In an autofocus image sensor, monitoring pixels are disposed adjacent to a pixel array over a length equal to that of the pixel array in each of a standard portion and a reference portion. Signals from the monitoring pixels of both of the standard portion and the reference portion are subjected to arithmetic operation to control accumulation of electric charges. Thus, an error in position detection is minimized.
1. An image sensor comprising a standard portion and a reference portion, each of the standard portion and the reference portion comprising a pixel array, a pixel array charge detector producing a signal corresponding to a light amount received in the pixel array, a control electrode controlling supply of charges from the pixel array to the pixel array charge detector, monitoring pixels monitoring an average light amount of the pixel array, and a monitoring charge detector producing a signal corresponding to a light amount received in the monitoring pixels, wherein:
the image sensor further comprises a signal control portion carrying out control so that each of the output signals from the pixel array charge detectors of the standard portion and the reference portion is kept constant;
the signal control portion comprising a minimum detector, a comparator, a reference voltage generator, and a control signal generating portion, the minimum detector producing an output signal corresponding to one of the two output signals from the monitoring charge detectors of the standard portion and the reference portion which has a lower level, the comparator comparing an output from the minimum detector with a reference voltage generated by the reference voltage generator, the control signal generating portion controlling the control electrodes of the standard portion and the reference portion on the basis of a result of comparison by the comparator.
2. An image sensor comprising a standard portion and a reference portion, each of the standard portion and the reference portion comprising a pixel array, a pixel array charge detector producing a signal corresponding to a light amount received in the pixel array, a control electrode controlling supply of charges from the pixel array to the pixel array charge detector, monitoring pixels monitoring an average light amount of the pixel array, and a monitoring charge detector producing a signal corresponding to a light amount received in the monitoring pixels, wherein:
the image sensor further comprises a signal control portion carrying out control so that each of the output signals from the pixel array charge detectors of the standard portion and the reference portion is kept constant;
the signal control portion comprising first and second comparators, a reference voltage generator, and a control signal generating portion, the first comparator comparing the output signal of the monitoring charge detector of the standard portion with a reference voltage generated by the reference voltage generator, the second comparator comparing the output signal of the monitoring charge detector of the reference portion with the reference voltage generated by the reference voltage generator, the control signal generating portion being supplied with outputs of the first and the second comparators and controlling the control electrodes of the standard portion and the reference portion on the basis of results of comparison by the first and the second comparators.
3. The image sensor as claimed in claim 1 , wherein the pixel arrays and the monitoring pixels of the standard portion and the reference portion are symmetrically disposed with respect to a center of an image area of the image sensor.
4. The image sensor as claimed in claim 2 , wherein the pixel arrays and the monitoring pixels of the standard portion and the reference portion are symmetrically disposed with respect to a center of an image area of the image sensor.
5. The image sensor as claimed in claim 1 , wherein, in each of the standard portion and the reference portion, the monitoring pixels monitoring the average light amount of the pixel array are disposed adjacent to the pixel array.
6. The image sensor as claimed in claim 2 , wherein, in each of the standard portion and the reference portion, the monitoring pixels monitoring the average light amount of the pixel array are disposed adjacent to the pixel array.
7. The image sensor as claimed in claim 1 , wherein, in each of the standard portion and the reference portion, the monitoring pixels monitoring the average light amount of the pixel array are arranged over a width same as that of the pixel array.
8. The image sensor as claimed in claim 2 , wherein, in each of the standard portion and the reference portion, the monitoring pixels monitoring the average light amount of the pixel array are arranged over a width same as that of the pixel array.
9. The image sensor as claimed in claim 1 , wherein the minimum detector comprises a first source follower circuit and a second source follower circuit, the first source follower circuit comprising a first constant current source disposed between a power source and an output line and a drive transistor having a gate supplied with the signal from the monitoring charge detector of the standard portion, the second source follower circuit comprising a second constant current source disposed between a power source and an output line and a drive transistor having a gate supplied with the signal from the monitoring charge detector of the reference portion, the output lines of the first and the second source follower circuits being connected to each other.
10. The image sensor as claimed in claim 1 , wherein the minimum detector comprises a constant current source disposed between a power source and an output line and a plurality of transistors connected between the output line and a ground voltage, the transistors having gates supplied with the outputs from the monitoring charge detectors of the standard portion and the reference portion, respectively.