Column amplifier reset circuit

A method for amplifying an input signal in an image sensor, comprising:

• receiving the input signal at an input of a common source amplifier, wherein the input signal is representative of an image charge from a pixel cell of the image sensor;
• coupling an auto-zero switch between the input of the common source amplifier and an output of the common source amplifier;
• coupling a feedback capacitor between the input of the common source amplifier and the output of the common source amplifier;
• coupling an offset capacitor to the input of the common source amplifier;
• coupling an offset switch coupled to the offset capacitor, to a reset voltage, and to ground;
• turning on the auto-zero switch during a reset operation of an amplifier circuit
• coupling the offset capacitor to the reset voltage through the offset switch while the auto-zero switch is turned on during the reset operation of the amplifier circuit;
• turning off the auto-zero switch after the reset operation of the amplifier circuit and
• coupling the offset capacitor to the ground through the offset switch while the auto-zero switch is turned off after the reset operation of the amplifier circuit.

An imaging system, comprising:

• a pixel array including a plurality of pixel cells;
• a column amplifier circuitry coupled to the pixel array to read out image charge information from the pixel array, wherein the column amplifier circuitry comprises:
• a common source amplifier coupled to receive an input signal representative of the image charge information from one of the pixel cells of the pixel array, wherein the common source amplifier comprises:
• a transistor having a gate coupled to receive the input signal and a source coupled to ground;
• a cascode device coupled between a drain of the transistor and the output of the column amplifier circuitry;
• and
• a current source coupled to the cascode device;
• a first auto-zero switch coupled between an input of the common source amplifier and an output of the common source amplifier, wherein the first auto-zero switch is further coupled between a gate of the transistor and a drain of the transistor;
• a second auto-zero switch coupled between a gate of the cascode device and a drain of the cascode device;
• a sample and hold capacitor coupled to the gate of the cascode device;
• a feedback capacitor coupled to the input of the common source amplifier;
• and
• an offset switch coupled to the feedback capacitor, wherein the offset switch is further coupled to a reset voltage and an output of the column amplifier circuitry, wherein the auto-zero switch and the offset switch are configured to couple the feedback capacitor to the reset voltage during a reset operation of the column amplifier circuitry, and wherein the offset switch is configured to couple the feedback capacitor to the output of the column amplifier circuitry after the reset operation of the column amplifier circuitry.

An amplifier circuit for use in an image sensor, comprising:

• a common source amplifier coupled to receive an input signal representative of an image charge from a pixel cell of the image sensor, wherein the common source amplifier comprises:
• a transistor having a gate coupled to receive the input signal and a source coupled to ground;
• a cascode device coupled between the transistor and the output of the amplifier;
• and
• a current source coupled to the cascode device;
• a first auto-zero switch coupled between an input of the common source amplifier and an output of the common source amplifier, wherein the auto-zero switch is further coupled between the gate of the transistor and a drain of the transistor;
• a second auto-zero switch coupled between a gate of the cascode device and a drain of the cascode device;
• a sample and hold capacitor coupled to the gate of the cascode device;
• a feedback capacitor coupled to the input of the common source amplifier;
• an offset switch coupled to the feedback capacitor, wherein the offset switch is further coupled to a reset voltage and an output of the amplifier circuit, wherein the auto-zero switch and the offset switch are configured to couple the feedback capacitor to the reset voltage during a reset operation of the amplifier circuit, and wherein the offset switch is configured to couple the feedback capacitor to the output of the amplifier circuit after the reset operation of the amplifier circuit.

An imaging system, comprising:

• a pixel array including a plurality of pixel cells;
• a column amplifier circuitry coupled to the pixel array to read out image charge information from the pixel array, wherein the column amplifier circuitry comprises:
• a common source amplifier coupled to receive an input signal representative of the image charge information from one of the pixel cells of the pixel array;
• an auto-zero switch coupled between an input of the common source amplifier and an output of the common source amplifier;
• a feedback capacitor coupled to the input of the common source amplifier;
• an offset switch coupled to the feedback capacitor, wherein the offset switch is further coupled to a reset voltage and an output of the column amplifier circuitry, wherein the auto-zero switch and the offset switch are configured to couple the feedback capacitor to the reset voltage during a reset operation of the column amplifier circuitry, and wherein the offset switch is configured to couple the feedback capacitor to the output of the column amplifier circuitry after the reset operation of the column amplifier circuitry;
• a level shifter coupled between the output of the common source amplifier and the auto-zero switch, wherein the level shifter comprises a source follower input device having an input coupled to the output of the column amplifier circuitry and a current source coupled to the source follower input device, wherein the current source comprises a source follower current source;
• a first power down switch coupled between the output of the column amplifier circuitry and a gate terminal of the source follower input device;
• and
• a second power down switch coupled between a voltage supply rail and a drain terminal of the source follower input device.