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Audio amplifier selection in hearable designs

The audio amplifier is a major building block in smart hearable devices—whether wired or wireless—inserted in the ear or placed behind the ear.

In a hearable device design, the analog circuitry converts audible sound via a microphone, amplifies, and conditioners and then sends it to a microcontroller, which further conditions and filters the signal and ultimately drives a tiny speaker that sends that signal to the eardrum. The MPU-based digital processing can facilitate background noise and acoustic feedback cancellation, enhance speech, and provide automatic gain control (AGC).

Figure 1 The block diagram shows the basic building blocks of a hearable design. Source: Texas Instruments

The audio amplifier employs transistors in linear mode to form an output voltage that is a scaled copy of the input voltage.

Audio systems commonly employ class A, class B, and class AB amplifiers. However, for tiny hearable devices, class B and class AB amplifiers are not practical choices because of significant power dissipation. Here, selecting a class A amplifier for the pre-amp, which facilitates low distortion, might seem a good first choice as long as design engineers make sure to operate it in the amplifier’s linear range.

Class A amplifiers have low distortion as well as euphonic sound. The problem is that class A has a high energy consumption and doesn’t offer a great dynamic range while on a small battery. So, the class H pre-amp, an energy-efficient version of the class A amplifier, maybe a better choice.

However, class D amplifiers are emerging as a highly suitable choice for hearable designs. Class D amplifiers, conceived in 1958, dissipate much less power than the amplifiers mentioned above. As a result, they offer energy efficiency and thus longer battery life.

Figure 2 The digital-input class D amplifiers make system design nearly plug and play. Source: Maxim Integrated

More importantly, class D amplifiers eliminate the need for a digital-to-analog converter (DAC) and thus can directly drive a speaker. The sound does not go through digital processing, and subsequently, the amplified sound is clearer as well as more natural and pleasant.

Class D amplifiers are playing a crucial role in bringing power-efficient amplification to small form-factor devices like hearables. Medical hearing aids are another design frontier where class D amplifiers’ power efficiency and low-noise features go a long way.

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