Variable-Bandwidth TIA Biasing for Stable Phase Characteristics
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Solution Overview
Problem
Existing trans-impedance amplifiers (TIAs) in digital coherent receivers face challenges in adjusting bandwidth without degrading signal quality, particularly in phase-gain tolerance, which affects the phase characteristic of the input signal.
Innovation Solution
The proposed TIA design includes an amplifying stage with a load resistor, a diode, and a variable current source that adjusts the bias current to the diode, allowing for variable bandwidth without altering the phase characteristic, achieved through a series connection of components between the power supply and ground, with a trans-impedance connected between the input and output terminals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the trans-impedance is adjusted to equalize the bandwidth of the TIAs in the coherent receiver, then the bandwidth is improved, but the phase-gain tolerance varies which degrades the signal quality
Solution Approach 1:
The patent changes the bias current parameter of the diode to adjust bandwidth while maintaining phase characteristics. By varying the bias current through a variable current source, the operating point of the diode changes, which adjusts the bandwidth without substantially affecting the phase-gain tolerance, thus resolving the contradiction between bandwidth improvement and signal quality maintenance
2Speed
If the trans-impedance is adjusted to equalize the bandwidth of the TIAs, then the bandwidth is improved, but other factors such as phase-gain tolerance vary
Solution Approach 1:
The patent selectively changes only the bias current parameter of the diode while keeping other parameters constant. This selective parameter change allows bandwidth adjustment through the variable current source without causing unwanted variations in phase-gain tolerance, thereby maintaining phase characteristic stability during bandwidth equalization
3Adaptability or versatility
If a variable bias current is provided to the diode, then the bandwidth can be changed without substantially varying other parameters, but the circuit complexity increases
Solution Approach 1:
The patent implements bandwidth adjustability by introducing a variable current source that modifies the bias current parameter of an existing diode in the TIA circuit. This approach provides adaptability through simple parameter change rather than requiring complex reconfiguration of the entire circuit, thus achieving bandwidth adjustment with minimal increase in circuit complexity
Data Source
AI summary
A trans-impedance amplifier (TIA) with an adjustable bandwidth is disclosed. The TIA of the present invention includes the amplifying stage and the emitter follower stage arranged in the downstream of the amplifying stage. The transistor in the amplifying stage includes a diode in the emitter thereof to provide a substantial emitter level to the transistor. This diode is biased by another current source with a variable function. The operating point of the diode, in particular, the differential resistance thereof, is variable by the current source, which adjusts the bandwidth of the TIA without affecting the phase characteristic of the TIA.