Amplifiers in Class D revolutionize underpowered systems, propelling them to greater performance.
In the realm of audio technology, Class-D audio power amplifiers (APAs) have made a significant impact, particularly in compact, battery-powered devices. First introduced nearly half a century ago, these efficient amplifiers are now becoming more common in mainstream applications such as portable computers, battery-operated music systems, wireless communication devices, and other low-power systems.
The performance of Class-D APAs is heavily influenced by the layout of the printed circuit board (PCB). A well-designed PCB is crucial for the overall efficiency and effectiveness of the amplifier.
When it comes to testing, real-world signals like music are used instead of the typical sine waves or tones to assess power efficiency. This approach provides a more accurate representation of the amplifier's performance in real-world conditions.
In a bridge-tied-load (BTL) configuration, the tolerance of inductors and capacitors is critical for good matching. Tight tolerances are necessary to ensure the inductors don't saturate during operation, which can significantly increase distortion. Similarly, the capacitors' tolerance should also be tight to maintain optimal performance.
Power losses must be taken into account during the design of the low-pass filter. The series resistance of the inductor reduces the power delivered to the speaker and decreases the efficiency of the amplifier.
A demodulation filter must be properly located to ensure effectiveness in Class-D APA systems. Adding decoupling capacitors close to the power supply and locating high-frequency bypass capacitors close to the power pins can reduce noise and help provide current to the amplifier.
Variations in components can lead to increased harmonic distortion and reduced filter performance through changes in the frequency response. The filter in Class-D APAs contributes a portion of the quiescent current used by the amplifier. Low-leakage, low equivalent series resistance capacitors should be used to minimize the power dissipated in the amplifier.
A stable power supply, a linear triangle signal, and an accurate and fast voltage comparator are important for minimizing pulse width modulation (PWM) distortion level in Class-D APAs. The corner frequency is set by the equation: F = 1/(21⁄4LC).
The low-pass filter in Class-D APAs should be carefully designed to avoid peaking at a specific frequency when a reactive load is present. The filter should have a maximally flat magnitude response within the passband to minimize ripple and introduce minimal distortion in the output.
Class-D APAs rely on pulse width modulation (PWM) to sample the input signal and recreate it as an audio signal at the load. The choice of components and the order of the filter will depend on the switching frequency, since the filter's sole purpose is to remove the switching frequency component from the output.
Integrated Class-D APAs are effective and highly efficient solutions for compact, battery-powered audio applications in the music bandwidth. Advances in semiconductor fabrication processes have made integrated Class-D audio amplifiers possible.
Key advantages and design considerations for Class-D APAs include high efficiency and low power dissipation, compact size and reduced cooling requirements, advanced modulation techniques, the use of GaN FETs, trade-offs in sound quality and load handling, EMI and filtering, and the importance of a well-designed low-pass filter.
In a laboratory test, a Class-D amplifier extended the life of a battery by 2.5 times compared to a linear Class-AB APA. Despite some historical concerns about sound quality, modern designs can perform near or above expectations for many applications, although they may still struggle with very low-impedance loads.
References: [1] Infineon Technologies. (2021). Class-D Audio Amplifiers. Retrieved from https://www.infineon.com/cms/en/product/power/audio/class-d-audio-amplifiers/ [2] Texas Instruments. (2021). Class-D Audio Amplifiers. Retrieved from https://www.ti.com/product/TAS5754 [5] Analog Devices. (2021). Class-D Audio Amplifiers. Retrieved from https://www.analog.com/en/products/amplifiers/audio-amplifiers/class-d-audio-amplifiers.html
- The efficient performance of Class-D audio power amplifiers (APAs) is heavily influenced by the design of the printed circuit board (PCB).
- To minimize noise and ensure effectiveness in Class-D APA systems, it's important to properly locate the demodulation filter and add decoupling capacitors close to the power supply and power pins.
