Transimpedance Amplifier RC Feedback for Bandwidth and SNR
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Solution Overview
Problem
Transimpedance amplifiers (TIAs) in integrated photonic systems face a gain-bandwidth trade-off, resulting in either better signal-to-noise ratio (SNR) with worse inter-symbol interference (ISI) or higher bandwidth with lower SNR, and they contribute noise due to their high input impedance, which is challenging to address effectively.
Innovation Solution
The proposed TIA design incorporates a data path with both feedback impedance and feedback capacitance, where the feedback capacitance is coupled across even numbers of inverter stages, and a biasing path to optimize gain and bandwidth, reducing DC input current and enhancing SNR and bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high input impedance is used in TIA, then signal-to-noise ratio is improved, but bandwidth deteriorates
Solution Approach 1:
The patent transforms the fixed input impedance into a frequency-dependent impedance by introducing a parallel RC network. The impedance magnitude changes with frequency according to |Zin| = sqrt(Rin^2 + (1/(2πfCin))^2), allowing high impedance at low frequencies for good SNR and lower impedance at high frequencies for extended bandwidth.
Solution Approach 2:
The patent introduces an intermediary RC network between the photodetector and the amplifier input. This network acts as a frequency-selective element that mediates between the conflicting requirements of high impedance (for SNR) and bandwidth, allowing both to be satisfied simultaneously through frequency-dependent behavior.
2Measurement precision
If high input impedance is used in TIA, then signal-to-noise ratio is improved, but inter-symbol interference worsens
Solution Approach 1:
The frequency-dependent input impedance created by the parallel RC network reduces the impedance magnitude at higher frequencies where inter-symbol interference occurs, thereby mitigating ISI while preserving the high impedance needed for good SNR at lower frequencies.
3Speed
If bandwidth is increased in TIA, then inter-symbol interference is reduced, but signal-to-noise ratio deteriorates
Solution Approach 1:
The patent achieves bandwidth extension not by uniformly increasing impedance across all frequencies, but by creating a frequency-dependent impedance that naturally provides high impedance at low frequencies (good SNR) and transitions to lower impedance at high frequencies (extended bandwidth), thus resolving the trade-off.
Data Source
AI summary
The disclosure provides an improved transimpedance amplifier (TIA) that can operate at a higher bandwidth and lower noise compared to conventional TIAs. The TIA employs a data path with both feedback impedance and feedback capacitance for improved performance. The feedback impedance includes at least two resistors in series and at least one shunt capacitor, coupled between the at least two resistors, that helps to extend the circuit bandwidth and improve SNR at the same time. The capacitance value of the shunt capacitor can be selected based on both the bandwidth and noise. In one example, the TIA includes: (1) a biasing path, and (2) a data path, coupled to the biasing path, including multiple inverter stages and at least one feedback capacitance coupled across an even number of the multiple inverter stages. An optical receiver and a circuit having the TIA are also disclosed.