CLASS D AMPLIFICATION. New horizons.

Since quite a few manufacturers are using Class D in their latest models, it is worth knowing the characteristics of this type of amplification in detail in order to understand its advantages and drawbacks compared with the far more common Class A and Class AB.

Nevertheless, as a reminder, we will also briefly review those types of electronics, so that our readers can assess in greater depth what Class D represents:

In Class A, current flows through a single transistor during the entire cycle of the input signal. The bias current of the output transistor is high and constant throughout the process, regardless of whether or not there is any audio output. The distortion introduced is very low, but so is the efficiency, which is always below 50%. This means that the other half of the amplified current is dissipated as heat.

Class AB is the type in which, during one half-cycle, the current flows through and is amplified by a single transistor. During the other half-cycle it flows through and is amplified by a different one, allowing each transistor to rest for one half-cycle and to work and dissipate power during the other. In addition, no current flows through the output when there is no audio signal.

The problem is that so-called "crossover distortion" occurs, since when the signal voltage in the first half-cycle falls below 0.6 V, the transistor turns off and stops amplifying, and the same happens in the other half-cycle while the voltage has not yet reached 0.6 V. In short, in the case of a sine wave, we would have 1.2 V that is not amplified.

Classes E, G and H are also mentioned. These are not as standardized as A and B. The Class E amplifier is a pulse amplifier (its efficiency can be very high) whose output is tuned to a particular frequency. It is usually used in radio applications when working on a single frequency or within a very narrow range. It is not used in audio. Class G refers to switched amplifiers that have two different power supplies. The amplifier's supply is switched to the lower voltage for weak signals and to the higher one for stronger signals. This provides greater efficiency without the need to switch output stages, so they can sound better than Class D amplifiers. Class H is based on using a Class D amplifier or a switch-mode power supply to power a Class AB or Class A amplifier. In this way, it offers excellent efficiency and has the sound of a good Class AB amplifier. Class H is widely used in professional power amplifiers.

Focusing now on Class D, it uses active elements operating in cutoff or saturation, so efficiency compared with the topologies described so far rises to values above 90%. This allows more compact and lighter designs. The use of a closed-loop control circuit is very important for achieving harmonic distortion levels similar to those of traditional designs. In this type of amplifier, the audio signal is encoded using PWM, or pulse-width modulation, so that the amplitude and frequency information is contained in the duty cycle of the switching of the power stage transistors.

The blocks that make up the amplifier are as follows:



- Input stage. This stage contains the gain control, low-pass filtering to prevent instability in the control circuit, and the unbalancing circuit.



- Control. To improve the system's THD (total harmonic distortion), closed-loop voltage control can be used. The filtered output voltage is fed back to compensate for the phase shift and imperfections of the power stage. This block is responsible for canceling out errors between the reference and the output signal.



- PWM generator. This is responsible for generating the PWM signal from the modulating signal and the carrier. A triangular signal with a frequency between 200 kHz and 400 kHz is used as the carrier, and the output signal of the control block as the modulating signal. This process is carried out by an analog comparator capable of operating at the frequency of the modulating signal.



- Half-bridge. This consists of two current-bidirectional switching cells. N-channel power MOSFET transistors have been used (allowing a more efficient design), powered by a properly decoupled DC supply to avoid the pumping effect inherent to the way Class D amplifiers operate.



- Filter. A correct output filter design offers multiple advantages, such as limiting current consumption, minimizing switching noise (EMI) and protecting the loudspeaker from high-frequency harmonics. These tones must be sufficiently attenuated. The amplitude of the carrier after the filter must be at most 5% of that of the amplified modulating signal (first harmonic), so that the passband (up to 20 kHz) is not altered in either amplitude or phase. Of the different types of alignment, one should be used that has a very flat response in the passband, damped resonance at the cutoff frequency (between 15 kHz and 25 kHz depending on the PWM frequency) and a limited number of components.



- Protection circuits. The most important are overcurrent, overvoltage and DC-at-the-output protection. These act on the modulation stage, preventing the cells from switching ON in the event of a fault.



- Feedback gain (ß). This gain makes it possible to adapt the output voltage level so that it can be fed into the control circuit.



