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WO-2026093422-A1 - IMAGE SENSOR WITH A PIXEL PROCESSOR INCLUDING A REPLICA TRANSISTOR

WO2026093422A1WO 2026093422 A1WO2026093422 A1WO 2026093422A1WO-2026093422-A1

Abstract

An image sensor includes a pixel processing circuit that converts radiation into digital pixel data. The pixel processing circuit includes a signal amplifier circuit, and an autozero transistor configured to temporarily connect a signal input of the signal amplifier circuit with a reset node. The image sensor further includes a replica amplifier circuit, a diode-connected replica transistor with a load path electrically connected in series with a current source, and a gate voltage circuit. The gate voltage circuit supplies an autozero switch signal AZSW to a gate of the autozero transistor, wherein an active signal level of the autozero switch signal AZSW for turning on the autozero transistor is a function of a voltage level at a gate of the replica transistor and an output voltage of the replica amplifier circuit.

Inventors

  • BERNER, Raphael
  • ZANNONI, Massimo
  • OGAWA, KOJI

Assignees

  • SONY SEMICONDUCTOR SOLUTIONS CORPORATION
  • SONY ADVANCED VISUAL SENSING AG

Dates

Publication Date
20260507
Application Date
20251030
Priority Date
20241031

Claims (16)

  1. 1. An image sensor, comprising: a pixel processing circuit configured to convert radiation into digital pixel data, the pixel processing circuit comprising a signal amplifier circuit, an autozero transistor configured to temporarily connect a signal input of the signal amplifier circuit with a reset node; a replica amplifier circuit and a diode-connected replica transistor with a load path electrically connected in series with a current source; and a gate voltage circuit configured to supply an autozero switch signal AZSW to a gate of the autozero transistor, wherein an active signal level of the autozero switch signal AZSW for turning on the autozero transistor is a function of a voltage level at a gate of the replica transistor and an output voltage of the replica amplifier circuit.
  2. 2. The image sensor according to claim 1, wherein at least one of temperature responses, supply voltage dependencies and manufacturing process dependencies of the signal amplifier circuit and the replica amplifier circuit are proportional per design.
  3. 3. The image sensor according to claim 1, wherein at least one of temperature responses, supply voltage dependencies and manufacturing process dependencies of the autozero transistor and the replica transistor are proportional per design.
  4. 4. The image sensor according to claim 1, wherein the current source is controllable.
  5. 5. The image sensor according to claim 1, wherein the active signal level of the autozero switch signal AZSW is higher than a sum of a nominal threshold voltage of the autozero transistor and a highest voltage at the reset node.
  6. 6. The image sensor according to claim 1, wherein the reset node is connected to a fixed potential.
  7. 7. The image sensor according to claim 1, wherein the autozero transistor is electrically connected between the signal input and a signal output of the signal amplifier circuit.
  8. 8. The image sensor according to the preceding claim, further comprising: a feedback/hold capacitance electrically connected between the signal input and the signal output of the signal amplifier circuit.
  9. 9. The image sensor according to claim 1, 73784 29 wherein the gate voltage circuit comprises a measurement circuit configured to obtain digital values of a voltage level at the gate of the replica transistor and the output voltage of the replica amplifier circuit, a memory circuit configured to store and output process values for a plurality of combinations of different voltage levels at the gate of the replica transistor and the output voltage of the replica amplifier circuit, a digital-to-analog converter configured to convert a selected process value output by the memory circuit into a ramp reference voltage REFR, and an autozero circuit configured to generate the autozero switch signal AZSW with an active high level based on the ramp reference voltage REFR.
  10. 10. The image sensor according to the preceding claim, wherein the gate voltage circuit further comprises a temperature measurement circuit configured to obtain a digital temperature value of the image sensor, and wherein the memory circuit is configured to store and output process values for a plurality of combinations of different temperatures of the image sensor, different voltage levels at the gate of the replica transistor and different voltage levels of the output voltage of the replica amplifier circuit.
  11. 11. The image sensor according to claim 1, wherein the signal amplifier circuit comprises a p channel MOSFET and an n channel MOSFET electrically connected in series between a low positive supply voltage VDDL and a supply reference potential GND, wherein a gate of one of the n channel MOSFET and the p channel MOSFET represents the signal input of the signal amplifier circuit and a node between the p channel MOSFET and the n channel MOSFET represents the signal output of the signal amplifier circuit.
  12. 12. The image sensor according to claim 1, wherein the gate voltage circuit comprises a first voltage coupling element configured to supply a first coupling voltage in series with the current source and the load path of the replica transistor, the coupling voltage being a function of the output voltage of the replica amplifier circuit.
  13. 13. The image sensor according to the claim 1, wherein the gate voltage circuit comprises a second voltage coupling element configured to supply a ramp reference voltage REFR, the ramp reference voltage REFR being a function of the gate voltage of the replica transistor.
  14. 14. The image sensor according to claim 1, wherein the gate voltage circuit comprises an input buffer circuit, an input of the input buffer circuit is configured to receive the output voltage of the replica amplifier circuit, and an output of the input buffer circuit is electrically connected with the load path of the replica transistor.
  15. 15. The image sensor according to claim 1, 30 wherein the gate voltage circuit further comprises an output buffer circuit, an input of the output buffer circuit is configured to receive the gate voltage of the replica transistor, and an output of the output buffer circuit is electrically connected with an input of an autozero circuit.
  16. 16. The image sensor according to claim 1, wherein the pixel processing circuit is configured to temporarily store a pixel charge on a feedback/hold capacitance electrically connected between the signal input and a signal output of the signal amplifier circuit, the pixel charge being a function of received light intensity.

