1. An active pixel sensor comprising: a plurality of pixels, wherein each pixel includes a light sensitive element and a transfer gate, and the plurality of pixels have at least one floating diffusion region; and a plurality of processing circuits associated with the plurality of pixels; wherein each processing circuit comprises a charge amplifier, wherein the charge amplifier comprises a feedback path, the charge amplifier is used for conversion gain, and the conversion gain is determined by the ratio of two capacitors in the feedback path, wherein the conversion gain equals 100 uV/e − , wherein the charge amplifier comprises an amplifier, a first reset gate, a first capacitor, a second capacitor, a third capacitor and a second reset gate, wherein the second capacitor is connected in series with the first capacitor between the input of the amplifier and the output of the amplifier, the first reset gate is connected between the input of the amplifier and the output of the amplifier, the second reset gate is connected in parallel with the second capacitor, and the third capacitor is connected between a connecting point of the first capacitor and the second capacitor and a fixed reference voltage, wherein the fixed reference voltage includes a power supply.