Abstract:
It is a principle object of the present invention to reduce a voltage drop of a common power supply wiring in a plurality of amplification circuits to suppress crosstalk generated in other signal output lines. A photoelectric conversion device includes: a plurality of pixels each having a photoelectric conversion area; a plurality of signal output lines through which electrical signals are to be read out from the plurality of pixels; and a plurality of amplification circuits provided in correspondence to the plurality of signal output lines for amplifying the electrical signals, respectively, the plurality of amplification circuits including at least one constant current circuit portion and being disposed in a predetermined direction of repetitive dispersion, in which a constant current circuit portion includes at least a source grounded field effect transistor (the gate electrode is designated by reference symbol  124 G), and a direction (X-axis direction) of a channel length of the source grounded field effect transistor is different from the direction of repetitive disposition of the amplification circuits.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a photoelectric conversion device and an image pickup system using the photoelectric conversion device, and more particularly to a photoelectric conversion device including a plurality of pixels each having a photoelectric conversion area, a plurality of signal output lines through which electrical signals are to be read out from a plurality of pixels, and a plurality of amplification circuits which are provided in correspondence to a plurality of signal output lines in order to amplify the electrical signals and which includes at least one constant current circuit portion, and an image pickup system using the photoelectric conversion device. 
         [0003]    2. Related Background Art 
         [0004]    A photoelectric conversion device for amplifying and reading out signals from a plurality of pixels each having a photoelectric conversion area by column amplifiers, respectively, to read out successively the amplified signals from the column amplifiers by a scanning circuit is described in Japanese Patent Application Laid-Open No. H02-296470 for example. 
         [0005]      FIG. 6  is a circuit diagram showing a configuration of a photoelectric conversion device described in Japanese Patent Application Laid-Open No. H02-296470. As shown in the drawing, a plurality of pixels for one column are connected to a column amplifier, as an amplification circuit, including a first MOS transistor M 1  and a second MOS transistor M 2 . Thus, a signal from the pixel is inputted and amplified to be accumulated in the form of an offset signal (noise signal) and a sensor signal in capacitors CT 1  and CT 2  as temporary accumulation means, respectively. 
         [0006]    Now, a large output resistance is required for a transistor constituting a constant current circuit in an inside of the column amplifier. 
         [0007]    This reason is that if an operating current of the column amplifier changes, then a voltage drop in a common GND wiring into which the operating current is caused to flow changes accordingly (in  FIG. 6 , a voltage drop in V vs  changes) to generate crosstalk in signals in other columns. For example, when amplifiers for 1,000 columns are disposed in a horizontal direction, and an operating current of each amplifier is 10 μA, then a current of 10 mA in total is caused to flow into the GND wiring. In this case, if the operating currents fluctuate in the amplifiers for 900 columns to be decreased by 1 μA, the current of 0.9 mA in total changes accordingly. A change in operating current is developed in the form of fluctuation of the voltage drop due to a parasitic resistance to exert an influence on an operation for reading the signal. 
       SUMMARY OF THE INVENTION 
       [0008]    It is a principle object of the present invention to reduce a voltage drop of a common power supply wiring in a plurality of amplification circuits to suppress crosstalk generated in other signal output lines. 
         [0009]    A photoelectric conversion device of the present invention includes: a plurality of pixels each having a photoelectric conversion area; a plurality of signal output lines through which electrical signals are to be read out from the plurality of pixels; and a plurality of amplification circuits provided in correspondence to the plurality of signal output lines for amplifying the electrical signals, respectively, the plurality of amplification circuits including at least one constant current circuit portion and being disposed in a predetermined direction of repetitive dispersion, in which the constant current circuit portion includes at least a field effect transistor, and a direction of a channel length of the field effect transistor is different from the direction of repetitive disposition of the amplification circuits. 
