Photodiode Sensor Circuit for Parasitic Capacitor Discharge
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Solution Overview
Problem
Optical proximity sensors in cell phones face issues due to parasitic capacitors in photodiodes, which affect the voltage of the cathode and prevent accurate sensing of photocurrent when the light signal is not present, leading to incorrect operation.
Innovation Solution
A sensor circuit is designed with a photodiode, operational amplifiers, current sources, and transistors to form a discharging path for the parasitic capacitor, ensuring the cathode voltage remains stable and only photocurrent is sensed, eliminating the influence of the parasitic capacitor's residual charge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a photodiode is used to detect light signals in an optical proximity sensor, then the sensor can detect light reflection from the user's face, but the parasitic capacitor in the photodiode causes voltage fluctuations at the cathode that prevent accurate sensing of photocurrent
Solution Approach 1:
The patent extracts and removes the harmful effect of the parasitic capacitor by introducing a discharge circuit that actively drains the parasitic capacitor's charge. The discharge circuit includes a discharge transistor connected between the photodiode cathode and ground, which periodically or continuously discharges the parasitic capacitor to prevent voltage fluctuations from interfering with photocurrent measurement.
Solution Approach 2:
The patent introduces an operational amplifier as an intermediary component to buffer and stabilize the photodiode cathode voltage. The op-amp acts as a voltage follower or buffer stage that isolates the photodiode's parasitic capacitor effects from the subsequent signal processing stages, maintaining a stable virtual ground at the inverting input and enabling accurate photocurrent sensing.
2Reliability
If the photodiode's parasitic capacitor is charged by photocurrent, then the photodiode can convert light signals into electrical signals, but the residual voltage from the charged parasitic capacitor affects the cathode voltage and prevents correct sensing when light is not present
Solution Approach 1:
The patent applies preliminary action by pre-charging the parasitic capacitor to a known voltage level using a dedicated charge pump or voltage source before the actual measurement process. This initial charging establishes a stable baseline voltage that eliminates uncertainty about the capacitor's state, ensuring that subsequent photocurrent measurements start from a known reference point and improving dark current sensing accuracy.
Solution Approach 2:
The patent implements feedback control through an operational amplifier that continuously monitors the photodiode cathode voltage and adjusts the discharge transistor's operation to maintain a stable voltage level. The feedback mechanism detects voltage deviations caused by parasitic capacitor charging and automatically compensates by increasing discharge current, thereby maintaining reliable operation and accurate dark current sensing.
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 discharging the parasitic capacitor, ensuring accurate photocurrent sensing and improving the sensor's ability to detect distances up to 11 cm, compared to the conventional sensor's limit of 5 cm.
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.