Optical Receiver Front End With FET-Controlled TIA Linearity
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Solution Overview
Problem
In optical receivers, especially those using digital coherent optical transmission systems, there is a challenge in effectively converting differential optical signals into usable voltage signals without introducing distortion or increasing power consumption, particularly at high signal light intensities.
Innovation Solution
A receiving circuit that includes a field effect transistor (FET) and transimpedance amplifier (TIA) circuits to convert current signals from photodetectors into differential voltage signals, with a control circuit to manage the FET's resistance and minimize voltage differences, thereby reducing signal amplitude and distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the amplitude of current signals from photodetectors is directly converted to voltage signals by TIA circuits, then the signal strength is maintained, but distortion increases at high signal light intensities
Solution Approach 1:
A FET is introduced as an intermediary component between the photodetectors and TIA circuits. The FET converts the large amplitude current signals into smaller amplitude current signals before they reach the TIA circuits, preventing distortion while maintaining signal integrity through controlled resistance modulation.
Solution Approach 2:
The FET's resistance is dynamically changed based on the amplitude of input current signals. When signal amplitude is high, the FET resistance increases to reduce the current signal amplitude; when signal amplitude is low, the FET resistance decreases to maintain signal strength, thereby optimizing linearity across different signal conditions.
2Manufacturing precision
If signal amplitude is reduced to prevent distortion, then linearity improves, but signal strength decreases
Solution Approach 1:
The FET resistance is made dynamic rather than fixed, allowing it to adapt to varying signal conditions. The resistance automatically adjusts based on the instantaneous amplitude of the current signal, enabling the system to maintain both linearity and signal strength across different operating conditions without manual intervention.
Solution Approach 2:
The control circuit monitors the voltage difference between differential voltage signals and uses this feedback to adjust the FET's resistance. This feedback mechanism ensures that the FET provides the appropriate amount of signal attenuation to maintain linearity while preserving sufficient signal strength for accurate reception.
3Manufacturing precision
If a FET is added to control signal amplitude, then distortion is reduced, but device complexity increases
Solution Approach 1:
The FET serves multiple functions within the receiving circuit: it acts as a variable resistor for signal amplitude control, provides impedance matching between stages, and enables automatic gain control through its resistance modulation. This multi-functionality reduces the need for additional dedicated components, thereby limiting the increase in overall circuit complexity.
4Manufacturing precision
If FET resistance is increased to reduce current signal amplitude, then distortion decreases, but power consumption increases
Solution Approach 1:
The FET resistance is modulated periodically or dynamically in response to signal conditions rather than being continuously high. The resistance increases only when and where needed to reduce signal amplitude and prevent distortion, and decreases when signal amplitude is already low, thereby minimizing unnecessary power consumption while maintaining linearity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enhances linearity of the reception signal, reduces power consumption, and maintains a compact size by minimizing the amplitude of current signals input to the TIA circuits, even at high signal light intensities, thereby preventing distortion and improving signal quality.
Implementation Method 1
The first TIA circuit is configured to convert a current signal received at the first input node to the first voltage signal and output the first voltage signal from the first output node. The second TIA circuit is configured to convert a current signal received at the second input node to the second voltage signal and output the second voltage signal from the second output node.
Implementation Method 2
The first FET has a first current terminal, a second current terminal, and a first control terminal. The first current terminal is electrically connected to the first input terminal. The second current terminal is electrically connected to the second input terminal. The first control terminal receives a first control signal.
Data Source
AI summary
A receiving circuit and an optical receiver including the receiving circuit are disclosed. The receiving circuit includes first and second input terminals, a FET, first and second TIA circuits, and a control circuit. The first and second input terminals each receive a current signal. The FET has first and second current terminals respectively connected to the first and second input terminals, and a control terminal. The first and second TIA circuits respectively are connected to the first and second current terminals, and convert the current signals to first and second voltage signals. The control circuit generates a control signal for application to the FET control terminal in accordance with a difference between the first and second voltage signals. The optical receiver includes the receiving circuit and each of first and second photodetectors for respectively supplying first and second current signals to the first and second input terminals of the receiver.