Sample-and-Hold Audio Amplifier for Headset Pop Noise
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Solution Overview
Problem
Audio amplifiers in headsets suffer from annoying 'pop' noises during power application and removal, particularly in headset applications, due to excessive settling times and the need for external filtering capacitors, which increase cost and power consumption.
Innovation Solution
A fully integrated solution using a sample-and-hold circuit to control the discharge of the AC coupling capacitor during power-down, reducing the slope of voltage variation and eliminating popping noises by employing a low drive output stage during power-up and a sample-and-hold circuit to gradually discharge the capacitor during power-down.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If an external filtering capacitor is used to reduce popping noises, then the popping noise is reduced, but the device complexity and cost increase due to requiring external components and extra pins
Solution Approach 1:
The patent merges the filtering capacitor function with the integrated circuit by providing an on-chip capacitor coupled to the output node through a switch. This combines what were previously separate external components into a single integrated device, eliminating the need for external filtering capacitors and extra pins while maintaining the noise reduction function.
Solution Approach 2:
The patent implements the filtering capacitor nested within the integrated circuit structure. The on-chip capacitor is embedded inside the IC along with the switch and other circuit elements, creating a compact nested arrangement that eliminates external components while preserving the filtering function.
2Object-affected harmful factors
If an external filtering capacitor is used to reduce popping noises, then the popping noise is reduced, but the power consumption increases
Solution Approach 1:
The patent merges the filtering capacitor function with the integrated circuit by providing an on-chip capacitor coupled to the output node through a switch. This combines what were previously separate external components into a single integrated device, eliminating the need for external filtering capacitors and extra pins while maintaining the noise reduction function.
3Loss of time
If the settling time of the output is reduced by decreasing the R1C1 filter settling time, then the power-up time is reduced, but the filtering effectiveness is compromised
Solution Approach 1:
The patent applies preliminary action by using the on-chip capacitor to gradually discharge the output node voltage before the amplifier fully powers down. This preliminary discharge action prevents the sudden voltage changes that cause popping noises, allowing for faster power-up and power-down times without sacrificing noise reduction effectiveness.
4Productivity
If the discharge phase is shortened to meet timing constraints, then the productivity is improved, but the popping noise may increase due to insufficient discharge time
Solution Approach 1:
The patent applies preliminary action by using the on-chip capacitor to gradually discharge the output node voltage before the amplifier fully powers down. This preliminary discharge action prevents the sudden voltage changes that cause popping noises, allowing for faster power-up and power-down times without sacrificing noise reduction effectiveness.
Data Source
AI summary
An amplifier (50) for voice or audio signals, and particularly for headset applications, uses a low gm amplifier (54) for initially charging an output node (OUT) at the beginning of a power-on phase. After charging the output node, a main amplifier (56) is enabled to amplify the voice or audio signal. At power-down, a sample-and-hold circuit (58) drives an output transistor to discharge an AC coupling capacitor (20). Thus, spikes at the output node are eliminated and an external filtering capacitor is not needed.