CMOS image sensors (complementary metal oxide semiconductors) operate at lower voltages than CCDs, reducing power consumption for portable applications. Each CMOS active pixel sensor cell has its own buffer amplifier, and can be addressed and read individually. A commonly used cell has four transistors and a photo-sensing element. The cell has a transfer gate separating the photo sensor from a capacitive "floating diffusion," a reset gate between the floating diffusion and power supply, a source-follower transistor to buffer the floating diffusion from readout-line capacitance, and a row-select gate to connect the cell to the readout line. All pixels on a column connect to a common sense amplifier.