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Mar 20 2018

Working Principle of Power Amplifier and Its Classification

Warm hints: The word in this article is about 3000 words and the reading time is about 15 minutes.

Summary

In this article, we will mainly talks about how power amplifier works and its classes and indicators. Nowadays, whether in the global mobile communication system, the third generation mobile communication system, the wireless local area network (WLAN) and other civilian fields, or in the radar, electronic warfare, navigation and other military fields, RF power amplifier is used as the front-end devices of these systems. The requirement for low consumption, high efficiency and small size increases rapidly.

A power amplifier is an amplifier capable of producing a maximum power output to drive a load (such as a loudspeaker) at a given distortion rate. The power amplifier plays a pivotal role in "organizing and coordinating" the entire audio system. To some extent, it determines whether the whole system can provide a good sound output. 


Article CoreIntroduction to power amplifierPurposeIntroduce how power amplifier works and its classes and indicator
English namePower amplifierCategoryElectronic amplifier
FunctionAn amplifier capable of producing a maximum power output to drive a loadFeatureLow distortion, low noise, wide dynamic range



Catalogs

CatalogsI. How Does Power Amplifier Work4. Class D amplifier3. Final Power amplifier
II. Power Amplifier Classes5. Class T amplifiersIV. Power Amplifiers Main indicators
1. Class A amplifiersIII. Basic Components of Power Amplifiers1. Technical Indicators
2. Class B amplifiers1. Technical Indicators2. Performance Indicators
3. Class AB amplifiers2. Performance Indicators



Introduction  

I. How Does Power Amplifier Work

The working principle of the power amplifier is to convert the power of the power supply to the current varying according to the input signal by using the current control action of the transistor or the voltage control action of the FET. Because the sound is waves of different amplitudes and different frequencies, that is, the AC signal current, the collector current of the triode is always β times that of the base current, and β is the AC amplification factor of the triode. If the small signal is injected into the base electrode, the current flowing through the collector will be equal to that of the base current. Then the signal is isolated with a straight capacitor, and the current (or voltage) is a large signal which is β times the original one. This phenomenon becomes the amplification of the triode. After constant current and voltage amplification, power amplification is completed.

Power amplifier--Working Principle of Power Amplifier and Its Classification

Power amplifier

It is well known that power amplifier has maximum power loss in many modules of RF circuit. As the core and front end of the system, its efficiency will directly affect the efficiency of the system, so efficiency has become a hot topic in the research of modern power amplifier. In most power amplifiers, the power loss is mainly caused by transistor loss, which is mainly caused by voltage and current. Therefore, the switching power amplifier is put forward, including D class E class and F class. Class F power amplifiers are designed to control drain voltage and current waveform by designing a harmonic network. Theoretically, the drain efficiency of Class F power amplifier is 100%, which is called the new generation power amplifier. 

Because of the power consumption in the output circuit, the efficiency of the traditional power amplifier is very low. In order to increase the efficiency of the traditional power amplifier, the output filter is used by the ideal Class F power amplifier to control the harmonic components in the output voltage or current of the transistor, and the output voltage and current waveform of the transistor are integrated. Then the angle parameter of collector current is 90 °, that is to say, the collector waveform is half sine wave, the collector voltage waveform is square wave, and the phase difference between them is λ / 4. In this way, the voltage and current waveforms of the collector do not overlap, thus achieving an ideal efficiency of 100%.



Detail

II. Power Amplifier Classes

The traditional digital speech playback system consists of two main processes:

1. Conversion of Digital speech data to Analog speech signal. ( Realized by using high precision digital-to-analog converter (DAC));

2.Amplification of analog signals by using analog power amplifiers, such as Class A, Class B and Class AB Amplifier. Since the early 1980s, many researchers have been working on developing different types of digital amplifiers. This kind of amplifier is used to amplify power directly from digital speech data without the need of analog conversion. This kind of amplifier is usually referred to as a digital power amplifier or a class D amplifier.

