Label: Fixed Bias Dynamic Range Compression
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If you talk about the seniors, it seems that there is no such thing as a "dynamic range compression" that creates distortions, but is praised. Of course, unless you are a fanatic audio "fan", you will never compromise on the quality of the sound.
More often, you will weigh the area, device cost, design cost and other factors, and then throw the olive branch to the dynamic range compression. The introduction of TI's first stereo Class D amplifier with dynamic range compression seems to be a matter of course.
The dynamic range we usually refer to is the difference between the strongest sound and the weakest sound. The unit is represented by "db" (ie 20Log10(P/Po)). The dynamic range of the general language is 20-40db, the dynamic range of songs and music is 40-60db, and the dynamic range of the symphony is 70-120db. The dynamic range of the audio amplifier must be greater than the dynamic range of the language and music.
"Want to sing and sing, and sing loudly" suddenly became popular in the north and south, and also ignited the portable market. Portable devices also require the amplifier to "sing loud", but this is limited by the speaker and power supply voltage. The higher the power, the greater the distortion, the lower the sound quality, and the smaller the size and lower the size. Power consumption is also a portable product that the industry is pursuing.
Therefore, during the design process, a high output volume is required throughout the life of the battery. For example, in the case of a lithium-ion battery, the voltage is converted between 3.0V and 4.2V. If the power is directly supplied, the sound of the new battery will be loud. When the battery is about to run out, the sound will gradually become smaller. In addition, you can also limit the maximum power consumption. Different audio sources (such as MP3, FM radio and other sources in mobile phones) signal input amplitude is not equal, which requires flexible control of output peripherals, such as headphones, speakers, etc., even the sensitivity of the two speakers is not the same. At the same time, it is necessary to maximize the sound level while ensuring a good signal to noise ratio.
Faced with these design challenges, the most simple way to maintain a sufficiently large sound output when the battery voltage changes is to provide a booster circuit. TI previously provided two products, TPA2014D1 and TPA2013D1, which provide constant voltage throughout this method. Maximum output power. Another way is the dynamic range compression that has been widely known for 20 years.
What is dynamic range compression?
As the name suggests, dynamic range compression is to artificially compress the dynamic range, that is, the small signal is amplified several times according to the demand, and the large signal is not amplified. The typical dynamic range of a normal CD is 96 db. In fact, the human ear processing, the noise has drowned the minimum signal, so the dynamic range that can be heard in a small portable device is 60 db (the dynamic range of human speech is only 20-40db) This extra dynamic range is useless, but it wastes system power consumption, accuracy, and makes it difficult for users to hear. And through dynamic compression, artificially control the signal, such as amplifying the small signal by 10 times, the large signal is not amplifying, and the dynamic range is from 1:1000 db to 1:100 db. Although this sacrifices the dynamic range, when the signal is a small signal, The relative signal strength does not become smaller. Zhang Hong, the business development manager of high-performance analog products in TI China, takes string and symphony as an example. If the dynamic range of the amplifier is 1 to 100db (100,000), and the dynamic range of the string is much smaller than this range, there is no distortion. When playing rock music, it will turn the violent drum gain into small music, and the person is not sensitive to the distortion of the drum, thus ensuring the user's appreciation of the music.
This dynamic range compression technology is mainly used for music appreciation and conference calls. "If you use your mobile phone or MP4 to learn English, the sound is too light and there are some details that can't be distinguished. If the sound is loud, you can distinguish it, but it will affect others. At this time, you need dynamic range compression," Zhang Hong said. one example.
TI is one of the early manufacturers to contact stereo amplifiers and has already accounted for approximately 40% of the Class D amplifier market share. The IMS Research study said that the Class D amplifier market will grow steadily in the next few years. "Because the mainstream mainstream portable consumer electronics products in the world have already adopted Class D amplifiers, there will be no large-scale mutations in market share," said Zhang Hongwei. Functionally, with the development of digital TV and digital broadcasting, digital audio sources will be more and more, and some pure digital amplifiers will gradually enter the market, but there will be no obvious changes in the next two or three years, he predicted.
State A, B, AB, D
What is the difference between TI's first stereo Class D amplifier with dynamic range compression and traditional A and B amplifiers? What is the secret behind such a naming convention?
Generally, the transistor power amplifier is composed of a triode, and the triode is formed by several groups of NP and NP structures. The structure is cut off when no voltage is applied, and only a bias voltage is applied and is higher than its threshold voltage (the silicon tube is 0.6). When V, the tube is 0.2V), the NP junction will be turned on and current will pass, and the transistor will start to work.
Class A power amplifiers set the forward bias at half the maximum output power, so that the power amplifier is also in full load operation when there is no signal input, so that the power amplifier is turned on and has current output during the entire signal period. Class A power amplifiers make the triode always work in the linear region. Therefore, the class A power amplifier has almost no distortion, and the texture is particularly good in the sense of hearing. Especially in the case of small signals, the entire sound balance, lubrication, and harmonics are abundant. However, Class A power amplifiers have low efficiency, generally no more than 25%, and Class A power amplifiers have a higher operating voltage when operating at the same output power, so their output peak voltage is limited and their input voltage is also output. The voltage amplifier is limited by the amplification factor. Therefore, the big dynamic performance of music is affected.
Class B amplifiers do not have a fixed bias voltage and are turned on by the signal voltage. Therefore, when the signal voltage is less than 0.6V (for example, a silicon tube), the transistor is in an off state and the output is zero. Only when the signal voltage is greater than or equal to 0.6V, the transistor is turned on, the amplifier starts to work, and the output has a signal output. Such a small signal is a straight line that coincides with the X-axis on the output waveform, that is, distortion is generated, and the more the small signal in the input signal, the more severe the distortion. In the sense of hearing, there will be loss of musical details, small signals become blurred and weak, and the whole music becomes incoherent. Moreover, the Class B power amplifier stage must use two transistors to form a push-pull circuit. One tube operates in the positive half cycle of the signal voltage, and the other operates in the negative half cycle of the signal voltage. In this circuit, when a tube is turned on, At the same time, the other one is in the cut-off state. When the other half of the signal voltage comes, the working state of the two tubes is just exchanged, and the crossover distortion is naturally inevitable. However, the efficiency of the Class B power amplifier is very high, which can reach more than 75%. Therefore, a smaller power tube can be used to output a larger power, and the push-pull circuit has an effect on suppressing even harmonics to reduce nonlinear distortion.
For the disadvantages of class B power amplifiers, the designer adds a preset fixed bias voltage slightly lower than the threshold voltage to the input terminal of the triode, so that the output current of the triode is slightly larger than zero when it is static, making it small. When the signal voltage is applied, the triode can also be turned on, and there is a current output, so that the transistor has a time greater than half a cycle of the signal, and the crossover distortion does not exist. This is the class AB, and the class AB power amplifier overcomes the class B power amplifier. There are problems, and the electrical efficiency is much higher than that of the Class A amplifier.
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