         [0010]    According to the photoelectric conversion device of the present invention, a sufficient output resistance is obtained in the source grounded field effect transistor in the amplification circuits, whereby a voltage drop in a common power supply wiring of a plurality of amplification circuits can be reduced, and hence the crosstalk generated in other signal output lines can be suppressed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a plan view of an amplification circuit of a photoelectric conversion device according to a first embodiment of the present invention; 
           [0012]      FIG. 2  is an equivalent circuit diagram of an amplification circuit of the photoelectric conversion device according to the first embodiment of the present invention; 
           [0013]      FIG. 3  is a block diagram showing a whole configuration of the photoelectric conversion device according to the first embodiment of the present invention; 
           [0014]      FIG. 4  is a plan view of an amplification circuit of a photoelectric conversion device according to a second embodiment of the present invention; 
           [0015]      FIG. 5  is an equivalent circuit diagram of an amplification circuit of the photoelectric conversion device according to the second embodiment of the present invention; 
           [0016]      FIG. 6  is a circuit diagram of an equivalent circuit of a photoelectric conversion device according to a related art; and 
           [0017]      FIG. 7  is a block diagram showing a case where the photoelectric conversion device according to the present invention is applied to a still video camera. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Preferred embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
         [0019]      FIG. 1  is a plan view of an amplification circuit of a photoelectric conversion device according to a first embodiment of the present invention;  FIG. 2  is an equivalent circuit diagram of an amplification circuit of the photoelectric conversion device according to the first embodiment of the present invention; and  FIG. 3  is a block diagram showing a whole configuration of the photoelectric conversion device according to the first embodiment of the present invention. While pixels for two rows are shown in  FIG. 3 , the pixels equal in number to or more than these pixels for two rows may be adopted. In addition, while the photoelectric conversion device is shown in the form of an area sensor in which the pixels are two-dimensionally disposed, a line sensor in which the pixels are one-dimensionally disposed may also be adopted as the photoelectric conversion device. 
         [0020]    Referring to  FIG. 3 , one pixel includes: a photodiode  101 ; a pixel transfer switch  102  for transferring electric charges accumulated in the photodiode  101  to a floating diffusion region (not shown); a driver MOS transistor  104 , as an amplification means, having an input portion common to the floating diffusion region; a reset switch  103  connected to the floating diffusion region; and a row selection switch  105  for selecting pixels belonging corresponding row among all the pixels. 
         [0021]    The pixel transfer switch  102 , the reset switch  103 , and the row selection switch  105  are controlled with control signals PTX, PRES and PSEL from a vertical scanning circuit  123 , respectively. Thus, signals from the pixels belonging to the row selected by the row selection switch  105  are outputted to vertical output lines  106 , as signal output lines, which are alternately provided vertically in the drawing. Each vertical output line  106  connects the corresponding pixels and one terminal (+) of an operational amplifier  120  as an amplification circuit through a clamp capacitor  108 . Reference numeral  107  designates a load MOS transistor and reference numeral  109  designates a clamp switch. 
         [0022]    The signals amplified by the operational amplifier  120  are accumulated in an accumulation capacity  112  through a transfer gate  110  and are then successively outputted to a horizontal output line  116  through a horizontal transfer switch  114  controlled by a horizontal scanning circuit  119  to be outputted through an output amplifier  118 . Reference numeral  132  designates a common GND wiring common to the operational amplifiers  120 . 
         [0023]    A configuration of an equivalent circuit of the operational amplifier  120  as the amplification circuit is shown in  FIG. 2 , and a plan view of the configuration of the operational amplifier  120  is shown in  FIG. 1 . In  FIGS. 1 and 2 , reference numeral  124  designates a source grounded field effect transistor of a constant current circuit portion; reference numeral  125  designates a field effect transistor as an input transistor of an input portion; reference numeral  128  designates a source grounded field effect transistor of a load portion; reference symbol  124 G designates a gate electrode of the source grounded field effect transistor  124 ; reference symbol  125 G designates a gate electrode of the field effect transistor  125 ; and reference symbol  128 G designates a gate electrode of the source grounded field effect transistor  128 . Also, reference numeral  129  designates a contact hole, reference numeral  130  designates an active region, and reference numeral  131  designates a P-type well region. 
         [0024]    As has already been described, if the operating current of the operational amplifier  120  changes, then the voltage drop of the common GND wiring  132  into which the operating currents are caused to flow changes accordingly to generate crosstalk in the signals of other columns. Hence, in the operational amplifier (amplification circuit)  120 , the source grounded field effect transistor  124  is required to have a sufficient output resistance. Consequently, it is important that a channel length of the source grounded field effect transistor is made long enough to be free from an influence of the channel length modulation. 
         [0025]    For this reason, it is desirable that the field effect transistors be disposed so as for their channel lengths to extend in a direction (corresponding to a direction vertical to a direction of repetitive disposition of the amplification circuits in this case, i.e., a Y-axis direction in the drawing) different from the direction of repetitive disposition of the amplification circuits (an X-axis direction in the drawing) limited by a pixel pitch. That is, the disposition is effective in which the direction of the channel length of the source grounded field effect transistor  124  is the direction (corresponding to the direction vertical to the direction of repetitive disposition of the amplification circuits in this case, i.e., the Y-axis direction in the drawing) different from the direction of repetitive disposition of the amplification circuits. Such a configuration can be grasped as causing a drain current of the source grounded field effect transistor  124  to flow in a direction different from the direction of repetitive disposition of the amplification circuits. 
