Due to the advantages of power consumption and volume, digital power amplifier is first applied to car audio with limited energy and active subwoofer speakers with high requirements. With the rapid development of consumer products such as DVD home theater, mini-speaker system, set-top box, personal computer, LCD TV, flat panel display and mobile phone, especially the appearance of some new sound source specifications with high sampling frequency such as SACD and DVDAudio, and the evolution of sound system from stereo to multi-channel surround sound system, the development of digital power amplifier has been accelerated. In recent years, the price of digital power amplifier has been declining, and patents in this field have emerged one after another.
Proportion of class D output power and consumption power to class AB power amplifier consumption
An amplifier with fixed high frequency pulse width modulated by low frequency audio signal is called a class D amplifier, and some people also call it a digital audio amplifier. Its greatest feature is its high efficiency (up to 65,438+000% in theory and over 85% in practice), and a very high-power audio amplifier can be manufactured by using very small electronic devices.
For low-power amplifiers, namely 1W-3W, the power efficiencies of class AB and class D amplifiers are about AB= 15% and D=75% respectively under the same broadcast content. In the case of playing 1W music, the class AB amplifier needs to consume 6.7W, while the class D amplifier only consumes 1.33W under the same playing conditions. Therefore, using class D amplifier can prolong the service life of the battery by 5 times (6.7W/ 1.33W). Besides mobile phones, DVD, MP3 and PMP, there are also some popular products, such as iPod, mobile phones and digital photo frames. Then in the case of medium power, that is, the power amplifier of 10W-30W, the power efficiency of class AB amplifier and class D amplifier is AB= 25% and D=80% respectively. In the case of playing 10W voice, class AB power amplifier needs to consume 40W, while class D power amplifier only consumes12.5w under the same conditions. Therefore, the power supply cost (40W/ 12.5W) can be reduced by nearly three times by using the class D power amplifier, and the 2.5W heat generated by the class D power amplifier can be handled by the general power supply package and PCB design, and no additional radiator is needed. In the case of high power output, that is, 100W-200W digital amplifier will also occupy a place in car audio. Under this high power, class D power amplifier will inevitably use heat sink, but the heat dissipation area and quantity are smaller than those required by class AB power amplifier. Because of its high efficiency, Class D power amplifier can be used for a long time without starting the automobile engine and will not consume too much battery power. Class d power amplifier has become the current car audio.
Teacher Lao Duo of Electronic Production Network thinks that because the technology of Class D digital audio amplifier is very mature, it is planned to design some representative circuits of Class D digital audio amplifier and special audio power supply for amplifier for everyone to learn and make.
Advantages of power supply cost and heat dissipation cost of class D digital audio power amplifier
When calculating power, the manufacturer does not take the sound content as the standard, but uses the traditional sine wave signal as the input. For sine wave signals, the power efficiency of class AB amplifier and class D amplifier is about 45% and 80% respectively. If 15W×2 is used to calculate the total power supply of class D amplifier is about 30W/80%=37.5W, and that of class AB amplifier is about 30W/45%= 66.7W, then using class D amplifier can save nearly 30W. 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 power device radiator and power amplifier radiator and the cost of circuit board space are greatly reduced.
Because the working mode of digital power amplifier is completely different from that of traditional analog power amplifier, it overcomes some inherent shortcomings of analog power amplifier and has some unique characteristics.
1. Overload capacity and power reserve
The overload capacity of digital power amplifier circuit is much higher than that of analog power amplifier. Analog power amplifier circuits are divided into Class A, Class B or Class AB power amplifier circuits, and the power amplifier tube works in the linear region during normal operation; After overload, the power amplifier tube works in the saturation region, resulting in harmonic distortion, doubled distortion and rapid deterioration of sound quality. However, digital power amplifier is always in saturation region and cutoff region during power amplification. As long as the power amplifier tube is not damaged, the distortion will not increase rapidly, as shown in figure 1.
