Image Sensor Row Driver Amplifiers With Reduced Inter-Circulation Currents
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Solution Overview
Problem
Designing satisfactory row control circuitry for image sensors is challenging due to issues with inter-circulation currents and power consumption in conventional class AB amplifiers used in row driver circuits, which can increase power consumption and affect image sensor readout performance.
Innovation Solution
The implementation of p-type and n-type row driver amplifiers with cross-coupled transistors and cascode switches, which provide a single dominant pole class AB operation, reducing inter-circulation currents and enhancing tolerance to DC mismatches by adjusting threshold voltage levels and using cascode transistors to boost output impedance and gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If conventional class AB amplifiers are used in row driver circuits, then sufficient gain and high slew currents are achieved, but inter-circulation currents increase power consumption
Solution Approach 1:
The amplifier circuit is segmented into separate p-type and n-type amplifiers, each handling different aspects of the signal drive. This segmentation allows independent optimization of each amplifier type to reduce inter-circulation currents while maintaining overall performance
Solution Approach 2:
Cascode transistors are introduced as intermediary elements between the input and output stages. These cascode transistors act as mediators that block DC offset inter-circulation currents while allowing AC signal currents to pass, thereby reducing power consumption without sacrificing slew current capability
2Speed
If conventional class AB amplifiers are used in row driver circuits, then high slew currents are provided, but DC offset inter-circulation currents affect readout performance
Solution Approach 1:
Cascode transistors serve as intermediary devices that selectively block DC offset currents while permitting AC signal currents to flow. This intermediary structure preserves the high slew rate needed for fast readout while eliminating DC offset inter-circulation currents that would otherwise degrade readout performance
Solution Approach 2:
The circuit utilizes different threshold voltage parameters for p-type and n-type transistors to optimize current flow characteristics. By carefully selecting and adjusting these voltage parameters, the circuit achieves high slew rates while minimizing DC offset effects on readout performance
3Reliability
If cross-coupled transistors and cascode switches are implemented, then tolerance to DC mismatches is enhanced, but device complexity increases
Solution Approach 1:
The circuit employs asymmetric cross-coupled transistor pairs with deliberately different threshold voltages and sizing ratios. This asymmetric design creates inherent DC mismatch tolerance by ensuring that offset currents in one direction are compensated by the other, reducing sensitivity to manufacturing variations without requiring excessive circuit complexity
Solution Approach 2:
The cross-coupled transistor structure serves multiple functions simultaneously: it provides DC offset cancellation, establishes bias currents, and contributes to the overall gain of the amplifier. This multi-functionality achieves enhanced DC mismatch tolerance without proportionally increasing circuit complexity
Data Source
AI summary
An image sensor may include an array of image sensor pixels. The array of image sensor pixels may be controlled by row driver circuitry. The row driver circuitry may include row drivers that receive power supply signals from transconductance amplifier circuitry. The transconductance amplifier circuitry may include multiple amplifiers with output ports shorted to one another. Each amplifier may include input transistors, cross-coupled transistors with a low threshold voltage, and additional transistors coupled in series with the cross-coupled transistors and having a moderate or high threshold voltage.