         [0026]    This importance increases since a pitch of repetitive disposition of the amplification circuits in the X-axis direction decreases along with reduction in pixel pitch. 
         [0027]    Note that the direction different from the direction of repetitive disposition of the amplification circuits means a direction which is inclined with respect to the direction of repetitive disposition of the amplification circuits. While the gradient of that direction is set as may be necessary, that direction is preferably set as the direction vertical to the direction of repetitive disposition of the amplification circuits as in this embodiment in terms of layout. While the pitch of repetitive disposition of the amplification circuits is designed so as to correspond to the pixel pitch in many cases, the pitch of repetitive disposition of the amplification circuits may be designed so as to be wider or narrower than the pixel pitch in some cases. 
         [0028]    As described above, the direction of the channel length of the source grounded field effect transistor  124  is set in the direction different from the direction of repetitive disposition of the amplification circuits, whereby the channel length of the source grounded field effect transistor  124  can be provided so as to be long without being limited by the pitch (indicated by X 1  in  FIG. 1 ) of repetitive disposition of the amplification circuits. In particular, this is suitable for a case where the channel length is set longer than the pitch of repetitive disposition of the amplification circuits. 
         [0029]    In addition, an open loop gain becomes larger as g m  (transconductance) of the input transistor  125  of the operational amplifier (amplification circuit) is larger. Thus, the direction (it can be grasped as a direction of flow of a drain current) of the channel length of the input transistor  125  is desirably set in the direction different from the direction (Y-axis direction) vertical to the direction of repetitive disposition of the operational amplifiers (amplification circuits)(this desirable direction is set in the direction of repetitive disposition of the amplification circuits in this embodiment). It is because g m  of a field effect transistor is proportional to W/L (L is a channel length and W is a channel width), and thus it is advantageous that the channel width is allowed to be ensured in the direction (Y-axis direction) different from the direction of repetitive disposition limited by the pixel pitch. Note that while the direction different from the direction (Y-axis direction) vertical to the direction of repetitive disposition of the operational amplifiers (amplification circuits) means the direction which is inclined with respect to the direction vertical to the direction of repetitive disposition of the operational amplifiers and its gradient is set as may be necessary, that direction different therefrom is preferably set in the direction (the X-axis direction in the drawing) of repetitive disposition of the amplification circuits as in this embodiment in terms of layout. 
         [0030]    When the input portion or the load portion has a cascode configuration, an output resistance of the amplification circuit is proportional to g m  of a gate grounded field effect transistor. Thus, the direction (it can be grasped as the direction as well of flow of the drain current) of the channel length of the gate grounded field effect transistor is desirably set in the direction (it is set as the X-axis direction as the direction of repetitive disposition of the amplification circuits in this embodiment) different from the direction vertical to the direction of repetitive disposition of the amplification circuits. An embodiment in such a case as well will hereinafter be described. 
         [0031]      FIG. 4  is a plan view of an amplification circuit of a photoelectric conversion device according to a second embodiment of the present invention, and  FIG. 5  is an equivalent circuit diagram of the amplification circuit of the photoelectric conversion device according to the second embodiment of the present invention. In  FIGS. 4 and 5 , the same constituent elements as those in  FIGS. 1 and 2  are designated with the same reference symbols. 
         [0032]    In  FIGS. 4 and 5 , reference numeral  124  designates a source grounded field effect transistor of a constant current circuit portion. Reference numeral  125  designates a field effect transistor as an input transistor, and reference numeral  126  designates a gate grounded field effect transistor. The field effect transistor  125  and the gate grounded field effect transistor  126  constitute an input portion having a cascode configuration. Reference numeral  127  designates a gate grounded field effect transistor, and reference numeral  128  designates a source grounded field effect transistor. The gate grounded field effect transistor  127  and the source grounded field effect transistor  128  constitute a load portion having a cascode configuration. Reference symbol  124 G designates a gate electrode of the source grounded field effect transistor  124 , reference symbol  125 G designates a gate electrode of the grounded field effect transistor  125 , reference symbol  126 G designates a gate electrode of the gate grounded field effect transistor  126 , reference symbol  127 G designates a gate electrode of the gate grounded field effect transistor  127 , and reference symbol  128 G designates a gate electrode of the source grounded field effect transistor  128 . Also, reference numeral  129  designates a contact hole, reference numeral  130  designates an active region, and reference numeral  131  designates a P-type well region. 
