Photodiode Sensor Circuit With Current Mirror for Residual Voltage Removal
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Solution Overview
Problem
The parasitic capacitor in photodiodes used in optical proximity sensors for cell phones affects the voltage of the cathode, leading to incorrect sensing of photocurrent when the light signal is not irradiated, causing operational issues.
Innovation Solution
A sensor circuit is designed with a current source connected to the cathode of the photodiode to form a discharging path for the parasitic capacitor, ensuring the voltage is maintained at a desired level, and including operational amplifiers and current mirror circuits to isolate the photocurrent from the discharge current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a photodiode is used to detect light signals in an optical proximity sensor, then the sensor can detect light blocking to determine user proximity, but the parasitic capacitor in the photodiode causes residual voltage that affects photocurrent sensing accuracy when light is not irradiated
Solution Approach 1:
The patent extracts the harmful residual voltage from the photodiode's parasitic capacitor by providing a dedicated discharge path through a transistor. This separates the discharge function from the normal photodetection function, allowing the parasitic capacitor to be emptied without affecting the photocurrent sensing accuracy when light is detected.
Solution Approach 2:
The patent applies preliminary action by discharging the parasitic capacitor before light detection occurs. The control circuit actively empties the parasitic capacitor when no light signal is present, preventing residual voltage from affecting subsequent photocurrent measurements. This preparatory discharge ensures accurate sensing when the light signal arrives.
2Measurement precision
If the parasitic capacitor is discharged to maintain voltage levels, then photocurrent sensing accuracy is improved, but the circuit complexity increases due to additional components needed for discharge control
Solution Approach 1:
The patent merges the discharge control function with the existing operational amplifier circuit. The same operational amplifier that processes the photodiode signal also controls the discharge transistor, eliminating the need for completely separate control circuitry. This integration reduces overall circuit complexity while maintaining measurement precision.
Solution Approach 2:
The operational amplifier serves multiple functions: it amplifies the photodiode signal during light detection and simultaneously controls the discharge transistor to empty the parasitic capacitor when no light is present. This multi-functionality reduces the total component count and simplifies the circuit structure while achieving both discharge and sensing objectives.
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 solution allows the photodiode to operate normally by eliminating residual voltage from the parasitic capacitor, ensuring that only the photocurrent converted from the light signal is sensed, improving the accuracy and range of distance detection.
Implementation Method 1
the light signal is reflected to a photodiode by the user and the photodiode converts the reflected light signal into a photocurrent
Data Source
AI summary
A sensor is provided. A first terminal of a first current source and a first terminal of a first transistor are connected to a cathode of the photodiode. A control terminal of a second transistor is connected to an output terminal of a first operational amplifier. A first terminal of the second transistor is connected to a second terminal of the first transistor through a first current mirror circuit. A second terminal of the second transistor is connected to a second current source, a second input terminal of a second operational amplifier and a first terminal of a third transistor. A first input terminal of the second operational amplifier is connected to the first terminal of the first transistor. A control terminal of the third transistor is connected to an output terminal of the second operational amplifier.