TAS-TIA Common-Mode Feedback Loops for Noise and Linearity
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Solution Overview
Problem
Existing transconductance-transimpedance amplifiers lack an effective common-mode feedback circuit design to adjust the output common-mode voltage to meet the requirements of subsequent circuits, leading to issues such as noise, mismatch, parasitic capacitance, bandwidth degradation, and linearity degradation.
Innovation Solution
A transconductance-transimpedance amplifier with one or more common-mode feedback loops, utilizing operational amplifiers, capacitors, and resistors to adjust the common-mode voltage and current sources, ensuring high speed, low noise, and high linearity by adaptively controlling the output and input common-mode voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If no common-mode feedback circuit is used, then the device complexity is reduced, but the output common-mode voltage cannot be adjusted to meet subsequent circuit requirements
Solution Approach 1:
The patent implements common-mode feedback circuits that sense the output common-mode voltage and adjust bias currents to maintain the desired common-mode voltage level. The feedback mechanism includes operational amplifiers that compare the actual common-mode voltage with a reference and adjust control signals accordingly, enabling precise voltage adjustment while maintaining system stability.
Solution Approach 2:
The common-mode feedback circuits are designed to work with the existing TAS-TIA amplifier architecture, where the feedback operational amplifiers also serve as part of the overall signal processing path. The bias current control mechanism simultaneously affects multiple stages of the amplifier, achieving common-mode voltage adjustment without requiring entirely separate control systems.
2Reliability
If conventional common-mode feedback designs are used, then the circuit can be implemented, but noise, mismatch, parasitic capacitance, bandwidth degradation, and linearity degradation occur
Solution Approach 1:
The patent applies different feedback strategies to different parts of the amplifier circuit. Separate feedback paths are implemented for the TAS stage and TIA stage, each optimized for its specific requirements. The feedback operational amplifiers are positioned to provide localized common-mode control at critical nodes, reducing the impact of parasitic effects and mismatch in specific circuit regions.
Solution Approach 2:
The common-mode feedback circuits dynamically adjust bias currents based on operating conditions to optimize performance. The feedback mechanism continuously monitors and adjusts control signals to maintain optimal common-mode voltage levels across varying signal conditions, thereby reducing noise and maintaining bandwidth and linearity under different operating scenarios.
Data Source
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Figure 2
AI summary
A transconductance-transimpedance, TAS-TIA, amplifier (100) includes a TAS amplifier (102), a TIA amplifier, (104) and a first common-mode feedback, CMFB, circuit (106). The TIA amplifier (104) includes a first transistor (M1N) and a second transistor (M2N). The first CMFB circuit (106) has a first operational amplifier (110), a first capacitor (112), and a first resistor (114). The first operational amplifier (110) has a first input node (+) for receiving a TIA output common-mode voltage (VCMO), a second input node (-), and a first output node coupled to control terminals of the first and second transistors (M1N, M2N). The first capacitor (112) is coupled between the first output node and the second input node (-) of the first operational amplifier (110). The first resistor (114) is coupled between the second input node (-) of the first operational amplifier (110) and a reference common-mode voltage (VREF).