Current Mirror Circuit for Image Sensor Power Supply Rejection
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Solution Overview
Problem
Conventional CMOS image sensors suffer from unsatisfactory power supply rejection ratios, leading to noise and unwanted ripple in captured images due to limited design and layout constraints of pixel cells, which affect the quality of image data read out from bit lines.
Innovation Solution
The implementation of a current mirror circuit with a first and second current path, where the second current path is coupled to an amplifier transistor, utilizing a first current source for a constant current component and a second current source responsive to power supply rail ripple, effectively compensating for noise and improving the power supply rejection ratio by matching the ripple phase and frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a source follower circuit is used in pixel cells to amplify the signal, then the signal can be output to the bit lines, but the power supply rejection ratio becomes unsatisfactory (e.g., -20 dB)
Solution Approach 1:
A current mirror circuit is introduced as an intermediary between the power supply and the source follower amplifier. The current mirror circuit receives the power supply voltage and generates a compensated current that is immune to power supply ripple, thereby mediating the harmful effect of power supply noise on the amplifier output.
Solution Approach 2:
The patent converts the harmful power supply ripple into a useful compensation signal. By sampling the power supply voltage through a capacitor and using it to control the current mirror, the circuit generates a current that automatically compensates for power supply variations, turning the harmful ripple into a beneficial correction mechanism.
2Productivity
If miniaturization and integration are pursued to meet higher resolution and lower power demands, then image sensor performance improves, but power supply rejection ratio deteriorates due to design constraints
Solution Approach 1:
The current mirror circuit is nested within the pixel cell structure, with the amplifier transistor's gate connected to the current mirror output. This nested configuration allows the power supply compensation mechanism to be integrated into the existing pixel architecture without requiring separate external circuits, thus maintaining miniaturization while improving power supply rejection.
3Device complexity
If conventional source follower circuits are used, then pixel cell design is simple, but noise from power supplies enters the output signal path causing horizontal ripple in captured images
Solution Approach 1:
The patent implements a feedback mechanism where the power supply voltage is sampled through a capacitor and fed back to control the current mirror circuit. This feedback loop continuously adjusts the current based on power supply variations, automatically compensating for noise and ripple in the output signal without requiring complex external correction circuits.
Data Source
AI summary
An image sensor read out circuit includes a first current mirror circuit in which a second current conducted through a second current path is controlled in response to a first current conducted through the first current path. The second current is conducted through an amplifier transistor of a pixel circuit. A first current source coupled to the first current path to provide a substantially constant current component of the first current. A second current source coupled to a power supply rail of the pixel circuit and coupled to the first current path to provide a ripple current component of the first current. The ripple current component provided by the second current source is responsive to a ripple in the power supply rail. The first current is responsive to a sum of the currents from the first and second current sources.