Amplifier classes--Working Principle of Power Amplifier and Its Classification

Amplifier classes

  • 1. Class A amplifiers

The main features of class A amplifier are that the operating point Q of the amplifier is set near the midpoint of the load line and the transistor is switched on throughout the period of the input signal. The amplifier can work on a single tube or push-pull. Because the amplifier operates in the linear range of the characteristic curve, the transient distortion and alternating distortion are smaller. The circuit is simple and easy to debug, but the efficiency is low, and the transistor power consumption is high. The theoretical maximum of efficiency is only 25% and there is a large nonlinear distortion.

  • 2. Class B amplifiers

The main features of the class B amplifier are that the static point of the amplifier is at the point of (VCC, 0). When there is no signal input, the output end almost consumes no power. In the positive half period of Vi, Q1 is on and Q2 is off. And the output terminates positive half cycle sine wave. Similarly, when Vi is a negative half wave sine wave, two tubes must be used to push-pull work. It is characterized by its higher efficiency (78%). However, because the amplifier has a section of work in the nonlinear region, its shortcoming is that the "cross-over distortion" is larger. That is, it is caused when the signal is between -0.6V ~ 0.6V, Q1Q2 can not be switched on. So this kind of amplifier is also gradually abandoned by designers.

  • 3. Class AB amplifiers

The main features of class AB amplifier are that the conduction time of the transistor is slightly longer than half period, it must be operated with two transistors push-pull, which can avoid cross-over distortion. The alternating distortion is large, which can counteract the even harmonic distortion. It is characterized by its higher efficiency and lower power consumption of the transistors.

A short video explaining the differences between class A, B and AB amplifiers.

  • 4. Class D amplifier

A Class D (Digital Audio Power) amplifier is a pulse signal that converts input analog audio signals or PCM digital information into PWM (Pulse Width Modulation) or PDM (Pulse Density Modulation). Then the pulse signal of PWM or PDM is used to control the on-off audio power amplifier of high power switch device. Class D amplifier has the outstanding advantage of high efficiency. The digital audio power amplifier also appears to be a one bit power digital-analog converter.The amplifier consists of four parts: input signal processing circuit, switch signal forming circuit, high power switch circuit (half bridge type and full bridge type) and low pass filter (LC). Class D amplifiers or digital amplifiers amplify the audio signal by using very high frequency switching circuits.

Advantages:

> Have very high efficiency, usually can reach more than 85%;

> Small size, can save a lot of space than analog amplifier circuit;

> No crack noise while connection;

> Low distortion, good frequency response curve, 

> Less peripheral components, easy to design and debug

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

Power amplifier--Working Principle of Power Amplifier and Its Classification

Power amplifier

  • 5. Class T amplifiers

The power output circuit and pulse width modulation of class T power amplifier are the same as those of class D power amplifier. And the power transistor also works in the switching state. The efficiency of the power transistor is the same as that of the class D power amplifier. However, it is different from the ordinary class D power amplifier.

First of all, instead of using pulse width modulation, Tripath has invented a digital power technology called Digital Power Processing (DPP), which is the core of Class T power amplifier. It uses the adaptive algorithm and prediction algorithm for small signal processing in communication technology. The input audio signal and the current entering the speaker are digitally processed by DPP to control the turn-on of the power transistor. Thus the sound quality can reach high fidelity linear amplification.

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

In addition, the dynamic range of class T power amplifier is wider and the frequency response is flat. DDP brings the power amplifier of the digital age to a new height. In terms of high fidelity, the linearity is better than that of class AB power amplifier.

III. Basic Components of Power Amplifiers

Power amplifiers usually consist of three parts: preamplifier, driver amplifier, and final power amplifier.

  • 1. Preamplifier

 The preamplifier acts as a matching role, whose input impedance is high (not less than 10k Ω), and can absorb most of the signal in front of it. Its output impedance is low (below tens of ohm), which can transmit most of the signal out. At the same time, it is a kind of current amplifier, which converts the input voltage signal into the current signal and amplifies it appropriately.

  • 2. Driver Amplifier

The driver amplifier acts as a bridge, which amplifies the current signal from the preamplifier and amplifies it to a medium power signal to drive the final power amplifier to work normally. If there is no driver amplifier, the final power amplifier can not send out high power sound signal.

