Classification of power amplifiers and their parameters

Power Amplifier (Power Amplifier) ​​"Power Amplifier", as its name suggests, is an amplifier that amplifies "power." Enter weak signals, such as microphones, VCDs, microwaves, etc., and send them to the preamplifier circuit, which is amplified enough to push the signal amplitude of the power amplifier. Finally, the power amplifier circuit of the latter stage pushes the speaker or other device. Its maximum function is as the output. "Level" is used. From another point of view, it is doing a large signal current amplification to achieve power amplification. In a broad sense, power amplifiers are not limited to audio amplification, and are used in many applications, such as RF, microwave, laser, and so on.

1, pure class A power amplifier

A pure class A power amplifier, also known as a Class A power amplifier (Class A), is a fully linear amplifier form of the amplifier. When a pure Class A power amplifier is operating, the positive and negative channels of the transistor are normally open regardless of the presence or absence of a signal, which means more power is dissipated as heat. Pure Class A power amplifiers are rare in automotive audio applications, such as the Italian Sinfoni high-quality series. This is because the efficiency of a pure class A power amplifier is very low, usually only 20-30%, and audiophiles are relishing its sound.

2. Class B power amplifier

Class B power amplifiers, also known as Class B power amplifiers (Class B), are also known as linear amplifiers, but they work in a completely different way than pure Class A power amplifiers. When class B power is put into operation, the positive and negative channels of the transistor are usually in the off state unless there is signal input, that is, only the positive phase channel works when the positive phase signal comes, and the negative phase channel is closed. It won't work at the same time, so there is no power loss in the part without signal. However, when the positive and negative channels are turned on and off, crossover distortion is often generated, especially in the case of low level, so the class B power amplifier is not a true high fidelity power amplifier. In practical applications, in fact, many of the early car audio amplifiers were Class B amplifiers because of their high efficiency.

3. Class A and B power amplifiers

Class A and B power amplifiers, also known as Class AB power amplifiers (Class AB), are a design that is compatible with Class A and Class B amplifiers. When there is no signal or the signal is very small, the positive and negative channels of the transistor are always open, and the power is lost, but there is no serious class A power amplifier. When the signal is positive, the negative phase channel is normally open before the signal becomes strong, but the signal is strong and the negative channel is closed. When the signal is a negative phase, the positive and negative channels work just the opposite. The defect of Class AB power amplifier is that it will produce crossover distortion, but it is better than Class A and Class B power amplifiers compared with its efficiency ratio and fidelity. Class AB power amplifier is also the most widely used design in car audio. .

4, class D power amplifier

Class D amplifiers differ from the above-mentioned A, B or AB amplifiers in that their operating principle is based on switching transistors that can be fully turned on or completely turned off in a very short period of time. The two transistors do not turn on at the same time, so there is very little heat generated. This type of amplifier is extremely efficient (about 90%), ideally 100%, compared to only 78.5% for Class AB amplifiers. On the other hand, however, the switching mode of operation also increases the distortion of the output signal. The circuit of the class D amplifier is divided into three levels: an input switching stage, a power amplification stage, and an output filtering stage. Class D amplifiers operate in pulse-width modulated (PWM) mode when switched. The PWM can convert the audio input signal into a high frequency switching signal, and compare the audio signal with the high frequency triangular wave through a comparator. When the voltage of the inverting terminal is higher than the voltage of the non-inverting terminal, the output is low; when the inverting end When the voltage is lower than the voltage at the non-inverting terminal, the output is high.

In a class D amplifier, the output of the comparator is connected to a power amplifier circuit that uses a metal oxide field effect transistor (MOSFET) instead of a bipolar transistor (BJT) because the former has a faster response time and is therefore suitable. In high frequency mode of operation. Class D amplifiers require two MOSFETs that can operate fully in an on or off state in a very short period of time. When a MOSFET is fully turned on, its tube voltage drop is low; when the MOSFET is completely turned off, the current through the tube is zero. The switching speed of the two MOSFETs operating in the on and off states 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 notations, such as T, which are all variants of Class D amplifiers. In practical applications, until the 1980s, this type of switching power amplifier was rapidly developed due to the emergence of MOSFETs. In the actual development process, although there is high efficiency, it also has high distortion, high noise and poor damping factor. With the development of technology, such defects will be less and less, and it is estimated that the future class D power will be more widely used in the field of car audio.

