Abstract:
The output voltages of dark pixels and optical black (OB) pixels are different between a row including a pixel which receives strong light and another row. An image formed upon receiving a strong light spot suffers whitish bands on the right and left of the spot. To solve this problem, this invention provides an image pickup apparatus including a pixel containing a photodiode which converts a photo-signal into a signal charge and accumulates the signal charge and an amplifier transistor which amplifies the signal charge accumulated in the photodiode, and a control element adapted to limit the output of the amplifier transistor so as to prevent the output from falling to below a predetermined voltage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a solid-state image pickup device widely used in image pickup apparatuses such as a video camera and digital still camera.  
         [0003]     2. Related Background Art  
         [0004]     In recent years, the pixel size has enthusiastically been reduced using a miniaturization process for the purpose of high resolution. Since this decreases the photoelectrically converted signal output, amplifier type solid-state image pickup device capable of amplifying and outputting a photoelectrically converted signal are receiving a great deal of attention. Such amplifier type solid-state image pickup device includes a MOS, AMI, CMD, and BASIS type device. Of these devices, the MOS type solid-state image pickup device accumulates photocarriers generated by a photodiode in the gate electrode of a MOS transistor, and amplifies the electric charges to output the potential change to an output unit in accordance with a driving timing from a scanning circuit. Of the MOS type solid-state image pickup device, attention is recently given particularly to a CMOS solid-state image pickup device in which all the components including a photoelectric conversion unit and peripheral circuit are implemented by a CMOS process.  
         [0005]      FIG. 1  is a block diagram showing the arrangement of a conventional CMOS solid-state image pickup device. In  FIG. 1 , the CMOS solid-state image pickup device comprises a pixel portion  1 , and a vertical scanning circuit unit  2  for vertically scanning pixels. The pixel portion  1  is constituted by photodiodes D 11  to D 33 , reset MOS transistors (to be referred to as MOSs) M 211  to M 233  for resetting electric charges in the photodiodes, amplifier MOSs M 311  to M 333  for amplifying electric charges in the photodiodes, selector MOSs M 411  to M 433  for selecting rows, and vertical output lines V 1  to V 3 . Load MOSs M 51  to M 53  serve as the loads of the amplifier MOSs, an input MOS M 50  sets a constant current to be supplied to the load MOSs, and a voltage input terminal  5  sets the gate voltage of the input MOS.  
         [0006]     The operation of the solid-state image pickup device in  FIG. 1  will be described. When light is incident on the photodiodes D 11  to D 33 , the respective photodiodes generate and accumulate photo-signal charges. In reading out signals, they are sequentially read out to the vertical output lines V 1  to V 3  in units of rows while the vertical scanning circuit unit  2  vertically scans the pixels. If the first row is selected, a signal on a second row selection line PSEL 1  connected to the gates of the selector MOSs M 411 , M 421 , and M 431  changes to high level, and the amplifier MOSs M 311 , M 321 , and M 331  are activated. As a result, signals on the first row are read out to the vertical output lines V 1  to V 3 . Then, a signal on a first row selection line PRES 1  connected to the gates of the reset MOSs M 211 , M 221 , and M 231  changes to high level, and the electric charges accumulated in the photodiodes D 11 , D 21 , and D 31  are reset. The second row is selected, and signals on the second row are similarly read out to the vertical output lines V 1  to V 3 . Similarly, signals on the third and subsequent lines are sequentially read out to the vertical output lines V 1  to V 3 .  
         [0007]     In a read-out operation in the solid-state image pickup device of  FIG. 1 , as a photo-signal is higher, the voltages on the vertical output lines V 1  to V 3  are lower. The vertical output lines V 1  to V 3  are respectively connected to the drains of the load MOSs M 51  to M 53 . On a column from which the signal of a pixel receiving very strong light is read out, the source-drain voltage of a load MOS drops to 0 V to turn it off. Hence, a current which flows through a common GND line  4  in reading out a given row changes depending on the number of load MOSs which are in an off-state (referred to hereinafter as “OFF load MOSs”). The line width of the GND line  4  can take only a finite value in terms of limitations on the chip size or the like, and has a certain impedance.  
