the image signal of the next line.

The present invention has been made in view of the above circumstances and provides a high resolution image sensor in which an afterimage phenomenon does not occur.

[0007]

That is, the present invention is
As shown in FIG. 1, a light-receiving element array 1 in which a plurality of light-receiving elements 2 in which a first photodiode 2a and a second photodiode 2b are connected in series so that their polarities are opposite to each other are arranged in line A charge reading means 3 to which the first photodiode 2a of each light receiving element 2 is connected via a common electrode, and the second photodiode 2 of each light receiving element 2.
Read pulse application in which a read pulse k is sequentially output from each output terminal and each output terminal is connected to the second photodiode 2b of each light receiving element 2 through an individual electrode. Means for reading the image signal G based on the output from the charge reading means 3 for each light receiving element 2 by sequentially applying the reading pulse k, and the first sensor is provided during the image signal reading period. The output level control means 5 for controlling the output level of the read pulse application means 4 is provided so that the potential at the connection point between the photodiode 2a and the second photodiode 2b becomes substantially constant. is there.

[0008]

According to the technical means as described above, the output level control means 5 controls the output level of the read pulse applying means 4 during the image signal read period, and the first photodiode 2a and the second photodiode 2a are controlled. The potential at the connection point with the photodiode 2b is kept substantially constant. Therefore, it is possible to avoid a situation in which the potential of the connection point between the first photodiode 2a and the second photodiode 2b decreases during the image signal reading period, and this decrease is superimposed on the image signal of the next line. To be done.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on the embodiments shown in the accompanying drawings. FIG. 2 shows an embodiment of an image sensor to which the present invention is applied. In the figure, the image sensor includes a plurality of light receiving elements 21 in which the photodiodes PD1 and PD2 are connected in series so that their polarities are opposite to each other (light receiving element array). , One end of the photodiode PD1 of each light receiving element 21 is connected to the charge reading circuit 24 via the common electrode line 23, and one end of the photodiode PD2 is for generating a drive pulse (readout pulse) via the individual electrode line 22 respectively. Output terminals 31, 3 of the shift register SR
2, 33 ...

Further, in this embodiment, the shift register SR is provided with an additional circuit 60 for adjusting the output level. The additional circuit 60 includes a diode 61, an O
It is composed of a P amplifier 62, an adder 63, and a reference voltage power supply 64, and the diode 61 is a photodiode PD.
2, which has the same performance as that of photodiode PD1
Is connected to the common electrode line 23 in the same direction as, and the negative input terminal of the OP amplifier 62 is also connected to the connection point. Also,
The positive input terminal of the OP amplifier 62 is grounded, and the other end of the diode 61 is connected to the output terminal of the OP amplifier 62 and the input terminal X of the adder 63. Further, the adder 6
The positive terminal of the reference voltage power supply 64 is connected to the other input terminal Y of the reference numeral 3, and the negative terminal of the reference voltage power supply 64 is grounded. Furthermore, the output terminal Z of the adder 64 is connected to the power supply terminal _Vcc of the shift register SR.

The diode 61 needs to have the same performance as that of the photodiode PD2. For example, when the photodiode array is manufactured, it is manufactured on the same substrate and separated from the photodiode array after the process is completed. Easily obtained.

Next, the operation of the image sensor according to this embodiment will be described focusing on the light receiving element at point A. now,
When the drive pulse is output from the terminal 32 of the shift register SR, the light receiving element 21 enters the signal reading period, the photodiode PD2 is forward biased, and the potential at the point A is V _cc.
The battery is charged so that

At this time, the photodiode PD2 can be regarded as a resistor R and the photodiode PD1 can be regarded as a capacitor C, and a transient current i determined by CR flows.

On the other hand, in the additional circuit 60, the transient current i is caused by the negative input terminal of the OP amplifier 62 and the diode 61.
Although it can be considered that the input impedance of the OP amplifier 62 is infinitely large, all the inflowing current i flows to the diode 61. in this case,
Since the diode 61 is forward biased and has the same characteristics as the photodiode PD2 of the light receiving element 21, it can be regarded as a resistor R as well, and V = − at the point B.
A voltage of iR is generated. This voltage has the opposite sign and the same magnitude as the potential at the point A of the light receiving element 21.

Further, a voltage obtained by adding the reference voltage from the reference voltage power source 64 and the potential at the point B is generated at the output terminal Z by the adder 63 in the next stage and applied to the power source terminal of the shift register SR. That is, the adder 63 has the output terminal Z
Constant value appropriately amplify the value of the reference voltage from the reference voltage source 64 so as to be power supply voltage V _cc of the shift register SR, which to add the voltage at the point B in a suitable gain. Therefore, as a result, the power supply voltage of the shift register SR is raised by an amount corresponding to the shortage with respect to V _{cc at the} point A, and the point A operates so as to always have a substantially constant value.

Since the above-mentioned operations are performed in time series in each light receiving element 21, all the light receiving elements 21 can be reliably charged to the reference voltage value during the signal reading period.

In evaluating the performance of the image sensor according to the example of the operation process, the reading performance of the example and the comparative example without the additional circuit 60 are compared. Although the afterimage phenomenon was observed in the first embodiment, it was confirmed that the afterimage phenomenon disappeared and the resolution of the read image was increased in the examples.

[0018]

As described above, according to the present invention, the potential at the connection point of the pair of photodiodes of the light receiving element is kept substantially constant by the function of the output level control means during the image signal reading period. Therefore, it is possible to avoid the potential decrease phenomenon at the connection point of the pair of photodiodes during the image signal reading period, thereby eliminating the afterimage phenomenon associated with the potential decrease and improving the resolution of the read image. You can

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a configuration of an image sensor according to the present invention.

FIG. 2 is an explanatory diagram showing an embodiment of an image sensor to which the present invention is applied.

FIG. 3 is an explanatory diagram showing an example of a conventional image sensor.

FIG. 4 is a timing chart showing an operation process of a conventional image sensor.

FIG. 5 is a waveform diagram for explaining an afterimage phenomenon of a conventional image sensor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Light receiving element array, 2 ... Light receiving element, 2a ... 1st photodiode, 2b ... 2nd photodiode, 3 ... Charge reading means, 4 ... Read pulse applying means, 5 ... Output level control means

Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H04N 5/335 E 8838-5C

Claims (1)

[Claims] 1. A plurality of light receiving elements (2) in which a first photodiode (2a) and a second photodiode (2b) are connected in series so that polarities thereof are opposite to each other are arranged in line. The light receiving element array (1), the charge reading means (3) to which the first photodiode (2a) of each light receiving element (2) is connected via the common electrode, and the second light receiving element (2) of each light receiving element (2). The second photodiode of each light receiving element (2) has an output terminal corresponding to the photodiode (2b), the read pulse (k) is sequentially output from each output terminal, and each output terminal is via an individual electrode. An image signal based on the output from the charge reading means (3) for each light receiving element (2) by sequentially applying the read pulse (k). Image sensor for reading (G) In, during the image signal reading period, the output level of the reading pulse applying means (4) is controlled so that the potential at the connection point of the first photodiode (2a) and the second photodiode (2b) becomes substantially constant. And an output level control means (5) provided therein.