How much do you know about Power Amplifiers?

                                       Basic Knowledge of Power Amplifiers

Power amplifiers are usually divided into five categories according to their working status. That is, Class A (A), Class B (B), Class A and B (AB), and Digital (D)

1. Class A (Class A) power amplifier:

The output power is small, and the power consumption is large, but the distortion is small, so it is rarely used.

The working point of the output transistor (or electron tube) of the class A amplifier is at the midpoint of its linear part. No matter how the signal level changes, the current it takes out from the power supply is always constant. It is inefficient and used for audio amplification. Since the signal amplitude is constantly changing, its actual efficiency cannot exceed 25%, and it can be operated by a single tube or push-pull. The advantage of Class A amplifiers is that there is no crossover distortion and switching distortion, and the harmonic components are mainly even-order harmonics. In the sense of hearing, the bass is thick, the midrange is soft and warm, the treble is clear and neat, and the sense of hierarchy is very pleasing. . However, it has not been widely used in high-power amplifiers because of high power consumption, low efficiency, easy heat generation and high heat dissipation requirements. Since the device works under high current and high temperature for a long time, it is easy to cause reliability and life problems, and the cost of the whole machine is high. Therefore, most of the famous manufacturers of Class A power amplifiers have stopped producing transistor Class A power amplifiers.

2. Class B (B) release:

The bias of the Class B (B) amplifier makes the push-pull transistor (or tube) in a low current state when there is no drive signal, and when the drive signal is applied, the current of one of the pair of tubes rises within a half cycle , while the other tube tends to cut off, and in the other half cycle, the situation is reversed. Since the two tubes work in turn, a push-pull circuit must be used to amplify the complete signal waveform. The advantage of the Class B amplifier is that it has high efficiency, which can reach 78% in theory, but the disadvantage is that it has large distortion.

3. Class A and B (AB) Class AB power amplifier:

The output power is large, the power consumption is medium, but the distortion is larger than that of class A, and we use this type of power amplifier in large quantities at present.

When the class AB power amplifier is driven at a low level, the amplifier works as a class A, and when the drive level is increased, it turns into a class B work. The advantage of Class A and B amplifiers is that it improves the efficiency of small signal input compared with Class A. As the output power increases, the efficiency also increases. Although the distortion is larger than Class A, it is still the most widely used transistor power amplifier program. , the trend is that more and more Class A and B with high bias current are used to reduce the distortion of low-level signals. Most car power amplifiers are now in this way.

4. Digital (D) Class D power amplifier:

It is a new type of power amplifier (digital power amplifier) that has emerged in recent years. Its biggest advantage is that the power consumption is extremely small. It usually uses a Yang effect tube that is resistant to secondary breakdown and has a high switching conversion efficiency. The efficiency can reach more than 90%. (The maximum efficiency of ordinary Class A and Class AB power amplifiers does not exceed 50%). The power consumption of the audio is small, the heat dissipation requirements are low, and it is easy to integrate and mass produce. Another advantage is that the distortion is small, the conversion time is fast, and the transient response is fast and accurate. It is especially suitable for heavy and low single power amplifiers with high explosive power requirements. It has a tendency to replace other power amplifiers in car audio.

Class D power amplifier and traditional analog power amplifier are two different working principles. It belongs to switching audio power amplifier. Class D uses PWM technology, which is an amplifier whose switching frequency changes with the clock pulse period. In high-power applications, digital power amplifiers also have wide frequency response, large dynamic range and good transient response. Its advantages are small distortion, strong anti-interference ability, small radiator area, small size, light weight, low power consumption, high conversion efficiency, and Class A and B sound quality. At present, the main reason is that the cost is relatively high. Now that the development cost of software and hardware technology is reduced, the application of digital power amplifier will be more and more

Five, the main performance indicators of the power amplifier:

The main performance indicators of the power amplifier include output power, frequency response, distortion, signal-to-noise ratio, input sensitivity, input impedance, output impedance, damping coefficient, load capacity, operating voltage, etc.

1. Output power: the unit is W. Because the measurement methods of each manufacturer are different, there are some names with different names. Such as rated output power, maximum output power, music output power, peak music output power.

