Variable-Impedance Audio Amplifier for Low-Volume Noise Rejection
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Solution Overview
Problem
Miniaturized audio amplifiers in devices like earbuds face challenges in managing power efficiently while minimizing noise interference from electromagnetically coupled sources, particularly at low volume levels.
Innovation Solution
An audio amplifier circuit that switches between voltage source and current source modes based on audio signal amplitude, with higher output impedance in current source mode to attenuate noise and lower output impedance in voltage source mode for higher power output, utilizing an H bridge configuration with pulse width modulation to achieve optimal noise rejection and power delivery.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If voltage source drive with lower output impedance is used, then power output capability is improved, but noise rejection deteriorates
Solution Approach 1:
The amplifier dynamically switches between voltage source mode and current source mode based on operating conditions. The output impedance is made variable rather than fixed, allowing the system to adapt to different signal levels and noise environments, thereby resolving the contradiction between power output and noise rejection
Solution Approach 2:
The output impedance parameter is changed based on operating conditions. At high signal levels, lower output impedance provides better power delivery. At low signal levels, higher output impedance provides better noise rejection. This parameter adaptation resolves the contradiction by optimizing for the current operating point
2Object-affected harmful factors
If current source drive with higher output impedance is used, then noise rejection is improved, but power output capability deteriorates
Solution Approach 1:
The amplifier transitions between operating modes dynamically. When high noise rejection is needed (low signal levels), current source mode is activated. When high power output is needed, voltage source mode takes over. This dynamic adaptation resolves the power-output contradiction
Solution Approach 2:
The output impedance parameter is increased in current source mode to achieve superior noise rejection, but only when power output requirements are low. This conditional parameter change resolves the contradiction by applying high impedance only when it doesn't limit power capability
3Device complexity
If amplifier operates in a single mode, then device complexity is reduced, but adaptability to different operating conditions deteriorates
Solution Approach 1:
The amplifier is designed to perform multiple functions through a single circuit topology that can operate in both voltage source mode and current source mode. This multi-functionality provides adaptability to different signal levels and noise conditions without requiring separate amplifiers for each mode
Solution Approach 2:
The amplifier uses dynamic mode switching based on signal amplitude and noise conditions. A control mechanism automatically selects the appropriate operating mode, providing adaptability while maintaining a relatively simple unified circuit structure rather than requiring multiple dedicated amplifiers
Data Source
AI summary
A hearable has an audio amplifier circuit coupled to a speaker as a load. The amplifier circuit has current source drive, which attenuates electromagnetically coupled noise of the speaker. In other instances, the amplifier circuit has a first amplifier mode and a second amplifier mode, wherein in the first amplifier mode the amplifier circuit becomes configured to drive the speaker as a voltage source, and in the second amplifier mode the amplifier circuit becomes configured to drive the speaker as a current source. Control logic varies the amplifier circuit between i) the first amplifier mode for larger amplitudes of the audio signal, and ii) the second amplifier mode for smaller amplitudes of the audio signal. Other aspects are also described and claimed.