5, class T amplifier

The power output circuit of the class T power amplifier is the same as the pulse width modulation class D power amplifier. The power transistor is also operating in the switching state, and the efficiency is comparable to that of the class D power amplifier. But unlike ordinary Class D power amplifiers: First, it is not a method of pulse width modulation. Tripath invented a digital power technology called Digital Power Processing (DPP), a digital power amplifier processor. It is the core of the class T power amplifier. It uses the adaptive algorithm and prediction algorithm for processing small signals in communication technology. The input audio signal and the current entering the speaker are digitally processed by DPP to control the turn-on and turn-off of the power transistor. This allows the sound quality to achieve high fidelity linear amplification.

Secondly, the switching frequency of its power transistor is not fixed, and the power spectrum of the unwanted component is not concentrated in a narrow frequency band on both sides of the carrier frequency, but spread over a wide frequency band. Make the details of the sound clear and audible throughout the entire frequency band. In addition, Class T power amplifiers have a wider dynamic range and a flat frequency response. The advent of DDP has pushed the power amplifiers of the digital age to a new level. In terms of high fidelity, the linearity is more than that of the traditional class AB amplifier.

RF power amplifier:

RF power amplifiers (RF PAs) are an important part of a variety of wireless transmitters. In the pre-stage circuit of the transmitter, the RF signal generated by the modulating oscillating circuit has a small power, and needs to pass through a series of amplification-buffering stage, intermediate amplification stage, and final stage power amplification stage, and after obtaining sufficient RF power, it can feed. Radiate out onto the antenna. In order to obtain a sufficiently large RF output power, an RF power amplifier must be used.

RF power amplifiers are an important part of the transmitting device. The main technical indicators of RF power amplifiers are output power and efficiency. In addition to this, the harmonic components in the output should be as small as possible to avoid interference with other channels.

High frequency power amplifier:

The high-frequency power amplifier is used for the final stage of the transmitting stage. The function is to amplify the high-frequency modulated signal to meet the requirements of the transmission power, and then radiate it to the space through the antenna to ensure that the receiving stage can be in a certain area. A satisfactory signal level is received and does not interfere with communication of adjacent channels. High frequency power amplifiers are an important component of transmitting devices in communication systems. According to the width of the working frequency band, it is divided into narrow-band high-frequency power amplifier and wide-band high-frequency power amplifier. The narrowband high-frequency power amplifier usually uses a frequency-selective circuit with frequency-selective filtering as the output loop, so it is also called a tuned power amplifier or Resonant power amplifier; the output circuit of the wideband high-frequency power amplifier is a transmission line transformer or other broadband matching circuit, so it is also called a non-tuned power amplifier. A high frequency power amplifier is an energy conversion device that converts the direct current energy supplied by a power source into a high frequency alternating current output.

It is known in the "Low Frequency Electronic Circuits" course that the amplifier can be divided into three types of working states: A, B, and C according to the different conduction angles of the current. Class A amplifier current has a flow angle of 360o and is suitable for small signal low power amplification. The flow angle of the class B amplifier current is approximately equal to 180o; the flow angle of the class C amplifier current is less than 180o. Both Class B and Class C are suitable for high power work. The output power and efficiency of the Class C operating state are the highest of the three operating states. Most of the high frequency power amplifiers work in Class C. However, the current waveform distortion of the Class C amplifier is too large to be used for low frequency power amplification, and can only be used for resonant power amplification using a tuning loop as a load. Since the tuning loop has filtering capability, the loop current and voltage are still very close to a sinusoidal waveform with little distortion. In addition to the above several operating states classified by current flow angle, there are D-type amplification and E-class amplification that enable the electronic device to operate in the switching state. The efficiency of the D-type amplifier is higher than that of the Class-C amplifier, theoretically up to 100%, but its maximum operating frequency is limited by the power consumption of the device (collector dissipated power or anode dissipated power) generated by the switching transient. .

