Image sensor and driving method thereof

A method for driving an image sensor, comprising:

• transferring a first photo charge generated from a photoelectric conversion element to a first node;
• amplifying a voltage caused by the first photo charge based on an operation of a common source transistor including a gate connected to the first node and connected between a second node and a reference voltage, and an operation of a mode transistor connected between the first node and the second node;
• transferring a second photo charge generated from the photoelectric conversion element to the first node;
• and
• buffering a voltage caused by the second photo charge based on an operation of a source follower transistor including a gate connected to the second node and having an end connected to a driving voltage, and an operation of the mode transistor.

An image sensor, comprising:

• a pixel circuit comprising
• a photoelectric conversion element configured to generate photo charge,
• a first node configured to accumulate the photo charge generated from the photoelectric conversion element,
• a mode transistor including a gate configured to receive a mode control signal and connected between the first node and a second node,
• a common source transistor including a gate connected to the first node and connected between the second node and a reference voltage, the common source transistor configured to amplify a voltage caused by the photo charge, and
• a source follower transistor including a gate connected to the second node and having an end connected to a driving voltage, the source follower transistor configured to buffer the voltage caused by the photo charge;
• a row driver configured to control a signal level of the mode control signal based on an illumination level;
• and
• a voltage generator configured to control a magnitude of the reference voltage based on the illumination level.