Lossless nonlinear analog gain controller in image sensor and manufacturing method thereof

An image sensor comprising:

• an active pixel sensor (APS) array including a plurality of pixels arranged in a two-dimensional matrix, wherein the APS array generates a reset signal and an image signal for each pixel of selected columns;
• a first analog-to-digital converter (ADC) including correlated double sampling (CDS) circuits for each column of the APS array, wherein the first ADC generates a digital code corresponding to a difference between the reset signal and the image signal using an output ramp signal that is applied to the CDS circuits for each column;
• and
• a ramp signal generator generating the output ramp signal in which a low illumination portion and a high illumination portion have different slopes,
• wherein the ramp signal generator includes:
• a second ADC generating a reference code using a feedback of a calibrated ramp signal;
• a tracking unit generating an analog ramp input signal as a result of a comparison of a digital target code with the reference code;
• a first ramp generator generating the calibrated ramp signal based on the ramp input signal;
• a non-linear analog gain controller generating folding currents for a voltage higher than a median voltage and a voltage lower than the median voltage, respectively, based on the first ramp signal and the median voltage equal to the median amplitude of the first ramp signal;
• and
• a second ramp generator generating the output ramp signal based on the folding current and the ramp input signal.

A method of driving an image sensor, comprising:

• generating a reset signal and an image signal for each pixel of a selected column in an active pixel sensor (APS) array including a plurality of pixels arranged in a two-dimensional matrix;
• generating a pulse width signal corresponding to a difference between the rest signal and the image signal using an output ramp signal in correlated double sampling (CDS) circuits of each column of the APS array, and generating digital data corresponding to the pulse width signal;
• generating a reference code that is a digital code corresponding to reference analog input voltages by receiving feedback of a calibrated ramp signal;
• generating the calibrated ramp signal as a result of a comparison of a digital target code, the digital target code being equal to a median value in the digital code range, with the reference code;
• generating a folding current using the calibrated ramp signal and a non-linear analog gain controller;
• and
• generating the output ramp signal by applying the folding current to a ramp generator.