If the circuit is modified to minimize the power consumption of the electronic device during the on-off conversion, the operating frequency can be increased. This is the Class A amplifier. In order to obtain a sufficiently large low-frequency output power in a low-frequency amplifier circuit, a low-frequency power amplifier must be used, and the low-frequency power amplifier is also an energy converter that converts the energy supplied from the DC power source into an AC output. The common characteristics of high-frequency power amplifiers and low-frequency power amplifiers are high output power and high efficiency, but the operating frequency and relative frequency bandwidth of the two are quite different, which determines the essential difference between them. Low frequency power amplifiers have low operating frequencies but are relatively wide in width. For example, from 20 to 20000 Hz, the ratio of high to low frequencies is 1000 times. Therefore, they all use untuned loads such as resistors, transformers, etc. High-frequency power amplifiers operate at high frequencies (from a few hundred kHz up to hundreds, thousands, or even tens of thousands of MHz), but the relative frequency band is narrow. For example, an AM radio station (band range of 535-1605 kHz) has a bandwidth of 10 kHz. If the center frequency is 1000 kHz, the relative bandwidth is only one hundredth of the center frequency. The higher the center frequency, the smaller the relative bandwidth. Therefore, high frequency power amplifiers generally use a frequency selective network as a load loop. Due to this latter feature, the operating states of the two amplifiers are different: the low-frequency power amplifier can work in Class A, Class A or Class B (limited to push-pull circuits); high-frequency power amplifiers generally work in C. Class (some special cases can work in class B).

A new type of broadband high-frequency power amplifier is widely used in each intermediate stage of the wide-band transmitter. Instead of using a frequency-selective network as a load loop, it uses a transmission line with a wide frequency response as a load. In this way, it can change the operating frequency over a wide range without having to re-tune. In summary, it can be seen that the common point of the high-frequency power amplifier and the low-frequency power amplifier is that the output power is required to be large and the efficiency is high; the difference between them is that the operating frequency and the relative bandwidth of the two are different, and thus the load network and the work The status is also different.

The main technical indicators of high-frequency power amplifiers are: output power, efficiency, power gain, bandwidth and harmonic rejection (or signal distortion). These indicators are contradictory. When designing an amplifier, some indicators should be highlighted according to specific requirements, taking into account other indicators. For example, some circuits in practice prevent interference as the main contradiction, require higher harmonic suppression, and appropriately reduce bandwidth requirements. The efficiency of the power amplifier is a prominent problem, and its efficiency is directly related to the operating state of the amplifier. The working state of the amplifier can be divided into Class A, Class B and Class C. In order to improve the efficiency of the amplifier, it usually works in Class B, Class C, that is, the transistor works to extend into the nonlinear region. However, there is a very serious nonlinear distortion between the output current and the output voltage of the amplifier under these operating conditions. The low-frequency power amplifier has a large frequency coverage factor for its signal and cannot use the resonant circuit as a load. Therefore, it generally works in the Class A state; it can work in Class B when using a push-pull circuit. Because the frequency coverage factor of the signal is small, the high-frequency power amplifier can use the resonant circuit as the load, so it usually works in Class C. Through the frequency selection function of the resonant circuit, the harmonic components in the collector current of the amplifier can be filtered out and selected. The fundamental component thus substantially eliminates nonlinear distortion.

Therefore, high frequency power amplifiers have higher efficiency than low frequency power amplifiers. Due to the non-linear state of large-signal power amplifiers, high-frequency power amplifiers cannot be analyzed by linear equivalent circuits. The analytical approximation analysis method, the fold line method, is commonly used in engineering to analyze its working principle and working state. The physical concept of this analytical method is clear, and the analytical work state is convenient, but the calculation accuracy is low. Among the various types of high-frequency power amplifiers discussed above, narrow-band high-frequency power amplifiers are used to provide a sufficiently strong narrow-band signal power centered on the carrier frequency, or to amplify narrow-band modulated signals or to achieve multipliers, usually working in B. Class, Class C status. Broadband high-frequency power amplifier: used for the frequency range of some carrier signals to be large, short-wave, ultra-short-wave radio intermediate stages of amplification, in order to avoid the tedious tuning of different fc. Usually works in the class A state.

1. Input sensitivity refers to the minimum input signal level required by the power amplifier. It is necessary to amplify the sound source signal enough to push the power amplifier.

