Image Sensor Biasing Circuit for Threshold Voltage Calibration
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Solution Overview
Problem
Image sensors operating at low voltages face challenges in maintaining satisfactory image quality due to process variations in transistor threshold voltages, leading to output variability and reduced dynamic range, which can render them ineffective.
Innovation Solution
A biasing circuit with a replica of the front-end circuit is used to control the biasing voltage applied to the semiconductor region, calibrating the threshold voltage and reducing the impact of variations across the image sensor, ensuring consistent output signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If process variations are present in transistor threshold voltages, then manufacturing is easier and cost is lower, but output signal variability increases and dynamic range is reduced
Solution Approach 1:
The patent creates a replica of the front-end circuit that copies the threshold voltage variations of the actual circuit. This replica is then used to generate a compensating bias voltage that counteracts the effects of process variations in the real circuit, thereby maintaining consistent output signals without requiring tight manufacturing tolerances.
Solution Approach 2:
The patent dynamically adjusts the bias voltage parameter in the front-end circuit based on measurements from the replica circuit. By changing the bias voltage parameter in response to detected threshold voltage variations, the system compensates for process variations and maintains consistent circuit operation across different manufacturing batches.
2Use of energy by stationary object
If low operating voltage is used, then power consumption is reduced, but image quality deteriorates due to increased output variability
Solution Approach 1:
The patent implements a feedback mechanism where the output signal from the replica circuit is fed back to control the bias voltage of the front-end circuit. This closed-loop feedback system continuously adjusts the bias voltage to compensate for threshold voltage variations, ensuring consistent image quality while operating at low voltages for reduced power consumption.
Solution Approach 2:
The replica circuit copies the characteristics of the front-end circuit including its response to low operating voltages. This allows the system to measure and compensate for the effects of low voltage operation on signal variability, thereby maintaining image quality consistency while benefiting from reduced power consumption.
3Device complexity
If no biasing voltage control is applied, then device complexity is lower, but dynamic range is reduced due to uncorrected threshold voltage variations
Solution Approach 1:
The patent divides the front-end circuit into two separate components: the actual front-end circuit that processes image signals and a separate replica circuit that measures threshold voltage variations. This segmentation allows independent optimization of each circuit's function while enabling compensation of process variations through the replica's measurements.
Solution Approach 2:
The replica circuit acts as an intermediary that measures the effects of threshold voltage variations without directly processing image data. By using the replica as a mediator to detect and quantify variations, the system can then apply appropriate bias voltage corrections to the front-end circuit, improving output consistency without adding complexity to the signal processing path.
Data Source
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AI summary
In an image sensor, a photo detector circuit (305) provides a detection quantity (308) having a level that depends on an amount of light to which the photo detector circuit has been exposed during a photo detection time interval. A front-end circuit (306) provides an output signal (309) indicative of the amount of light in response to at least one input signal, among which is the detection quantity (308). The front-end circuit comprises a transistor (315) having a threshold voltage that affects a relationship between the output signal (309) and the level of the detection quantity (308). This transistor (315) has a biasing node (319) via which a biasing voltage (320) can be applied to a semiconductor region (318) that affects the threshold voltage of the transistor. A replica (1401) of the front-end circuit receives at least one defined input signal (1406, 1407), among which is a substitute of the detection quantity (308) having a defined level (1406). The replica (1401) is arranged in a control loop (1408) that controls the biasing voltage (320) that is applied to the biasing node in the replica, so that the replica (1401) provides a defined output signal (1412). This biasing voltage (320) is then also applied to the biasing node (319) in the front-end circuit.