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Examining the advances in audio amplifiers

25 Mar 2013  | Jaehong Oh

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The proliferation of mobile devices such as smartphones and tablets has necessitated a need for power efficiency to extend battery life. Hitherto the focus has primarily been on the processors which are the biggest power consumption device on the board. There is now an emerging market need for power efficient audio delivery. The mobile devices are now essentially a multimedia consumption device. In the "good old days" (circa 2000s!), the mobile phone was primarily used as a phone—The "on" time of the audio signal path as a percentage of the "on" time of the device was very small. Consequently the efficiency of the audio signal path did not matter as it made no significant impact on the overall efficiency of the device. Contrast that with today where the tablet/smartphone is used as the primary device for audio/multimedia consumption (music, movies, radio, audiobooks etc.), and one appreciates the impetus for the market drive for power efficient audio solutions. This paper will briefly survey the history and the current state of the art for audio amplifiers and then describe architectures and circuit techniques incorporated in Nuvoton's new generation of ultra-low power consumption audio amplifiers.

Figure 1: Class AB and Class D Output Power v/s efficiency for 10W.

The story so far
Up until the late 90's the familiar analogue amplifiers topologies from EE101 Class A, Class B, Class AB formed the bulk of the audio amplifier topologies used. There was a performance (THD etc...)/ power efficiency/cost trade-off that dictated the engineering choice of topology for a particular application. The power efficiency varied from as low as 25% (Class A) to a theoretical high of 78.5% but at the expense of performance (Class AB). As the end devices were predominantly AC powered (Receivers, Boom boxes, TVs) the power efficiency in itself was not a big concern.

In around the mid 90's multiple market forces converged in providing a need for higher efficiency audio amplifiers. On the high power end, the home theatre was becoming increasingly popular. Providing six channel of hi power at low efficiencies led to use of bulky power supplies and large heat sinks with the associated costs and form factors. Around that same time Flat Panel TVs made their market debut. These TVs by the fact that their selling point was "thin" did not even have the space for heat sinks to dissipate the excessive power. Further along in that decade, the advent of mobile phones, IPods drove the need for longer battery life. All these market trajectories were looking for higher efficiency in audio amplifiers without sacrificing audio performance. The answer lay in a new class of amplifiers—the Class D. Class D amplifiers touted theoretical efficiencies of 100% and practical efficiencies of over 90%. Though Class D as a topology has been around for many decades, its mainstream adoption was plagued by cost, performance and EMI consideration, and hence was limited to high end subwoofers markets.

The graph figure 1 illustrates the typical power efficiency v/s output power curves normalized for a 10Watt amplifier.

The power efficiencies offered by Class D quickly made them the topology of choice for markets that needed the efficiencies, whether for battery savings or enabling ever smaller form factors. The widespread adoption of the first generation of Class D technologies led to an industry focus on some of the shortcomings of the topology and on new ways to overcome them.

Class D amplifier topologies
The most basic Class D Amplifier topology uses pulse-width modulation (PWM) with a triangle-wave (or sawtooth) oscillator. Figure 2 shows a simplified block diagram of a half-bridge Class D amplifier. It consists of a pulse-width modulator, two output MOSFETs, and an external low-pass filter to recover the amplified audio signal. The MOSFETs operate as current-steering switches by alternately connecting the output node to VDD and ground. Because the output transistors switch the output to either VDD or ground, the resulting output of a Class D amplifier is a high-frequency square wave. The switching frequency for most Class D amplifiers is typically between 250kHz to 1MHz. The output square wave is pulse width modulated by the input audio signal by comparing the input audio signal to an internally generated triangle-wave oscillator. The resulting duty cycle of the square wave is proportional to the level of the input signal. When no input signal is present, the duty cycle of the output waveform is equal to 50% and increases or decreases in proportion to the input signal

Figure 2: Class D amplifier in half bridge mode.

Figure 3: Class D amplifier in Full Bridge/BTL mode.

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