BJT Pixel Circuit Reducing Pre-Flash Time in Optical Mice
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Solution Overview
Problem
Bipolar junction transistor (BJT) pixel circuits in optical mice require a long pre-flash time, leading to increased power consumption and reduced battery life, as the base-emitter voltage takes time to stabilize, affecting image capture quality.
Innovation Solution
A BJT pixel circuit with a current generating unit that generates a pulsed base current to quickly boost the base-emitter voltage to a predetermined level when the light source turns on, shortening the pre-flash time and reducing power consumption by using a feedback amplifier circuit and shutter circuit to control exposure time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the BJT transistor uses conventional base current generation, then the beta stabilizes reliably, but the pre-flash time becomes too long (220 us) causing high power consumption
Solution Approach 1:
The patent applies preliminary action by generating a pulsed base current before the main exposure to quickly establish the necessary base-emitter voltage. This pre-action current pulse (lasting approximately 30 us) prepares the BJT transistor in advance, allowing beta to stabilize much faster than conventional methods, thereby reducing the light source on-time and power consumption while maintaining reliable beta stability during the actual exposure period.
2Reliability
If the pre-flash time is extended to ensure stable beta, then image quality improves, but the light source must remain on longer increasing power consumption and reducing battery life
Solution Approach 1:
The patent separates the beta stabilization process from the main exposure by applying a preliminary pulsed base current that quickly establishes stable beta conditions (achieving stability in approximately 30 us compared to conventional methods). This preliminary action ensures image capture quality is maintained while dramatically reducing the total light source on-time, thereby extending battery life without compromising image quality.
3Loss of time
If a pulsed base current is applied to quickly boost base-emitter voltage, then pre-flash time is reduced (by 86.3%), but additional circuit components are required
Solution Approach 1:
The patent employs periodic action by using a pulsed base current signal with specific timing characteristics (pulse width of approximately 30 us) to quickly establish the base-emitter voltage. This periodic current pulse, generated by additional circuit components including a current generating unit and timing control circuitry, achieves an 86.3% reduction in pre-flash time compared to conventional continuous current methods, despite the increased circuit complexity.
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 the pre-flash time by 86.3%, lowering power consumption and enhancing battery life while maintaining image capture quality by stabilizing the BJT transistor quickly at the start of each frame.
Implementation Method 1
The photodetector operatively generates a base current IB responsive to the light intensity of a light illuminated from a light source
Data Source
AI summary
A bipolar junction transistor (BJT) pixel circuit, an image sensor and a driving method thereof are provided. The BJT pixel circuit includes a BJT, a photodetector, a feedback amplifier circuit, a shutter circuit, and a current generating unit. The photodetector generates a first base current to a base of the BJT responsive to a light incident on the photodetector. The feedback amplifier circuit is operative to increase an emitter voltage of the BJT according to the light intensity. The shutter circuit controls an exposure time of the photodetector according to a shutter signal. The current generating unit generates the second pulsed base current responsive to a trigger signal causing a base voltage of the BJT to drop while the feedback amplifier circuit operates to increase the emitter voltage of the BJT so as to build-up the base-emitter voltage to a predetermined voltage level when a light source turns on.