         [0008]     The value of a constant current which is supplied to the load MOS is set to apply the voltage of the input terminal  5  between the gate of the input MOS M 50  and an absolute GND (e.g., ground potential of an external substrate). For this reason, the set current value changes owing to a voltage drop determined by the impedance of the GND line  4  and the current flowing there through. Since the number of OFF load MOSs is larger as the number of pixels receiving strong light is larger, the voltage drop across the GND line  4  decreases to increase the set current of the load MOS, resulting in a large gate-source voltage of the amplifier MOS. Due to this phenomenon, the output voltages of dark pixels and optical black (OB) pixels are different between a row including a pixel receiving strong light and another row. An image receiving a strong light spot suffers whitish bands on the right and left of the spot.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of the present invention to provide an image pickup apparatus capable of obtaining a clear image.  
         [0010]     To achieve the above object, according to an aspect of the present invention, there is provided an image pickup apparatus comprising: 
        a plurality of pixels, each of the pixels including a photoelectric conversion element adapted to perform photoelectric conversion and an amplifier element adapted to amplify and output a signal from the photoelectric conversion element; and     a control element adapted to limit an output level of the amplifier element so that the output level is prevented from falling to below a predetermined level.        
 
         [0013]     According to another aspect of the present invention, there is provided an image pickup apparatus comprising: 
        a plurality of pixels, each of the pixels including a photoelectric conversion element adapted to perform photoelectric conversion and an amplifier element adapted to amplify and output a signal from the photoelectric conversion element;     a load element adapted to serve as a load of the amplifier element; and     a control element adapted to control to prevent the load element from being turned off regardless of quantity of signal generated by the photoelectric conversion element.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a block diagram showing a conventional solid-state image pickup device;  
         [0018]      FIG. 2  is a block diagram showing the first embodiment of the present invention;  
         [0019]      FIG. 3  is a block diagram showing the second embodiment of the present invention;  
         [0020]      FIG. 4  is a block diagram showing the third embodiment of the present invention;  
         [0021]      FIG. 5  is a block diagram showing the fourth embodiment of the present invention;  
         [0022]      FIG. 6  is a block diagram showing the fifth embodiment of the present invention;  
         [0023]      FIG. 7  is a block diagram showing the sixth embodiment of the present invention;  
         [0024]      FIG. 8  is a block diagram showing the seventh embodiment of the present invention; and  
         [0025]      FIG. 9  is a block diagram showing the eighth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the following embodiments, circuit elements constituting a solid-state image pickup device are formed on one semiconductor substrate of single-crystal silicon by a semiconductor integrated circuit manufacturing technique (e.g., CMOS process), but the present invention is not limited to this. In the following embodiments, a 3×3 pixel array is adopted for descriptive convenience, but the present invention is not limited to this. In the following embodiments, a MOS transistor will be simply referred to as a MOS.  
         [0027]      FIG. 2  is a block diagram showing a solid-state image pickup device according to the first embodiment of the present invention. In  FIG. 2 , the same reference numerals as in the conventional element of  FIG. 1  denote the same parts. That is, the solid-state image pickup device comprises a pixel portion  1 , vertical scanning circuit unit  2 , and GND line  4 , which are identical to those in  FIG. 1 . Clipping element  3  clips vertical output lines, which will be described later. The pixel portion  1  will be explained. In this embodiment, the anodes of photodiodes D 11  to D 33  for generating photo-signal charges are grounded, and their cathodes are connected to the gates of amplifier MOSs M 311  to M 333 . The gates of the amplifier MOSs M 311  to M 333  are connected to the sources of reset MOSs M 211  to M 233  for resetting the gates, and the drains of which are connected to a reset power supply. The drains of the amplifier MOSs M 311  to M 333  are connected to selector MOSs M 411  to M 433  for supplying a power supply voltage.  
