Comparison of advantages and disadvantages of digital power amplifier and analog power amplifier

The basic circuit of the "digital power amplifier" is an already existing Class D amplifier (referred to as Class D amplifier in China). Previously, due to price and technical reasons, this amplifier circuit was only used in laboratories or high-priced test instruments. The technological developments in recent years have made the components of digital power amplifiers integrated into one or two chips, and the price has been declining. The theory proves that the efficiency of class D amplifier can reach 100%. However, no ideal switching element has been found so far, and it is inevitable that some power loss will occur. If the device used is poor, the loss will be greater. But no matter what, its amplification efficiency still reaches more than 90%.

Due to the advantages of power consumption and volume, digital power amplifiers are first used in car audio with limited energy and active subwoofers with higher requirements. 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 new high-sampling frequency audio source specifications such as SACD and DVDAudio, as well as audio The evolution of the system from stereo to multi-channel surround systems has accelerated the development of digital power amplifiers. In recent years, the price of digital power amplifiers has been declining, and patents in this area are also emerging.

Proportion of Class D output power and consumed power to Class AB power amplifier consumption

An amplifier that uses a low-frequency audio signal to modulate a fixed high-frequency pulse width is known as a class D amplifier and some are called a digital audio amplifier. Its biggest feature is its high efficiency (theoretically, it can reach 100%, and the actual 85% or more), using very small electronic devices can make a very powerful audio amplifier.

For low-power, 1W-3W power amplifiers, the power efficiency of Class AB power amplifiers and Class D power amplifiers is about AB = 15% and D = 75% under the same content. Under the condition of playing 1W music, Class AB power amplifier needs to consume 6.7W of power, but Class D power amplifier consumes only 1.33W under the same playback conditions. Therefore, the use of Class D power amplifiers can extend battery life by up to 5 times (6.7W / 1.33W). In addition to mobile phones, DVD, MP3 and PMP, there are some popular products such as iPods, mobile phones, and digital photo frames. In the case of medium power, that is, the power of 10W-30W power amplifiers with the main content of the same playback content as voice, the power efficiency of class AB power amplifiers and class D power amplifiers is AB = 25% and D = 80 %.

In the case of playing 10W voice, Class AB power amplifiers need to consume 40W of power, but Class D power amplifiers only consume 12.5 Watts under the same conditions. Therefore, the use of class D power amplifiers can reduce the cost of power supply by nearly 3 times (40W / 12.5W), and the 2.5W heat generated by class D power amplifiers can be handled by general power packaging and PCB design without additional heat sinks. In the case of high power output, that is, 100W-200W Class D digital power amplifiers will also have a place in car audio. Under this high power, Class D power amplifiers still have to use heat sinks, but the heat dissipation area and heat dissipation ratio are AB Class A power amplifiers need to be small. Due to high efficiency, Class D power amplifiers can have a longer use time without starting the car engine without consuming too much battery power. Class D power amplifiers have become the current car The main application products of audio.

The power cost and heat dissipation cost advantage of Class D digital audio power amplifier

Manufacturers do not use sound content as a standard when calculating power, but use traditional sine wave signals as input. For example, in terms of sine wave signal, the power efficiency of Class AB power amplifier and Class D power amplifier is about 45% and 80% respectively. If 15W & TImes; 2 is used to calculate the total supply power of Class D power amplifier is about 30W / 80% = 37.5W, the total supply power of Class AB power amplifier is about 30W / 45% = 66.7W, so using Class D power amplifier can Save nearly 30W of power. Because 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.

Digital power amplifiers have completely different working methods from traditional analog power amplifiers, so they overcome some of the inherent shortcomings of analog power amplifiers and have some unique characteristics.

