"Audio power amplifiers are everywhere. Where there is music, there will be audio amplifiers. Generations of electronic engineers have worked hard and spread wisdom in this field. The audio amplifier is to reproduce the sound signal truly and efficiently on the speaker or headset with a certain volume and power. Authenticity and efficiency have always been the driving force of technological progress in the field of power amplifier. The audio frequency range is about 20Hz ~ 20kHz, and the amplifier must have good frequency response within this frequency range; According to different output power, the amplifier can be subdivided into different output power specifications, such as from a few hundred MW headphone amplifier to a low-power amplifier applied to portable devices of about 2W, and then to a medium power and high-power amplifier for home audio of 10W and 20W.
Class AB amplifier is the most commonly used audio power amplifier. It is different from the earliest Class A or class B amplifiers in that class AB amplifiers adopt complementary output stages, which can prevent crossover distortion by adding a certain bias current at the output end, so as to provide good sound quality on the basis of improving the efficiency of class a amplifiers. However, it is still a linear amplifier. The output stage transistor works in the linear amplification state to provide instantaneous continuous output current for the load. When the output transistor works in a linear state, the voltage drop between the source and drain is very large. The instantaneous power consumption of the output transistor can be expressed as VDS & times; IDS, a considerable amount of energy will be consumed on the output transistor and converted into heat. Even the most effective class AB amplifier, its efficiency is usually only about 60 ~ 70%.
Class D amplifier adopts a new working mode, its output stage transistor works in the switching state, and the output terminal switches between positive power supply and negative power supply to produce a series of voltage pulses. When the output transistor is not turned on, the output stage does not consume any current. The on resistance of the output stage transistor is usually 0.2 Ω, so the VDS is very low when on, and the power consumption on the transistor (VDS & times; IDS) is also small. The switching mode of low power consumption makes the class D amplifier have significant advantages in many applications, such as saving the metal area for heat dissipation on the printed circuit board, saving the special heat sink, and prolonging the battery life of portable devices.
When class D amplifier works, the input audio signal must first be modulated into a series of voltage pulses. Nowadays, there are many methods to modulate the input audio signal into voltage pulses. The most commonly used technology is pulse width modulation (PWM). In principle, PWM modulation generates a series of voltage pulses with the same frequency as the carrier by comparing the input audio signal with the triangular wave of fixed carrier frequency. The duty cycle of PWM pulse is directly proportional to the amplitude of audio signal in the carrier cycle. The change of duty cycle includes the change of input audio signal. Since the carrier frequency is usually more than 10 times that of the audio signal, PWM modulation has no distortion in the audio range.
Usually, the output of class D amplifier is connected to the speaker through an LC filter. LC filter is used to filter the high-frequency part of voltage pulse signal and reconstruct audio signal. We know that the speaker itself has a certain frequency response range, and the human ear is basically insensitive to signals outside the audio frequency range. From this point of view, the LC filter can be deleted from the class D amplifier. Let's see what happens when the LC filter is deleted. When there is no audio input signal, the comparator will generate a series of pulses with 50% duty cycle, which are directly added to both ends of the speaker, filtered inside the speaker and reconstructed to the DC state of zero input. This process will produce great power consumption on the resistive load of the loudspeaker and reduce the efficiency of the amplifier.
The three-level H-bridge output structure can completely eliminate the LC filter. In the three-level H-bridge output structure, the input audio signal and its inverse signal are compared with the triangular wave at the same time to generate two different voltage pulses added to the two half bridges of the H-bridge. The differential pulse of two series of voltage pulses is the voltage pulse actually added to both ends of the speaker. Let's also look at the situation when there is no audio input signal. When there is no input signal, the two series of voltage pulses are in phase and have a duty cycle of 50%, so there will be no differential pulse and no power loss. When the forward direction of the input signal becomes larger, a series of forward differential pulses will be generated, and there will be forward current flowing through the speaker; When the input signal becomes larger in reverse, a series of reverse differential pulses will be generated, and there will be reverse current flowing through the speaker. The current on the speaker is generated as needed when the input signal changes, and no excess power consumption will be lost.
