Self-Biasing Output Booster Amplifier for Stable Filter Cutoff
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Solution Overview
Problem
Existing electronic filter arrangements in hearing devices suffer from a non-constant cut-off frequency with input signal level, leading to poor sound quality in the low-frequency range due to resistance biasing and high noise levels, particularly in high-pass filters using NMOS transistors.
Innovation Solution
A self-biasing output booster amplifier with current copying circuits that maintain a stable frequency response by balancing internal bias currents through a negative feedback path, allowing for a fixed external bias current source and reducing supply current efficiency, which is ideal for lower power applications.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If resistance biasing is used in the input stage of the amplifier, then the amplifier can operate with simple biasing circuitry, but the cut-off frequency becomes non-constant with input signal level
Solution Approach 1:
The patent implements a feedback mechanism where the output signal is fed back to the input stage through a feedback resistor. This feedback stabilizes the bias current in the input stage, ensuring that the cut-off frequency remains constant regardless of input signal level variations. The feedback loop compensates for signal-level-dependent variations by adjusting the bias conditions dynamically.
Solution Approach 2:
The patent changes the operating parameters of the input stage by using a constant current source instead of resistance biasing. This constant current source maintains a fixed bias current through the input transistor, thereby stabilizing the cut-off frequency. The parameter change from resistance-based biasing to current-source-based biasing eliminates the signal-level dependence of the cut-off frequency.
2Power
If a common source NMOS output stage is used, then the amplifier can provide high output current, but the output noise level increases
Solution Approach 1:
The patent employs a current mirror circuit that copies the bias current from a reference transistor to the output transistor. This current mirror ensures that the output transistor operates at a controlled current level that provides sufficient output drive capability while minimizing noise. The copying mechanism allows precise control of the output current to optimize the noise-performance tradeoff.
Solution Approach 2:
The patent introduces a current mirror as an intermediary between the biasing circuit and the output stage. This current mirror acts as a mediator that translates a stable reference current into the required output current, providing isolation between the noise sources in the biasing circuit and the output stage, thereby reducing output noise while maintaining high current capability.
3Adaptability or versatility
If bias current varies with output signal level, then the amplifier can adapt to different signal conditions, but the high-pass filter function is lost for large low frequency signals
Solution Approach 1:
The patent uses feedback to maintain a constant bias current in the input stage independent of output signal level. This feedback mechanism ensures that the high-pass filter characteristics remain stable and reliable across all signal levels, preventing the loss of filter function even when large low frequency signals are present. The feedback loop continuously adjusts to maintain the intended filter behavior.
4Stability of the object's composition
If fixed current sources are used inside the amplifier circuit, then the amplifier can maintain stable operation, but supply current efficiency decreases
Solution Approach 1:
The patent implements a self-biasing scheme where the amplifier circuit generates and regulates its own bias currents using the available supply voltage and external biasing components. The circuit automatically adjusts its internal current distribution to maintain stable operation without requiring additional fixed current sources, thereby improving supply current efficiency while preserving operational stability.
Data Source
AI summary
A self-biasing output booster amplifier having an input amplifier stage, an output amplifier stage being operatively connected to an output of the input amplifier stage, and first and second current copying circuits. The second current copying circuit is biased from an output of the self-biasing output booster amplifier. The first and second current copying circuits are configured to copy at least a portion of the current through the output amplifier stage. The sum of the output of the second current copying circuit and the output of the output amplifier stage provides the output current of the self-biasing output booster amplifier, Finally, the input amplifier stage is biased from the output of the second current copying.