         [0028]     The gate of the reset MOS M 211  is connected to a first row selection line (vertical scanning line) PRES 1  which runs in the lateral direction. The gates of the reset MOSs M 221  and M 231  of the remaining pixel cells on the same row are also commonly connected to the first row selection line PRES 1 . The gate of the selector MOS M 411  is connected to a second row selection line (vertical scanning line) PSEL 1  which runs in the lateral direction. The gates of the selector MOSs M 421  and M 431  of the remaining pixel cells on the same row are also commonly connected to the second row selection line PSEL 1 . The first and second row selection lines are connected to the vertical scanning circuit unit  2 , and receive signal voltages based on operation timings (to be described later).  
         [0029]     On the remaining rows in  FIG. 2 , pixel cells having the same arrangement, and row selection lines are laid out. These row selection lines receive signals on row selection lines PRES 2 , PRES 3 , PSEL 2 , and PSEL 3  from the vertical scanning circuit unit  2 . The source of the amplifier MOS M 311  is connected to a vertical output line V 1  which runs in the longitudinal direction. The sources of the amplifier MOSs M 312  and M 313  of pixel cells on the same column are also connected to the vertical output line V 1 . The vertical output line V 1  is connected to a load MOS M 51  serving as a load element, and also to a corresponding clipping element  3  for clipping the vertical output line V 1 .  
         [0030]     The clipping element  3  is made up of a clipping MOS M 71 , and a MOS M 81  for activating clipping operation. The source of the MOS M 71  is connected to the vertical output line V 1 . The drain of the MOS M 71  is connected to the power supply via the MOS M 81 , and its gate is connected to a power supply line  7  for setting a clipping voltage. The gate of the MOS M 81  is connected to a control pulse line  6  for controlling clipping operation. Remaining vertical output lines V 2  and V 3  in  FIG. 1  are similarly connected to the amplifier MOSs, load MOSs, and clipping element  3 . The sources of the load MOSs M 51  to M 53  are connected to the common GND line  4 , and their gates are connected to the gate of the input MOS M 50  and a voltage input terminal  5 .  
         [0031]     The operation of the first embodiment will be explained. When light is incident on the photodiodes D 11  to D 33 , the respective photodiode generate and accumulate photo-signal charges. In reading out signals, they are sequentially read out to the vertical output lines V 1  to V 3  in units of rows while the vertical scanning circuit unit  2  vertically scans the pixels. If the first row is selected, the second row selection line PSEL 1  connected to the gates of the selector MOSs M 411 , M 421 , and M 431  changes to high level, and the amplifier MOSs M 311 , M 321 , and M 331  are activated. At the same time, a signal on the control pulse line  6  for driving the clipping element  3  also changes to high level, and clipping MOSs M 71  to M 73  are also activated. As a result, signals on the first row are read out to the vertical output lines V 1  to V 3 . Then, the first row selection line PRES 1  connected to the gates of the reset MOSs M 211 , M 221 , and M 231  changes to high level, and electric charges accumulated in the photodiodes D 11 , D 21 , and D 31  are reset. The second row is selected, and signals on the second row are similarly read out to the vertical output lines V 1  to V 3 . Similarly, signals on the third and subsequent lines are sequentially read out to the vertical output lines V 1  to V 3 .  
         [0032]     While, e.g., signals on the first row are read out in this operation, the sources of the amplifier MOS M 311  and clipping MOS M 71 , those of the amplifier MOS M 321  and clipping MOS M 72 , and those of the amplifier MOS M 331  and clipping MOS M 73  are commonly connected to each form a differential arrangement. For example, when the gate voltage of the amplifier MOS M 311  is much higher than a set clipping voltage  7 , the clipping MOS M 71  is OFF, and thus a signal voltage read out to the vertical output line VI is a voltage based on the signal voltage of the gate of the amplifier MOS M 311 . If the gate voltage of the amplifier MOS M 311  comes close to the set clipping voltage  7 , the clipping MOS M 71  is turned on to activate clipping operation, thereby starting clipping effect. When the gate voltage of the amplifier MOS M 311  is much lower than the set clipping voltage  7 , the vertical output line V 1  does not decrease below a voltage determined by the set clipping voltage  7  or less.  
