1 Introduction
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Audio amplifiers have been around for a century, and in recent years, electronic products are rapidly becoming thinner and portable. Class D amplifiers with good sound quality, high power efficiency, and low heat generation have become the market demand. And because of the low power consumption and low heat generation of Class D amplifiers, it is increasingly recognized by the market that is increasingly emphasizing environmental protection. At the same time, the working hours of portable electronic devices have always been the most important performance indicators that manufacturers are pursuing. The new filterless Class D amplifiers are replacing the previously fixed Class AB devices at a power level of several watts. Compared with the traditional linear amplifiers, the use of Class D amplifiers does not affect the sound quality of audio signals, but it can realize the miniaturization of portable products. Therefore, the market demand for thinner and portable electronic products has led to the development of traditional amplifiers to digital amplifiers. Conversion. Simply put, three generations of Class D amplifier designs have appeared in history:
The first generation of the example was TacTMillennium, designed by Toccata, which confirmed the concept of a Class D amplifier, but the technology did not provide sufficient performance, which made the first generation of Class D amplifiers move toward practicality.
The second generation Class D amplifier combines a PWM signal for the analog source signal with an integrated output stage and an off-chip filter. These amplifiers require complex front-end functions such as source selection, volume, balance, and tone control, and these additional features add extra complexity. But first of all, this generation of amplifiers has become affordable, and secondly, it has approached or even surpassed Class AB amplifiers in terms of low-power performance, thus achieving certain applications.
The third generation is that in the recent past, existing Class D digital amplifiers have improved over previous technologies, and they have made significant improvements in sound quality, packaging, performance, price, and core technology. To produce accurate audio, the input transistors need to work equally well on both ends of the dynamic range to help accurately achieve accurate power distribution. Improve audio output with a simple yet powerful internal control logic system with an additional set of input transistors that allow for finer control of audio signal input. Finally, the new architecture technology cannot be ignored.
2 Basic structure of Class D amplifier
The circuit of the class D amplifier is divided into three levels: the input switching stage, the power amplification stage, and the output filtering stage.
Class D amplifiers operate in pulse-width modulated (PWM) mode when switched. The PWM can be used to convert the audio input signal into a high frequency switching signal. The audio signal is compared with the high frequency triangular wave through a comparator. When the voltage of the inverting terminal is higher than the voltage of the non-inverting terminal, the output is low; when the voltage of the inverting terminal is lower than the voltage of the non-inverting terminal, the output is high.
In a class D amplifier, the output of the comparator is connected to a power amplifier circuit that uses a metal oxide field effect transistor (MOSFET) instead of a bipolar transistor (BJT) because:
(1) The power MOSFET is a high input impedance, voltage controlled device, and the BJT is a low impedance, current controlled device.
(2) From the driving circuit of the two, the driving circuit of the power MOSFET is relatively simple. The BJT may require up to 20% of the rated collector current to ensure saturation, while the MOSFET requires a much smaller driving current, and usually It can be driven directly by CMOS or open collector TTL driver circuits.
(3) The switching speed of the MOSFET is relatively fast. It is a majority carrier device with no charge storage effect and can operate at a higher speed.
(4) The MOSFET has no secondary breakdown failure mechanism. The higher the temperature, the stronger the endurance and the lower the possibility of thermal breakdown. He also offers better performance over a wide temperature range.
(5) The MOSFET has parallel operation capability and has a positive temperature coefficient of resistance. Higher temperature devices tend to direct current to other MOSFETs, allowing parallel circuit configurations. Moreover, the parasitic diode formed between the drain and source of the MOSFET can act as a clamping diode, which is especially useful in inductive load switches.
The FET has two modes of operation, switch mode or linear mode. The so-called switch mode means that the device acts as a simple switch that switches between on and off states. The linear mode of operation refers to the linear portion of the device operating in a characteristic curve, but this is not necessarily the case. "Linear" herein refers to the MOSFET's continuous state of operation, where leakage current is a function of the voltage applied between the gate and source. The difference between his linear mode of operation and the mode of operation of the switch is that in a switching circuit, the leakage current of the MOSFET is determined by external components, but not in linear circuit design. Class D amplifiers require two MOSFETs that can be fully turned on or off in a very short period of time. When a MOSFET is fully turned on, its tube voltage drop is low; when the MOSFET is completely turned off, the current through the tube is zero. The switching speed of the two MOSFETs operating in the on and off states is very fast, so the efficiency is extremely high, and the generated heat is very low, so the class D amplifier does not require a heat sink.
3 pulse width modulation (PWM)
There is an important conclusion in the sampling control theory: when narrow impulses with equal impulses and different shapes are added to the link with inertia, the effect is basically the same. The PWM control technology is based on this conclusion, and controls the on and off of the semiconductor switching device, so that the output end obtains a series of pulses of equal amplitude and unequal width, and these pulses are used instead of sine waves or other places. The waveform you need. Modulating the width of each pulse according to a certain rule can change the output voltage of the inverter circuit or change the output frequency. The biggest difference between the class D digital audio power amplifier and the above various types of analog power amplifiers is that it is not based on a linear amplified audio signal, but a digital signal amplification technique based on the principle of amplifying a digital signal. The Class D digital power amplifier first converts the analog audio signal into a pulse width modulation (PWM) signal, as shown in Figure 1.
