DC Offset Cancellation in Trans-Impedance Amplifier
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Solution Overview
Problem
Conventional trans-impedance amplifier circuits face challenges in accurately canceling the wide range of DC offset current from photodiodes, requiring large gain and capacitance in the feedback loop, which increases complexity and cost due to the need for substantial gain to account for the 40 dB range of input current.
Innovation Solution
A photodetector amplifier circuit utilizing a power monitor to generate a replica current, which is used to subtract the DC component of the photodiode current through a current mirror circuit, reducing the input range for the offset cancellation feedback loop and allowing for a smaller feedback capacitor, thereby reducing the burden on the offset cancellation circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the gain of the feedback circuit is increased to account for the wide range of photodiode input current (40 dB range), then the DC component can be subtracted with sufficient accuracy, but the filtering capacitor size must be increased by 100 times to maintain the same low-frequency response
Solution Approach 1:
The patent divides the DC offset cancellation into two segments: (1) a power monitor circuit that handles the bulk of the DC offset current (10-1000 μA range) independently, and (2) a feedback circuit that only needs to handle the remaining small offset current. This segmentation allows the feedback capacitor to be 100 times smaller while maintaining accuracy across the full input range.
Solution Approach 2:
The power monitor circuit acts as an intermediary that pre-processes the DC offset current before it reaches the feedback circuit. By introducing this intermediate stage, the feedback circuit is relieved of the burden of handling the full 40 dB range, enabling significant reduction in capacitor size.
2Measurement precision
If the gain of the feedback circuit is increased to account for the wide range of photodiode input current, then the DC component can be subtracted with sufficient accuracy, but the feedback loop becomes more complex and expensive to integrate
Solution Approach 1:
The patent segments the DC cancellation function between a power monitor circuit and a simplified feedback circuit. This segmentation reduces the integration complexity and cost by allowing the feedback circuit to use a much smaller capacitor (100 times smaller), while the power monitor handles the heavy lifting of DC offset compensation.
Solution Approach 2:
The power monitor circuit creates a copy or representation of the photodiode current to generate the DC offset cancellation signal. This copying approach allows accurate DC cancellation without requiring the main feedback circuit to be overly complex or expensive.
3Adaptability or versatility
If a conventional feedback circuit is used to remove DC component, then the circuit can handle the full range of photodiode current, but the low-frequency cutoff becomes too high for both Telecom and Data com applications
Solution Approach 1:
The patent segments the frequency response requirements: the power monitor circuit handles the DC and low-frequency components independently, while the feedback circuit with its small capacitor maintains a low low-frequency cutoff suitable for both Telecom and Data com applications. This segmentation allows the system to handle full current range while maintaining low-frequency performance.
Data Source
AI summary
The invention relates to a DC offset cancellation circuit for a trans-impedance amplifier, which is typically used for converting an input current from a photodiode into an output voltage. The DC offset cancellation circuit utilizes the monitor current from a photodiode monitoring device to cancel the DC offset from the photodiode input current, enabling the conventional feedback circuit in the trans-impedance amplifier to be fully integrated.