Image pickup circuit, cmos sensor, and image pickup device

Abstract

Disclosed herein is an image pickup circuit including: amplifying means for amplifying a charge corresponding to an amount of light received by a photodetector, and outputting a pixel signal; ramp signal generating means for generating a ramp signal whose voltage drops with a fixed slope from a predetermined initial voltage; and comparing means for comparing the pixel signal output by the amplifying means with the ramp signal output by the ramp signal generating means. A reference potential of the pixel signal output by the amplifying means and a reference potential of the ramp signal output by the ramp signal generating means are at a same level.

1 . An image pickup circuit comprising:
 amplifying means for amplifying a charge corresponding to an amount of light received by a photodetector, and outputting a pixel signal;   ramp signal generating means for generating a ramp signal whose voltage drops with a fixed slope from a predetermined initial voltage; and   comparing means for comparing the pixel signal output by said amplifying means with the ramp signal output by said ramp signal generating means;   wherein a reference potential of the pixel signal output by said amplifying means and a reference potential of the ramp signal output by said ramp signal generating means are at a same level.   
     
     
         2 . The image pickup circuit according to  claim 1 ,
 wherein said ramp signal generating means includes:   gain changing means for changing gain of an image picked up by said image pickup circuit by changing the slope of said ramp signal; and   a transistor forming a current mirror circuit with said gain changing means, and when said gain is a minimum, a ratio between conductance of said gain changing means and conductance of said transistor is about one.   
     
     
         3 . The image pickup circuit according to  claim 2 ,
 wherein said gain changing means increases said gain by decreasing a current produced by the current mirror circuit of said gain changing means and said transistor.   
     
     
         4 . The image pickup circuit according to  claim 2 ,
 wherein said ramp signal is in a form having a first section in which the voltage drops with the fixed slope from a first initial voltage and a second section in which the voltage drops with the fixed slope from a second initial voltage, and   said ramp signal generating means further includes:
 normal ramp signal generating means for generating a ramp signal in which said first initial voltage and said second initial voltage are equal to each other; and 
 offset means for superimposing an offset component for making said first initial voltage higher than said second initial voltage onto said ramp signal at an offset time when said first initial voltage and said second initial voltage are offset from each other. 
   
     
     
         5 . The image pickup circuit according to  claim 4 ,
 wherein said gain changing means and said offset means use a common constant-current circuit.   
     
     
         6 . A CMOS (Complementary Metal Oxide Semiconductor) sensor formed by disposing an image pickup circuit on a semiconductor chip,
 wherein said image pickup circuit includes:   amplifying means for amplifying a charge corresponding to an amount of light received by a photodetector, and outputting a pixel signal;   ramp signal generating means for generating a ramp signal whose voltage drops with a fixed slope from a predetermined initial voltage; and   comparing means for comparing the pixel signal output by said amplifying means with the ramp signal output by said ramp signal generating means, and   a reference potential of the pixel signal output by said amplifying means and a reference potential of the ramp signal output by said ramp signal generating means are at a same level.   
     
     
         7 . An image pickup device having a CMOS (Complementary Metal Oxide Semiconductor) sensor formed by disposing an image pickup circuit on a semiconductor chip,
 wherein said image pickup circuit includes:   amplifying means for amplifying a charge corresponding to an amount of light received by a photodetector, and outputting a pixel signal;   ramp signal generating means for generating a ramp signal whose voltage drops with a fixed slope from a predetermined initial voltage; and   comparing means for comparing the pixel signal output by said amplifying means with the ramp signal output by said ramp signal generating means, and   a reference potential of the pixel signal output by said amplifying means and a reference potential of the ramp signal output by said ramp signal generating means are at a same level.   
     
     
         8 . (canceled)