Amplifier, the abbreviation of power amplifier. Its basic function is to amplify the signal from the preamplifier about 1V, so that it produces enough undistorted output power to drive the speaker to work properly. The power amplifier generally consists of six parts: the input stage, the pre-drive stage, the push stage, the output stage, the negative feedback circuit, and the protection circuit.
The basic requirements for power amplifiers are: have sufficient output power and good dynamic characteristics, and require various distortions as small as possible, the signal-to-noise ratio S/N is as high as possible, and has flat frequency response characteristics, etc. .
Power amplifiers can be divided into two categories: tube power amplifiers and transistor power amplifiers according to their main electronic components, which are commonly known as "amplifiers" and "stone machines". The power amplifier circuit can be divided into several types, such as a class A power amplifier, a class B power amplifier, and a class A power amplifier, depending on the bias conditions.
1. Characteristics of tube amp
Transistor power has dominated the market today, but in Hi-Fi high-fidelity playback systems, tube amps still have a place. This is because: the signal overload tolerance of the tube amp is obviously better than that of the transistor amp, so the required power reserve is more than double that of the transistor amp. Compared with the same power transistor amplifier, the low-frequency sound of the tube amp is softer, the high-frequency sound is slender, and the transistor amp has obvious "transistor sound" or "metal sound", that is, the sound is somewhat stiff and burr.
In addition, the negative feedback depth of the tube power amplifier is not large, so there is generally no transient intermodulation distortion, and the gain of the transistor power amplifier is often large, and the electroacoustic index is achieved by increasing the negative feedback depth, making the transistor power amplifier easy. Produces transient intermodulation distortion. In addition, the protection circuit of the tube power amplifier is simple, the thermal stability of the electron tube is good, and generally the damage is not easy, and the damage rate of the high power transistor is relatively high. The weakness of the tube amp is the large power consumption, anti-vibration and anti-mechanical impact performance.
2. Characteristics of transistor amplifier
For Class A, Class B, Class A and Class B transistor amplifiers, Class A power amplifiers have a fixed transistor bias, so that collector current flows throughout the entire period of the sinusoidal input signal, even if there is no input. At the time of the signal, there is still a certain value of the collector quiescent current. The Class B power amplifier adjusts the fixed bias of the transistor almost so that the collector current is just off when there is no input signal. Only the positive half cycle of each alternating period of the applied input signal has the collector current. Circulation. The Class A and Class B power amplifiers work between the Class A and Class B amplification states, so that the applied signal flows in a period of time greater than half a cycle and less than the full cycle. If only one transistor is used as the output stage of the amplifier, in order to avoid distortion caused by the amplified signal, only the amplifier is designed to operate in Class A, and most tube amplifiers and some enthusiast-level transistor amplifiers do. For Class B and Class A power amplifiers, in order to finally obtain an undistorted output signal, a push-pull amplifier circuit composed of two transistors must be used.
The main advantage of Class A power amplifier is that the circuit is simple and easy to operate, and the nonlinear distortion is small. It is suitable for low-power linear audio amplifiers. Now Class A power amplifiers are mainly used in high-end power amplifier products. The main difference between Class B power amplifier and Class A power amplifier is that the quiescent current is small, so the power consumption is small when there is no signal, and higher efficiency can be obtained. However, when Class B power is put into operation, the two transistors are alternately turned on and As a result, at the junction of the output signals of the two tubes, crossover distortion will occur; and when the power amplifier tube is switched from reverse bias to zero offset to positive offset, the output signal will be as the signal frequency increases. Delayed in time, so-called switching transition distortion occurs. Therefore, in the actual Hi-Fi high-fidelity playback system, the class B power amplifier is generally not used, and the class A power amplifier or the class A power amplifier with small linear distortion is used. Class A and Class B power amplifiers reduce the crossover distortion by changing the bias method. It compromises the high fidelity of Class A power amplifiers with Class B power amplifiers, thus solving the contradiction between high efficiency and large distortion to some extent.
The main circuit form of Class A and Class B power amplifiers is single-ended push-pull circuit with complementary or quasi-complementary output form. Common OTL no output transformer circuit, OCL no output capacitor circuit and BTL balanced transformerless circuit. The following mainly introduces the OCL no-output capacitor circuit. OCL no-output capacitor circuit is a transistor power amplifier circuit without output coupling capacitor. It is developed on the basis of OTL circuit. The main difference is that OCL has no output capacitor, adopts a set of symmetrical power supply, and saves it. An output capacitor has the main advantage that the lower limit frequency is low, the entire frequency characteristic curve is flat, and the distortion is small.
3. The basic protection circuit of the power amplifier
a. horn protection circuit
For the direct coupling of the OCL power amplifier and the speaker, a speaker protection circuit is generally required. The circuit is to prevent the amplifier from experiencing a high DC voltage at the output, causing DC current to flow into the horn, which causes the horn of the horn to shift, and then burns the horn. The detection point of the protection circuit is the output end of the OCL, that is, the midpoint. When the point-to-ground DC potential exceeds a certain specified value (such as positive or negative 1V), the protection circuit operates to cut off the circuit to the horn. It plays a role in protecting the speakers.
The horn protection circuit generally does not share the DC power supply with the power amplifier, so that the protection circuit can work normally when the power amplifier circuit fails. The horn protection circuit generally also has a delay function. When the power amplifier is turned on, the horn is turned on after a delay of a few seconds, which protects the horn from being damaged by a large surge current when the power is turned on, and also avoids the moment when the horn is turned on. The "å˜å˜" sound that appears in it.
b. Overload protection circuit
In a transistor power amplifier, a high-power output tube operates under high voltage, high current, and heavy load, and is easily damaged. When the input signal is extremely large in the instantaneous amplitude or the power amplifier output is short-circuited with the feed signal, the high-power tube will over-current, and sometimes a high reverse voltage will occur, causing high-power tube over-voltage damage. . Set the overvoltage and overcurrent protection circuits for this reason. The protection circuit is generally electronically protected, does not require any mechanical contacts and relays, acts quickly, can effectively protect the high-power tube, and has the function of automatically returning to normal.
Class A
Also known as class a, a class of amplifiers with no current cutoff (ie, stop output) occurs in any power output component of the amplifier over the entire period of the signal (positive and negative sinusoidal two and a half cycles). Class A amplifiers generate high heat during operation and are inefficient, but the inherent advantage is that there is no crossover distortion. Single-ended amplifiers are Class A working methods, and push-pull amplifiers can be Class A or Class B or Class A.
Class B
Also known as class b, the positive and negative sinusoidal signals are amplified by the two "arms" of the push-pull output stage. The conduction time of each "arm" is half a cycle of the signal. Class B amplifiers have the advantage of high efficiency and the disadvantage of crossover distortion.
Class A and B
Also known as ab class, between Class A and Class B, each "arm" conduction time of push-pull amplification is greater than half a cycle of the signal and less than one cycle. Class A and Class B amplification effectively solve the crossover distortion problem of Class B amplifiers, and the efficiency is higher than that of Class A amplifiers, so it has been widely used.
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