2, harmonic distortion, this is a very important indicator of the power amplifier, harmonic distortion is a kind of nonlinear distortion, it is caused by the nonlinear characteristics of the amplifier during operation, the distortion result is the generation of new harmonics The component makes the sound lose its original tone. In severe cases, the sound is broken and harsh. Harmonic distortion is also divided into odd and even times. The odd harmonics can make people feel annoyed and disgusted, and they are easily perceived by people. Some amps sound irritating and feel tired, which is caused by large distortion. The most influential factor on the power amplifier is the distortion. Generally, the high-fidelity requires that the harmonic distortion be below 0.05%. The lower the better. In addition to harmonic distortion, there are intermodulation distortion, cross distortion, clipping distortion, transient distortion, phase distortion, etc., which are the main culprit affecting the quality of the amplifier. The merits of the assessment function, first of all depends on its distortion, like the total harmonic distortion of the Italian Sinfoni amplifier is below 0.01%.

3, output power, power problems are most unclear for car audio practitioners, here we need to explain one by one:

A. Rated output power, called (RMS), refers to the higher power that the amplifier's output audio signal can output in the total harmonic distortion range. It is typically 0.707 times the peak value of the AC signal.

B. Average power, average power generally refers to the average power consumption at each frequency point. It is similar to the rated output power, but it generally refers to time.

C, peak output power, the larger music power that the power amplifier can output is called peak output power. It does not consider distortion, and is usually about 1.414 times of (RMS) power.

D. Peak-to-peak power, which is the power of the peak value of the positive voltage to the negative voltage, which is four times the peak output power. Its appearance is that the manufacturer has no practical significance for commercial purposes.

4. Signal-to-noise ratio, the larger the value, the better, generally expressed by (S/N), expressed in decibels of the ratio of signal power Ps to noise power Pn, S/N=10lgPs/Pn=20lgVs/Vn(db), In the formula, Vs and Vn are signal voltage and noise voltage, respectively.

The signal-to-noise ratio and the input signal level increase, the signal-to-noise ratio also gradually increases, but when the input signal level reaches a certain value, the signal-to-noise ratio remains basically unchanged. According to the current high fidelity requirements, the signal-to-noise ratio should be above 90dB, and the imported high-quality power amplifier can often reach 110-120dB, and its performance can be imagined. Some signal-to-noise ratios are followed by A-weighted words. A-weighting refers to the result of passing the noise signal through the weighting network. Since people are relatively insensitive to the noise of high and low frequency bands, this has occurred. The method of weighting. The weighted noise is more intuitively representative of the noise signal conditions that people actually experience. In short, the greater the signal-to-noise ratio, the smaller the noise mixed in the signal, the better the playback quality, and the replayed music is clear, clean and layered.

5, frequency response, early common known as power bandwidth, when the harmonic distortion does not exceed the specified value, the power amplifier's 1/2 rated power band width, that is, the frequency band included between the two frequency points where the high and low ends fall -3dB, It is called power bandwidth.

6, the damping coefficient, mainly for the low frequency, is a very important technical parameter that directly affects the bass sound quality. As we all know, the larger the diameter of the speaker, the better the bass is, but the larger the cone, the greater the inertia of the motion. This inertia makes it difficult to synchronize with the audio signal, and the sound is often opaque, especially At the low frequency of 100-400 Hz, it is easy to cause acoustic dyeing, which makes people sound blurred and unnatural. Some modified car woofer, the low frequency signal is strong when the flutter is not only, the bass tail is serious, which is caused by the inertia of the cone.

In the design of the power amplifier, the engineer takes some technical measures on the power amplifier, such as selecting multiple tubes and parallels, low internal resistance (milliohms) high power tube, increasing working voltage, selecting high quality wire, etc., and trying to increase the damping coefficient so that it can be used for the speaker. The inertial motion produces a "resistance" effect, so that the motion of the cone is synchronized with the audio signal, so that the cone is restored to the zero position (ie, the center position) as soon as possible after the end of the drive signal. This blocking effect is the damping coefficient. (Damp Factor), D=Rs/Ri, Rs=horn impedance, Ri=power amplifier output internal resistance, the larger D, the better the sound basin and signal synchronization effect, the purer and cleaner the bass, the better the playback effect. .

7, slew rate (Slew rate), the conversion rate of the amplifier greatly affects the quality and performance of high-pitched playback. The faster the conversion rate, the better the treble sound quality, and the more accurate the high-frequency information can be captured. High-quality power amplifiers can be used for dozens to tens of V/us. Low- and medium-range power amplifiers are generally not marked. The value of this conversion rate is closely related to the design and materials, but it should not be too high or too high. Producing a supersonic signal above 20KHz that the human ear can't hear, not only has no effect on improving the sound quality, but it is easy to burn the tweeter.

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