How much do you know about HIFI power amplifier ?

                          How much do you know about HIFI power amplifier ?

Power amplifier, the abbreviation of power amplifier. Its basic function is to amplify the signal from the preamplifier around 1V, so that it can generate enough undistorted output power to drive the speaker to work normally. A power amplifier is generally composed of six major parts, including an input stage, a pre-promoter stage, a push stage, an output stage, a negative feedback circuit, and a protection circuit.

The basic requirements for power amplifiers are: sufficient output power and good dynamic characteristics, and require its various distortions to be as small as possible, signal-to-noise ratio S/N as high as possible, and have flat frequency response characteristics, etc. .

According to the different main electronic components, power amplifiers can be divided into two categories: tube power amplifiers and transistor power amplifiers, which are commonly known as "tube amplifiers" and "stone amplifiers". Power amplifier circuits can be divided into several different types such as Class A power amplifiers, Class B power amplifiers, and Class A and B power amplifiers due to different bias conditions.

1. The characteristics of the tube power amplifier

Transistor power amplifiers have dominated the market today, but in Hi-Fi high-fidelity playback systems, tube power amplifiers still have a place. This is because: the signal overload tolerance of the tube power amplifier is obviously better than that of the transistor power amplifier, so its required power reserve is more than twice that of the transistor power amplifier. Compared with transistor power amplifiers of the same power, the low-frequency sound of the tube power amplifier is softer, and the high-frequency sound is thinner, while the transistor power amplifier obviously has "transistor sound" or "metal sound", that is, the sound is a bit harsh and glitchy.

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 its electroacoustic index is achieved by increasing the depth of negative feedback, making it easier for the transistor power amplifier. Transient intermodulation distortion is produced. In addition, the protection circuit of the electronic tube power amplifier is simple, the thermal stability of the electronic tube is good, and it is generally not easy to be damaged, while the damage rate of the high-power transistor is relatively high. The weakness of tube power amplifiers is high power consumption, poor anti-vibration and anti-mechanical shock performance.

2. Characteristics of transistor power amplifier

For three different types of transistor power amplifiers, Class A, Class B, and Class A and B, the Class A power amplifier has a fixed transistor bias, so there is a collector current flowing during the entire period of the sinusoidal input signal, even when there is no input Signal, there is still a certain value of collector quiescent current. The Class B power amplifier adjusts the fixed bias of the transistor almost so that the collector current is just in the cut-off state when there is no input signal, and only in the positive half cycle of each alternating cycle of the external input signal, there is collector current. circulation.

The Class A and B power amplifiers work between the Class A and Class B amplification states, so that the external signal has a collector current flowing during the period that is greater than half a cycle and less than a full cycle. If only one transistor is used as the output stage of the power amplifier, in order to avoid distortion caused by the amplified signal, the only way to design the amplifier is a Class A working state, which is the case for most tube power amplifiers and some audiophile transistor power amplifiers. For Class B and Class A and B 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 advantages of Class A power amplifiers are that the circuit is simple and easy to operate, and the nonlinear distortion is small, which 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 the Class B power amplifier and the 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 the Class B power amplifier is working, because the two transistors are alternately turned on and Therefore, crossover distortion will occur at the junction of the output signal waveforms of the two tubes; and when the power amplifier tube is converted from reverse bias to zero bias and then to forward bias, as the signal frequency increases, the output signal will be in the Delayed in time, so-called switching distortion occurs.

Therefore, in the actual Hi-Fi high-fidelity playback system, Class B power amplifiers are generally not used, but Class A power amplifiers or Class A and B power amplifiers with small linear distortion are used. Class A and B power amplifiers reduce crossover distortion by changing the bias. It compromises the high fidelity of Class A power amplifiers and Class B power amplifiers, thus solving the above-mentioned contradiction between high efficiency and large distortion to a certain extent.

The main circuit forms of class A and B power amplifiers are single-ended push-pull circuits with complementary or quasi-complementary output forms. The common ones are OTL non-output transformer circuits, OCL non-output capacitor circuits, and BTL balanced non-transformer circuits. The following mainly introduces the OCL circuit without output capacitor.

OCL no output capacitor circuit is a transistor power amplifier circuit without output coupling capacitor. It is developed on the basis of OTL circuit. An output capacitor, its main advantages are low lower limit frequency, flat entire frequency characteristic curve, and low distortion.

3. Basic protection circuit of power amplifier

a. Horn protection circuit

In view of the characteristics of direct coupling between the OCL power amplifier and the speaker, it is generally necessary to install a speaker protection circuit. The purpose of this circuit is to prevent the high DC voltage at the output terminal when the power amplifier fails, causing DC current to flow into the speaker, which may cause the voice coil of the speaker to shift, or burn the speaker if it is serious. The detection point of this protection circuit is the output terminal of OCL, that is, the midpoint. When the DC potential of the midpoint to ground exceeds a certain value (such as positive or negative 1V), the protection circuit will act and cut off the circuit leading to the speaker. Play the role of protecting the horn.

The speaker protection circuit generally does not share the DC power supply with the power amplifier, so that the protection circuit can still work normally when the power amplifier circuit fails. The speaker protection circuit generally also has a delay function. When the power amplifier is turned on, the speaker is switched on after a delay of a few seconds, which protects the speaker from being damaged by the large surge current at the moment of turning on, and also avoids the speaker being damaged at the moment of turning on. The "bang bang" sound that appears in the

b. Overload protection circuit

In the transistor power amplifier, the high-power output tube works under the condition of high voltage, high current and heavy load, and it is easy to be damaged. When the input signal has a particularly large amplitude in an instant or the output terminal of the power amplifier is short-circuited when there is a feed-in signal, it will cause the high-power tube to over-current, and sometimes a high reverse voltage will appear, causing the high-power tube to be damaged by overvoltage. . Overvoltage and overcurrent protection circuits are provided for this reason. The protection circuit generally adopts electronic protection, does not need any mechanical contacts and relays, operates quickly, can effectively protect high-power tubes, and has the function of automatically returning to normal.

Class A

Also known as class A, during the entire period of the signal (the positive and negative two half cycles of the sine wave), any power output element of the amplifier will not have a current cutoff (that is, stop the output) of a class of amplifiers. Class A amplifiers generate high heat and have low efficiency when operating, but have the inherent advantage of no crossover distortion. Single-ended amplifiers all work in Class A mode, and push-pull amplifiers can be Class A, Class B or Class A and B.

Class B

Also known as class B, the positive and negative two half cycles of the sinusoidal signal are amplified and output by the two "arms" of the push-pull output stage in turn, and the conduction time of each "arm" is half a cycle of the signal. The advantage of Class B amplifiers is high efficiency, but the disadvantage is that it will produce crossover distortion.

Class A and B

Also known as class ab, it is between class A and class B. The conduction time of each "arm" of push-pull amplification is greater than half a cycle of the signal and less than one cycle. Class A and B amplification effectively solves the crossover distortion problem of Class B amplifiers, and its efficiency is higher than that of Class A amplifiers, so it has been widely used.