         [0033]     This also applies to the remaining vertical output lines V 2  and V 3  in  FIG. 2 . The clipping voltage  7  is set such that the voltages of the vertical output lines V 1  to V 3  do not drop below a drain voltage for operating the load MOSs M 51  to M 53  within the saturation range. This setting can prevent the load MOSs M 51  to M 53  from being turned off even in case of reading out very high signal charges. The voltage drop across the GND line  4  remains the same regardless of the number of pixels which receive strong light, so the set currents of the load MOSs M 51  to M 53  stay constant regardless of read from any row. In the first embodiment, the output voltages of dark pixels and OB pixels are equal between a row including a pixel receiving strong light and another row. Thus, an image receiving a strong light spot is free from any whitish band, and a clear image can be obtained.  
         [0034]      FIG. 3  is a block diagram showing a solid-state image pickup device according to the second embodiment of the present invention. The second embodiment is different from the first embodiment in that a pixel portion  1  additionally comprises transfer MOSs M 111  to M 133  for transferring photo-signal charges accumulated in photodiodes, between the cathodes of photodiodes D 11  and D 33  and the gates of amplifier MOSs M 311  to M 333 . The gate of the amplifier MOS M 311  is connected to a third row selection line (vertical scanning line) PTX 1  which runs in the lateral direction. The gates of the transfer MOSs M 121  and M 131  of the remaining pixel cells on the same row are also commonly connected to the third row selection line PTX 1 . Similar to the first and second row selection lines, the third row selection line is also connected to a vertical scanning circuit unit  2 , and receives a signal voltage based on an operation timing (to be described later). The remaining arrangement of the pixel portion is the same as in  FIG. 1 , and the same reference numerals denote the same parts.  
         [0035]     A vertical signal line V 1  is simultaneously connected to a capacitance CTN 1  for temporarily holding a noise signal via a noise signal transfer switch M 11 , and to a capacitance CTS 1  for temporarily holding a photo-signal via a photo-signal transfer switch M 21 . The opposite terminals of the noise signal holding capacitance CTN 1  and photo-signal holding capacitance CTS 1  are grounded. The node between the noise signal transfer switch M 11  and the noise signal holding capacitance CTN 1 , and the node between the photo-signal transfer switch M 21  and the photo-signal holding capacitance CTS 1  are respectively grounded via holding capacitance reset switches M 31  and M 32 , and connected via horizontal transfer switches M 41  and M 42  to a differential circuit block  8  for calculating the difference between a photo-signal and a noise signal.  
         [0036]     The gates of the horizontal transfer switches M 41  and M 42  are commonly connected to a column selection line H 1  and horizontal scanning circuit unit  10 .  
         [0037]     Similar read circuits are arranged in remaining columns V 2  and V 3  in  FIG. 3 . The gates of noise signal transfer switches M 11  to M 13  connected to respective columns, and those of photo-signal transfer switches M 21  to M 23  are commonly connected to PTN and PTS, respectively, and receive signal voltages according to operation timings (to be described later).  
         [0038]     The operation of the second embodiment will be described. A first row selection line PRES 1  connected to the gates of reset MOSs M 211 , M 221 , and M 231  changes to high level prior to read of photo-signal charges from the photodiodes D 11  to D 33 . Then, the gates of the amplifier MOSs M 311 , M 321 , and M 331  are reset to a reset power supply. After the first row selection line PRES 1  connected to the gates of the reset MOSs M 211 , M 221 , and M 231  returns to low level, a second row selection line PSEL 1  connected to the gates of selector MOSs M 411 , M 421 , and M 431 , a control pulse on a control pulse line  6  connected to clipping element  3 , and PTN connected to the gates of the noise signal transfer switches M 11  to M 13  change to high level. Accordingly, reset signals (noise signals) on which reset noise is superposed are read out to noise signal holding capacitances CTN 1  to CTN 3 .  