         [0033]    In this embodiment, the direction of the channel lengths of the gate grounded field effect transistor  126  and the gate grounded field effect transistor  127  is set in the direction (the X-axis direction in the drawing) of repetitive disposition of the amplification circuits. 
         [0034]    Next, a description will be given with respect to an image pickup system using the photoelectric conversion device according to the first and second embodiments of the present invention. As for the image pickup system using such a photoelectric conversion device, a still camera, a video camera, a copying machine, a facsimile and the like are given. Then, a detailed description will hereinafter be given with respect to an embodiment when the photoelectric conversion device according to the present invention is applied to a still camera with reference to  FIG. 7 . 
         [0035]      FIG. 7  is a block diagram showing a case where a solid state image pickup device of the present invention is applied to “a still video camera”. 
         [0036]    In  FIG. 7 , reference numeral  1  designates a barrier serving both as a protector and as a main switch; reference numeral  2  designates a lens for forming an optical image of a subject on a surface of a solid state image pickup device  4 ; reference numeral  3  designates a diaphragm for making a quantity of light passed through the lens  2  variable; reference numeral  4  designates the solid state image pickup device for capturing an image of the subject formed by the lens  2  in the form of an image signal; reference numeral  6  designates an A/D converter for A/D-converting the image signal outputted from the solid state image pickup device  4 ; reference numeral  7  designates a signal processor for subjecting image data outputted from the A/D converter  6  to various corrections or compressing data; reference numeral  8 , a timing generator for outputting various timing signals to the solid state image pickup device  4 , a circuit processing image pickup signal  5 , the A/D converter  6  and the signal processor  7 ; reference numeral  9 , an arithmetic operation unit controlling whole structure for carrying out various arithmetic operations and controlling the whole still video camera; reference numeral  10 , a memory for temporarily storing therein image data; reference numeral  11 , an I/F unit controlling processing medium for recording or reading out data in or from a recording medium; reference numeral  12 , a detachable recording medium, such as a semiconductor memory, in or from which the image data is to be recorded or read out; and reference numeral  13 , an external I/F unit through which the still video camera is to communicate with an external computer or the like. 
         [0037]    Next, a description will be given with respect to an operation of the still video camera during the photographing with the above-mentioned configuration. 
         [0038]    Upon open of the barrier  1 , a main power supply is turned ON, a power supply of a control system is next turned ON, and moreover a power supply of an image pickup system circuit such as the A/D converter  6  is turned ON. 
         [0039]    Thereafter, in order to control the exposure, the arithmetic operation unit controlling whole structure  9  releases the diaphragm  3 , and a signal outputted from the image pickup device  4  is then A/D-converted in the A/D converter  6  to be inputted to the signal processor  7 . An arithmetic operation for the exposure is carried out based on the resultant data in the arithmetic operation unit controlling whole structure  9 . 
         [0040]    The brightness is judged based on the results of the photometry, and the arithmetic operation unit controlling whole structure  9  controls the diaphragm  3  in correspondence to the judgment results. 
         [0041]    Next, high frequency components are taken out from the signal outputted from the solid state image pickup device  4  to carry out the arithmetic operation for obtaining a distance up to the subject in the arithmetic operation unit controlling whole structure  9 . Thereafter, the lens  2  is driven to judge whether or not the focus is obtained. When the judgment results show that the focus is not yet obtained, the lens  2  is driven again to carry out the distance measurement. 
         [0042]    After the focus is verified, the main exposure is started. After completion of the exposure, the image signal outputted from the solid state image pickup device  4  is A/D-converted in the A/D converter  6  to pass through the signal processor  7  to be written to a memory  10  by the arithmetic operation unit controlling whole structure  9 . Thereafter, the data accumulated in the memory  10  passes through the I/F unit controlling recording medium  11  to be recorded in the detachable recording medium  12  such as a semiconductor memory in accordance with the control by the arithmetic operation unit controlling whole structure  9 . Or, the data concerned may pass through the external I/F unit  13  to be directly inputted to a computer in order to process the image. 
         [0043]    The present invention is applied to the photoelectric conversion device used in a still camera, a video camera, a copying machine, a facsimile or the like, and especially is suitably used in the photoelectric conversion device of high resolution for which the reduction in pixel pitch is required. 
         [0044]    This application claims priority from Japanese Patent Application No. 2004-021542 filed on Jan. 29, 2004, which is hereby incorporated by reference herein.