In PWM conversion, A/D (analog/digital) conversion is performed at a sampling frequency of 44.1 kHz or 48 kHz and a quantization rate of 8 b or 16 b (that is, a readout scale of an analog signal amplitude value), and then the PWM number is applied. The signal is amplified with high efficiency (Class D amplification). Since the information of the audio signal is all included in the variation of the width of the pulse, it is independent of the amplitude variation of the pulse. Therefore, the analog audio signal can be demodulated by using a low-pass filter with a cutoff frequency of 30 to 40 kHz. Figure 2 is a schematic diagram of a class D digital power amplifier. The PCM signal input directly for each digital sound source is also provided with a PCM/PWM two-pulse code conversion device.
In order to adapt to the pulse code modulation (PCM) digital signal input directly from a digital sound source such as a CD disc, the digital power amplifier is provided with a PCM conversion to PWM modulation conversion device. The power utilization of the class D digital power amplifier can reach more than 80%, and its delay (phase shift) is about 1/6 of that of the analog power amplifier, but the demodulated audio signal has a large distortion.
4 circuit design of class D amplifier
4.1 Composition of Class D Amplifier
The architecture of Class D amplifiers is symmetrical and asymmetrical. The Class D amplifiers discussed here are for portable applications that are very sensitive to power and volume. Therefore, a full-bridge symmetrical amplifier is used to make full use of its single Features of power supply and system miniaturization. The class D amplifier is generally composed of an integrator, a PWM circuit, a switching power amplifier circuit and an output filter. The block diagram is shown in FIG.
He uses a fixed-frequency PWM circuit composed of a comparator and a triangular wave generator to modulate the triangular wave with the amplitude of the input audio signal to obtain a square wave whose duty cycle varies with the amplitude of the audio input signal, and drives the upper and lower phases in opposite phases. The power tube of the bridge arm turns off the power tube when it is turned on, and then converts the square wave into an audio signal through the output filter to push the speaker to sound. Balanced outputs are achieved with a full-bridge Class D amplifier, which improves the output filter characteristics of the amplifier and reduces interference. The peak-to-peak voltage of the full-bridge circuit load is close to twice the supply voltage and can be powered from a single supply. In the implementation, usually the two output pulse phase is opposite. The output voltage is superimposed and larger. After the low-pass filter, there is still a large load current. Especially when the filter design is not good, the current flowing through the load will be larger, resulting in large load loss and lowering. Amplifier efficiency.
4.2 Improved Class D Power Amplifier Circuit Design
4.2.1 Pulse Width Modulation (PWM) Design
The H full bridge circuit is shown in Figure 4.
Adopt improved PWM modulation scheme: PWM output is in phase with 2 outputs when zero signal input, the voltage on the load is 0. When the input signal is positive, the duty ratio of the first output pulse is greater than 50%, and the output pulse of the other is The duty cycle is less than 50%. When the input signal is negative, the duty cycle of the first output pulse is less than 50%, and the duty cycle of the other output pulse is greater than 50%.
When one signal is determined, the second output of the improved PWM scheme differs from the second output of the conventional PWM scheme by half a cycle. This PWM method can suppress the static loss at the time of zero signal input, thereby contributing to the improvement of the amplifier efficiency.
4.2.2 Improved simulation method for full-bridge PWM scheme
In the full-bridge Class D power amplifier structure with improved PWM scheme, the PWM controller uses the audio signal as the reference signal to modulate the high-frequency (300 kHz) triangular wave to obtain a pulse signal whose pulse width varies with the audio amplitude. The comparator can be realized by a high-speed comparator, and the inverting input terminal is connected to the high-frequency triangular wave, and the non-inverting input terminal is respectively connected to the audio signal of the opposite phase of the input voltage amplifier output. When the input audio signal voltage is 0, two pulse waves with a duty ratio of 50% are output; when the input signal voltage is positive, one output is a pulse wave with a duty ratio greater than 50%, and the other output is a duty cycle smaller than 50% of the pulse wave; when the input signal voltage is negative, the opposite is true. This program has been well applied in the design of high-efficiency audio power amplifiers for the national undergraduate electronic design competition. Practice has shown that the system performs well and reduces the filter performance requirements.
4.2.3 Improved digital implementation of full-bridge PWM scheme
The improved full-bridge PWM scheme is implemented by the digital method based on CPLD. The block diagram of the PWM converter is shown in Figure 5.
When different pulse width data D8 to D0 are input, the converter outputs PWM1 and PWM2 signals of different pulse widths. The clock signal is obtained by the 512-ary counter with the carry pulses C0 and Cy2 (delay C0 half cycle) to determine the frequency of the PWM signal. The rising edge sets the D flip-flop Q end to 1; the value of the 512-ary counter is 0. Start incrementing. When the count value is equal to the input pulse width, the comparator outputs a negative pulse and clears the trigger C to zero. This achieves the output of the PWM signal corresponding to the input pulse width data. In the implementation of the circuit, the regularity of the two PWM outputs can be utilized to reduce the required circuit resources. Split the 9 b value comparator into 8 b comparator and 1 b comparator, so that the 2 PWM outputs can share the 8 b comparator, but the comparison of the high comparator is different, because the clearing time of PWM2 is clearer than PWM1. 0 lags half a cycle.
5 Conclusion
Class D amplifiers are a very promising development direction for audio players today. He is better suited to the development of high efficiency and low distortion of power amplifiers for portable electronic audio equipment.
This design uses a full-bridge improved PWM scheme to achieve Class D amplifiers with high efficiency and reduced filter requirements. The improved full-bridge PWM scheme is implemented by CPLD, and combined with DSP to realize serial-to-parallel conversion, digital interpolation and noise shaping, etc., can realize high-fidelity audio amplifier. The designed digital power amplifier can directly amplify the audio signal output by the digital audio source. Provides a complete solution for the integration of digital audio and power amplification.
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