         [0039]     PTN connected to the gates of the noise signal transfer switches M 11  to M 13  returns to low level. PTX 1  connected to the gates of the transfer MOSs M 111 , M 121 , and M 131  changes to high level, and the photo-signal charges in the photodiodes D 11 , D 21 , and D 31  are transferred to the gates of the amplifier MOSs M 311 , M 321 , and M 331 . After PTX 1  connected to the gates of the transfer MOSs M 111 , M 121 , and M 131  returns to low level, PTS connected to the gates of the photo-signal transfer switches M 21  to M 23  changes to high level. Thus, photo-signals are read out to the photo-signal holding capacitances CTS 1  to CTS 3 . Subsequently, PSEL 1  connected to the gates of the selector MOSs M 411 , M 421 , and M 431 , the control pulse line  6  connected to the clipping element  3 , and PTS connected to the gates of the photo-signal transfer switches M 21  to M 23  return to low level. By this operation, noise signals and photo-signals in pixel cells connected to the first row are held by the noise signal holding capacitances CTN 1  to CTN 3  and photo-signal holding capacitances CTS 1  to CTS 3  connected to respective columns.  
         [0040]     The first row selection line PRES 1  connected to the gates of the reset MOSs M 211 , M 221 , and M 231 , and PTX 1  connected to the gates of the transfer MOSs M 111 , M 121 , and M 131  change to high level, and the photo-signal charges in the photodiodes D 11 , D 21 , and D 31  are reset. Thereafter, the gates of horizontal transfer switches M 41  to M 46  on respective columns sequentially change to high level in accordance with signals H 1  to H 3  from the horizontal scanning circuit unit  10 . The voltages held in the noise holding capacitances CTN 1  to CTN 3  and photo-signal holding capacitances CTS 1  to CTS 3  are sequentially read out to the differential circuit block  8 . The differential circuit block  8  calculates the differences between photo-signals and noise signals, and sequentially outputs the differences to an output terminal OUT. Then, read of pixel cells connected to the first row is completed. After that, PCTR connected to the gates of reset switches M 31  to M 36  for the noise signal holding capacitances CTN 1  to CTN 3  and photo-signal holding capacitances CTS 1  to CTS 3  changes to high level prior to read of the second row, and is reset to GND. Similarly, signals in pixel cells connected to the second and subsequent rows are sequentially read out in accordance with signals from the vertical scanning circuit unit  2 , and read-out of all the pixel cells is completed.  
         [0041]     While, e.g., signals on the first row are read out by this operation, the sources of the amplifier MOS M 311  and a clipping MOS M 71 , those of the amplifier MOS M 321  and a clipping MOS M 72 , and those of the amplifier MOS M 331  and a clipping MOS M 73  are commonly connected to form a differential arrangement. For example, when the gate voltage of the amplifier MOS M 311  is much higher than a set clipping voltage  7 , the clipping MOS M 71  is OFF, and thus a photo-signal voltage read out to the vertical output line V 1  is a voltage based on the signal voltage of the gate of the amplifier MOS M 311 . If the gate voltage of the amplifier MOS M 311  comes close to the set clipping voltage  7 , the clipping MOS M 71  is turned on to activate clipping operation. When the gate voltage of the amplifier MOS M 311  is much lower than the set clipping voltage  7 , the vertical output line VI does not decrease below a voltage determined by the set clipping voltage  7  or less.  
         [0042]     This also applies to the remaining vertical output lines V 2  and V 3  in  FIG. 3 . The clipping voltage  7  is set such that the voltages of the vertical output lines V 1  to V 3  do not drop below a drain voltage for operating load MOSs M 51  to M 53  in the saturation range. This setting can prevent the load MOSs M 51  to M 53  from being turned off even in case of reading out very high signal charges. The voltage drop across a GND line  4  remains the same regardless of the number of pixels which receive strong light, and the set currents of the load MOSs also stay constant regardless of read from any row. Also in the second embodiment, the output voltages of dark pixels and OB pixels are equal between a row including a pixel receiving strong light and another row. Thus, an image receiving a strong light spot is free from any whitish band, and a clear image can be obtained.  
