3T Pixels

These pixels are an example of Active Pixel Sensor (APS). The term active comes from the source follower amplifier inside the pixel circuit, differentiating it from prior passive pixels. The source follower buffers the voltage developed at the sense node to an ADC placed outside of the array. Each pixel is multiplexed to a shared column using the read transistor operating as a switch.

Circuit Diagram

../_images/3T_0_0.svg

Single Pixel Timing Diagram

Operation

  1. Initially assume that the sense node (SN) is discharged.

  2. Then reset signal RST is pulled high which pulls the sense node to a threshold voltage below VRST.

  3. The reset is then released and the photodiode begins to integrate photocarriers, discharging the sense node according to the light level seen by the diode.

  4. At the end of the integration period the READ signal is raised which connects the in-pixel source follower to the shared column bus and the current sink located at the edge of the array. The source-follower buffers the sense node voltage to VOUT with an offset equal to the threshold voltage of the SF transistor.

  5. The voltage buffered to VOUT is sampled by an ADC at SAMPLE1.

  6. The sense node is then reset and a second sample is taken at SAMPLE2. The difference between the two samples is the true signal level corresponding to the number of photogenerated carriers during the exposure time.