Introduction
In general, an amplifier (Amplifier) ​​refers to any device that can use a small amount of energy to control large amounts of energy. Nowadays, in everyday use, the term is often referred to as an amplifier circuit, often used in audio applications. The input-output relationship of an amplifier—often expressed as a function related to the input frequency—this relationship is called the transfer function of the amplifier, and the coefficient of this transfer function is defined as the gain.
In general, an amplifier (Amplifier) ​​refers to any device that can use a small amount of energy to control large amounts of energy. Nowadays, in everyday use, the term is often referred to as an amplifier circuit, often used in audio applications.
The input-output relationship of an amplifier—often expressed as a function related to the input frequency. This relationship is called the transfer function of the amplifier, and the coefficient of this transfer function is defined as the gain.
Purchase
When selecting a power amplifier, first pay attention to some of its technical indicators: 1. Input impedance: usually indicates the anti-interference ability of the power amplifier, generally in the range of 5000-15000 Ω, the larger the value, the stronger the anti-interference ability; Distortion: refers to the degree of distortion of the output signal compared with the input signal. The smaller the value, the better the quality, generally below 0.05%. 3. The signal-to-noise ratio: refers to the ratio between the music signal and the noise signal in the output signal. The bigger the voice, the cleaner the sound.
In addition, when purchasing a power amplifier, you must also specify your willingness to purchase. If you want to install a subwoofer, it is best to buy a 5-channel amplifier. Usually 2-channel and 4-channel speakers can only push the front and rear speakers. The subwoofer can only be equipped with a power amplifier. The 5-channel power amplifier can solve this problem. The output power of the power amplifier should be as large as possible.
principle
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. It is divided into narrow-band high-frequency power amplifier and wide-band high-frequency power amplifier according to the width of its working frequency band. The narrow-band 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 amplifiers 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 currents have a flow angle of 360o and are 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 operation. 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 a switching state. The efficiency of a D-type amplifier is higher than that of a 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 switching transients. .
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. We already know that 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 by 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. The low frequency power amplifier has a low operating frequency but a wide relative bandwidth. 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 broadcaster (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).
In recent years, a new type of broadband high-frequency power amplifier has been widely used in the intermediate stages of broadband transmitters. 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, 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 a resonant circuit as a load. Therefore, it generally works in a 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, the short-wavelength, ultra-short-wave radio intermediate level amplification stage, so as to avoid the tedious tuning of different fc. Usually works in the class A state.
Performance
Regardless of the AV amplifier and Hi-Fi power amplifier, the power amplifier is very strict, and there are clear requirements in terms of output power, frequency response, distortion, signal-to-noise ratio, output impedance and damping coefficient.
Output Power
Output power refers to the power delivered by the power amplifier circuit to the load. At present, people's measurement methods and evaluation methods for output power are not uniform, so pay attention when using them.
1. Rated power (RMS)
It refers to the maximum power (strictly speaking, sine wave signal) that can be output by the power amplifier for a long period of time in a certain harmonic range. The average power when the harmonic distortion is often 1% is called the rated output power or the maximum useful power, the continuous power, the undistorted power, and the like. It is obvious that the nominal power values ​​will be different when the specified distortion preconditions are different.
2, the maximum output power
When the distortion size is not considered, the output power of the power amplifier circuit can be much higher than the rated power, and can output a larger value of power. The maximum power that can be output is called the maximum output power, and the aforementioned rated power and maximum output power are two. Output power under different preconditions
3, music output power (MPO)
The music output power MPO is the abbreviation of English Music Power Outpur, which refers to the output power of the power amplifier circuit when it works on the music signal, that is, the instantaneous maximum output power of the power amplifier to the music signal under the condition that the output distortion does not exceed the specified value.
The music output power can be used to evaluate the dynamic listening effect of the power amplifier. For example, a strong impact instrument sound suddenly appears behind a smooth music process. Some power amplifier circuits can provide a large output power in an instant to give a sense of strength. There is no end to the strength; some of the power amplifiers seem to be powerless. The ability to reflect this instantaneous burst of output power can be measured in terms of music output power.
4, peak music output power (PMPO)
It is the maximum music output power and is another dynamic indicator of the power amplifier circuit. If the distortion is not taken into account, the maximum music power that can be output by the power amplifier circuit is the peak music output power.
Generally, the peak music output power is greater than the music output power, the music output power is greater than the maximum output power, and the maximum output power is greater than the rated output power. According to practice statistics, the peak music output power is 5-8 times of the rated output power.
Frequency response
The frequency response reflects the amplification capability of the power amplifier for each frequency component of the audio signal. The frequency response range of the power amplifier should not be lower than the hearing frequency range of the human ear. Therefore, in an ideal situation, the operating frequency range of the main channel audio power amplifier is 20 -20kHz. Internationally, the frequency range of a typical audio power amplifier is 40-16 kHz ± 1.5 dB.
distortion
Distortion is a phenomenon in which the waveform of the reproduced audio signal changes. There are many reasons and types of waveform distortion, including harmonic distortion, intermodulation distortion, and transient distortion.
Dynamic Range
The ratio of the amplified minimum signal to the maximum signal level without distortion of the amplifier is the dynamic range of the amplifier. In practical use, the ratio uses dB to indicate the level difference between the two signals. The dynamic range of the high-fidelity amplifier should be greater than 90 dB.
The various noises in nature form the surrounding background noise, and the surrounding background noise and the sound intensity appearing in the performance vary greatly. Under normal circumstances, this intensity difference is called the dynamic range, and the excellent sound system should not input a strong signal. Overload distortion is generated, and when the weak signal is input, it should not be overwhelmed by the noise generated by itself. For this reason, a good sound system should have a large dynamic range, and the noise can only be minimized, but it is impossible to generate no noise.
