In the previous era of audio amplifiers, performance was a trade-off for efficiency, size and thus cost. By using an output power device in a switching state, Class D topology brings the benefits of digital device evolution to audio power amplification.
The latest silicon technology with finer device geometry enables lower power loss and improves switching speed at the same time. Consequently, the newer Class D technology enables less heat, and better audio performance to enable a better amplifier. To demonstrate these benefits, this article illustrates a design with fully integrated control IC with application tailored power MOSFET, all in the size of an iPod.
Class AB: the founder of high efficient power amplification
Single-ended Class A was the dominant circuit topology used in audio signal amplification since Lee De Forest invented the triode in 1906. This simple circuit configuration offers good linearity, but with the device always conducting current the topology is very inefficient.
The single-ended Class A amplifier has 25% maximum efficiency at maximum output power. The most problematic characteristic of Class A in terms of efficiency was the large amount of idling current that kept high power consumption in the output power device even at no output power.
The double-ended Class B amplifier was invented to solve the idling power consumption deficiency by introducing a concept of switching two output devices to carry only half the audio cycles per device. Due to the elimination of idling current, the efficiency of Class B can reach 78.5% at maximum output power and no power consumption at idling.
A typical Class AB amplifier applies a small idling current (in the range of mA) through both devices. While similar in efficiency to Class B, Class AB eliminates crossover distortion by adding a conduction angle overwrap. Hence, Class AB became the dominant power amplification topology.
The efficiency of linear amplification is determined by its load condition and bus voltage, not from device parameters such as current gain hfe or transition frequency fT. The power loss is induced by the product of voltage across the power device and current flowing through the device. Because the topology is in the linear operation region, power losses are high and efficiency is typically less than 25%. Class AB audio amplifiers require large thermal management systems because more power is dissipated as heat than delivered to the speakers.
There are topologies with dynamic supply voltage modulation designed to obtain higher efficiency than Class AB, such as Class H and Class G. These approaches sacrifice audio performance and rely on switching power supply topologies.
Class D: the topology with no efficiency limitation
Class D topology introduces the pulse width modulation (PWM) concept, essentially eliminating linear mode operation. An ideal switching device would generate no power loss in either state used in this topology, therefore, delivering no power dissipation and offering 100% efficiency. The gain in the Class D switching stage is proportional to the duty cycle of the MOSFET, thus it can be controlled with great linearity
In Class D, the input audio signal is converted into a series of pulses whose instantaneous average value is proportional to the input signal. This binary signal controls the power MOSFET to create an amplified version of the PWM. An LC passive low pass filter removes high frequency carrier signal components and recovers amplified audio signal. It is interesting to realize that once the audio signal is converted to a PWM signal, the rest of signal path can be digital logics, making circuit integration and level shifting easier.
Figure 1: Class AB vs. Class D topology comparison
Class D topology is perfect in theory. It is free from non-linearity, meaning there is zero distortion. It is also 100% efficient, meaning there is no power dissipation. The greatest benefit from the text book perfection of Class D is that the degradation of performance comes from the figure of merit in the power device. For this reason, Class D keeps evolving with power MOSFET technology advancements.