         [0043]      FIG. 4  is a block diagram showing a solid-state image pickup device according to the third embodiment of the present invention. The third embodiment is the same as the second embodiment except that clipping elements  3  are respectively made up of clipping diodes D 41  to D 43 , and MOSs M 81  to M 83  for activating clipping operation. The cathodes of the clipping diodes D 41  to D 43  are respectively connected to vertical output lines V 1  to V 3 , and their anodes are connected via the MOSs M 81  to M 83  to a power supply line  7  for setting a clipping voltage. The gates of the MOSs M 81  to M 83  are connected to a control pulse line  6  for controlling clipping operation. The operation of the third embodiment is the same as in  FIG. 3 . The use of the diodes for the clipping element enables steeper clipping of vertical output lines.  
         [0044]      FIG. 5  is a block diagram showing a solid-state image pickup device according to the fourth embodiment of the present invention. The fourth embodiment is different from the second embodiment in the arrangements of a pixel portion  1  and clipping element  3 . The drains of amplifier MOSs M 311  to M 333  are directly connected to the power supply. The source of the amplifier MOS M 311  is connected via a selector MOS M 411  to a vertical output line V 1  which runs in the longitudinal direction. The sources of the amplifier MOSs M 312  and M 313  of pixel cells on the same column are also connected to the vertical output line V 1  via selector MOSs M 412  and M 413 , respectively.  
         [0045]     Each clipping element  3  is made up of a clipping MOS M 71 , and a MOS M 81  for activating clipping operation. The source of the MOS M 81  is connected to the vertical output line V 1 . The clipping MOS M 71  is directly connected to the power supply, similar to the arrangement of the pixel portion  1 , and its gate is connected to a power supply line  7  for setting a clipping voltage. The gate of the MOS M 81  is connected to a control pulse line  6  for controlling clipping operation. Amplifier MOSs, selector MOSs, and the clipping element  3  are similarly connected to remaining vertical output lines V 2  and V 3  shown in  FIG. 4 . The operation of the fourth embodiment is the same as that of the second embodiment, and a description thereof will be omitted. The fourth embodiment can attain the same effects as those of the second embodiment.  
         [0046]      FIG. 6  is a block diagram showing a solid-state image pickup device according to the fifth embodiment of the present invention. The fifth embodiment is different from the first embodiment in the arrangement of clipping element  3 . In the fifth embodiment, the clipping elements  3  are respectively formed from clipping MOSs M 71  to M 73 , the sources of which are respectively connected to vertical output lines V 1  to V 3 . The gates of a MOS M 70  and the MOSs M 71  to M 73  are commonly connected to the input terminal  5 , and their drains are connected to the power supply.  
         [0047]     The operation will be described. Similar to the first embodiment, light is incident on photodiodes D 11  to D 33 , and accumulated photo-signal charges are sequentially read out to the vertical output lines V 1  to V 3 . In the fifth embodiment, for example, when the gate voltage of an amplifier MOS M 311  is much higher than the voltage of an input terminal  5 , the clipping MOS M 71  is OFF, and thus a signal voltage read out to the vertical output line V 1  is a voltage based on the signal voltage of the gate of the amplifier MOS M 311 . If the gate voltage of the amplifier MOS M 311  comes close to the voltage of the input terminal  5 , the clipping MOS M 71  is turned on to activate clipping operation. When the gate voltage of the amplifier MOS M 311  is much lower than the voltage of the input terminal  5 , the vertical output line V 1  does not decrease below a voltage determined by the voltage of the input terminal  5  or less. This also applies to the remaining vertical output lines V 2  and V 3  shown in  FIG. 6 . Therefore, the voltages of the vertical output lines V 1  to V 3  do not drop below a drain voltage for operating load MOSs M 51  to M 53  in the saturation range. Accordingly, the load MOSs M 51  to M 53  can be prevented from being turned off even in case of reading out very high signal charges. The fifth embodiment can obtain the same effects as those of the first embodiment. In addition, the fifth embodiment need not set a clipping voltage  7  of the first embodiment independently of the voltage of the input terminal  5 .  
