Green energy standards, the continuing need to reduce costs and the demand for higher audio fidelity are driving the adoption of Class D amplifiers in high-power audio applications. Traditional analog implementations, such as Class AB topology, are more complex and less efficient, and yet have dominated the high end of the audio market due to their high-fidelity performance. Class D systems are quickly narrowing this gap with simpler, more efficient designs that offer fidelity capable of surpassing that of analog amplifiers.
A typical Class D audio system converts audio signals at its input to a digital PWM signal, adds power amplification in the digital domain, and then converts the digital signal back to analog at its output. As shown in Figure 1, the incoming audio signal is applied to a pulse-width modulator (PWM), which consists of an operational amplifier and a comparator. This modulator digitizes the audio input by varying the modulator duty cycle in direct proportion to the instantaneous value of the audio input signal.
Figure 1: Basic block diagram of a Class D amplifier
This PWM signal is then appropriately level shifted and applied to a gate driver that switches a two-state power circuit consisting of MOSFETs (M1 and M2). The resulting amplified signal is then passed through an output filter, which removes the PWM carrier frequency, leaving only the amplified analog audio signal that ultimately drives the speakers. Audio output fidelity is further enhanced by the outer feedback loop from the filter input to the error amplifier input thereby reducing distortion and noise.
Class D amplifier design
Historically, analog power amplifiers have relied on linear amplification circuits that are prone to high-power losses. By comparison, Class D amplifier power efficiencies can be 90% or higher, depending on the design. This high efficiency benefit is intrinsic in Class D technology where binary switches, usually power MOSFETs, are the amplification mechanism. These switches are either fully on or off, and very little time is spent transitioning between those two states.
This discrete switching action, and low MOSFET on-resistance, minimize I2R losses and increase efficiency. In practice however, the switch transition time (dead time) must be long enough to avoid efficiency-killing shoot-through currents from both switches being on simultaneously.
Audio fidelity can be defined as the faithfulness with which sound is reproduced. For audio systems, fidelity is a proxy for the all-enveloping term “sound quality.” While various specifications are used to quantify fidelity, some of these measures are especially challenging for designers. Two of the most challenging specifications are total harmonic distortion (THD) and noise (N), collectively referred to as THD + N.
THD is a measure of accuracy of an audio system, very much akin to high fidelity itself. Inaccuracies in signal reproduction create additional signal components at harmonic multiples of the input frequencies, which obviously distract from the purity of the output signal. THD is the ratio of the unwanted energy of all harmonic frequencies to the energy of the fundamental frequencies of the input, typically measured at half of full power for a given system. While THD performance of less than 0.1% is adequate for most non-audiophile audio applications, discerning listeners usually go for THD levels as low as 0.05% or even lower.
Output noise level is a measure of the noise floor level of the amplifier outputs with no signal input. For most speakers, a noise floor of 100-500 µV is inaudible from most normal listening distances, while a noise floor as high as 1 mV will prove to be quite annoying. Combined, THD+N is a very good indicator of audio fidelity of a given amplifier.