  • 3. Final Power amplifier

The final power amplifier plays a key role. The current signal sent by the driver amplifier is formed into a high power signal, which drives the loudspeaker to sound. Its technical specifications determine the technical specifications of the whole power amplifier.



Analysis

IV. Power Amplifiers Main indicators

  • 1. Technical Indicators

> Rate Power: 

Rated power refers to the power output of a continuous sine wave. Under the input of 1 kHz sine wave and a certain load, the harmonic distortion is less than 1% of the output power, which is expressed as W / Ch (Watt / Channel). Generally speaking, the higher the rated power is, the higher the cost will be

> Total harmonic distortion (THD): 

Total harmonic distortion refers to the percentage of high order harmonics in the fundamental wave. The smaller the total harmonic distortion is the better. The total harmonic distortion of a good power amplifier can reach 0.02%.

> Slew rate: 

Slew rate refers to the voltage range increased by unit time. The unit is V/μs . It reflects the tracking ability of the power amplifier to the transient sound signal and is a transient characteristic index.

> Damping factor: 

Damping factor is defined as the load impedance of the power amplifier (Internal resistance of high power tube plus wiring resistance of loudspeaker). Eg. 8Ω:0.04Ω=200:1. The general requirement ratio is bigger, but cannot be too big. If too big,the loudspeaker will sound thin and if too small, it will make the sound muddy, the level of sound is poor, and the distribution of sound and image is not good.

> Output impedance (Rated load impedance): The general value of the output impedance is 8Ω,4Ω,2Ω. The smaller the value is, the stronger the load capacity of the power amplifier will be. For a single channel, a power amplifier with a rated load of 2Ω can drive 4 speakers with an impedance of 8Ω, and the distortion is very small.

  • 2. Performance Indicators

Both the AV amplifier and the Hi-Fi power amplifier have strict requirements on the power amplifier, such as output power, frequency response, distortion, signal-to-noise ratio, output impedance and damping coefficient.

> Output power:

The output power refers to the power transmitted to the load by the power amplifier circuit. The measuring method and the evaluation method of the output power are not uniform, so attention should be paid to the use of the 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. The operating frequency range of the main channel audio power amplifier is 20-20 kHz. The frequency range of the international standard audio power amplifier is 40-16 kHz ±1.5 dB.

> Distortion:

Distortion is the phenomenon of waveform change of playback audio signal. There are many causes and types of waveform distortion, including harmonic distortion, intermodulation distortion, transient distortion and so on.

> Dynamic range:

The ratio of the 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, and the dynamic range of the high fidelity amplifier should be greater than 90 dB.

All kinds of noise in nature form the background noise around it, and the intensity of the background noise and the sound intensity of playing is very different. In general, the difference in intensity is called the dynamic range. A good sound system should not produce overload distortion when inputting a strong signal, but when inputting a weak signal, it should not be submerged by the noise produced by itself. For this reason, a good sound system should have a larger dynamic range, and the noise can only be reduced as much as possible. But it is impossible not to produce noise.

> Signal-to-noise ratio (SNR):

The signal-to-noise ratio (SNR) 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 output sound signal level to the output noise level of the attack and amplifier circuit is called the size of the signal to noise ratio.

> Output impedance:

Output impedance refers to the equivalent internal impedance of the output end of the amplifier and the load (loudspeaker).

> Damping coefficient:

Damping coefficient refers to the ability of a power amplifier circuit to carry out resistive nitriding on a load.



Book Recommendation

  • Audio Power Amplifier Design

This is the essential book reference for amplifier designers. Douglas Self covers all the design issues of noise, distortion, power supply rejection, protection, reliability, and layout. He describes advanced forms of compensation that give dramatically lower distortion.

This edition is much expanded, and packed with new information. It is a must-have for audio power amplifier professionals and audiophiles, amateur constructors and anyone with intellectual curiosity about the struggle towards technical excellence.

