Lookup-Table PWM Class D Amplifier for Low-Distortion Audio
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Solution Overview
Problem
Class D amplifiers face limitations in achieving high fidelity and efficiency due to distortion issues in direct PCM to PWM conversion, requiring complex pre-processing and high switching frequencies, and are prone to noise and distortion in open-loop output power stages.
Innovation Solution
A class D amplifier incorporating a noise-shaping modulator, pulse width modulator, and pulse amplifier with a lookup table for PCM to PWM conversion, along with a compensation circuit to optimize performance, which reduces total harmonic distortion and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If direct PCM to PWM conversion is used in class D amplifiers, then the circuit complexity is reduced, but distortion and noise increase significantly
Solution Approach 1:
The patent applies preliminary action by implementing a noise-shaping modulator that pre-processes the PCM signal before PWM conversion. The modulator shapes the quantization noise spectrum in advance, pushing noise to higher frequencies where it can be filtered more easily, thereby improving signal fidelity before the actual PWM conversion occurs.
Solution Approach 2:
The patent introduces an intermediary element - a lookup table-based PWM circuit that acts as a mediator between the PCM signal and the final PWM output. This lookup table stores pre-calculated PWM duty cycle values that correspond to different PCM input levels, enabling accurate conversion without complex real-time calculations, thus maintaining signal fidelity while keeping the circuit relatively simple.
2Manufacturing precision
If high switching frequencies are used to reduce distortion, then signal fidelity improves, but power consumption increases
Solution Approach 1:
The patent applies parameter changes by optimizing the switching frequency to a specific range (20 kHz to 100 kHz) rather than using uniformly high frequencies. The lookup table is designed to work effectively at these moderate frequencies, allowing the system to achieve good signal fidelity without the excessive power consumption associated with much higher switching frequencies.
Solution Approach 2:
The patent replaces complex analog signal processing mechanisms with a digital lookup table approach. Instead of using complex analog filters and high-frequency switching circuits to achieve low distortion, the system uses pre-stored digital values that can be retrieved and applied directly, reducing the need for high switching frequencies and associated power consumption.
3Loss of energy
If open-loop output power stage is used, then the amplifier efficiency is high, but distortion and noise performance deteriorates
Solution Approach 1:
The patent implements feedback by using the lookup table to store PWM values that are derived from considering both the desired output signal and the expected characteristics of the power stage. This form of feedforward feedback allows the open-loop power stage to operate efficiently while still achieving good distortion and noise performance through pre-compensation in the lookup table.
Data Source
AI summary
A class D amplifier includes a noise-shaping modulator, a pulse width modulator, and a pulse amplifier. The noise-shaping modulator receive a pulse code modulated (PCM) signal and produces an oversampled PCM signal. The pulse width modulator produce a pulse width modulated (PWM) signal from the oversampled PCM signal. The pulse amplifier amplifies the PWM signal to produce an amplified PWM signal. The PWM uses a lookup table to convert from PCM to PWM. A compensation circuit optimizes amplifier performance. An optional demodulator filter converts the amplified PWM signal to an analog signal. The amplifier is ideal for integrated audio applications.