Basic Introduction to Power Amplifier

Catalog

Ⅰ Working principle

Use the current control function of the transistor or the voltage control function of the field-effect tube to convert the power of the power supply into a current that changes according to the input signal. Because sound is waves of different amplitudes and frequencies, that is, AC signal current, the collector current of the transistor is always β times the base current in the amplification area, and β is the current amplification factor of the transistor. Applying this point, if the small signal is injected into the base, the current flowing through the collector will be equal to β times the base current, and then isolate this signal with a DC blocking capacitor to obtain a large signal with a current (or voltage) that is β times the original. This is the amplifying effect of the transistor. After continuous current amplification, the power amplification is completed.

Figure 1. Power amplifier

Ⅱ Main classification

Class A amplifier:

The main feature of the Class A amplifier is: the operating point Q of the amplifier is set near the midpoint of the load line, and the transistor is turned on during the entire cycle of the input signal. The amplifier can work in a single tube or push-pull. Since the amplifier works in the linear range of the characteristic curve, transient distortion and alternating distortion are small. The circuit is simple and the debugging is convenient. However, the efficiency is low, the transistor power consumption is large, the theoretical maximum efficiency is only 25%, and there is a large nonlinear distortion. Therefore, the efficiency is relatively low.

Class B amplifier:

The main characteristic of a Class B amplifier is: the static point of the amplifier is at (VCC, 0), when there is no signal input, the output end consumes almost no power. In the positive half cycle of Vi, Q1 turns on and Q2 turns off, and the output is a positive half-cycle sine wave; in the same way, when Vi is a negative half-wave sine wave, two tubes must be used for push-pull work. Its characteristic is higher efficiency (78%), but because the amplifier has a section of work in the nonlinear region, its disadvantage is that the "crossover distortion" is larger. That is when the signal is between -0.6V~0.6V, both Q1 and Q2 cannot be turned on. So this type of amplifier is gradually abandoned by designers.

Class AB amplifier:

The main feature of Class AB amplifiers is: the on-time of the transistor is slightly longer than half a cycle, and two tubes must be used for push-pull operation. It can avoid crossover distortion. The alternating distortion is large, which can offset even harmonic distortion. It has the characteristics of higher efficiency and lower transistor power consumption.

Class C amplifier:

The main feature of Class C amplifiers is that the transistors only work within a short period of each cycle of the input signal. When the circuit is working, a negative bias is usually provided to the amplifier tube to ensure that the transistor does not work in a Class B state. Its collector load is not a resistor but an LC parallel resonant circuit, so the class C amplifier is also called a resonant amplifier circuit. The frequency selection function is achieved by adjusting the capacitance value of the capacitor or the inductance value of the inductor. The conversion efficiency of class C amplifiers is extremely high, reaching 98%. However, because the load is a resonant circuit, the circuit often works at high frequency, so the distortion is large. Therefore, the class C amplifier is not suitable as an audio power amplifier. On the contrary, it is widely used in the radio world because of its optional frequency characteristics, so it is usually used as a radio frequency amplifiers, tuning amplifiers and frequency multipliers.

Class D amplifier:

Class D (digital audio power) amplifier is a kind of switching amplifier that turns the input analog audio signal or PCM digital information into PWM (pulse width modulation) or PDM (pulse density modulation) pulse signal, and then use the PWM or PDM pulse signal to control the on/off of the large power switching devices’ audio power amplifiers. It has outstanding advantages of high efficiency. The digital audio power amplifier also looks like a one-bit power digital-to-analog converter. The amplifier consists of an input signal processing circuit, a switching signal forming circuit, a high-power switching circuit (half-bridge and full-bridge) and a low-pass filter ( LC) and other four parts. Class D amplifier or digital amplifier uses a very high-frequency switch circuit to amplify the audio signal.

Advantage:

1) It has high efficiency, usually above 85%;

2) Small size, which can save a lot of space than analog amplifier circuits;

3) Connection without crack noise;

4) Low distortion and good frequency response curve. Few peripheral components, easy to design and debug.

Class A, B, and AB amplifiers are analog amplifiers, and class D amplifiers are digital amplifiers. Class B and Class AB push-pull amplifiers are more efficient and have less distortion than Class A amplifiers. The power amplifier transistors consume less power and have better heat dissipation. However, Class B amplifiers will have alternate distortion due to poor switching characteristics or improper selection of circuit parameters during the transition between transistor on and off states. The Class D amplifier has high efficiency and low distortion, and a good frequency response curve. Class AB amplifier and Class D amplifier are the basic circuit forms of audio power amplifiers.

Class T amplifier:

The power output circuit of the T class power amplifier is the same as the pulse width modulation class D power amplifier. The power transistor also works in the on-off state, and its efficiency is equivalent to that of a Class D power amplifier. But it is different from an ordinary class D power amplifier:

First of all, it is not the method of pulse width modulation. Tripath company invented a digital power technology called Digital Power Processing (DPP), which is the core of Class T power amplifiers. It uses adaptive algorithms and prediction algorithms for small signal processing in communication technology. The input audio signal and the current entering the speaker are digitally processed by DPP and used to control the turn-on and turn-off of the power transistor. So that the sound quality can achieve high-fidelity linear amplification.

