Using a suitable amplifier can reduce the weight, cost, and size of the system.

Many companies are releasing a wide range of Bluetooth speakers, from inexpensive speakers with basic systems to high-spec, expensive ones. To develop products with reasonable price-to-performance ratios, design constraints such as solution size, number of components, cost, efficiency, and battery size must be considered. Figure 1 shows the basic internal components of a Bluetooth speaker.


Batteries are essential in portable applications. They must be small and lightweight, and utilize cells to provide maximum usage time. The power management block supplies the necessary power to the rest of the circuit. Due to battery constraints, the voltage provided by one or two battery cells is very low. Therefore, a boost converter is required to increase the available voltage for the rest of the system.

The Bluetooth block must provide wireless communication to the speakers of other devices, such as smartphones and tablets. The module provides a complete solution for portable audio systems and supports both wired and wireless audio by default. The audio block includes all the electronic devices required to drive the system's speakers. Since the incoming signal from the module may be low voltage and low current, an audio amplifier is used. In addition, it includes a Digital-to-Analog Converter (DAC) to convert incoming digital audio signals into analog. As a high-spec system for audio processing added to the digital domain, it enables the provision of a rich user experience.

Class AB and Class D amplifiers are available as audio amplifiers for Bluetooth speaker systems.

Class AB audio amplifiers are linear amplifiers that do not generate electromagnetic interference (EMI) and do not require many external electronic components; however, because they are highly inefficient, they require significant passive or active thermal management in the form of heat sinks or fans. On the other hand, Class D audio amplifiers are highly efficient switching amplifiers that require almost no thermal management, but they require an output inductor, which has the disadvantage of not being free from EMI issues.

Portable systems must present a method to reduce costs while adopting batteries required to constitute one or more individual cells of the battery material. Class AB audio amplifiers are suitable for use in price-sensitive systems, such as Bluetooth speakers, because they do not generate EMI and do not require many external electronic components. However, since Class AB audio amplifiers have very low efficiency, most of the charge from the battery backup is wasted as heat when used in Bluetooth speakers. As such low efficiency leads to high costs, systems using Class AB amplifiers require additional battery cells to meet requirements.

Class D amplifiers are suitable for use in portable audio systems due to their high efficiency. Due to its high efficiency, a Bluetooth speaker system can be powered using a battery consisting of a very small number of cells (a single battery cell is also possible if suitable materials are selected), which can significantly reduce the overall system cost, weight, and size.

Audio systems are selected based on output and peak power ratings, but these specifications do not accurately reflect typical usage patterns. Most users do not listen to music at such high output levels with home audio systems, and this is even more true for portable applications like Bluetooth speakers. In the case of Bluetooth speakers, the factor consumers care most about is battery life.

While it is true that a Class D amplifier was designed with high efficiency for a Bluetooth speaker system, this efficiency was not considered solely for maximizing usage time. Furthermore, factors such as the power consumption of all system blocks and the idle power loss of the audio amplifier itself, which are not clearly apparent when the system is operated and shut down, must also be taken into account.

A typical music waveform exhibits amplitude fluctuations, and as shown in Figure 2, one can observe "loud" music (high amplitude) and "quiet" music (low amplitude) sections. Looking at this waveform, it can be seen that when an audio system plays typical music, it stays in the "quiet" music band for most of the time. Therefore, the audio amplifier outputs low-power sound for most of the time.

Most Class D amplifiers on the market, including TI’s widely popular previous-generation Class D amplifier solution TPA3110D2, are not optimized for efficiency at low output power. As shown in Figure 3, the supply current of previous-generation Class D amplifiers remains constant even when the output power is low or even zero. This constant current wastes battery charge, which reduces usage time, increases the number of battery cells, and consequently raises system costs.



TI's next-generation Class D amplifier, the TPA3128D2, utilizes an innovative hybrid modulation mode to minimize idle power loss and maximize power savings. Figure 4 illustrates that the supply current to the amplifier decreases dramatically as output power drops. These power savings extend operating time and reduce the number of battery cells, thereby lowering system costs.



By migrating from the TPA3110D2 to the TPA3128D2, it is possible to fully leverage these benefits and reduce costs. Since these two solutions are pin-compatible, developers can conveniently redesign.