Current-Mode Feedback Source Follower Linearity
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Solution Overview
Problem
High-performance analog-to-digital converters require input buffers that maintain linearity at both high and low frequencies, but existing source follower buffers compromise high-frequency linearity as the limits of the feedback loop are approached.
Innovation Solution
The implementation of a current-mode feedback source follower with one or two bypass capacitors in the feedback loop improves linearity, particularly for high-bandwidth designs, by compensating phase rotation and reducing current peaking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If feedback loops are employed to enhance low frequency linearity, then low frequency linearity is improved, but high frequency linearity becomes compromised
Solution Approach 1:
The patent changes the electrical parameters of the feedback path by introducing capacitors with specific capacitance values. These capacitors create frequency-dependent impedance changes, allowing the feedback loop to provide enhanced low-frequency linearity while maintaining high-frequency stability. The parameter change in capacitive coupling enables differential treatment of frequency components.
Solution Approach 2:
The patent introduces capacitor elements as intermediary components in the feedback path. These capacitors act as mediators that selectively couple different frequency components, allowing the feedback mechanism to improve linearity at low frequencies without adversely affecting high-frequency performance. The intermediary capacitors isolate the feedback loop's effect to specific frequency ranges.
2Measurement precision
If bypass capacitors are added to the feedback loop, then linearity at high frequencies is enhanced, but device area increases
Solution Approach 1:
The patent optimizes the capacitance values of the bypass capacitors to achieve the minimum necessary capacity for high-frequency linearity enhancement. By carefully selecting capacitor parameters, the design achieves improved high-frequency linearity while minimizing the area occupied by these components. The parameter optimization balances performance requirements with area constraints.
Data Source
AI summary
An example apparatus includes a first transistor coupled between a supply node and a first node, a current mirror having a first side and a second side, and a second transistor coupled between the first node and the first side of the current mirror. The input buffer further includes a third transistor coupled between the first node and the second side of the current mirror, and a first capacitor coupled between a source and a drain of the second transistor.