--Douglas Self

  • Audio Power Amplifiers: Towards Inherently Linear Amplifiers Paperback

Most audio amplifiers are designed and measured using well established practices that have hardly changed during last thirty or more years. Despite their widespread use, these practices are not necessarily the best available. Harmonic distortion levels are in many cases below a realistically audible limit. Still, many people claim to hear differences between amplifiers with similar THD performance. Therefore, it is clear that harmonic distortion is not the metric that should be optimized for best subjective quality. That indicates that there are other errors to be considered in amplifier design, which do not receive enough attention. In this study, Dr. Kolinummi analyses many sources of distortion and measurement methods. His focus is specifically on dynamic and transient errors, which are not normally considered in measurements, and what Kolinummi calls ‘memory errors’1. Optimization of internal linearity is thought to be the best way to optimize dynamic and transient performance and to maintain good linearity and predictable characteristics over a wide frequency and dynamic signal range. 

--Mr Arto Kolinummi



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8 comments

    • sam on 2018-3-20 14:02:39

    Hi~blogger,thank you for offering such a useful pages. Could you help me? I am new to electrical stuff so bare with me and please answer as simply as possible. How would I go about this problem that I am having? I want to power my raspberry pi, my pre-amplifier (AC 12V), and my amplifier (DC 12V) all through one power supply. How would I do this? Thank you in advance!

      • author on 2018-3-20 14:29:24
        author

      Re:

      Your Raspberry is looking for 5V DC and these are often powered from a USB connection. Your preamp wants 12V AC and your amp 12V DC.This is not too difficult:
      First get a transformer to generate 12V AC from whatever your local mains is, Not knowing where you are I cant help there its 100V in Japan, 110 in the US and 230 in most, if not all of Europe.Use a full bridge rectifier on the output of the transformer to generate a DC rail it will be around 15V or so allowing for diode drops then regulate this down to 12V and 5V.You can use a 7805 for your 5V output but it will need a heatsink.
      I can not be more specific as I do not know your power requirements. I know a Pi is less than 100mA as I have one but I have no idea how much power your amp and pre-amp need. It could be a simple IC such as a 7812 or much more complicated at higher powers. The power level also dictates the size of your transformer.
      A 7805 will be sufficient for your 5V rail but will require a heatsink.


    • powerful on 2018-3-20 14:04:48

    Excellent pages! It introuduces power amplifier in detail! This website like an electronic encyclopedia.

    • vivitern on 2018-3-20 14:08:45

    This is very helpful for me, thank u!

    • Sanyantan on 2018-3-20 14:18:34

    Good. A power amplifier is an amplifier capable of producing a maximum power output to drive a load (such as a loudspeaker) at a given distortion rate. The power amplifier plays a pivotal role in "organizing and coordinating" the entire audio system. To some extent, it determines whether the whole system can provide a good sound output.

    • Stratton on 2018-3-20 14:33:27

    Hi~Author I am confusing about your answer for @Sam. A 7805 probably won't be sufficient for a typical modern pi? And especially not with 12+ volts in. With that much headroom a switching regulator is all but unavoidable.

      • author on 2018-3-20 14:35:21
        author

      Re:

      Hi~@Stratton

      You could add a series resistor. In series with the input. Wire wound resistors get hot but are OK with it Say 82R or 91R @ 1W or do the latest Pi models draw significantly more current than the old one I have?

    • Stratton on 2018-3-20 14:36:29

    @author You're still going to nearly triple the amount of heat produced by the system and the amount of power the supply must provide. For something like a pi3 with a tft LCD or a few USB peripherals even a 2.5 amp supply can be marginal - with a linear regulator that means you actually need something like a 7 amp supply at 12v just for the pi, even before you get to the other audio amps!

      • author on 2018-3-20 14:38:07
        author

      Re:

      I stand corrected, my original pi draws less than 100mA but if newer models are drawing 2A+ then yes you need a switch mode and I would go with a buck design。I'm now confused if I use a linear regulator why do I need a 7A supply for my pi? It wants 2.5A @ 5V so what ever my input voltage it wants 2.5A, No? I accept that this is a massive power loss, and not sustainable in a linear regultor but I don't see where 7A comes from.

    • stratton on 2018-3-20 16:18:49

    You are right,author. I misunderstood.

    • nasa on 2018-4-9 15:51:15

    I have read the book "Audio Power Amplifier Design" you recommended

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