Music power (PMPO): refers to 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 output power calculated by the peak voltage that the music signal can reach instantaneously has more commercial significance than practical effect. The PMPO power can be 3 to 4 times higher than the internationally recognized RMS rated output power (RMS). For example, the RMS power per channel of the early portable radio recorders is only 4 or 5 watts, but it is marked with PMPO, and the value can be increased by one click. Up to about 20W.

Peak power: refers to the maximum music power that the amplifier can output when the volume of the amplifier is turned to the maximum without distortion.

Rated output power: the average output power when the harmonic distortion is 10%. Also known as the maximum useful power. Rated power For power amplifiers, rated power generally refers to the effective value (RMS) power that can be continuously output; for speakers, rated power generally means that the speaker can withstand the power of this value for a long time without damage. Only such a high-power power amplifier can be pushed, and the difficulty of driving the speaker is mainly determined by its sensitivity and impedance characteristics. It also does not mean that a power amplifier with an output power greater than the rated power of the speaker cannot be equipped. Just like driving a car, driving a sports car with a speed of 300 kilometers per hour does not mean that there will be a car accident, you don't have to drive so fast. Similarly, as long as the volume is not increased blindly, high-power amplifiers can also be matched with low-power speakers. Generally speaking, the peak power is greater than the music power, and the music power is greater than the rated power. Generally speaking, the peak power is 5-8 times the rated power.

2. Frequency response: Indicates the frequency range of the power amplifier and the unevenness within the frequency range. The flatness of the frequency response curve is generally expressed in decibels [db]. The frequency response of household HI-FI amplifiers is generally 20HZ--20KHZ plus or minus 1db. The wider the range, the better. The frequency response of some top-quality power amplifiers has reached 0--100KHZ.

Distortion: The ideal power amplifier should amplify the input signal and restore it faithfully without any change. However, due to various reasons, compared with the input signal, the signal amplified by the power amplifier often produces different degrees of distortion, which is called distortion. Expressed as a percentage, the smaller the value, the better. The total distortion of the HI-FI power amplifier is between 0.03% and 0.05%. The distortion of the power amplifier includes harmonic distortion, intermodulation distortion, crossover distortion, clipping distortion, transient distortion, and transient intermodulation distortion.

3. Signal-to-noise ratio: refers to the ratio of various noise levels output by the power amplifier to the signal level, expressed in db, the larger the value, the better. Generally, the signal-to-noise ratio of household HI-FI amplifiers is above 60db.

Input Sensitivity: This is the adjustment level set for different manufacturers, different brands of hosts and pre-amplifier audio sources. The range is from 100mv to 4V or even higher. When tuning, it must match the audio source.

Input impedance: It is generally required that the input impedance of the power amplifier should be high, and the output impedance should be low. The higher the input impedance, the more effective it is to block various noises. The common value is 10KΩ or higher.

4. Output impedance: The equivalent internal resistance presented to the speaker is called output impedance.

Damping coefficient: This coefficient is calculated from the output impedance of the rated load (4Ω). It is generally believed that the smaller the output impedance and the higher the damping coefficient, the better the amplifier. In fact, the ratio of high-quality power amplifiers is mostly above 50, and a few even exceed 500. Although some experts believe that a damping coefficient of around 50 is sufficient, judging from experience, if the coefficient is high, the requirements for cables can be relaxed. Conversely, if the coefficient is too high, it will affect the timbre, but it will help the bass performance.

Load capacity: home power amplifiers are generally 8Ω/4Ω; car power amplifiers are 2Ω to 8Ω in stereo; 4Ω to 8Ω in bridge connection. However, for some specially designed power amplifiers, the impedance can be as low as 0.1Ω, which is extraordinary. At this time, a power amplifier can connect dozens of woofers in parallel to create an ideal sound pressure level (sound pressure refers to the pressure generated by sound on the human ear, which is a standard for measuring the ability of the sound system, because the higher the sound pressure is, the higher the sound pressure is. The higher the system requirements, the higher the domestic record is 141.5dB, and the foreign is 176.5dB). Just like those vans with dozens of woofers in the car, which can smash the glass, but this scene may only be seen in the audio competition.