Class-D Loudspeaker Current Sampling for Low Distortion and DC Offset
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Solution Overview
Problem
Current loudspeaker driving circuits with current sensing resistors coupled to the ground suffer from poor second-order harmonic distortion and DC offset due to resistor mismatch, which can damage the loudspeaker.
Innovation Solution
A driving circuit and method that includes a periodic signal generation circuit, signal processing circuit, class-D amplifier circuit, current sensing circuit, and sample and hold circuit, where the current sensing signal is sampled and held at the peak value of a triangle wave signal, ensuring all resistors are equally loaded, and an average computation is performed to generate a current sampling signal, thereby improving harmonic distortion and avoiding DC offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If current sensing resistors are coupled to the ground in a class-D amplifier, then the noise floor is improved, but the second-order harmonic distortion becomes poor and DC offset occurs due to resistor mismatch
Solution Approach 1:
The patent applies periodic action by using a triangle wave signal to periodically switch the connection of current sensing resistors. The switching circuit alternates between connecting different sensing resistors to the output node, ensuring that each resistor is equally loaded over a complete cycle. This periodic switching eliminates the need for precise resistor matching while maintaining low noise floor performance.
Solution Approach 2:
The patent changes the operational parameters of the current sensing circuit by dynamically switching which resistor is connected to which node based on the triangle wave signal phase. This parameter change approach allows the system to achieve balanced loading conditions without requiring fixed, precisely matched resistor values, thereby resolving the contradiction between noise performance and harmonic distortion.
2Measurement precision
If current sensing resistors are not matching, then the noise floor is improved by coupling to ground, but DC offset is caused
Solution Approach 1:
The switching circuit implements periodic action by alternately connecting different current sensing resistors to the output node in synchronization with the triangle wave signal. This ensures that over one complete cycle, each resistor experiences equal loading conditions, which eliminates DC offset even when resistor values are not precisely matched.
Solution Approach 2:
The patent uses the triangle wave signal as a reference pattern that is copied and used to control the switching of multiple current sensing paths. This copying approach ensures symmetrical operation where each sensing resistor experiences identical temporal patterns of connection, thereby canceling out any DC offset that would result from resistor mismatches.
Data Source
AI summary
A driving circuit of a loudspeaker includes a periodic signal generation circuit, a signal processing circuit, a class-D amplifier circuit, a current sensing circuit, and a sample and hold circuit. The periodic signal generation circuit is arranged to generate a periodic signal and a control signal. The signal processing circuit is coupled to the periodic signal generation circuit, and is arranged to generate a pre-driving signal. The class-D amplifier circuit is coupled to the signal processing circuit, and is arranged to drive the loudspeaker according to the pre-driving signal. The current sensing circuit is coupled to the class-D amplifier circuit, and is arranged to generate a current sensing signal. The sample and hold circuit is coupled to the periodic signal generation circuit and the current sensing circuit, and is arranged to sample and hold the current sensing signal according to the control signal, to generate a current sampling signal.