Figure 1 Comparison of Overload Distortion between Full Digital Power Amplifier and Ordinary Power Amplifier
Because the digital power amplifier adopts switching amplifier circuit, the efficiency is extremely high, which can reach 75%~90% (the efficiency of analog power amplifier is only 30%~50%), and it basically does not generate heat when working. Therefore, it has no static current consumption of analog power amplifier, almost all the energy is reserved for audio output, and there is no analog amplification and negative feedback before and after, so it has better "dynamic" characteristics, good transient response and strong "explosive feeling".
2. Cross distortion and mismatch distortion
Analog Class B power amplifier is distorted at the zero-crossing point, which is caused by the nonlinear characteristics of the transistor at the positive and negative intersection of the output waveform under small current (when the signal is small, the transistor will work in the cut-off area and no current will pass, resulting in serious output distortion). However, the digital power amplifier only works in the on-off state and will not produce cross distortion.
Because the characteristics of push-pull transistors are inconsistent, there is mismatch distortion of asymmetric output waveform in analog power amplifier, so the requirements for power amplifier tubes are very strict when designing push-pull amplifier circuits. However, digital power amplifier has no special requirements for the matching of switch tubes, and it can be used basically without strict selection.
3. Matching of power amplifier and loudspeaker
Because of the large internal resistance of the amplifier tube in the analog power amplifier, the working state of the analog power amplifier circuit will be affected by the size of the load (speaker) when matching speakers with different resistance values. However, the internal resistance of digital power amplifier does not exceed 0.2 Ω (switch internal resistance plus filter internal resistance), and the resistance value (4 ~ 8 Ω) relative to the load (speaker) can be completely ignored, so there is no matching problem with the speaker.
4. Transient intermodulation distortion
Almost all analog power amplifiers use negative feedback circuits to ensure their electro-acoustic indicators. In the negative feedback circuit, the phase compensation circuit is used to suppress parasitic oscillation, which will produce transient intermodulation distortion. Digital power amplifier does not use any analog amplification feedback circuit in power conversion, thus avoiding transient intermodulation distortion.
5. Acoustic positioning
For analog power amplifiers, there is generally a phase difference between the output signal and the input signal, and the phase distortion is different when the output power is different. Digital power amplifier uses digital signal amplification, so that the phase of the output signal is exactly the same as that of the input signal, and the phase shift is zero, so the sound image positioning is accurate.
upgrade
Digital power amplifier can obtain high power by simply replacing the switching amplifier module. High-power switching amplifier module has low cost and broad development prospects in professional fields.
7. Production debugging
There are debugging problems at all working points of analog power amplifier, which is not conducive to mass production. Digital power amplifiers are mostly digital circuits, which can work normally without debugging, especially suitable for mass production.
Third, the difference between digital power amplifier and "digital" power amplifier
The so-called "digital" power amplifier only uses digital signal processing in the previous stage. After analog audio signal or digital audio signal is input, the existing digital audio processing integrated circuit is used to realize sound field processing, digital delay, reverberation and other functions. Finally, the audio is amplified by the analog power amplifier module. Its typical circuit block diagram is shown in Figure 2. As can be seen from Figure 2, the interface of each module is analog. The general principle block diagram of the digital sound field processing module is shown in Figure 3.
Fig. 2 composition block diagram of digital power amplifier circuit Fig. 3 principle block diagram of digital sound field processing module.
Although various integrated circuit manufacturers have introduced digital sound field processing, digital karaoke and digital Dolby decoding integrated circuits. However, at present, most power amplifiers can only receive analog audio signals, so the interfaces of integrated circuits are mostly analog, which requires repeated analog-to-digital and digital-to-analog conversion, which will introduce quantization noise and deteriorate the sound quality.
In all digital power amplifiers, audio signals are processed in the form of digital signals (including power amplification) except for the interface used for speakers (this is because speakers can only accept analog audio signals at present); Analog audio signals must be converted into digital signals before being processed.