Description

73784 1 IMAGE SENSOR WITH PIXEL PROCESSING CIRCUIT AND REPLICA TRANSISTOR The present disclosure relates to a pixel processing circuit for converting incoming radiation into digital image data and a replica transistor emulating an autozero transistor of the pixel processing circuit. More particularly, the present disclosure relates to an emulating a portion of the pixel processing circuit to track process comers, temperature variations, and voltage fluctuations. BACKGROUND Complementary metal oxide semiconductor imaging sensors (CMOS Imaging Sensors, CIS) convert radiation into analog voltage signals. In event-based vision sensors (EVS), luminance changes detected by each pixel are filtered to extract only those that exceed a preset threshold value. This event is then combined with the pixel coordinate, time, and polarity information to digital event data. In both CIS and EVS, signal amplifier circuits amplify an analog voltage, compare analog voltages, and/or charge capacitances to temporary store electric charge. Amplifier circuits usually have an input offset voltage. Autozeroing is an offset-cancellation technique that samples the input offset voltage of an amplifier circuit in an autozero phase and then subtracts the sampled voltage from the input voltage in a working phase of the amplifier circuit. Further, in EVS the autozero operation is not only to cancel non-idealities, but it is part of the specific principle of operation: the voltage is stored on a capacitor in order to be sensitive only to luminance changes. Metal oxide field effect transistors (MOSFETs) can be used for autozeroing amplifier circuits. Typically, some parts of a pixel processing circuit operate with a low supply voltage than other parts. SUMMARY A gate voltage effective between the gate terminal and the MOSFET transistor channel must be sufficiently high to fully turn on the MOSFET and is therefore selected higher than the transistor channel voltage plus the transistor threshold voltage. In a pixel processing circuit containing a signal amplifier circuit, the transistor channel voltage of the autozero transistor can assume the positive supply voltage for the signal amplifier circuit. Due to variations in the manufacturing process, the positive supply voltage may vary between different pixel processing circuits of the same image sensor and between corresponding pixel processing circuits of different image sensors of the same lot. In addition, the positive supply voltage can change with temperature. The transistor threshold voltage depends on the individual characteristics of the autozero transistor and may vary between different autozero transistors of the same image sensor and between corresponding autozero transistors of different image sensors of the same lot. In addition, the transistor threshold voltage can change with temperature. A common approach is to provide a sufficiently high gate voltage to ensure that the autozero transistor is fully switched on even in the worst case. However, even if a sufficiently high fixed gate voltage is used to switch on the autozero transistor even in the worst case, this fixed gate voltage may leave the stated good region across PVT due to varying characteristics of the amplifier circuit and the autozero transistor. Further, if unnecessarily high gate voltages are used to switch on the autozero transistor, negative effects can occur in the part of the circuit generating the gate voltage signal. Higher gate voltages may also make the charge injection worse at transistor turn-off phase. This makes variations across PVT also worse, and charge injection of a MOS switch may introduce an 73784 2 offset. Capacitive couplings to other circuits may be larger if the voltage swing is larger, so negative effects may not only remain in the part of the circuit that generates the gate voltage signal. The present disclosure inter alia mitigates deficiencies in pixel processing circuits with autozeroed signal amplifier circuits. For this purpose, an image sensor includes a pixel processing circuit that converts radiation into digital pixel data. The pixel processing circuit includes a signal amplifier circuit and an autozero transistor configured to temporarily connect a signal input of the signal amplifier circuit with a reset node. The image sensor further includes a replica amplifier circuit, a diode-connected replica transistor with a load path electrically connected in series with a current source, and a gate voltage circuit. The gate voltage circuit supplies an autozero switch signal AZSW to a gate of the autozero transistor, wherein an active signal level of the autozero switch signal AZSW (active autozero switch signal AZSW) for turning on the autozero transistor is a function of a voltage level at a gate of the replica transistor and an output voltage of the replica amplifier circuit. The diode-connected replica transistor operates in full saturation. Provided sufficient similarity between the d