What Is a Power Amplifier?

Power amplifier (English name: power amplifier), referred to as "power amplifier", refers to an amplifier that can produce a maximum power output to drive a certain load (such as a speaker) under a given distortion rate. The power amplifier plays a pivotal role of "organization and coordination" in the entire audio system, and to a certain extent, it dominates whether the entire system can provide good sound quality output.

Power amplifier

Power amplifier (English name: power amplifier), referred to as "power amplifier", refers to an amplifier that can produce a maximum power output to drive a load (such as a speaker) under a given distortion rate. The power amplifier plays a pivotal role of "organization and coordination" in the entire audio system, and to a certain extent, it dominates whether the entire system can provide good sound quality output.

Chinese name

Power amplifier

Foreign name

power amplifier

Features

"Amplifier"

Main types

A, B, C, D, T

use

Traditional digital voice playback systems consist of two main processes:

1. The conversion of digital voice data to analog voice signal (using high-precision digital-to-analog converter DAC);

2.Using an analog power amplifier

Power amplifier usually consists of 3 parts: preamp, drive amplifier,

The active devices used in power amplifiers are mainly transistors (bipolar or field-effect transistors). When operating at high frequencies or requiring high output power, electronic tubes (including high-power transmitting tubes) are also used; traveling waves are used in the microwave band tube. Amplifiers can be divided into Class A (A), Class A (AB), Class B (B), Class C (C) and Class D (D), etc. according to the operating points of their active devices. The table lists the highest efficiency that power amplifiers of different operating types can achieve with sine waves.

When choosing a power amplifier, we must first pay attention to some of its technical indicators:

1,

RF power amplifier:

For amplifier circuits whose main function is to provide power to a load, it is often called

Power amplifiers are referred to as power amplifiers. For their main purposes, power amplifiers can be divided into two main categories, namely dedicated power amplifiers and civilian power amplifiers. Amplifiers used in sports venues, theaters, song and dance halls, conference halls, public spaces, and sound recording and monitoring, etc., generally have some unique requirements in terms of their technical parameters. Amplifier or professional amplifier. ^[1]

Power amplifiers dominate the system's ability to provide good sound output to some extent.

For many people, the amplifier is not very well understood, it is not clear which accessories are needed to configure the amplifier speakers in order to play the effect of the amplifier to an optimal state.

The first type is to install a resistor under the speaker for current sampling. In fact, the voltage signal is fed back, which is an analog current feedback. The most people do it, but this circuit is defective for two reasons:

1. His output gain will change with the change of impedance. As a result, the constant voltage is not applied to the end of the horn 2 as if it can make the power applied to the horn constant.

Because the sound pressure characteristic curve of the speaker is tested under constant voltage output, this simple circuit is not good, it is not good to listen to, it is just fun, but there are improved circuits that mainly take negative voltage feedback and add An appropriate amount of this type of negative current feedback can make a good sound, but at this time the role of negative current feedback is to change the damping coefficient of the power amplifier, which has little effect on the amplitude-frequency characteristics.

2. The sampling point is below the horn. The horn is an inductor. The phase of the current flowing through the inductor will change. The low frequency is fine. The high frequency can be phase-shifted by 90 degrees. The phase characteristics are extremely poor.

The second type: negative resistance amplifier. Except for some special occasions, the first one to be used for acoustics and to achieve success is YAMAHA. Its main function is to improve the extension of low frequencies, but to 200Hz The frequency will have the effect of degrading the sound quality, so it is generally used on ultra-low frequency active speakers.

In fact, this circuit is used in conjunction with the speaker, and it has no practical significance alone. The working principle is: If the speaker is a rigid body, then adding a tube can become an ideal Holmoz resonance box, so no matter what the size of the box and the thickness of the tube, as long as it meets the Holmoz resonance Calculation formula. Even the resonance point of 20Hz can be achieved. The size of the box is just a high or low efficiency. Due to the presence of the speaker on the speaker, the speaker is moving when the sound is emitted. The speaker is not a rigid body, so the box will not produce Holmoz resonated.

Therefore, if the diaphragm of the horn is stationary during sounding. Then, the box is close to a rigid body, and it can meet the conditions of Holmitz resonance. The resonance point of this box can be arbitrarily designed. The job of keeping the horns stationary when making sounds is the task of negative resistance amplifiers. The working principle of the negative resistance power amplifier is that when the speaker is working in a low frequency band, its impedance characteristic changes sharply. The amplifier circuit takes out this change through current sampling to feed back the power amplifier, so that the power amplifier controls the speaker in the form of current. Equivalent analysis of the circuit can be found that the internal resistance of the power amplifier has a negative resistance characteristic in calculation.

During dynamic amplification, the internal resistance of the speaker plus amplifier is close to zero. As a result, this circuit makes the speaker strongly damped in any direction. As long as the sound ends, the horn will not move and the box will become rigid.

The third type: current-mode feedback amplifier circuit. This is a practical current amplifier circuit. It is also a true current-type negative feedback. The feedback signal is current, not voltage, that is, it is not added to the negative feedback terminal, but added. There is current flowing in. This kind of circuit was first used in video transmission, or equipment such as oscilloscopes.

Because it is a low-impedance negative current feedback input, this circuit has excellent high-frequency characteristics, and the driving capacity of the capacitive load is very strong. The open-loop frequency response is low, the closed-loop transient frequency response is distorted, and the capacity of the capacitive load is very weak. The disadvantage is that the open-loop gain of this circuit is relatively low, and the distortion after the closed-loop will be an order of magnitude higher than that of the voltage-type amplifier. However, the total distortion will not exceed 0.01%. ^[2]

Whether in the global mobile communication system, the third generation mobile communication system, wireless local area networks and other civil fields, or in the military fields such as radar, electronic warfare, navigation, etc., RF power amplifiers are used as front-end devices in these systems. The demand for small size is increasing rapidly.

As we all know, the power amplifier has the largest power loss among the many modules of the RF circuit. As the core and front-end part of the system, its efficiency will directly affect the system efficiency. Therefore, the issue of efficiency has become a research hotspot of modern power amplifiers. In most power amplifiers, the main power loss is the transistor loss, which is mainly caused by voltage and current. Therefore, switching power amplifiers are proposed, mainly class D, class E and class F. Among them, Class F power amplifiers specifically design a harmonic network to achieve drain voltage and current waveform control. In theory, the drain efficiency of a Class F power amplifier is 100%, which is called a new generation of power amplifier.

Traditional power amplifiers have low operating efficiency due to power consumption on the output circuit. In order to increase the working efficiency of traditional power amplifiers, the ideal class F power amplifier uses an output filter to control the harmonic components in the transistor output voltage or current, and to normalize the voltage and current waveforms output by the transistor. Therefore, the angle parameter of the collector current is 90 °, that is, the collector waveform is kept as a half sine wave, the collector voltage waveform is a square wave, and the phase difference between the two is / 4, so the waveform of the collector voltage and current There is no overlap area, thus achieving the ideal efficiency of 100%. ^[3]