Optical Sensor Photodiode Capacitance Reduction High Speed
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Solution Overview
Problem
Existing optical sensors, such as those using photodiodes, face limitations in achieving high-speed operation and sufficient signal-to-noise (SN) ratio, especially when photocurrent is small, due to fluctuations in bias voltage and capacitance values.
Innovation Solution
An optical sensor configuration incorporating a photodiode, a first MOS transistor, a second MOS transistor forming a current mirror circuit, and a third MOS transistor with a threshold voltage equal to or above the first MOS transistor's threshold, along with a current-voltage conversion element, amplifies photocurrent and reduces photodiode capacitance, ensuring high-speed operation and improved SN ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a bias voltage is applied to the photodiode to reduce capacitance and improve operating speed, then the operating speed increases, but the capacitance value fluctuates and SN ratio decreases
Solution Approach 1:
A MOS transistor is introduced as an intermediary element between the photodiode and the signal processing circuit. The transistor's gate is connected to the photodiode's cathode, and its source is connected to GND. This configuration allows the transistor to buffer and stabilize the voltage at the photodiode cathode, preventing capacitance fluctuations while maintaining high-speed operation. The transistor acts as a mediator that decouples the photodiode from direct voltage variations, thereby preserving both speed and SN ratio.
2Speed
If a photodiode is reverse-biased to decrease capacitance value, then the operating speed improves, but it becomes difficult to improve SN ratio when photocurrent is small
Solution Approach 1:
The patent replaces the conventional direct voltage amplification method with a transistor-based current control mechanism. Instead of directly amplifying the small photocurrent voltage signal (which worsens SN ratio), the MOS transistor converts the voltage signal at its gate into a controlled current at its source, which is then amplified by the current mirror circuit. This substitution of voltage amplification with current control allows the system to maintain low photodiode capacitance (for high speed) while achieving sufficient SN ratio through current-based signal processing.
3Measurement precision
If the detection signal depends on photocurrent, then the sensor can detect light, but the sensor response becomes limited and cannot operate at high speed
Solution Approach 1:
The patent changes the operational parameters of the photodiode by applying a reverse bias voltage, which fundamentally alters its capacitance characteristics. This parameter change reduces the photodiode's junction capacitance, enabling faster response times. Simultaneously, the introduction of the MOS transistor and current mirror circuit changes the signal processing mechanism from direct voltage amplification to current mirroring, which operates more quickly and maintains detection capability while achieving high-speed response.
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 configuration enables high-speed and high-accuracy optical sensing by maintaining a sufficient SN ratio even with small photocurrents, allowing for precise detection in applications requiring rapid object detection.
Implementation Method 1
a photodiode which generates a photocurrent upon receipt of light
Data Source
AI summary
A light receiving sensor (1) includes: a photodiode (PD) which generates a photocurrent (Ipd) upon receipt of light; a transistor (Tr11) through which the photocurrent (Ipd) flows; a transistor (Tr12) which forms, together with the transistor (Tr11), a first current mirror circuit (CM1); a transistor (Tr9) whose channel type is different from that of the transistor (Tr11), and a resistor (R10) which converts, to a voltage, a current flowing through the transistors (Tr11 and Tr12). The first current mirror circuit (CM1) amplifies the photocurrent (Ipd), the transistor (Tr11) has a source connected with a gate of a MOS transistor (Tr9), and the MOS transistor (Tr9) has a threshold voltage that is set to be equal to or above a threshold voltage of the transistor (Tr11). This decreases a capacity of the photodiode (PD) and therefore allows the light receiving sensor (1) to operate at a high speed while the photodiode (PD) is biased.