Analysis of pixel circuits in CMOS image sensors

CMOS image sensors (CIS) have lower power consumption, lower cost and smaller size than CCD image sensors. However, generally CCDs have higher performance than CIS mainly due to lower noise. The pixel circuit used in CIS is the first part of the signal processing circuit and connected to photodiode directly, so its performance will greatly affect the CIS or even the whole imaging system. To achieve high performance, CMOS image sensors need advanced pixel circuits. There are many pixel circuits used in CIS, such as passive pixel sensor (PPS), 3T and 4T active pixel sensor (APS), capacitive transimpedance amplifier (CTIA), and passive pixel sensor (PPS). At first, the main performance parameters of each pixel structure including the noise, injection efficiency, sensitivity, power consumption, and stability of bias voltage are analyzed. Through the theoretical analysis of those pixel circuits, it is concluded that CTIA pixel circuit has good noise performance, high injection efficiency, stable photodiode bias, and high sensitivity with small integrator capacitor. Furthermore, the APS and CTIA pixel circuits are simulated in a standard 0.18-μm CMOS process and using a n-well/p-sub photodiode by SPICE and the simulation result confirms the theoretical analysis result. It shows the possibility that CMOS image sensors can be extended to a wide range of applications requiring high performance.