In the actual circuit, the output stage and drive circuit always contain various unsatisfactory factors, resulting in nonlinear distortion of the amplifier output. First, the output stage transistor has a very low on resistance. If the upper and lower output stage transistors are turned on at the same time, a low resistance path from VDD to VSS will pass through the transistor, resulting in a large impulse current. When the voltage pulse changes from high to low or from low to high, it is very easy to turn on the output stage transistors at the same time. Therefore, it is important to avoid simultaneous conduction of output stage transistors. To prevent this, both transistors must be forcibly turned off before one transistor is turned on. The time interval when both transistors are turned off is called dead time. The dead time will change the duty cycle of the original PWM pulse and cause distortion of the amplifier output. The shortest dead time is usually used in the design, which can not only prevent the output transistors from conducting at the same time, but also reduce the distortion as much as possible. In addition, the mismatching of the rise time and fall time of the output pulse and the mismatching of the parameters of the transistor driving circuit of the output stage will also change the duty cycle of the original PWM pulse, which will also produce distortion on the output of the amplifier.
Let's take another look at what happens on the power supply when playing music. The power supply is directly connected to the speaker through an output stage transistor with a resistance of 0.2 Ω. The actual power supply always has a certain internal resistance. When playing music, the power supply will produce a ripple twice the signal frequency, and the ripple amplitude will vary with the internal resistance of the power supply. The ripple is directly coupled to the speaker through the output stage transistor with 0.2 Ω resistance, and even audio noise is generated in the output signal. From the circuit point of view, this audio noise comes from poor power rejection ratio performance, that is, through power coupling, the power ripple will produce high-order even order distortion on the amplifier output.
In order to reduce the distortion, a method similar to class AB amplifier can be used to improve the circuit performance through closed-loop negative feedback. In principle, the closed-loop negative feedback is to shape the error noise between the input and output of the amplifier, attenuate the distortion in the audio range and improve the linearity of the amplifier by adding a noise shaping filter in front of the comparator. This error comes not only from the output transistor and driving circuit, but also from the direct coupling of power ripple, so it not only improves the linearity of the amplifier, but also improves the power rejection ratio of the amplifier. Similar to linear amplifiers, noise shaping filters actually provide a large open-loop gain for the loop in the audio range. The optimized closed-loop class D amplifier can achieve good performance of PSRR > 60dB and thd < 0.1%.
Pt5306 / 26 are two high-performance class D amplifiers recently launched by China Resources Siwei technology. Pt5306 is a single channel 2.5W class D amplifier, and pt5326 is a dual channel 2.1W class D amplifier. Both products adopt closed-loop negative feedback three-level H-bridge output PWM modulation structure, and the output can be directly connected to the speaker without LC filter. By carefully designing the internal noise shaping filter, good performance is achieved. From the typical test data, the performance parameters are by no means inferior to foreign products of the same type. For example, under 3.6V power supply voltage, 8 ohm load and 0.5W output power, 1kHz frequency thd + n is 0.1% and 500Hz frequency thn + n is 0.06%. Four output transistors with 0.25 Ω on resistance are internally integrated to form an H-bridge output stage. Under 5V power supply voltage, 8 ohm load and 1W output power, the efficiency can reach more than 88%. These two products can be widely used in various portable multimedia devices, such as mobile phones, MP3, MP4, digital photo frames and low-power USB portable speakers.
The pt5306 / 26 contains a complete "crackle" suppression scheme to turn off or wake up the amplifier quietly. In terms of protection circuit, pt5306 / 26 also carefully integrates overheating and overcurrent protection. Although the power consumption of the output stage of class D amplifier is much lower than that of linear amplifier, if the amplifier provides very high power for a long time, it will still cause the device to overheat. To prevent the danger of overheating, when the temperature exceeds the thermal shutdown safety threshold, the output stage is turned off and maintained until the device cools down. In addition, if there is a short circuit between the two output terminals, a huge current will be generated. If protective measures are not taken, the large current will damage the output stage transistor. Therefore, it is necessary to add a maximum current limit on the output transistor. If the output current exceeds the safety threshold, the output stage will turn off automatically.
In the application of amplifier, the input amplitude is often too large. This condition will cause the amplifier output to reach the limiting state. For the closed-loop modulator in the limiting state, even if the input signal is lower than the overload input amplitude, the modulator output still lags behind the input signal and is in the output overload state for a long time. With this hysteresis, additional nonlinearity will be introduced at the output of the amplifier when a large amplitude signal is input. Pt5306 / 26 can quickly keep up with the change of input signal after the modulator overload condition disappears through the carefully designed modulator anti overload circuit., Read the full text, technical section
Hardware simulation and debugging based on Embedded System
Brief introduction of ms1830 chip HDMI to CVBS function analysis
The function of ms9282 chip is VGA to HDMI
Explain the swap solution brought by integrated transceiver
How to use DFT app for hardware simulation“
Our other product:
