Transimpedance Amplifier Input Isolation via Current Mirror
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Solution Overview
Problem
Existing optical receivers face performance degradation due to parasitic resistance and capacitance introduced by isolation switches, which limit their frequency response and require DC-balanced data patterns for calibration, increasing overhead and complexity.
Innovation Solution
An optical receiver with a transimpedance amplifier (TIA) that operates in both normal and calibration modes, using a feedback path and strategically placed switches to isolate the input during calibration while maintaining frequency response, eliminating the need for DC-balanced data patterns and reducing parasitic effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an isolation switch is inserted between the PD and TIA to isolate the TIA from input current during calibration, then the TIA can be calibrated without input current interference, but the parasitic resistance and capacitance of the switch degrade the frequency response
Solution Approach 1:
The patent introduces a current mirror circuit as an intermediary mechanism between the PD and TIA. During calibration, the current mirror isolates the TIA from the PD output current while maintaining a controlled current through the feedback resistor, enabling accurate TIA calibration without direct connection to the PD. This resolves the contradiction by providing isolation functionality without the parasitic degradation associated with direct switch insertion.
Solution Approach 2:
The patent dynamically changes the operating parameters of the TIA by controlling the current mirror ratio. By adjusting the mirror ratio, the system can switch between calibration mode (where TIA is isolated and receives controlled current) and normal operation mode (where TIA receives full PD output current). This parameter control enables precise calibration while maintaining optimal frequency response during normal operation.
2Speed
If the switch size is increased to reduce on-resistance, then the RC time constant decreases improving frequency response, but the parasitic capacitance of the switch increases degrading the frequency response
Solution Approach 1:
The current mirror circuit serves as an intermediary that eliminates the need for large-sized isolation switches. By using the current mirror to control current flow during calibration, the patent avoids the trade-off between switch resistance and capacitance, as the current mirror can be implemented with small transistors that introduce minimal parasitic capacitance while maintaining low effective resistance through active control.
3Speed
If DC coupling is used between PD and TIA, then the frequency response is maintained, but the TIA cannot be properly calibrated without input current isolation
Solution Approach 1:
The patent implements a dynamic calibration mechanism where the current mirror ratio is controlled to switch between calibration and normal operation modes. During calibration, the current mirror provides isolation while maintaining a controlled current through the feedback resistor. During normal operation, the current mirror transitions to transparent mode, maintaining DC coupling and optimal frequency response. This dynamic control resolves the contradiction between maintaining frequency response and enabling accurate calibration.
Data Source
AI summary
An optical receiver is described. This optical receiver has two operating modes: a calibration mode and a normal mode. During the normal mode, switches are used to electrically couple an input of a transimpedance amplifier (TIA) to an optical-to-electrical (OE) converter that receives an optical signal and provides a corresponding analog electrical signal. Moreover, during the calibration mode, the switches are used to electrically isolate the input of the TIA from the OE converter while maintaining a feedback path from an output of the TIA to the input of the TIA, thereby ensuring proper bias of the TIA during calibration. Furthermore, a frequency response of the TIA during the normal mode is substantially unchanged over an operating bandwidth of the TIA by the capability to electrically isolate the input of the TIA from the OE converter during the calibration mode.