What are the technical principles of power amplifiers? And buying guide?

In general, an amplifier is any device that can use less energy to control more energy. Nowadays, in everyday use, the term often refers to amplifier circuits, often used in audio applications. The input-output relationship of an amplifier is often expressed as a function related to the input frequency. This relationship is called the amplifier's transfer function, and the coefficient of this transfer function is defined as the gain.

In general, an amplifier is any device that can use less energy to control more energy. Nowadays, in everyday use, the term often refers to amplifier circuits, often used in audio applications.

The input-output relationship of an amplifier is often expressed as a function related to the input frequency. This relationship is called the amplifier's transfer function, and the coefficient of this transfer function is defined as the gain.

Buy

When selecting a power amplifier, you must first pay attention to some of its technical indicators: 1. Input impedance: usually indicates the size of the power amplifier's anti-interference ability, generally in the range of 5000-15000Ω, the larger the value, the stronger the anti-interference ability; 2. Distortion: refers to the degree of distortion of the output signal compared to the input signal. The smaller the value, the better the quality, generally below 0.05%; 3. Signal-to-noise ratio: the ratio between the music signal and the noise signal in the output signal. The larger the sound, the cleaner the sound.

In addition, when purchasing a power amplifier, you must also make your purchase intention clear. 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 drive the front and rear speakers, and the subwoofer can only be equipped with another power amplifier. The 5-channel power amplifier can solve this problem. The output power of the power amplifier must be greater than the rated power of the speaker.

Principle

The high-frequency power amplifier is used for the final stage of the transmitting stage. Its role is to amplify the high-frequency modulated signal to meet the transmission power requirement, and then radiate it to space through the antenna to ensure that the receiving stage in a certain area can Receive a satisfactory signal level without interfering with communication on adjacent channels.

A high-frequency power amplifier is an important component of a transmitting device in a communication system. According to the width of its operating frequency band, it is divided into two types: narrow-band high-frequency power amplifier and wide-band high-frequency power amplifier. Resonant power amplifier; the output circuit of a broadband 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 DC energy supplied by a power source into a high-frequency AC output. It is known in the course of "low-frequency electronic circuits" that amplifiers can be classified into three types of working states according to the current conduction angle. Class A amplifier current flow angle is 360 degrees, suitable for small signal low power amplification. Class B amplifier current flow angle is approximately 180 degrees; Class C amplifier current flow angle is less than 180 degrees. Both B and C are suitable for high power work. The output power and efficiency of Class C working state are the highest of the three working states. Most high-frequency power amplifiers work in Class C. However, the current waveform distortion of the Class C amplifier is too large, so it cannot be used for low-frequency power amplification, and can only be used for resonance power amplification using a tuning loop as a load. Due to the filtering capability of the tuning loop, the loop current and voltage are still very close to the sinusoidal waveform, and the distortion is very small. In addition to the above types of working states classified by the angle of current flow, there are D-type amplifiers and E-type amplifiers that make electronic devices work in the switching state. The efficiency of Class D amplifiers is higher than that of Class C amplifiers, theoretically up to 100%, but its maximum operating frequency is limited by the device power consumption (collector dissipated power or anode dissipated power) generated by the switching transition .

If the circuit is improved to minimize the power consumption of the electronic device at the moment of on-off transition, the operating frequency can be increased. This is a class E amplifier. We already know that in order to obtain sufficient 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 provided by a DC power supply to an AC output. The common characteristics of high-frequency power amplifier and low-frequency power amplifier are high output power and high efficiency, but the working frequency and relative frequency bandwidth of the two are very different, which determines that there is an essential difference between them. Low-frequency power amplifiers operate at low frequencies but have a relatively wide frequency band. For example, from 20 to 20000 Hz, the ratio of high and low frequencies is 1000 times. Therefore, they all use non-tuned loads, such as resistors and transformers. High-frequency power amplifiers have high operating frequencies (from hundreds of kHz to hundreds, thousands, or even tens of thousands of MHz), but the relative frequency band is very narrow. For example, the AM radio station (535-1605 kHz frequency range) has a frequency bandwidth of 10 kHz. If the center frequency is set to 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 selection network as the load circuit. Due to this latter characteristic, the working states selected for these two amplifiers are different: low-frequency power amplifiers can work in Class A, Class B, 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 is widely used in each intermediate stage of a wideband transmitter. It does not use a frequency selection network as a load circuit, but 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 retune. In summary, it can be seen that the common point of high-frequency power amplifiers and low-frequency power amplifiers is that they require high output power and high efficiency; the difference between them is that their operating frequencies and relative bandwidths are different. So the load network and working status are also different.

