Audio content has evolved over the years. It’s gone from the days of buying 12-inch LP vinyl at the local record store to being able to stream almost any content from the cloud instantaneously. Although some audiophiles are bringing back the days of vinyl as a novelty, the vast majority of audio today is consumed digitally. Even though the way consumers receive audio has changed, one thing has remained constant: a desire to enjoy the content as it was originally recorded. A key component of making this a reality is the audio amplifier in the sound system. With one piece of silicon having such a large impact on the audio quality, it’s important to understand the tradeoffs and how to choose the right one for a design.

An end product’s features are usually defined before it is designed — an example being the target loudness for a voice-controlled smart speaker that should be audible across the room. In terms of electrical system design, the loudness correlates with output power and speaker/voice coil efficiency. Without knowing a lot of details about the end system, output power is the closest specification one could use to determine the loudness (usually approximated in dB) of the finished product. Hence, the output power level is usually one of the first criteria used to select an audio amplifier. The industry standard for specifying the power level of an amplifier is the output power at 1% or 10% total harmonic distortion plus noise (THD+N). Typically, the number of channels at that power level is also referenced, so a stereo device could be specified as 2 x 100 W at 10% THD+N. One important thing to note when referring to an amplifier's power level is whether it is specified as a peak or continuous value.

Class-D amplifiers are usually up to about 90% efficient and the approximate 10% loss of energy is converted to heat. To protect the integrated circuit, Class-D audio amplifiers have a temperature threshold at which they shut down. So a device may be able to hit a certain peak output power level, say 30 W, but not be able to sustain this level for an extended period of time without going into over-temperature shutdown. The highest output power level a system could support and sustain without reaching the over-temperature state is considered to be the maximum continuous output power level.

Figure 2-1 Continuous and Peak Power in an Audio Signal

Since the continuous output power level relies on heat dissipation, it depends on factors of the end system; for example: PCB thickness, enclosure size, and ventilation. A major attribute that does impact the continuous output power level from the amplifier is the thermal pad orientation. Devices with a thermal pad on the bottom are soldered directly to a PCB, while devices with the thermal pad on the top require a heatsink. Using a heatsink can increase the continuous output power level, but may in turn require extra space and heatsink material.

Figure 2-2 Pad-Down vs. Pad-Up Audio Amplifier

An important piece of the output power equation is the supply voltage for the amplifier. Some Class-D amplifiers use one rail and others (bipolar or split-rail amplifiers) use two of equal magnitude, but opposite polarity rails to amplify the audio signal. Aside from this, another rail is typically required for internal logic regulators and gate-drive circuitry, such as 1.8 V, 3.3 V, 5 V, or 12 V. Some newer devices like the TPA322x family integrate a linear drop-off regulator (LDO) internally to produce this rail from the main supply rail. While using the internal LDO isn’t as efficient, it can provide large cost and design savings if the necessary supply voltage isn’t already available in the system. A great use case for single-rail supply amplifiers with an LDO are battery-operated speakers. For systems with a very low supply voltage, from a 1S battery for example, often a boost converter is used to drive the necessary output power. Class-D amplifiers such as TAS2562 even integrate a boost converter for overall system cost and space savings.

A speaker’s output power is correlated to the power supply and the load connected to the audio amplifier. The amplifier itself also limits the output power for an audio system, as the core of an audio amplifier is a MOSFET amplifying the analog signal, restricted by voltage, current, and thermal limits. Ohm’s law dictates that current is a function of voltage and resistance, making power inversely proportional to resistance (the load). Since an amplifier has limitations on its voltage and current, the resistance of the speaker is important to consider when selecting an amplifier. Drivers typically range between 2-8 Ω. A lower impedance driver draws more current and produces higher output power for as long as the amplifier is able to support it. Lower impedances are typically used in subwoofers (2-4 Ω) while higher impedances (4-8 Ω) are used for woofers and tweeters.

Generally, a Class-D amplifier can be manipulated to provide different levels of power by topology. For example, a Class-D amplifier with four half-bridges can drive 4, 2 or even just 1 channel. These topologies are referred to as single-ended (SE), bridge-tied load (BTL), and parallel bridge-tied load (PBTL). Bridging two outputs effectively doubles the voltage across the load, while configuring them in parallel allows double the current. In theory, doubling the voltage and current in mode PBTL should quadruple the power, but since amplifiers have voltage and current limitations, running in PBTL typically only allows for more power into smaller loads. These output configurations can be seen in Figure 4-1. For more information on output configurations, visit section one of the LC Filter Design application note.

Figure 4-1 Various Output Configurations