Audio amplifier selection in hearable device designs

Article By : Steve Taranovich

Class D amplifiers are playing a crucial role in bringing power-efficient amplification to small form-factor devices like hearables.

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.

This article was originally published on Planet Analog.

Steve Taranovich is the editor in chief of EE Time’s sister site, Planet Analog and also serves as senior technical editor at EDN. Steve has 40 years of experience in the electronics industry. He received his MSEE from Polytechnic University, Brooklyn, New York, and his BEEE from New York University, Bronx, New York. He is also chairman of the Educational Activities Committee for IEEE Long Island.  His expertise is in analog with a diverse embedded processing education as it relates to analog design from his years at Burr-Brown and Texas Instruments. Steve was a circuit design engineer for his first 16 years in electronics. He then served as one of the first field application engineers with Burr-Brown Corp and also became one of their first global account managers, traveling to Europe, India and China.

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