CMOS Image Sensor Readout with In-Pixel Current Sensing

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Conventional CMOS image sensors face limitations in readout speed and noise due to small in-pixel transistors, leading to high fixed pattern noise and reduced dynamic range, while existing solutions either require additional transistors or increase silicon area, and are not scalable for high pixel counts.

Innovation Solution

The current-sensing active pixel (CSAP) sensor design employs an external circuit to sense and feedback current through the in-pixel readout transistor, allowing for a reconfigurable amplifier that operates in both readout and reset phases, using a source-follower configuration during readout and common-source during reset, with negative feedback to maintain a constant current and reduce noise.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Area of stationary object

If in-pixel transistors are made small to maintain high fill factor, then pixel area efficiency is improved, but readout speed decreases and fixed pattern noise increases

Engineering Contradiction:

Improvefill factorVSAvoidreadout speed

Core Design Contradiction:

Area of stationary objectVSSpeed

Solution Approach 1:

The patent divides the transistor sizing function between two locations: small in-pixel transistors maintain high fill factor, while a separate external transistor (Q_ext) provides the current driving capability. This segmentation allows each transistor to be optimized for its specific function without compromise.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces an external current sensing and feedback circuit as an intermediary between the in-pixel transistor and the readout amplifier. This intermediary measures the in-pixel transistor current and uses feedback to drive the readout amplifier, enabling fast readout without requiring large in-pixel transistors.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Area of stationary object

If in-pixel transistors are made small to maintain high fill factor, then pixel area efficiency is improved, but fixed pattern noise increases

Engineering Contradiction:

Improvefill factorVSAvoidfixed pattern noise

Core Design Contradiction:

Area of stationary objectVSObject-generated harmful factors

Solution Approach 1:

The patent implements a feedback mechanism where the external circuit senses the current through the in-pixel readout transistor and feeds back a control signal to maintain constant current operation. This feedback stabilizes the operating point and reduces gain variation-related fixed pattern noise.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The external current sensing circuit acts as an intermediary that measures and stabilizes the in-pixel transistor current, preventing the noise generation that would occur with directly coupled small transistors to the readout amplifier.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Speed

If additional transistors are added to improve readout speed, then readout speed is improved, but device complexity and silicon area increase

Engineering Contradiction:

Improvereadout speedVSAvoidtransistor count

Core Design Contradiction:

SpeedVSDevice complexity

Solution Approach 1:

The patent extracts the current driving function from the in-pixel transistor and places it in an external circuit. This allows the in-pixel transistor to remain simple and small while the external circuit provides the necessary current driving capability for fast readout.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The external transistor Q_ext serves multiple functions: it acts as the current driving element for the readout amplifier, provides the sensing function for current measurement, and implements the feedback control. This multi-functionality reduces the need for separate dedicated components.

Inventive Principle:

Principle #6Universality (Multi-functionality)

4Device complexity

If conventional readout circuits are used, then device simplicity is maintained, but readout speed is limited

Engineering Contradiction:

Improvecircuit simplicityVSAvoidreadout speed

Core Design Contradiction:

Device complexityVSSpeed

Solution Approach 1:

The patent replaces the conventional direct coupling mechanism with a current sensing and feedback control mechanism. This substitution enables the system to achieve high-speed operation through active control rather than relying on passive transistor sizing.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Data Source

PatentUS7847846B1CMOS image sensor readout employing in-pixel transistor current sensing

Publication Date: 2010.12.07 UNIVERSITY OF ROCHESTER

AI summary

In an image sensor, the current through the in-pixel readout transistor is sensed by a circuit that is external to the pixel, and according to the measured current value a feedback current is supplied to charge the read-line parasitic capacitance. The feedback current is supplied by a circuit that also is external to the pixel area. The amplifier structure is reconfigurable so that it can be used both to read out and to reset the pixel.