Class D Audio Amplifier Feedback Loop for Low Distortion
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Prior art pulse width modulated class D audio amplifiers suffer from high output impedance, slew-rate distortion due to active components, and aliasing errors, leading to increased distortion and reduced power efficiency, especially at high frequencies and with varying loads.
Innovation Solution
A passive loop filter system is introduced in the amplifier circuit, taking feedback signals after the LC filter to reduce output impedance and minimize aliasing errors, using a pulse code modulation to pulse width modulation (PCM-PWM) converter and feedforward filters to generate feedback signals with a loop transfer function that minimizes aliasing error, thereby reducing harmonic distortion and eliminating the need for a Zobel network.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a prior art modulator with a PWM output signal is used, then the amplifier can achieve high power efficiency through switching mode operation, but the output impedance becomes high and distortion increases
Solution Approach 1:
The patent implements feedback control by feeding back the output signal through the LC filter to the modulator input. This feedback mechanism allows the system to maintain high power efficiency through switching mode operation while simultaneously reducing output impedance and distortion by actively correcting output deviations.
Solution Approach 2:
The patent changes the operating parameters of the amplifier by using pulse width modulation instead of traditional linear amplification. By varying the duty cycle of the PWM signal based on feedback, the system achieves high efficiency while controlling output impedance through parameter modulation rather than fixed operation.
2Ease of operation
If active components are used in the amplifier circuit, then the amplifier can provide signal processing functionality, but slew-rate distortion increases
Solution Approach 1:
The patent extracts and removes active components from the amplifier circuit, replacing them with passive elements. By taking out the active components that cause slew-rate distortion, the system maintains signal processing capability through the PWM modulation and feedback control while eliminating the harmful distortion effects.
Solution Approach 2:
The patent substitutes active electronic components with passive circuit elements combined with digital signal processing. The signal processing functionality previously provided by active components is achieved through PWM modulation and feedback control, replacing the mechanical/electronic active system with a control-based approach that avoids slew-rate distortion.
3Object-generated harmful factors
If a Zobel network is added to reduce distortion, then the amplifier performance improves, but the device complexity increases
Solution Approach 1:
The patent uses feedback control to achieve distortion reduction without adding a Zobel network. By feeding back the output signal and adjusting the PWM duty cycle accordingly, the system compensates for distortion effects inherently, eliminating the need for additional passive compensation networks and reducing overall circuit complexity.
Solution Approach 2:
The feedback control mechanism serves multiple functions simultaneously: it reduces distortion, controls output impedance, and maintains power efficiency. This multi-functionality eliminates the need for separate distortion compensation circuits like the Zobel network, reducing device complexity while achieving the same or better performance.
4Object-generated harmful factors
If feedback control is implemented, then distortion is reduced, but aliasing errors increase
Solution Approach 1:
The patent applies preliminary filtering to the feedback signal before it is processed by the modulator. By pre-filtering the feedback signal to remove high-frequency components that would cause aliasing, the system can implement feedback control to reduce distortion while preventing aliasing errors from being introduced into the control loop.
Solution Approach 2:
The patent introduces an intermediary filtering stage between the output and the feedback path. This intermediary filter acts as a mediator that allows feedback control to reduce distortion while blocking the transmission of high-frequency aliasing components back to the modulator, thus resolving the contradiction between distortion reduction and aliasing prevention.
Data Source
AI summary
System for pulse-width-modulated class D audio amplifiers. In one preferred embodiment an adder is described to generate a difference signal responsive to an input signal and a feedback signal, a pulse-width-modulator coupled to the adder to compare the difference signal to a reference signal and produce a pulse-width-modulated signal based on the comparing, a filter coupled to an output of the pulse-width-modulator, and a loop filter having a first input coupled to the output of the filter and a second input coupled to the input of the filter, the loop filter to generate a feedback signal by applying transfer functions to signals at its inputs. The loop transfer function of the amplifier is minimum aliasing error transfer function. The minimum aliasing error properties provide low distortion and taking the feedback from the output of the filter reduces high frequency output impedance.