Audio Amplifier Coupling Inductors for High-Frequency Ripple Attenuation
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Solution Overview
Problem
Existing high-fidelity, low-distortion audio amplifiers face efficiency limitations and heat dissipation issues due to high-frequency triangular current ripple in class D amplifiers, which stress class A amplifiers and restrict bandwidth and linearity, particularly above 10 kHz.
Innovation Solution
The audio amplifier design incorporates a coupling impedance with two inductors in series, connected to ground by a resistor, and a chain of autotransformers to reduce high-frequency ripple, using a control unit with phase-shifted class D amplifiers to cancel ripple and maintain signal stability, effectively attenuating high-frequency currents before they reach the class A amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a low inductance value is used in the coupling impedance, then the current slew rate in the audio band is improved, but the triangular current amplitude at the switching frequency increases
Solution Approach 1:
A resonator circuit (RLC) tuned to the operating frequency of the class D PWM control is introduced as an intermediary element. This resonator is placed at the output of the amplifier supplying the load, and it selectively attenuates the triangular current ripple at the switching frequency while allowing the audio signal to pass through with minimal distortion.
Solution Approach 2:
The resonator circuit parameters (inductance, capacitance, and resistance values) are specifically designed and tuned to match the PWM switching frequency. By adjusting these parameters, the resonator achieves maximum impedance at the switching frequency, thereby maximizing the attenuation of triangular current ripple while maintaining low impedance for audio frequencies.
2Loss of energy
If a resonator circuit (RLC) is used to cancel high-frequency triangular current, then the fundamental of the ripple is reduced, but phase shift affecting harmonics impairs control loop stability
Solution Approach 1:
The quality factor (Q) of the resonator circuit is carefully optimized to achieve the right balance between ripple attenuation and stability. By adjusting the resistance component and the L/C ratio, the resonator provides sufficient attenuation of the triangular current while introducing minimal phase shift in the audio band, thereby maintaining control loop stability.
Solution Approach 2:
The resonator circuit is designed to replicate the impedance characteristics needed for ripple cancellation without introducing harmful phase shifts. The circuit parameters are copied from proven designs and fine-tuned to match the specific PWM frequency and load conditions of the amplifier system.
3Speed
If class A amplifier is used to reproduce high frequencies, then high-frequency reproduction is achieved, but heat dissipation increases significantly
Solution Approach 1:
The harmful triangular current ripple generated by the class D amplifier's PWM switching is converted into a beneficial filtering opportunity. By placing the resonator circuit at the output, the ripple that would otherwise be absorbed and dissipated as heat by the class A amplifier is instead attenuated by the resonator, converting a harmful effect into a design feature that protects the class A amplifier from excessive heat generation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration reduces heat dissipation in class A amplifiers, increases the useful bandwidth, and enhances linearity by attenuating high-frequency ripple, allowing efficient current supply to the load without distorting the audio signal.
Implementation Method 1
One first solution consists of using a resonator circuit (RLC) tuned to the operating frequency of the class D PWM control
Implementation Method 2
The combination of a class A amplifier and a class D amplifier aims to create an amplifier with very high efficiency and very high linearity
Data Source
Figure 1~2
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Figure 4
AI summary
This audio amplifier (10) comprises: - an input (12) for an audio signal to be amplified and an output (14) for feeding a load (16) from the amplified audio signal; - a reference voltage generator (18) for generating voltage of very high linearity and low output impedance, capable of receiving as an input the audio signal to be amplified; - a power current generator (19), comprising a power voltage generator (21), the output of which is connected to the output of the reference voltage generator (18), via a coupling impedance (30). The coupling impedance (30) comprises two coupling inductances (32A, 32B) mounted in series between the output of the reference generator (18) and the output of the power voltage generator (21) and an attenuation impedance (44) linking a mid-point between the two coupling inductances (32A, 32B) and a reference potential. The attenuation impedance (44) comprises an attenuation inductance (44A).