Signal to noise ratio
The signal-to-noise ratio refers to the proportional relationship between the size of the sound signal and the size of the noise signal. The number of decibels of the ratio of the output sound signal level of the attack and discharge circuit to the various noise levels of the output is called the signal-to-noise ratio.
Output impedance and damping coefficient
Output impedance
The equivalent internal impedance exhibited by the amplifier output and the load (speaker) is called the output impedance of the amplifier.
2. Damping coefficient
The damping coefficient refers to the ability of the power amplifier circuit to perform a resistance to the load.
Detailed terminology
The scope of work
The working range refers to the operating frequency bandwidth of the power amplifier under the specified distortion and rated output power, that is, the range between the lowest operating frequency of the power amplifier and the highest operating frequency, in Hz (hertz). The actual operating frequency range of the amplifier may be greater than the defined operating frequency range.
Operating mode
The working modes of the power amplifier mainly include the following:
Time Division Duplex (TDD) mode:
In a TDD mode mobile communication system, different time slots on the same frequency channel (i.e., carrier) are received and transmitted, and the reception and transmission channels are separated by the guaranteed time.
The TDD system has the following characteristics:
(1) No need for paired frequencies, various frequency resources can be used, suitable for asymmetric uplink and downlink data transmission rates, especially suitable for IP type data services.
(2) The uplink and downlink work at the same frequency, and the symmetrical characteristics of the wave propagation make it easy to use new technologies such as smart antennas to achieve performance and cost reduction.
Time Division Multiple Access (TDMA) mode:
TDMA is the abbreviation of Time Division Multiple Access. The carrier of the same frequency is divided into several equal small time segments within a certain time, and the users of multiple different numbers use different small time segments to implement the connected communication mode. In short, it is a digital wireless technology that divides a narrow wireless channel into frame-like time segments (especially 3 and 8) and assigns each time segment to each user.
Transmission gain
Refers to the ratio of the output power of the amplifier to the input power, expressed in units of “dB†(decibel). The output gain of the power amplifier increases or decreases as the frequency of the input signal changes. This indicator is the most important basis for assessing the quality of power amplifiers. The smaller the decibel value, the flatter the frequency response curve of the power amplifier, the smaller the distortion, and the stronger the degree of reproduction and reproducibility of the signal.
Output Power
The power amplifier's power specification is strictly divided into nominal output power and maximum instantaneous output power. The former is the rated output power, which can be interpreted as the maximum value of the output power when the harmonic distortion changes within the standard range and can work safely for a long time; the latter refers to the "peak" output power of the power amplifier, which is interpreted as the power amplifier accepts When the electrical signal is input, the maximum output power that can be withstand instantaneously under the premise of ensuring that the signal is not damaged.
Receive gain
Gain is one of the main indicators of the antenna. It is the product of the direction coefficient and the efficiency, and is the performance of the antenna radiation or the size of the received wave. The choice of gain size depends on the requirements of the system design for the coverage area of ​​the radio wave. Simply put, under the same conditions, the higher the gain, the farther the distance the radio wave travels. The larger the receiving gain value of the power amplifier, the stronger the receiving performance.
Lightning protection
Common direct lightning protection measures:
1 lightning rod: lightning rod is used to protect industrial and civil high-rise buildings and power distribution units of power plants, transformer stations, individual sections of transmission lines, and in the process of extending the lightning pilot circuit to the ground, due to the influence of the lightning rod distortion circuit, Gradually turn and hit the lightning rod, thus avoiding the possibility of lightning leading to the protected equipment and destroying the protected equipment and buildings. It can be seen that the lightning rod is actually a lightning rod, which leads lightning to itself, thereby protecting other equipment from lightning strikes.
2 Lightning line: The lightning protection line is also called the overhead ground line. It is a metal wire that is placed along the line at the top of the tower and has good grounding. The lightning protection line is the main lightning protection measure for the transmission line.
3 Lightning protection belts and lightning protection nets: Metal grids placed on the building along the corners, ridges, corners and eaves that are vulnerable to lightning strikes are mainly used to protect tall civil buildings.
Surge protection
Surge is also called a surge, as the name suggests is a transient overvoltage that exceeds the normal operating voltage. In essence, a surge is a violent pulse that occurs in just a few millionths of a second. Possible causes of surges are: heavy equipment, short circuits, power switching, or large engines. Products containing surge arresters can effectively absorb sudden bursts of energy to protect connected equipment from damage.
Surge protector, also known as signal lightning protection device, is an electronic device that provides security protection for various electronic devices, instruments, and communication lines. When a sudden current or voltage is suddenly generated in an electrical circuit or a communication line due to external interference, the surge protector can conduct the shunt in a very short time, thereby preventing the damage of the surge to other devices in the circuit.
Performance
Regardless of the AV amplifier and Hi-Fi power amplifier, the power amplifier is very strict, and there are clear requirements in terms of output power, frequency response, distortion, signal-to-noise ratio, output impedance and damping coefficient.
Characteristics
The power amplifier is called the power amplifier. It can be said to be the largest family of all kinds of audio equipment. There are so many brands and models. Although they are all called power amplifiers, for their main purposes, power amplifiers can be divided into two main categories, which are dedicated amplifiers and civilian amplifiers. In the stadiums, theaters, dance halls, conference halls, public places, sound recording, and recording monitors, etc., there are often some unique requirements on their technical parameters. These amplifiers are usually called special. Amplifier or professional amplifier.
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