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A relatively new technology called the pure digital or Class-D amplifier has been introduced into the audio world. The Class-D architecture allows the input audio signal to stay in its digital format throughout the audio signal path.

While the theory behind Class-D amplifiers has been around for some time, their distortion levels were generally considered to be too high for standard audio applications. However, the technology has now matured to the stage where a Class-D amplifier can be fully implemented within a single device capable of producing high-quality audio. This significantly lowers manufacturing costs while also providing tremendous gains in power efficiency. This, in turn, has led to smaller, lighter and more efficient audio systems based on the Class-D architecture. Not surprisingly, these systems are emerging as possible replacements for the inefficient linear amplifiers that have dominated the audio market, including those suited for automotive applications.

Evolution of Audio Amplification
Audio signals have been electronically amplified since 1925, but the goal has always been to reproduce the input audio signal at the output with low distortion. This function was first performed by audio amplifiers that employed vacuum tubes, but though this was a functional approach, the vacuum tubes in these first audio amplifiers made them fragile and unreliable.

However, audio amplifier technology advanced when transistors replaced vacuum tubes as the main amplifying component. The result was the proliferation of the nowfamiliar linear amplifier architectures, including Class A, Class B, and Class AB. Yet, while more reliable than vacuum-tube technology, these transistor-based analog architectures were still highly inefficient, and required bulky heatsinks (unlike the vacuum tubes they replaced). Furthermore, audio quality in the transistor-based architectures was still affected by the susceptibility to noise inherent in any analog system. While audio quality was significantly improved in systems using digital audio signals that could be stored on CDs, these audio signals still had to be fed into a DAC and converted into analog signals that were then boosted by analog audio amplifiers in order to produce sound. Therefore, inefficiency remained a concern in these first digital audio systems.

Comparison of Amplifier Architecture Classes
A Class-A amplifier is known for its high sound quality. In this architecture, the output-stage transistors operate continuously in return for excellent linearity. Therefore, this architecture always consumes current, even when there is no audio signal, because a dc-bias current component continuously flows in the two output transistors but bypasses the speaker completely. Therefore, this amplifier has a high power dissipation and low efficiency (approximately 25%), which causes the amplifier to generate significant heat.

The output transistors in the Class-B amplifier operate alternately in a swing mode, and together alternately source positive current to the speaker and synch negative current from the speaker. This action eliminates the bias current in the Class-A amplifier. The Class-B amplifier, therefore, has much higher efficiency (approximately 65%). However, this gain in efficiency comes at the expense of audio quality, and the Class- B amplifier has a much higher distortion than a Class-A amplifier.

Class-AB amplifiers allow current to run through the output transistors when there is no audio signal, but only at a very low level. The small bias current improves linearity and thus lowers distortion, yet remains relatively efficient, enabling both good sound quality and low power dissipation. As such, this architecture provides a good engineering compromise between the audio performance of the Class-A amplifier, and the power efficiency of the Class-B amplifier.

In contrast to these linear amplifiers, the Class-D amplifier (Figure 1) is a switched-mode design based on a digital-modulation technique that operates the output transistors as switches—rather than as variable resistors—to control power distribution. This provides high efficiency, because an ideal transistor has no voltage drop in the on state, and no current flow in the off state. By the equation P = I x V, no power is dissipated by an ideal transistor in either state.

Because, the output transistors are either on or off, losses in a digital audio system are relatively small and mainly due to the non-ideal nature of the components. Specifically, the main contributors of power loss are transistor on resistances, transistor switching losses, and the parasitic resistances of the circuit elements (i.e., interconnects, lead frame and PCB traces). Therefore, Class-D amplifiers exhibit much greater power efficiency than linear amplifiers, resulting in reduced heatsink requirements, smaller PCB sizes and lower costs.

Unfortunately, Class-D amplifiers also produce higher distortion than Class-AB designs due to the high switching frequencies of the transistors (usually outside the audio range). However, this distortion is easily removed by a low-pass filter. The filter’s cut-off frequency is selected to suppress the distortion and noise over a frequency threshold that is just above the desired audio bandwidth. The result is that the instantaneous voltage at the output of the low-pass filter becomes the supply voltage (the specific voltage feeding the transistor output stage) multiplied by the duty cycle of the transistorswitching signal. Therefore, the duty cycle can be modulated as a function of the audio input signal, replicating a scaled, low-distortion version of this input signal at the output of the low-pass filter.

Audio Power Ratings
It is important to note that ratings for audio power and continuous output power are not identical. Figure 2 illustrates the difference. The audio power is the peak power that the amplifier can supply for about 10 ms. It is real, instantaneous and higher than the continuous power. Continuous output power is the averaged electrical power of the output signal. Practically, the amplifier cannot continuously maintain its full-rated peak power. Therefore, an amplifier’s performance should not be based on its peak power rating.

A common method of evaluating an amplifier is to connect it to a known resistive load, apply a continuous sinusoidal input signal, and then monitor its power output. The continuous output power is the resulting product of the rms voltage and the rms current at the output, taken over the measurement time interval. The rms value for any ac voltage or current signal is the equivalent dc signal that dissipates the same power in the same load (in this case, a fixed resistor). Therefore, a dc signal has an rms factor of one by definition. However, the rms value for an ac signal depends on the shape of the input signal, and a sinusoidal waveform is often selected because it is a convenient waveform to generate and analyze. Specifically, for a sine wave, V_RMS = V_PEAK/√2, and I_RMS = I_PEAK/√2 , so the peak power is twice the rms power, or P_RMS = P_PEAK/2.

As a review exercise, consider the following comparison for a dc signal and a sinusoidal signal applied across the simplest load, a fixed resistor. To evaluate the difference between a dc and sinusoidal input signal, apply 12 Vdc across a 15 Ω resistor. The continuous power will be V²/R, or (12 V)2/15 Ω = 9.6 W. However, if a sine wave with the amplitude of 17 V is applied to the 15 Ω resistor, the corresponding continuous power is (17 V)2/(2 x 15 Ω), which is also 9.6 W. Hence, a 17 V amplitude is required for a sine wave to produce the same output power generated by 12 Vdc for the case of this simple load (and any other constant resistive load).

For an ac signal other than a sine wave, such as a real audio signal, the rms voltage and current must be measured to obtain the true rms power reading. In general, a real audio signal will have a higher peak power than a sinusoidal signal, but its true rms power is usually only about half that of a sine wave with the same average amplitude. Therefore, the thermal effect of a real audio signal on a Class-D amplifier are much lower than that produced by a sine wave. Yet, it is a common engineering practice to use a continuous sine wave to evaluate an audio amplifier’s audio performance, since all waveforms are composed of sine waves having distinct frequencies, amplitudes and phases (as defined by the Fourier series). However, a sinusoidal signal is more likely to drive the Class-D amplifier near its maximum output power (where the amplifier enters into a thermal shutdown) than a real audio signal. Therefore, it is imperative to test the amplifier performance with a realistic audio signal instead of a sinusoidal signal.