Compare the advantages and disadvantages of digital amplifiers and analog amplifiers

The basic circuit of "digital power amplifier" is the long-existing Class D amplifier (called Class D amplifier in China). In the past, due to price and technical reasons, this amplifier circuit was only used in laboratories or high-priced test instruments. In recent years, technological development has enabled the components of digital power amplifiers to be integrated into one or two chips, and the price has been continuously falling. Theoretically, the efficiency of Class D amplifiers can reach 100%. However, the ideal switching element has not been found so far, and some power loss is inevitable. If the device used is poor, the loss will be greater. But in any case, its amplification efficiency is still above 90%.

Due to the advantages of power consumption and size, digital amplifiers were first used in car audio with limited energy and active speakers with high requirements for heavy bass. With the rapid development of consumer products such as DVD home theaters, mini audio systems, set-top boxes, personal computers, LCD TVs, flat-panel displays and mobile phones, especially the emergence of some new audio source specifications with high sampling frequencies such as SACD and DVDAudio, and the evolution of audio systems from stereo to multi-channel surround systems, the development of digital amplifiers has been accelerated. In recent years, the price of digital amplifiers has been declining, and patents in this area have emerged in an endless stream.

The ratio of class D output power and power consumption to class AB power amplifier consumption

An amplifier that uses a low-frequency audio signal to modulate a fixed high-frequency pulse width is called a Class D amplifier or a digital audio amplifier. Its greatest feature is its extremely high efficiency (theoretically up to 100%, and actually over 85%), and it can produce a very high-power audio amplifier using very small electronic devices.

For low-power, i.e. 1W-3W power amplifiers, under the same playback content, the power efficiency of class AB power amplifiers and class D power amplifiers are approximately AB=15% and D=75% respectively. When playing 1W music, the class AB power amplifier needs to consume 6.7W of power, but the class D power amplifier only consumes 1.33W under the same playback conditions. Therefore, using a class D power amplifier can extend the battery life by 5 times (6.7W/1.33W). In addition to mobile phones, DVDs, MP3s, and PMPs, there are also some popular products such as iPods, mobile phones, and digital photo frames that use low power. In the case of medium power, i.e. 10W-30W power amplifiers, under the same playback content dominated by voice, the power efficiency of class AB power amplifiers and class D power amplifiers is AB=25% and D=80% respectively.

When playing 10W voice, the AB class power amplifier needs to consume 40W of power, but the D class power amplifier only consumes 12.5W under the same conditions. Therefore, the use of the D class power amplifier can reduce the cost of the power supply by nearly 3 times (40W/12.5W), and the 2.5W of heat generated by the D class power amplifier can be handled by the general power package and PCB design without the need for an additional heat sink. In the case of high power output, that is, 100W-200W D class digital power amplifiers will also have a place in car audio. Under this high power, the D class power amplifier still cannot avoid the use of heat sinks, but the heat dissipation area and heat dissipation are smaller than those required by the AB class power amplifier. Due to its high efficiency, the D class power amplifier can be used for a longer time without starting the car engine without consuming too much battery power. The D class power amplifier has become the main application product of car audio.

Mr. Lao Duo from the Electronic Production Network believes that since the Class D digital audio amplifier technology is very mature, he plans to design some relatively representative Class D digital audio amplifier circuits and amplifier-specific audio power supplies for everyone to learn and make within a period of time.

Power cost and heat dissipation cost advantages of Class D digital audio power amplifier

When calculating power, manufacturers do not use the sound content as the standard, but continue to use the traditional sine wave signal as input. For example, in terms of sine wave signals, the power efficiency of class AB power amplifiers and class D power amplifiers is approximately 45% and 80% respectively. If the total supply power of the class D power amplifier is calculated as 15W×2, it is approximately 30W/80%=37.5W, and the total supply power of the class AB power amplifier is approximately 30W/45%=66.7W, so using a class D power amplifier can save nearly 30W of power. Since the power supply of the power amplifier is provided by the power supply device, the cost of the power supply device of the class D power amplifier will be greatly reduced. At the same time, the cost of the heat sink of the power supply device and the power amplifier heat sink and the cost of the circuit board space are greatly reduced.

Since digital amplifiers work in a completely different way from traditional analog amplifiers, they overcome some of the inherent shortcomings of analog amplifiers and have some unique characteristics.

