Novel Operation Scheme for Realizing Combined Linear-Logarithmic Response in Photodiode-Type Active Pixel Sensor Cells

Abstract

A novel operation scheme that realizes a combined linear-logarithmic response in conventional photodiode-type (PD) threetransistor (3-Tr) active pixel sensor (APS) cells is presented. It has been revealed that, by setting a reset transistor in weak inversion through properly selected bias voltages, conventional PD 3-Tr APS cells can be operated with a seamlessly combined linear-logarithmic response. A modification in the conventional APS cells that enhances their dynamic ranges in a linear response mode is also described.

A Wide-Dynamic-Range Photodiode-Type Active Pixel Sensor Cell with Seamlessly Combined Logarithmic–Linear–Logarithmic Response

“…Thus the authors have proposed, first, an APS cell with selectable linear/logarithmic response, 3) and second, the operation scheme that realizes seamlessly combined linearlogarithmic response in photodiode-type (PD) APS cells. 4) Similar proposals have been made by other groups. 5,6) Although these previous studies reveal the availability of combined linear-logarithmic response in APSs, ''logarithmic-linear-logarithmic response'', which shall achieve very precise resolution in the desired illuminance region while maintaining the response continuity, has not been proposed yet.…”

“…1(b)], the cathode voltage (V PD ) of the photodiode reaches to V DD À V th M1 (V th M1 : threshold voltage of M1), and RESET is set to V M , an intermediate value between V DD and 0 V (hereafter 0 V is also referred to as Gnd) as in our previous operation scheme. 4) V th M4 must be selected so that M4 is ''ON'' at that V PD . During exposure (t 2 -t 3 ), V PD decreases as the stored charge is discharged by the photogenerated current.…”

“…It has two additional metal-oxide-semiconductor field-effect transistors (MOSFETs) stacked in parallel to the reset MOSFET in conventional three-transistor (3-Tr) PD APS cells. By utilizing our previously proposed operation scheme which achieves automatic linear-logarithmic response switching, 4) the proposed 5-Tr APS cell realizes seamlessly combined logarithmic-linear-logarithmic response. The proposed operation scheme is as follows: At the end of the reset period [t 2 in Fig.…”

“…Once V PD becomes lower than V M À V th M1 , where M1 is biased in weak inversion, logarithmic response due to M1 is achieved as described in our previous report. 4) Thus two additional MOSFETs (M4 and M5) enable both logarithmic response under low-illumination conditions and seamless transition to linear response under medium-illumination conditions.…”

Response Mode Detection of a Linear-Logarithmic Image Sensor Using a Current-Mode Readout Circuit

“…However, at low-illumination levels, their sensitivity is reduced. To alleviate this problem, pixels that combine a logarithmic response at high-illumination with a linear response at low-illumination levels have been designed [10]. Thus, a wide dynamic range is achieved using a combined linear-logarithmic response Active Pixel Sensor (APS).…”