Reference voltage circuit and image-capture circuit

Abstract

A reference voltage circuit for generating a reference voltage to be referred when a pixel signal is digitally converted, includes ramp voltage generating means for generating a ramp voltage which drops from a predetermined initial voltage at a certain gradient, a transistor for forming, together with the ramp voltage generating means, a current mirror circuit, and gain change means for changing a current value of a current flowing from a predetermined power supply via the transistor to change the gradient of the ramp voltage generated by the ramp voltage generating means.

1 . A reference voltage circuit for generating a reference voltage to be referred when a pixel signal is digitally converted, comprising:
 ramp voltage generating means for generating a ramp voltage which drops from a predetermined initial voltage at a certain gradient;   a transistor for forming, together with the ramp voltage generating means, a current mirror circuit; and   gain change means for changing a current value of a current flowing from a predetermined power supply via the transistor to change the gradient of the ramp voltage generated by the ramp voltage generating means.   
   
   
       2 . The reference voltage circuit according to  claim 1 , wherein;
 the gain change means includes a variable resistance for adjusting the current flowing via the transistor.   
   
   
       3 . The reference voltage circuit according to  claim 1 , wherein;
 the gain change means includes;   a plurality of gain changing transistors whose drains are connected to the transistor;   resistances each connected to sources of the gain changing transistors; and   switches each connected to gates of the gain changing transistors; and   the current flowing via the transistor is adjusted by switching the switch.   
   
   
       4 . The reference voltage circuit according to  claim 1 , wherein;
 the gain change means includes;   amplifying means for amplifying the reference voltage; and   reference voltage change means for changing the reference voltage to be inputted to the amplifying means; and   the current flowing via the transistor according to a voltage outputted by the amplifying means is adjusted.   
   
   
       5 . The reference voltage circuit according to  claim 4 , wherein;
 the reference voltage change means includes;   a plurality of resistances for connecting in series between a predetermined power supply and a ground level;   reference voltage changing switches for selecting respective connection points of the plurality of resistances; and   reference voltage generating amplifying means, in which an input terminal is connected to the reference voltage changing switches, for outputting the reference voltage; and   the reference voltage changing switches are switched and the voltage to be inputted from the predetermined power supply to the reference voltage generating amplifying means are changed in order to change the reference voltage.   
   
   
       6 . The reference voltage circuit according to  claim 5 , wherein;
 the gain change means further includes a capacitor for connecting an output terminal of the amplifying means and a ground level.   
   
   
       7 . The reference voltage circuit according to  claim 1 , wherein;
 the ramp voltage has a segment in which voltage drops from a first initial voltage at a predetermined gradient and a segment in which voltage drops from a second initial voltage at a predetermined gradient; and the reference voltage circuit further comprising:   offset generating means for offsetting the second initial voltage with respect to the first initial voltage.   
   
   
       8 . The reference voltage circuit according to  claim 7 , wherein;
 a current source for the gain change means to generate a current component for changing the gradient of the ramp voltage and a current source for the offset generating means to generate a current component for offsetting the second initial voltage with respect to the first initial voltage are shared.   
   
   
       9 . An image-capture circuit for capturing an image, comprising:
 a pixel array in which a plurality of pixels for outputting a pixel signal are arrayed; and   a reference voltage circuit for generating a reference voltage to be referred when the pixel signal outputted from the pixels of the pixel array is digitally converted, wherein;   the pixel array and the reference voltage circuit are configured on an identical semiconductor chip, and   the reference voltage circuit includes;   ramp voltage generating means for generating a ramp voltage which drops from a predetermined initial voltage at a certain gradient;   a transistor for forming, together with the ramp voltage generating means, a current mirror circuit; and   gain change means for changing a current value of a current flowing from a predetermined power supply via the transistor in order to change the gradient of the ramp voltage generated by the ramp voltage generating means.