1. Overload capacity and power reserve

The overload capacity of digital power amplifier circuits is much higher than that of analog power amplifiers. Analog power amplifier circuits are divided into Class A, Class B or Class AB power amplifier circuits. When working normally, the power amplifier tube works in the linear region; when overloaded, the power amplifier tube works in the saturation region, harmonic distortion occurs, the degree of distortion increases exponentially, and the sound quality deteriorates rapidly. However, digital power amplifiers are always in the saturation region and cutoff region during power amplification. As long as the power amplifier tube is not damaged, the distortion will not increase rapidly.

Comparison of overload distortion between full digital amplifier and ordinary amplifier

Since the digital amplifier uses a switching amplifier circuit, the efficiency is extremely high, up to 75%~90% (the efficiency of analog amplifiers is only 30%~50%), and it basically does not generate heat during operation. Therefore, it does not have the static current consumption of analog amplifiers, and almost all energy is reserved for audio output. In addition, there is no analog amplification and no negative feedback before and after, so it has better "power" characteristics, good transient response, and a strong "bursting feeling".

2. Crossover distortion and mismatch distortion

Analog Class B amplifiers have zero-crossing distortion, which is caused by the nonlinear characteristics of transistors at low currents, causing distortion at the intersection of the positive and negative output waveforms (when the signal is small, the transistor will work in the cutoff region, no current will pass, resulting in severe output distortion). Digital amplifiers only work in the switching state and will not produce crossover distortion.

Analog power amplifiers have inconsistent push-pull tube characteristics, which causes mismatch distortion in the output waveform, so when designing push-pull amplifier circuits, the requirements for power amplifier tubes are very strict. Digital power amplifiers have no special requirements for the pairing of switch tubes, and basically do not require strict selection before use.

3. Matching of amplifier and speaker

Since the internal resistance of the power amplifier tube in the analog power amplifier is relatively large, when matching speakers with different resistance values, the working state of the analog power amplifier circuit will be affected by the size of the load (speaker). However, the internal resistance of the digital power amplifier does not exceed 0.2Ω (the internal resistance of the switch tube plus the internal resistance of the filter), which can be completely ignored relative to the resistance value of the load (speaker) (4~8Ω), so there is no matching problem with the speaker.

4. Transient intermodulation distortion

Analog power amplifiers almost all use negative feedback circuits to ensure their electroacoustic indicators. In the negative feedback circuit, in order to suppress parasitic oscillation, a phase compensation circuit is used, which will produce transient intermodulation distortion. Digital power amplifiers do not use any analog amplification feedback circuits in power conversion, thus avoiding transient intermodulation distortion.

5. Sound and image localization

For analog amplifiers, there is usually a phase difference between the output signal and the input signal, and the phase distortion is different when the output power is different. Digital amplifiers use digital signal amplification to make the output signal and the input signal phase completely consistent, with zero phase shift, so the sound and image positioning is accurate.

6. Upgrade

Digital amplifiers can achieve high power by simply replacing the switch amplifier module. High-power switch amplifier modules have low costs and broad development prospects in professional fields.

7. Production debugging

Analog power amplifiers have debugging problems at all levels of working points, which is not conducive to mass production. However, digital power amplifiers are mostly digital circuits, which generally do not require debugging to work normally, and are particularly suitable for large-scale production.

3. The difference between digital power amplifier, "digital" power amplifier and "digital" power amplifier

The so-called "digital" amplifier only uses digital signal processing in the pre-stage. After the analog audio signal or digital audio signal is input, the existing digital audio processing integrated circuit is used to realize some functions such as sound field processing, digital delay, reverberation, etc., and finally the audio is amplified through the analog power amplifier module. Its typical circuit block diagram is shown in Figure 2. As can be seen from Figure 2, the interfaces of its various modules are all in analog mode. The general principle block diagram of the digital sound field processing module is shown in Figure 3.

Although various integrated circuit manufacturers have launched digital sound field processing, digital karaoke and digital Dolby decoding integrated circuits, since the current power amplifiers can only receive analog audio signals, the interfaces of various integrated circuits are mostly analog, which requires repeated analog/digital and digital/analog conversions, which will introduce quantization noise and deteriorate the sound quality.

Except for the interface for the speaker (this is because currently speakers can only accept analog audio signals), the audio signals are processed as digital signals inside the amplifier (including power amplification); for analog audio signals, they must be converted into digital signals before they can be processed.

The introduction of digital amplifiers in an era where digital audio is already available is likely to have a significant impact on the development of audio technology.