1. Overload capability and power reserve

The overload capacity of digital power amplifier circuits is much higher than that of analog power amplifiers. The analog power amplifier circuit is divided into Class A, Class B or Class AB power amplifier circuits. During normal operation, the power amplifier tube works in the linear region; when overloaded, the power amplifier tube works in the saturation region, harmonic distortion occurs, and the degree of distortion increases exponentially. The sound quality quickly deteriorates. The digital power amplifier is always in the saturation area and the cut-off area when the power is amplified. As long as the power amplifier tube is not damaged, the distortion will not increase rapidly.

Comparison of overload distortion between full digital power amplifier and ordinary power amplifier

Because the digital power amplifier uses a switching amplifier circuit, the efficiency is extremely high, up to 75% ~ 90% (the analog power amplifier efficiency is only 30% ~ 50%), and it basically does not generate heat during work. Therefore, it does not have the static current consumption of the analog power amplifier. Almost all of the energy is reserved for audio output. There is no analog amplification and no negative feedback before and after it is added, so it has better "power" characteristics and good transient response. "Sense" is very strong.

2. Crossover distortion and mismatch distortion

Analog class B power amplifier zero-crossing distortion, which is caused by the nonlinear characteristics of the transistor at low current and caused by the distortion at the positive and negative crossings of the output waveform (the transistor will work in the cut-off region when there is a small signal, no current passes, resulting in output Severely distorted). The digital power amplifier only works in the switching state, and will not produce crossover distortion.

Analog power amplifiers have inconsistent push-pull tube characteristics that cause asymmetrical mismatch distortion in the output waveform. Therefore, when designing a push-pull amplifier circuit, the requirements for the power amplifier tube are very strict. The digital power amplifier has no special requirements for the matching of the switch tubes, and basically can be used without strict selection.

3. Matching of amplifier and speaker

Because the internal resistance of the power amplifier tube in the analog power amplifier is large, when matching speakers of different resistances, the working state of the analog power amplifier circuit will be affected by the size of the load (speaker). 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), and the resistance value (4 ~ 8Ω) relative to the load (speaker) is completely negligible, so there is no matching problem with the speaker.

4. Transient intermodulation distortion

Almost all analog power amplifiers use a negative feedback circuit to ensure their electro-acoustic index. In the negative feedback circuit, in order to suppress parasitic oscillations, a phase compensation circuit is used, which will cause transient intermodulation distortion. The digital power amplifier does not use any analog amplifier feedback circuit for power conversion, thus avoiding transient intermodulation distortion.

5. Sound image localization

For analog power amplifiers, there is generally a phase difference between the output signal and the input signal, and when the output power is different, the phase distortion is also different. The digital power amplifier uses digital signal amplification to make the output signal and input signal phase exactly the same, and the phase shift is zero, so the sound image positioning is accurate.

6. Upgrading

The digital power amplifier can obtain high power by simply replacing the switching amplifier module. High-power switch amplifying modules have low cost and broad development prospects in professional fields.

7. Production debugging

The simulation power amplifier has debugging problems at all levels of work points, which is not conducive to mass production. The digital power amplification part is a digital circuit, which can work normally without debugging, and is particularly suitable for large-scale production.

Third, the difference between digital power amplifier and "digital" power amplifier, "digital" power amplifier

The so-called "digital" amplifier only adopts the method of digital signal processing on the pre-stage. After the analog audio signal or digital audio signal is input, the existing digital audio processing integrated circuit is used to implement some such as sound field processing, digital delay, Reverberation and other functions, and finally through the analog power amplifier module for audio amplification, the interface of each module is in analog mode, the digital power amplifier is different.

Although various integrated circuit manufacturers have launched digital sound field processing, digital karaoke and digital Dolby decoding integrated circuits. However, since the current power amplifier can only receive analog audio signals, the interfaces of various integrated circuits are also mostly analog. This requires repeated analog-to-digital and digital-to-analog conversion, which introduces quantization noise and deteriorates the sound quality.

In addition to the interface for the speaker (this is because the speaker can only accept analog audio signals), the audio signal is processed in the form of a digital signal (including power amplification); for analog audio signals, it must be It can be processed only after being converted into a digital signal.

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

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