Class AB and Class D amplifiers
Class D amplifier ?It is also called digital power amplifier, which is characterized by high working efficiency and small size. Structure of Class D amplifier ??????The first part is the modulator. The simplest one is to complete the comparator with only one op amp. Put the original audio signal with a certain DC offset and put it on the positive input of the op amp, and then generate a triangle wave through the self-oscillation and add it to the negative input of the op amp. When the potential on the positive terminal is higher than the triangular wave potential on the negative terminal, the output of the comparator is high, otherwise the output is low. If the audio input signal is zero and 1/2 of the peak value of the DC offset triangle wave, the high and low levels of the output of the comparator last for the same time, and the output is a square wave with a duty ratio of half. When there is an audio signal input, during the positive half cycle, the comparator output high level time is longer than the low level, the duty cycle of the square wave is greater than one half; during the negative half cycle, due to the DC bias, so the comparison The level of the positive input of the device is still greater than zero, but the time when the amplitude of the audio signal is higher than the amplitude of the triangular wave is greatly reduced, and the duty cycle of the square wave is less than half. In this way, the waveform output by the comparator is a waveform whose pulse width is amplitude modulated by the audio signal, which is called PWM (Pulse Width Modulation) or PDM (Pulse Duration Modulation) waveform. Audio information is modulated into a pulse waveform. ?????The second part is the class-D power amplifier, which is a pulse-controlled high-current switching amplifier that turns the PWM signal output by the comparator into a high-voltage, high-current high-power PWM signal. The maximum power that can be output is determined by the load, the supply voltage, and the current allowed by the transistor. ?????The third part needs to restore the sound information in the high-power PWM waveform. The method is simple and only requires a low-pass filter. However, due to the large current at this time, the resistance of the low-pass filter of the RC structure will consume energy and cannot be used, and an LC low-pass filter must be used. When a pulse with a duty ratio greater than one-half comes, the charging time of C is greater than the discharge time, and the output level rises; when a narrow pulse comes, the discharge time is long, and the output level falls, which is exactly the same as the amplitude change of the original audio signal Consistent, so the original audio signal is recovered. The output can be driven by H-bridge Because the output end is PWM output, many of them contain high frequency components, and the frequency of PWM is usually 50k-1MHz Therefore, part of the filtering process needs to be added to the general output terminal, and the typical value of the output filter inductance is 10uH-50uH. And a power amplifier with a slightly higher power usually adds a Schottky diode to the power supply and ground at the output end to clamp the overshoot voltage caused by inductive load The principle is basically as above Digital power amplifier layout ??The ground wire is more important than the power cord. The main method to overcome electromagnetic interference is the design of ground wire. The wiring of the ground wire is particularly particular. Usually, the single-point grounding method is used to separate the analog ground, the digital ground and the high-power device ground, and finally all are collected to the power ground. The power amplifier ground wire structure includes system ground, chassis ground, digital ground and analog ground. The design principles of the ground wire are: (1) Separate digital ground and analog ground. The power amplifier has both a logic circuit and a linear circuit, they should be separated as much as possible, connected to the ground wire of the power supply end, and the ground area of ??the linear circuit should be increased as much as possible. The ground of analog audio should use single-point parallel ground as much as possible. (2) Correctly select single-point grounding and multi-point grounding. The analog part of the power amplifier has a low operating frequency, its wiring and the inductance between the devices have little effect, and the circulating current formed by the grounding circuit has a great influence on the interference, so a single point of grounding should be used. However, when the working frequency of the digital part is greater than 10MHz, the impedance of the ground wire becomes very large. At this time, the impedance of the ground wire should be reduced as much as possible, and the ground should be grounded at multiple points nearby. When the operating frequency is between 1MHz and 10MHz, if a single point is used for grounding, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise the multi-point grounding method should be used. Due to the influence of harmonics, the digital part of the power amplifier adopts multi-point grounding. (3) Make the ground wire form a closed loop. In the digital power amplifier PCB, designing the ground wire as a closed loop can significantly improve the noise resistance. The reason is that there are many power-consuming components in the circuit, and due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground wire, causing a reduction in the ability to resist noise.If the ground is formed into a loop, the potential difference will be reduced , Improve the anti-noise ability of the power amplifier circuit.