One of the greatest attractions of Class D is that its operation can be considered (not without certain reservations) digital, since the audio signal is amplified by sampling the analog signal, obtaining pulse-width encoding with 1-bit quantization. In general, this encoding is carried out in the analog domain by comparing the input signal with a triangular signal of a much higher frequency than the maximum frequency of the audio signal, so that aliasing can be avoided. Since today's audio storage and playback systems are based on digital processes, the digital data stream must be converted to analog in order to carry out amplification using any of the methods described so far. The use of D/A converters adds distortion to the reference signal, so a carefully designed preamplifier and power amplifier may prove useless when paired with a poor-quality D/A converter. The advantage of Class D is that the digital input signal can be amplified without the need for converters, using various PCM-to-PWM conversion techniques. In this case, noise-shaping techniques are very commonly used to reduce the noise due to signal quantization, which significantly improves THD. This method is based on a deterministic technique that uses a fine quantizer inserted in a feedback loop to alter the distribution of the quantization noise inherent to the digital signal, resulting in a smaller error. Using DSP, it is therefore possible to integrate the usual digital processing (filters, delays, crossovers...) and amplification in a single device without the need for D/A converters. This has an impact on both the quality and the cost of the final product. The last natural step in the process is to find transducers capable of reproducing the amplified digital signal. Although loudspeaker design has remained relatively unchanged for many years, except as far as construction materials are concerned, work is currently under way to reproduce the digital signal directly, using basically two methods: multiple voice coils connected to the same cone (MVC) or multiple drivers forming a matrix or array (DTA). The first consists of using as many voice coils as there are bits in the digital signal, with the digital-to-analog conversion taking place in the loudspeaker's magnetic field, while the second requires multiple drivers, their number being the main problem, although various techniques can be used to reduce it to a reasonable quantity. In the latter case, the D/A conversion takes place in the acoustic domain.

To produce a robust design capable of meeting the demands of the professional market, a number of important considerations must be taken into account, such as those listed below:



- Output load. Although the amplifier is usually designed for a constant load modeled as a resistor, the impedance it has to handle is not at all resistive. Specifically, it depends on the driver and the enclosure in which it is mounted, and varies greatly from the nominal value given by the manufacturer. In addition, the amplifier must be prepared to cope with open-circuit and short-circuit conditions, and under no circumstances must these damage either the amplifier or the loudspeaker. A very important point is that any malfunction of the amplifier can result in unpleasant sounds that would disturb the listener. This is an important aspect that the designer must always bear in mind.



- Output power. A study of the average power of an audio signal reveals that it is up to 12 dB lower than the peak power. This makes it possible to relax the thermal design of the amplification stage and determines that of the power supply, which must be able to keep the voltage stable despite power peaks. Bearing in mind what has been said so far, there is no need to design a power supply capable of delivering peak power for prolonged periods, although it must be able to do so for a reasonable time, which will depend on several factors, including the quality intended for the final product.



- Size and weight. Once the decision has been made to design a Class D amplifier stage, one of the main objectives must be to obtain a compact, lightweight design that visibly justifies the use of a technology different from the usual one.



- Digital signal processing using DSP. If digital modulation is chosen, it is very worthwhile to include in the power amplifier functions typical of standalone digital processors, such as parametric filters, limiters, compressors, crossovers, delay, etc.



Finally, by way of conclusion, we can say that Class D amplifiers have high energy efficiency, in some cases above 95%, which reduces the size of the heat sinks required and, therefore, the overall size and weight of the circuit.

In the past, they were limited to portable devices or subwoofers, where distortion or bandwidth are not decisive factors. With more modern technologies, there are full-range Class D amplifiers with distortion levels similar to those of Class AB.

Class D amplifiers require meticulous design to minimize the electromagnetic radiation they emit, and thus prevent them from interfering with nearby equipment, typically in the FM band. Today, bearing in mind that these limitations have now been overcome, many manufacturers are choosing this path, which still has things to prove, especially to the most purist ears, which are reluctant to use this type of amplification. Nevertheless, given the improvements they have undergone, we may well see several prestigious manufacturers using these systems in the medium term.