Introduction and comparison of Class A, Class B

by:Gisen     2020-06-17
First, the types of power amplifiers can be divided into the following types of power amplifiers: 1. Pure Class A power amplifier Pure Class A power amplifier is also called Class A power amplifier (Class A), which is a completely linear amplifier. When the pure class A power amplifier is working, the positive and negative channels of the transistor are always open whether there is or no signal, which means that more power is consumed as heat. Pure Class A power amplifiers are relatively rare in car audio applications, such as the Italian Sinfoni high-quality series. This is because the efficiency of the pure class A power amplifier is very low, usually only 20-30%, audio enthusiasts are relished about its sound performance. 2. Class B power amplifier Class B power amplifier, also called Class B power amplifier (Class B), it is also called linear amplifier, but its working principle is completely different from pure Class A power amplifier. When the class B power amplifier is working, the positive and negative channels of the transistor are usually closed unless there is a signal input, that is, when the positive phase signal comes, only the positive phase channel works, and the negative phase channel is closed, the two channels are absolutely It will not work at the same time, so there is no power loss at the part where there is no signal. However, when the positive and negative channels are turned on and off, crossover distortion often occurs, especially in the case of low levels, so Class B power amplifiers are not true high-fidelity power amplifiers. In practical applications, in fact, many early car audio amplifiers are Class B amplifiers because of their relatively high efficiency. 3. Class A and B power amplifiers Class A and B power amplifiers are also called Class AB power amplifiers (Class AB), which is a design compatible with the advantages of Class A and Class B power amplifiers. When there is no signal or the signal is very small, the positive and negative channels of the transistor are always open, then the power is lost, but there is no serious class A amplifier. When the signal is in positive phase, the negative phase channel is normally open before the signal becomes stronger, but the negative channel is closed when the signal becomes stronger. When the signal is in negative phase, the operation of the positive and negative channels is just the opposite. The disadvantage of class AB power amplifier is that it will produce crossover distortion, but relative to its efficiency ratio and fidelity, it is superior to class A and B amplifiers. Class AB amplifiers are also the most widely used designs in car audio . 4. Class C amplifier Frankly speaking, in fact, it works in a distorted state! The early stage C is used for RF power amplification. Because the FM type RF output can be used, the carrier is adjusted by adjusting the frequency. Even if it is distorted, it does not affect its frequency. There are also studies on the direction of applying it to audio. However, the recently popular D-type efficiency is also very high ~ There is also a T-type amplifier, find it yourself ~ If you do n’t understand, you can ask me ~ I hope my answer can help you 5, Class D power amplifier Class D amplifiers are different from the above Class A, B or AB amplifiers, and their working principle is based on switching transistors, which can be completely turned on or off in a very short time. The two transistors will not turn on at the same time, so little heat is generated. This type of amplifier is extremely efficient (around 90%), ideally up to 100%, compared to 78.5% for class AB amplifiers. On the other hand, the switching mode also increases the distortion of the output signal. The circuit of Class D amplifier is divided into three stages: input switch stage, power amplifier stage and output filter stage. Class D amplifiers can use pulse width modulation (PWM) mode when they are switched. Use PWM to convert the audio input signal into a high-frequency switching signal, and compare the audio signal with a high-frequency triangle wave through a comparator. When the voltage at the inverting terminal is higher than the voltage at the in-phase terminal, the output is low; When the voltage is lower than the voltage of the non-inverting terminal, the output is high. In the class D amplifier, the output of the comparator is connected to the power amplifier circuit. The power amplifier circuit uses a metal oxide field effect transistor (MOSFET) instead of a bipolar transistor (BJT). This is because the former has a faster response time, so it is suitable for In high frequency working mode. Class D amplifiers require two MOSFETs, and they can be fully turned on or off in a very short time. When a MOSFET is fully turned on, the tube voltage drop is very low; when the MOSFET is completely turned off, the current through the tube is zero. The switching speed of the two MOSFETs alternately working in the on and off state is very fast, so the efficiency is extremely high and the heat generated is very low, so the class D amplifier does not require a large heat sink. Class D amplifiers have many other names, such as class T, etc., which are a variant of class D amplifiers. In practical applications, until 1980, due to the emergence of MOSFET, this switching power amplifier was able to develop rapidly. In the actual development process, although it has high efficiency, it also has high distortion, high noise and poor damping factors. With the development of technology, there will be fewer and fewer such defects. It is estimated that Class D amplifiers will be more widely used in the field of car audio in the future. Class A, Class B, Class AB, Class C, Class D power amplifier introduction and comparison Class A, B, AB, C, D Class 5 amplifier introduction 1. Class A amplifier (also known as Class A amplifier) The two (or two sets of) transistors in the output stage of the Class A amplifier are always in a conductive state, that is to say they maintain the