Class D amplifier: higher volume and power efficiency

Class A and B amplifier circuits are true analog amplifier circuits, but their efficiency is relatively low, 50% and 78.5% respectively. Especially when used as a power amplifier, the efficiency directly affects the volume of the power supply and the heat sink of the power amplifier stage. In order to improve the efficiency, the class D amplifier circuit adopts modulation switch and frequency selection filtering technology, which increases the efficiency of the amplifier circuit to more than 90%. Therefore, from the perspective of the transistor working area, it is actually in a switching state.

There are two types of Class D amplifier circuits: voltage switching type and current switching type. It is not easy to explain the working principle of Class D amplifier circuit in a few words. Here is a brief introduction to the design steps of Class D amplifier:

1. Consider the operating frequency;

2. Determine the output power;

3. Working reliability design, mainly considering the design of additional protection circuit;

4. Power supply method and modulation issues.

1. Class A amplifier refers to an amplifier in which current flows continuously through all output devices. This amplifier avoids the nonlinearity caused by device switching. As long as the bias and dynamic range are properly controlled, it can be considered a good linear amplifier from the perspective of distortion. Class A is essentially a single emitter follower with an active emitter load to achieve appropriate current discharge. When this type is used as an output stage, it is necessary to figure out how low the impedance to be driven is before starting the design.

2. Class B amplifiers refer to a type of amplifier in which the device is on for 50% of the time. This type of amplifier can be said to be the most popular type of amplifier, and perhaps 99% of the amplifiers currently produced belong to this category.

3. Class D (Class D) amplifiers. This type of amplifier is characterized by intermittent switching of the device, with a frequency exceeding the audio frequency. The duty cycle of the signal can be controlled so that its average value can represent the instantaneous level of the audio signal. This situation is called pulse width modulation (PWM), and its efficiency is very high in theory. However, the actual difficulties are still very large, because it is not clear whether the 200kHz high-power square wave is a good starting point; from the perspective of distortion, in order to ensure the effectiveness of the sampling frequency, a low-pass filter with a steep cutoff frequency must be inserted between the amplifier and the speaker to eliminate most of the RF components, which requires at least 4 inductors (considering stereo), and the cost is naturally not low. In addition, in terms of frequency response, it can only guarantee a flat frequency response for a certain load impedance.

Why are Class D amplifiers popular today?

We will not elaborate on the detailed operation principle of the circuit here, but only explain the result characteristics. Class A amplification has the best signal fidelity (the voltage waveform is almost distortion-free), but it consumes a lot of power. Generally speaking, the power utilization rate is only 20% to 30%. For example, if 100W of power is supplied to a Class A amplifier (amplifier), only 25W of power is actually output to the speaker, and the remaining 75W is consumed during the operation of the amplification system. Moreover, this high energy consumption will also generate high waste heat, and it is necessary to install a thick heat sink on the amplification transistor to help dissipate heat. Although Class A has poor power utilization, signal integrity is its advantage, so it is still used in high-end professional audio. In order to enjoy the perfect sound quality without distortion, audiophiles will not care too much about consuming 3 times more power.

As for Class B amplification, its power utilization rate is higher, ideally up to 75%, but there is a problem of crossover distortion, one of the upper and lower waveforms will be partially cut off, and the full wave cannot be fully amplified. If used in an audio system, the sound will be obviously rough and deteriorated. As for Class C amplification, it is worse than Class B, and both the upper and lower waveforms are distorted, so it cannot be used in fax amplification applications, and is mostly used only in RF radio frequency systems for wireless communications.

Since Class A has a good waveform and high power consumption, while Class B has a good power consumption and a slightly worse waveform (between Class A and Class C), people came up with the idea of ​​truncating and using two Class B amplifier circuits at the same time to combine the remaining complete half-waves of the two to achieve the same full-wave effect as Class A. This is the so-called Class AB amplification (the operating circuit comes from two Class Bs, but the presentation effect is close to Class A), and the power consumption is still lower than Class A. If you want to achieve a system with an output amplification of 25W, Class A requires 100W as a whole, while Class AB only requires about 66W, so even the size of the heat sink can be simplified. Today, most consumer audio and audio-visual equipment uses Class AB.

Class D amplifier: striving for higher volume and power efficiency

Obviously, Class AB is a compromise design that takes into account both power consumption (including heat dissipation and volume) requirements and sound quality requirements. The same is true for Class D discussed in this article, but this time it focuses more on circuit volume and power utilization.

Here we need to explain the principle of Class D amplification in a little more detail. Unlike Class A, B, and C, Class D does not use the linear operating range characteristics of power transistors for amplification, nor does it use analog principles for amplification. Instead, it uses voltage comparison, pulse width modulation and other technologies for amplification. Therefore, some people call Class D amplification digital power amplification or digital power amplifier.

First, the Class D amplifier will compare the voltage of the original analog signal waveform with a triangle wave (or sawtooth wave) with a higher frequency than it (through a voltage comparator). In this way, the signal represented by the amplitude can be modulated into a signal represented by the pulse width. This is pulse width modulation (PWM). The PWM signal is then output to the gate of the MOSFET field effect transistor to control the on/off of the transistor. At the same time, the signal power is also amplified at this stage. Finally, the output end of the MOSFET is connected to the LC (inductor, capacitor) low-pass filter circuit to filter out the PWM carrier and restore the original signal waveform.

After understanding the principle, we can further understand the advantages and disadvantages of the Class D method. The disadvantage is that the amplification formed by the modulation process is bound to be somewhat different from the original signal, but it is still acceptable in general consumer music playback. Relatively speaking, Class D amplification provides more benefits, mainly extremely high power utilization, which is 100% in theory and often 80% to 90% in practice, which is better than Class AB. Therefore, the dependence on heat sinks can be further reduced, and even the heat sink can be completely abandoned at low power. In addition, the circuit area and volume occupied by its related components, as well as the simplicity of the circuit, are also superior to Class D.

To put it simply, Class D, like Class AB, is a compromise design that achieves significant power savings and size savings while still providing good sound quality. This is exactly the quality that today's handheld, mobile, and handheld devices desire most. Currently, the vast majority of mobile phones, digital walkmans, pocket TVs, PDAs, PMPs, etc., use Class D amplifiers for their audio components.

At this point, you may say: I don't design handheld devices, so I don't need to pay attention to power consumption and circuit size, so I can still use Class AB amplifiers. But in fact, there is a gradual trend of implementing Class D amplifiers for non-mobile applications. In the past, traditional analog CRT TVs were very large, and there was still ample space inside them to design audio circuits. Heat dissipation and power consumption were also considered together with the CRT system. However, now digital flat-panel LCD TVs and flat speakers are popular, and they strive to be short, light, and low in power consumption. At this time, it is difficult to continue using Class AB amplifiers, and Class D amplifiers are also needed. The same is true for in-car audio and car entertainment systems. Although the power of car batteries is more than that of handheld devices, it is less than that of household power sockets. In addition, the limitations of the car body and interior space also have the pressure to save power and volume. At this time, Class D amplifiers will also be considered.

In fact, the market has developed in this way. Handheld devices are the most urgent in need of Class D amplifiers. Therefore, the early Class D amplifiers were all low-power, that is, 1W to 3W. Later, medium-power (10W to 30W, tens of watts) began to appear, and now they have reached high power of 100W to 200W. Class D amplifiers are becoming more and more popular. They are no longer just needed for mobile designs, but are also needed for consumer products that are increasingly energy-saving, short and light.