Low-noise CMOS active pixel
OwnerLaurent Simonyinventor
Life of this patent
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Abstract
A low-noise CMOS active pixel for image sensors comprises a photosensitive element (PD), a feedback capacitive element (CF) with a capacitance CF, and four transistors, namely a first transistor (M1), two reset transistors (M3, M4) and one pixel selection transistor (M2). These components are laid out and controlled in such a way that the first transistor (M1) is mounted as an amplifier during the pixel reset phase and as a follower during the read phase. The reset transistors (M3, M4) are parallel-connected so that one of them (M4) compensates for the negative effects of the other transistor (M3) on the node common to the two transistors.
Claims20
A CMOS active pixel for an image sensor comprising:
- a photosensitive element whose electrical charge is reset during a reset phase and read during a read phase, the photosensitive element connected between a photosensitive node and a ground;
- a first amplifier, active during the reset phase, the first amplifier including an output, an inverter input connected to the photosensitive node, and a non-inverter input connected to a reference voltage source;
- a first switch, connected between the inverter input and the output of the first amplifier, the first switch controlled by a first control signal that is active during a first period of the reset phase and inactive during a second period and a third period of the reset phase and during a read phase;
- a capacitive element connected between the output of the first amplifier and an intermediate node of the pixel;
- a second switch connected between the photosensitive node and the intermediate node of the pixel, controlled by a second control signal that is active during the start of the first period of the reset phase and during the totality of the second period of the reset phase and inactive during the third period of the reset phase and during the read phase, and
- a second amplifier mounted as a follower, active during the read phase, the second amplifier including an input connected to the photosensitive node and an output delivering a signal representing the electric charge of the photosensitive element during the read phase.
A method for resetting a CMOS active pixel comprising at least one photosensitive element with a capacitance having a photosensitive node, an amplifier with an inverter input connected to the photosensitive node, a non-inverter input and an output, a first switch initially in an open state connected between the inverter input and the output of the amplifier, a capacitive element with a capacitance connected between the output of the amplifier and an intermediate node of the pixel, and a second switch initially in an open state connected between the photosensitive node and the intermediate node of the pixel, the method comprising:
- during a first period
- applying a reference voltage to the non-inverter input of the amplifier;
- connecting the output of the amplifier with an non-inverter input of the amplifier by closing the first switch during the first period;
- and
- briefly applying the reference voltage to the two terminals of the capacitive element by momentarily closing the second switch at the beginning of the first period to reset the charge of this switch;
- during a second period
- setting up a feedback loop between the output of the amplifier and the inverter input by means of the feedback capacitive element by closing the second switch until a state of equilibrium is reached, the first switch being open;
- and
- during a third period
- opening the second switch.
Dependencies are inferred from the claim text.