         [0048]      FIG. 7  is a block diagram showing a solid-state image pickup device according to the sixth embodiment of the present invention. In the sixth embodiment, the clipping elements  3  are replaced by an identical function by setting the low-level voltage of a pulse for driving the gates of reset MOSs M 211  to M 233  from a VRESL terminal  9 . In this case, the VRESL terminal  9  is connected to the ground power supply terminals of BUFs (buffers)  1  to  3 , and the voltage of the VRESL terminal  9  sets the low-level voltage of the pulse for driving the gates of the reset MOSs.  
         [0049]     The operation of the sixth embodiment will be explained. Similar to the first embodiment, light is incident on photodiodes D 11  to D 33 , and accumulated photo-signal charges are sequentially read out to vertical output lines V 1  to V 3 . When, for example, the source voltage of the reset MOS M 211  determined by the photo-signal charges of the photodiode D 11  is higher than the gate voltage (voltage of the VRESL terminal  9 ) while signals on the first row are read out by this operation, the reset MOS M 211  is OFF, and thus a voltage based on the signal voltage of the gate of an amplifier MOS M 311  that is determined by the photo-signal charges of the photodiode D 11  is read out.  
         [0050]     If the source voltage of the reset MOS M 211  determined by the photo-signal charges of the photodiode D 11  falls to below the voltage subtracted Vth (threshold voltage of the reset MOS M 211 ) from a gate voltage (voltage of the VRESL terminal  9 ), the reset MOS M 211  is turned on to clip the gate voltage of the amplifier MOS M 311 . For this reason, the vertical output line V 1  does not decrease below a voltage determined by subtracting the Vth from the voltage of the VRESL terminal  9  or less. This also applies to the remaining vertical output lines V 2  and V 3  in  FIG. 7 . The voltage of the VRESL terminal  9  is set such that the voltages of the vertical output lines V 1  to V 3  do not drop below a drain voltage for operating load MOSs M 51  to M 53  in the saturation range. As a result, the load MOSs M 51  to M 53  can be prevented from being turned off even in reading out very high signal charges. Thus, the sixth embodiment can obtain the same effects as those of the first embodiment.  
         [0051]      FIG. 8  is a block diagram showing a solid-state image pickup device according to the seventh embodiment of the present invention. In the seventh embodiment, a pixel portion  1  is constituted by a one-dimensional line sensor. Compared to the first embodiment, the pixel portion  1  does not use any selector MOS for selecting a row, and the drains of amplifier MOSs M 313 , M 323 , and M 333  are directly connected to the power supply. Clipping elements  3  for clipping the outputs of the amplifier MOSs M 313 , M 323 , and M 333  are respectively formed from only clipping MOSs M 71 , M 72 , and M 73 , the drains of which are directly connected to the power supply. When light is incident on photodiodes D 13 , D 23 , and D 33 , these photodiodes generate and accumulate photo-signal charges, and at the same time output them to output lines V 4 , V 5 , and V 6  of the amplifier MOSs M 313 , M 323 , and M 333 . Then, PRES connected to the gates of reset MOSs M 213 , M 223 , and M 233  changes to high level, and the charges accumulated in the photodiodes D 13 , D 23 , and D 33  are reset.  
         [0052]     With this operation, the sources of the amplifier MOS M 313  and clipping MOS M 71 , those of the amplifier MOS M 323  and clipping MOS M 72 , and those of the amplifier MOS M 333  and clipping MOS M 73  are commonly connected to form a differential arrangement. For example, when the gate voltage of the amplifier MOS M 313  is much higher than the set clipping voltage of a power supply line  7 , the clipping MOS M 71  is OFF, and a signal voltage read out to the output line V 4  is a voltage based on the signal voltage of the gate of the amplifier MOS M 313 . If the gate voltage of the amplifier MOS M 313  comes close to the set clipping voltage, the clipping MOS M 71  is turned on to activate clipping operation. When the gate voltage of the amplifier MOS M 313  is much lower than the set clipping voltage, the vertical output line V 4  does not decrease below a voltage determined by the set clipping voltage or less.  