The main technical indicators of high-frequency power amplifiers are: output power, efficiency, power gain, bandwidth, and degree of harmonic suppression (or signal distortion). These several index requirements are contradictory. When designing the amplifier, some indicators should be highlighted, and other indicators should be taken into consideration. For example, in some circuits, preventing interference is the main contradiction, requiring higher levels of harmonic suppression, and reducing the bandwidth requirements appropriately. The efficiency of a power amplifier is a prominent issue, and its efficiency is directly related to the working state of the amplifier. The working status of the amplifier can be divided into Class A, B and C. In order to improve the working efficiency of the amplifier, it usually works in class B and C, that is, the transistor operation extends to the non-linear region. However, there are serious non-linear distortions between the output current and output voltage of the amplifiers in these operating states. Low-frequency power amplifiers, because of their large frequency coverage factor, cannot use a resonant circuit as a load, so they generally work in Class A status; they can work in Class B when a push-pull circuit is used. The high frequency power amplifier has a small frequency coverage factor of the signal, and can use a resonant circuit as a load, so it usually works in class C. With the frequency selection function of the resonant circuit, the harmonic components in the collector current of the amplifier can be filtered and selected. The fundamental component thus substantially eliminates non-linear distortion.

Therefore, high-frequency power amplifiers have higher efficiency than low-frequency power amplifiers. Because high-frequency power amplifiers work in the non-linear state of large signals, linear equivalent circuit analysis cannot be used. In engineering, the analytical approximation analysis method—polyline method is generally used to analyze its working principle and working state. The physical concept of this analysis method is clear, and the analysis work status 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 sufficiently strong narrow-band signal power centered at the carrier frequency, or to amplify narrow-band modulated signals or achieve frequency doubling functions, usually working at Class, C status. Broadband high-frequency power amplifier: It is used to amplify short-wave frequencies of certain carrier signals, and mid-level amplifier stages of ultra-short-wave radio stations to avoid tedious tuning of different fc. Usually works in a Class A state.

Performance 

No matter the AV amplifier and Hi-Fi amplifier have strict requirements on the power amplifier, they have 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 to the load by the amplifier circuit. At present, the measurement methods and evaluation methods of output power are not uniform, so pay attention when using them.

1.Rated power (RMS):

It refers to the high power (strictly speaking a sine wave signal) that can be output by the power amplifier in a certain harmonic range for a long time. The average power when the harmonic distortion is 1% is often referred to as the rated output power or the maximum useful power, continuous power, and undistorted power. Obviously, when the prerequisites for the specified distortion are different, the rated power values ​​will be different.

2.Maximum output power

When the distortion is not considered, the output power of the power amplifier circuit can be much higher than the rated power, and it can also output a larger amount of power. The high power it can output is called the high power output. The aforementioned rated power and high output power Is the output power of two different prerequisites

3.Music output power (MPO)

Music output power MPO is the abbreviation of English Music Power Outpur. It refers to the output power of the power amplifier circuit when it works on music signals, that is, the instantaneous output power of the power amplifier to the music signal under the condition that the output distortion does not exceed the specified value.

Music output power can be used to evaluate the dynamic listening effect of the amplifier. For example, after a smooth music process, a strong percussion instrument sound suddenly appears. Some amplifier circuits can provide a large output power to the sense of strength. Unstoppable; some power amplifiers seem to be inadequate. In order to reflect the ability of sudden output power at this moment, it can be measured by the output power of music.

4.Peak music output power (PMPO)

It is the maximum music output power, and it is another dynamic indicator of the power amplifier circuit. If you do not consider the distortion, the maximum music power that the power amplifier circuit can output is the peak music output power.

Usually 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 practical statistics, the peak music output power is 5-8 times the rated output power.

Frequency response 

The frequency response reflects the power amplifier's ability to amplify 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 the ideal case, the operating frequency range of the main channel audio power amplifier is 20 -20kHz. The international regulation of the general audio power amplifier frequency range is 40-16 kHz ± 1.5dB.

Distortion 

Distortion is a phenomenon in which the waveform of a reproduced audio signal changes. There are many causes and types of waveform distortion, including harmonic distortion, intermodulation distortion, and transient distortion.

Dynamic Range 

The ratio of the amplifier's undistorted amplified small signal to the large signal level is the dynamic range of the amplifier. In practice, 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 90dB.

Various noises in nature form the surrounding background noise, and the surrounding background noise and the sound intensity of the performance are very different. In normal cases, this intensity difference is called dynamic range. A good sound system should not input a strong signal. Overload distortion occurs, and when a 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. The noise can only be reduced as much as possible, but it is impossible to not generate 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 decibel number of the ratio of the sound signal level output to the various noise levels output by the attack-amplifier circuit is called the size of the signal-to-noise ratio.

Output impedance and damping coefficient

1.Output impedance

The equivalent internal impedance shown by the output end of the power amplifier and the load (speaker) is called the output impedance of the power amplifier.

 2.Damping coefficient

The damping coefficient refers to the ability of the power amplifier circuit to resistively load the load.

 

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