Secondly, the switching frequency of its power transistor is not fixed, and the power spectrum of the useless components is not concentrated in the narrow frequency bands on both sides of the carrier frequency but scattered over a wide frequency band. So that the details of the sound can be "heard" throughout the frequency band.

In addition, Class T power amplifiers have a wider dynamic range and flat frequency response. The emergence of DDP has pushed the power amplifier of the digital age to a new level. In terms of high fidelity, the linearity is better than traditional AB power amplifiers.

Ⅲ Basic components

The power amplifier usually consists of three parts: preamplifier, driver amplifier, and final power amplifier.

Figure 2. power amplifier

1. The preamplifier plays a matching role. Its input impedance is high (not less than 10kΩ;), which can absorb most of the previous signals, and the output impedance is low (below tens of Ω;), and most of the signals can be transmitted. At the same time, it is also a current amplifier, which converts the input voltage signal into a current signal and gives it a proper amplification.

2. The driver amplifier acts as a bridge. It further amplifies the current signal sent by the preamplifier and amplifies it into a medium power signal to drive the final power amplifier to work normally. If there is no driver amplifier, the final power amplifier cannot send high-power sound signals.

3. The final power amplifier plays a key role. It forms a high-power signal from the current signal sent by the drive amplifier to drive the speaker to produce sound. Its technical indicators determine the technical indicators of the entire power amplifier.

Ⅳ Main indicators

Technical index

Figure 3. power amplifier

1. Rate power: refers to the continuous sine wave power. Under 500Hz sine wave input and a certain load, the harmonic distortion is less than 1% of the output power, expressed as W/CH (watts per channel). Generally speaking, the higher the rated power, the higher the cost.

2. Total harmonic distortion (THD): refers to the percentage of high-order harmonics to the fundamental wave. The smaller the total harmonic distortion, the better. The total harmonic distortion of a good power amplifier can reach 0.02%

3. Slew rate: the voltage amplitude rising per unit time, in volts/microsecond. It reflects the power amplifier's ability to track transient sound signals and is a transient characteristic index.

4. Damping factor: It is defined as the load impedance of the power amplifier (the internal resistance of the high-power tube plus the wiring resistance of the speaker), such as 8Ω: 0.04Ω=200:1. Generally, the ratio is relatively large, but it cannot too large, or it will make the speaker sound thin.

5. Output impedance (or rated load impedance): usually 8Ω, 4Ω, 2Ω, etc. The smaller the value, the stronger the load capacity of the power amplifier. In terms of a single channel, a power amplifier with a rated load of 2Ω can drive 4 speakers with an impedance of 8Ω to produce sound with very little distortion.

Performance index

Regardless of the AV amplifier and Hi-Fi power amplifier, the requirements for the power amplifier are very strict. 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 to the load by the power amplifier circuit. People have very different measurement methods and evaluation methods for output power, so pay attention to them when using them.

1. Rated power (RMS): It refers to the maximum power that the power amplifier can output for a long time within a certain harmonic range (strictly speaking, it is a sine wave signal). The average power when the harmonic distortion is 1% is often referred to as rated output power or maximum useful power, continuous power, undistorted power, etc. Obviously, when the prescribed distortion conditions are different, the rated power value 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 value of power. The maximum power it can output is called the maximum output power.

3. Music power output (MPO): MPO is the abbreviation of Music Power Output. It refers to the power output of the power amplifier circuit when working with music signals, that is, when the output distortion does not exceed the specified value, the instantaneous maximum output power of the music signal.

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

4. Peak music power output (PMPO): It is the maximum music power output and another dynamic indicator of the power amplifier circuit. If the distortion degree is not considered, the maximum music power that the power amplifier circuit can output is the peak music output power.

Usually, the peak music power output is greater than the music output power, the music power output is greater than the maximum output power, and the maximum power output is greater than the rated power output.  According to practical statistics, the peak music power output is 5-8 times the rated power output.

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 auditory frequency range of the human ear. Therefore, under ideal circumstances, the operating frequency range of the main channel audio power amplifier is 20 -20000Hz. According to international regulations, the frequency range of a general audio power amplifier is 40-16000Hz±1.5dB.

Distortion:

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

Dynamic Range:

The ratio of the minimum signal to the maximum signal level of the amplifier without distortion is the dynamic range of the amplifier. In actual application, the ratio uses dB to represent the level difference between the two signals, and 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 is very different from the sound intensity of the performance. Under normal circumstances, this difference in intensity is called the dynamic range. A good sound system should not input strong signals. Overload distortion is generated, and when a weak signal is an 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 noise can only be minimized, but it is impossible not to produce 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 decibels of the ratio of the output sound signal level of the attack amplifier circuit to the various output noise levels is called 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 resist the load.

Ⅴ Main application

Whether in civil fields such as global mobile communication systems, fourth-generation mobile communication systems, wireless local area networks, or military fields such as radar, electronic warfare, and navigation, RF power amplifiers are used as front-end devices in these systems to provide low power consumption, high efficiency, and the requirement for small size is increasing rapidly.

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

The traditional power amplifier has very low working efficiency due to the power consumption on the output circuit. In order to increase the working efficiency of the traditional power amplifier, the ideal class F power amplifier uses an output filter to control the harmonic components in the output voltage or current of the transistor, and normalize the voltage and current waveforms output by the transistor. In this way, 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 λ/4, so that there is no overlap zone between the collector voltage and current waveforms, thus achieving the ideal efficiency of 100%.