Photodiode Amplifier Feedback Filtering for Low-Noise Signal Separation
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Solution Overview
Problem
Existing amplifier circuits for photodiodes face challenges in reducing noise characteristics, particularly when dealing with signals that have superimposed DC or low-frequency components from ambient light, which can overwhelm the useful AC signal and lead to increased complexity and susceptibility to noise in signal processing systems.
Innovation Solution
An amplifier circuit with a measurement path and feedback path, utilizing two filters to filter out direct voltage and noise components, ensuring stable phase shifts and preventing overshoots, thereby reducing noise and maintaining circuit stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If filters are used to filter out DC or low-frequency components on the input side, then the useful signal can be separated from ambient light interference, but the circuit becomes highly susceptible to noise due to low supply voltage and integrated circuit constraints
Solution Approach 1:
The patent implements a feedback path that takes the amplifier output, filters it with a first filter, converts it to current via an auxiliary amplifier, and feeds it back to the amplifier input through a second filter. This feedback mechanism allows the circuit to actively compensate for DC and low-frequency components while maintaining stability and reducing noise susceptibility.
Solution Approach 2:
The patent introduces an auxiliary amplifier as an intermediary component that converts the filtered output voltage to a feedback current. This intermediary transformation allows for proper impedance matching and signal conditioning in the feedback path, enabling effective noise reduction while maintaining measurement precision.
2Device complexity
If active feedback of amplified signals is used to compensate for DC and low-frequency components, then signal processing requirements are reduced, but noise signals are added to the amplifier input
Solution Approach 1:
The patent applies preliminary filtering action by placing the first filter in the feedback path before the auxiliary amplifier. This filter removes DC and low-frequency components from the output signal before it is converted to feedback current, preventing these components from being reintroduced as noise to the amplifier input.
Solution Approach 2:
The patent extracts harmful noise components from the feedback signal by using the second filter to remove noise from the output current before it is fed back to the amplifier input. This extraction process ensures that only the useful feedback signal is reintroduced, while noise is eliminated.
3Measurement precision
If the amplifier is designed to detect small AC signals near the noise level, then photodiode sensitivity is utilized, but the useful signal is overwhelmed by superposed DC components from ambient light
Solution Approach 1:
The feedback mechanism continuously monitors the amplifier output and feeds back a compensated signal that actively cancels out superposed DC components from ambient light. This allows the amplifier to maintain its ability to detect small AC signals from the photodiode while rejecting the harmful DC offset.
Solution Approach 2:
The patent segments the signal processing into distinct frequency components by using filters that separate DC/low-frequency components (handled by feedback) from AC signal components (processed by the main amplifier). This segmentation allows each processing path to be optimized for its specific function.
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
The solution effectively reduces noise components at the amplifier input, preventing signal distortion and allowing for accurate measurement of small photoelectric currents across multiple orders of magnitude, while maintaining circuit stability and preventing saturation.
Implementation Method 1
the first filter has the function of filtering a direct voltage from the output voltage
Implementation Method 2
The auxiliary amplifier in turn has the function to convert an input voltage into an output current
Implementation Method 3
The second filter in turn has the function to filter noise from the output current
Implementation Method 4
As a function of this photoelectric current, the amplifier generates an output voltage which, as a rule, comprises both DC and AC components
Implementation Method 5
the low-noise current thus filtered is impressed in the form of feedback current into the measurement path and thus onto the first amplifier input
Data Source
AI summary
An amplifier circuit comprises a measurement path with an amplifier (1) for providing an output voltage (Vout) depending on a measuring current (Ipd) with a first and a second amplifier input (11, 12), and an amplifier output (13). A return path of the amplifier circuit comprises a first filter (2), an auxiliary amplifier (3) and a second filter (4). In this case, the first filter (2) is designed to filter a DC voltage from the output voltage (Vout) and is connected to the amplifier output (13). The auxiliary amplifier (3) serves to convert an input voltage (Vfil) into an output current (Ifil) and has a first and a second auxiliary amplifier input (31, 32) and an auxiliary amplifier output (33). In this case, the first auxiliary amplifier input (31) is connected to the first filter (2). The second filter (4) is designed to filter noise from the output current (Ifil) and couples the auxiliary amplifier output (33) to the first amplifier input (11).