conduction current regardless of whether there is a signal input, and make these two currents equal to the peak value of the alternating current. In case of load. When there is no signal, the two transistors flow the same amount of current, so there is no unbalanced current or voltage at the output center point, so no current is input to the speaker. When the signal tends to be positive, the output transistor above the line allows more current to flow, and the output transistor below lowers the current relatively. Because the current begins to be unbalanced, it flows into the speaker and pushes the speaker to sound. The working mode of the Class A amplifier has the best linearity. Each output transistor amplifies the full wave of the signal, and there is no switching distortion (Switching DistorTIon) at all. Even if negative feedback is not applied, its open-loop distortion is still very low, so It is said to be the most ideal amplifier circuit design for sound. However, this design has advantages and disadvantages. The biggest disadvantage of Class A amplifiers is low efficiency, because there is still full current flowing when there is no signal, and all the energy is converted to high heat. When the signal level increases, some power can enter the load, but many still turn into heat. Class A amplifier is ideal for replaying music. It can provide very smooth sound quality, round and warm tone, transparent and open treble, these advantages are enough to compensate its shortcomings. Class A power amplifiers generate amazing amounts of heat. In order to effectively deal with heat dissipation, class A power amplifiers must use large radiators. Because of its low efficiency, the power supply must provide sufficient current. A 25W Class A amplifier power supply has a capacity of at least 100 watts Class AB amplifier. Therefore, the size and weight of the class A machine are larger than the class AB, which makes the manufacturing cost increase and the price is more expensive. Generally speaking, the selling price of Class A amplifier is about twice or more of the same power class AB amplifier. 2. Class B amplifier (Class B amplifier) The working mode of Class B amplifier is that when there is no signal input, the output transistor is not conductive, so it does not consume power. When there is a signal, each pair of output tubes amplifies half of the waveform, turns on and off in turn to complete a full-wave amplification, and crossover distortion occurs when the two output transistors work alternately, thus forming nonlinearity. Pure Class B amplifiers have less, because the distortion is very serious when the signal is very low, so the crossover distortion makes the sound rough. The efficiency of Class B amplifiers is about 75% on average, and the heat generated is lower than that of Class A machines, allowing the use of smaller radiators. The usual working methods of Class B amplifiers are divided into OCL and BTL. BTL can provide greater power. At present, most power integrated circuits can use two blocks to form a BTL circuit. 3, Class AB amplifier Compared with the previous two types of power amplifiers, AB power amplifiers can be said to be a compromise in performance. Class AB amplifiers usually have two bias voltages, and a small amount of current flows through the output transistor when there is no signal. It uses the A-type working mode when the signal is small to obtain the best linearity. When the signal increases to a certain level, it automatically switches to the B-type working mode to obtain higher efficiency. The 10 watt class AB amplifier of the ordinary machine works with class A within about 5 watts. Since the power required to listen to music is only a few watts, the class AB amplifier uses the class A amplifier working mode most of the time. It only changes to category B when there is a strong transient sound. This design can obtain excellent sound quality and improve efficiency to reduce heat, which is a quite logical design. Some Class AB amplifiers adjust the bias current so high that they work in Class A in a wider power range, making the sound close to a pure Class A machine, but the heat generated is also relatively increased. 4, Class C amplifier (Category C amplifier) This type of power amplifier is rarely heard, because it is a very high distortion power amplifier, only suitable for use in communications. The output efficiency of Class C is extremely high, but it is not suitable for HI-FI amplification. 5. Class D amplifier (class D amplifier) This design is also called digital power amplifier. The transistor of the class D amplifier is connected to the load directly when it is turned on. The current flows but the transistor has no voltage, so there is no power consumption. When the output transistor is turned off, all the power supply voltage appears on the transistor, but there is no current, so no power is consumed, so the theoretical efficiency is 100%. The advantage of Class D amplifier is that it has the highest efficiency, the power supply can be reduced, and almost no heat is generated. Therefore, a large radiator is not needed, and the body volume and weight are significantly reduced. However, the work of this kind of power amplifier is complicated, and the added circuit itself is inevitably biased, so there are few really successful products and the price is not cheap. PS: At present, most multimedia speakers use Class B (Category B) power amplifiers, and due to cost and space reasons, the power amplifier circuits of multimedia speakers mostly use integrated circuits, and power transformers and filter capacitors cannot be made very large . This directly affects the sound quality and dynamics of the multimedia box. Of course, the power amplifier of the multimedia speakers (such as Dajidian) that uses the tube power amplifier is working in Class A, but the price of the speaker is not acceptable to most people. Amplifiers are divided into output channels to be divided into: Mono, stereo dual channel, 2.1 channel, 5.1 surround sound
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