         [0053]     This also applies to the remaining output lines V 5  and V 6  shown in  FIG. 8 . The clipping voltage is set such that the voltages of the output lines V 4  to V 6  do not drop below a drain voltage for operating load MOSs M 51  to M 53  in the saturation range. This setting can prevent the load MOSs M 51  to M 53  from being turned off even in case of reading out very high signal charges. The voltage drop across a GND line  4  remains the same regardless of the number of pixels which receive strong light, so the set currents of the load MOSs M 51  to M 53  stay constant. In the seventh embodiment, the output voltages of dark pixels and OB pixels remain the same regardless of the number of pixels which receive strong light, and thus the circuit can be simplified without adopting any circuit for clamping OB on the output stage.  
         [0054]     The eighth embodiment when the solid-state image pickup device described in any one of the first to seventh embodiments of the present invention is applied to a still camera (image pickup apparatus) will be described in detail with reference to  FIG. 9 .  
         [0055]     In  FIG. 9 , a barrier  101  serves as both a lens protector and main switch. A lens  102  forms an optical image of an object on a solid-state image pickup device  104 . An iris  103  varies the light quantity having passed through the lens  102 . The solid-state image pickup device  104  captures the object image formed on the lens  102  as an image signal. An A/D converter  106  analog-to-digital-converts the image signal output from the solid-state image pickup device  104 . A signal processing unit  107  executes various correction processes for the image data output from the A/D converter  106 , or compresses data. A timing generation unit  108  outputs various timing signals to the solid-state image pickup device  104 , an image pickup signal processing circuit  105 , the A/D converter  106 , and the signal processing unit  107 . A system control and operation unit  109  controls various operations and the whole still camera. A memory unit  110  temporarily stores image data. An I/F unit  111  is used to record/read out data on/from a recording medium. Image data is recorded/read out on/from a detachable recording medium  112  such as a semiconductor memory. An I/F unit  113  is used to communicate with an external computer or the like.  
         [0056]     The operation of the still camera in image pickup with the above arrangement will be explained.  
         [0057]     When the barrier  101  is opened, the main power supply is turned on, the power supply of the control system is turned on, and the power supply of the image pickup system circuit including the A/D converter  106  is turned on.  
         [0058]     To control the exposure amount, the system control and operation unit  109  sets the iris  103  to a full-aperture F-number. A signal output from the solid-state image pickup device  104  is converted by the A/D converter  106 , and then input to the signal processing unit  107 . The system control and operation unit  109  calculates the exposure amount on the basis of the data.  
         [0059]     The brightness is determined from the results of photometry, and the system control and operation unit  109  controls the iris in accordance with the results.  
         [0060]     A high-frequency component is extracted from the signal output from the solid-state image pickup device  104 , and the system control and operation unit  109  calculates the distance to the object. The lens is driven to check whether the image is in focus or not. If the image is out of focus, the lens is driven again to measure the distance.  
         [0061]     After an in-focus state is confirmed, actual exposure starts.  
         [0062]     After exposure, an image signal output from the solid-state image pickup device  104  is A/D-converted by the A/D converter  106 , and then written in the memory unit by the system control and operation unit  109  via the signal processing unit  107 .  
         [0063]     Data accumulated in the memory unit  110  are recorded on the detachable recording medium  112  such as a semiconductor memory via the recording medium control I/F unit under the control of the system control and operation unit  109 .  
         [0064]     Data may be directly input to a computer or the like via the external I/F unit  113  to process an image.  
         [0065]     As described above, an output from an amplifier element for amplifying photo-signal charges from a photodiode is limited not to decrease below a predetermined voltage or less. This equalizes the output voltages of dark pixels and OB pixels between a row including a pixel which receives strong light and another row. An image which receives a strong light spot is free from any whitish band, and a clear image can be achieved. With the use of an image pickup apparatus according to the present invention, outputs from horizontal OB pixels do not change between a row including a pixel which receives strong light and another row, so the horizontal OBs need not be clamped. This can prevent any lateral stripe generated by variations in horizontal OB clamping level in units of rows, and a high-quality image pickup apparatus can be constructed with a simple block arrangement.  
         [0066]     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.