Transimpedance Amplifier Shared Input Reduces Die Size and Noise
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Solution Overview
Problem
Traditional transimpedance amplifiers (TIAs) in optical receiver circuits face issues such as complex design, increased die size, high power consumption, excessive thermal noise, dark current noise, and impedance matching challenges due to multiple connections and feedback resistors, which limit their dynamic range and efficiency.
Innovation Solution
A circuit design where the anode of one photodiode is connected to the cathode of another via a conductor with specific width and length configurations, reducing the number of connections and feedback resistors, and utilizing a single input for the TIA to minimize thermal noise and dark current noise, eliminating the need for additional biasing circuitry and impedance matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple connections and feedback resistors are used in traditional TIA designs, then the TIA can process optical signals from multiple photodiodes, but the device complexity and die size increase
Solution Approach 1:
The patent merges the anode of the first photodiode with the cathode of the second photodiode through a conductor, creating a shared node that eliminates the need for separate connections to each photodiode. This combining approach maintains the ability to process signals from both photodiodes while reducing the number of separate connections and feedback resistors required, thereby simplifying the overall circuit architecture
Solution Approach 2:
The shared conductor and single TIA input serve multiple functions: it collects current from both photodiodes, provides a common reference potential, and eliminates the need for separate biasing circuits for each photodiode. This multi-functional design reduces device complexity while maintaining full signal processing capability
2Ease of operation
If multiple feedback resistors are used in traditional TIA designs, then each photodiode connection can be independently controlled, but thermal noise increases
Solution Approach 1:
The patent combines multiple feedback resistors into a single effective feedback path through the shared conductor configuration. By merging the feedback paths, the number of resistive elements generating thermal noise is reduced, while the circuit maintains its ability to independently respond to signals from either photodiode through the differential nature of the balanced detector configuration
3Adaptability or versatility
If multiple inputs are provided to the TIA, then each photodiode can be individually connected, but the die size increases
Solution Approach 1:
The patent merges multiple input connections into a single shared input node by connecting the anode of one photodiode to the cathode of another through a conductor. This consolidation reduces the number of separate TIA inputs required from multiple to essentially one, significantly reducing the die area occupied by connection structures while maintaining the flexibility to connect multiple photodiodes
4Reliability
If DC offset is provided for each input, then the dynamic range of the TIA is maintained, but additional biasing circuitry is required
Solution Approach 1:
The patent merges the biasing requirements of multiple photodiode inputs into a single shared biasing arrangement. The shared conductor configuration allows a single DC offset to be applied at the common node, which then distributes the biasing effect to all connected photodiodes. This eliminates the need for separate biasing circuitry at each input while maintaining the dynamic range through the unified biasing approach
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 design reduces power consumption, thermal noise, and dark current noise, allowing for smaller die integration, simplified circuitry, and improved dynamic range by effectively canceling out dark current and eliminating the need for additional biasing and impedance matching.
Implementation Method 1
a first photodiode that receives a first optical signal portion and generates a first current in response to the first optical signal portion, and a second photodiode that receives a second optical signal portion and generates a second current in response to the second optical signal portion
Data Source
AI summary
Consistent the present disclosure, a receive circuit is provided that includes a balanced detector portion and a transimpedance amplifier (TIA). The anode of one photodiode is connected to the cathode of the other by a bonding pad, which supplies the sum of the currents generated in each photodiode to an input of the TIA. Thus, the TIA may, for example, have a single input, as opposed to multiple inputs, thereby reducing the number of connections so that the photodiodes and the TIA may be integrated onto a smaller die. In addition, since there are few connections, fewer TIAs are required and differential stages are unnecessary. Power consumption is thus reduced, and, since the photodiode current is fed through one input to the TIA, fewer feedback resistors are required, thereby reducing thermal noise. In addition, since the anode of one photodiode is connected to the cathode of the other, the dark current generated in each flows in opposite directions, and is therefore effectively cancelled out. Since one input is provided, impedance matching with other inputs is unnecessary, nor is additional DC biasing circuitry needed. As described in greater detail below, an example of the present disclosure includes a bonding pad, which connects the two photodiodes and provides the input current to the TIA.