Amplifier Classes from A to H.docx

8/16/2019 Amplifier Classes from A to H.docx

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Amplifer Classes rom A to H

Engineers and audiophiles have one thing in common when it comes to amplifers. They want a design that

provides a strong balance between perormance, eciency, and cost.

I you are an engineer interested in choosing or designing the amplifer best suited to your needs, you’ll fnd

columnist Robert acoste’s article in Circuit Cellar ’s !ecember issue helpul. "is article provides a

comprehensive loo# at the characteristics, strengths, and wea#nesses o di$erent amplifer classes so you

can select the best one or your application.

The article, logically enough, proceeds rom %lass & through %lass " 'but only touches on the more nebulous

%lass T, which appears to be a developer’s custom(made creation).

*Theory is easy, but diculties arise when you actually want to design a real(world amplifer,+ acoste says.

*hat are your particular choices or its fnal ampliying stage-+

The ollowing article ecerpts, in part, answer that /uestion. '0or uller guidance, download Circuit

Cellar ’s !ecember issue.)

CLASS A

The frst and simplest solution would be to use a single transistor in linear mode (see Figure 1 )… Basically

the transistor must be biased to have a collector voltage close to V CC /2 when no signal is applied on the

input This enables the output signal to swing

Figure 1—A Class-A amplifier can be built around a simple transistor. The transistor must be biased in so it stays in the

linear operating region (i.e., the transistor is always conducting.

either above or below this !uiescent voltage depending on the input voltage polarity…

This solution"s advantages are numerous# simplicity$ no need %or a bipolar power supply$ and e&cellent

linearity as long as the output voltage doesn"t come too close to the power rails This solution is considered

as the per%ect re%erence %or audio applications But there is a serious downside

Because a continuous current 'ows through its collector$ even without an input signal"s presence$ this

implies poor eciency n %act$ a basic Class*+ amplifer"s eciency is barely more than ,-.…

CLASS B

ow can you improve an amplifer"s eciency0 1ou want to avoid a continuous current 'owing in the output

transistors as much as possible

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Class*B amplifers use a pair o% complementary transistors in a push*pull confguration (see Figure 2 ) The

transistors are biased in such a way that one o% the transistors conducts when the input signal is positive and

the other conducts when it is negative Both transistors never conduct at the same time$ so there are very

%ew losses The current always goes to the load…

+ Class*B amplifer has more improved eciency compared to a Class*+ amplifer This is great$ but there is

a downside$ right0 The answer is un%ortunately yes

The downside is called crossover distortion…

Figure !—Class-" amplifiers are usually built around a pair of complementary transistors (at left. #ach transistor conducts

$%& of the time. This minimi'es power losses, but at the epense of the crosso)er distortion at each 'ero crossing.

CLASS AB

+s its name indicates$ Class*+B amplifers are midway between Class + and Class B ave a loo at the

Class*B schematic shown in Figure 2. % you slightly change the transistor"s biasing$ it will enable a small

current to continuously 'ow through the transistors when no input is present This current is not as high as

what"s needed %or a Class*+ amplifer owever$ this current would ensure that there will be a small overall

current$ around 3ero crossing

4nly one transistor conducts when the input signal has a high enough voltage (positive or negative)$ but

both will conduct around - V There%ore$ a Class*+B amplifer"s eciency is better than a Class*+ amplifer

but worse than a Class*B amplifer 5oreover$ a Class*+B amplifer"s linearity is better than a Class*B

amplifer but not as good as a Class*+ amplifer

These characteristics mae Class*+B amplifers a good choice %or most low*cost designs…

CLASS C

There isn"t any Class*C audio amplifer 6hy0 This is because a Class*C amplifer is highly nonlinear ow can

it be o% any use0 +n 78 signal is composed o% a high*%re!uency carrier with some modulation The resulting signal is o%ten

!uite narrow in terms o% %re!uency range 5oreover$ a large class o% 78 modulations doesn"t modi%y the

carrier signal"s amplitude

8or e&ample$ with a %re!uency or a phase modulation$ the carrier pea*to*pea voltage is always stable n

such a case$ it is possible to use a nonlinear amplifer and a simple band*pass flter to recover the signal9…

+ Class*C amplifer can have good eciency as there are no lossy resistors anywhere t goes up to :-. or

even ;-.$ which is good %or high*%re!uency designs 5oreover$ only one transistor is re!uired$ which is a ey


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CLASS G AND CLASS H

Class D and Class are !uests %or improved eciency over the classic Class*+B amplifer Both wor on the

power supply section The idea is simple 8or high*output power$ a high*voltage power supply is needed 8or

low*power$ this high voltage implies higher losses in the output stage

6hat about reducing the supply voltage when the re!uired output power is low enough0 This scheme is

clever$ especially %or audio applications 5ost o% the time$ music re!uires only a couple o% watts even i% %ar

more power is needed during the %ortissimo agree this may not be the case %or some teenagers" music$ but this is the concept

Class D achieves this improvement by using more than one stable power rail$ usually two Figure 4shows

you the concept

Figure 0—A Class- amplifier uses two pairs of power supply rails. b—2ne supply rail is used when the output signal has a

low power (blue. The other supply rail enters into action for high powers (red. +istortion could appear at the crosso)er.

What is Switch-mode power supply? The electronic power supply integrated with the switching regulator or converting

the electrical power eciently rom one orm to another orm with desired

characteristics is called as 1witch(mode power supply. It is used to obtain regulated

!% output voltage rom unregulated &% or !% input voltage.


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Switch mode power supply

1imilar to other power supplies, switch(mode power supply is a complicated circuit

that supplies power rom a source to loads. switch(mode power supply is essential

or power consuming electrical and electronic appliances and even or

building electrical and electronic pro2ects.

Topologies of Switch Mode Power Supply

There are di$erent types o topologies or 1341, among those, a ew are as ollows

• !% to !% converter

• &% to !% converter

• 0ly bac# converter

• 0orward converter

Switch Mode Power Supply’s Working Principle

The wor#ing o a ew types o switch(mode power supply topologies is as ollows5

1. ! to ! !on"erter SMPS Working Principle

In a !%(to(!% converter, primarily a high(voltage !% power is directly obtained rom

a !% power source. Then, this high(voltage !% power is switched at a very high

switching speed usually in the range o 67 8"9 to 7: 8"9.

&nd then it is ed to a step(down transormer which is comparable to the weight and

si9e characteristics o a transormer unit o 7:"9. The output o the step(down

transormer is urther ed into the rectifer. This fltered and rectifed output !%

power is used as a source or loads, and a sample o this output power is used as a

eedbac# or controlling the output voltage. ith this eedbac# voltage, the ;< time

o the oscillator is controlled, and a closed(loop regulator is ormed.

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DC to DC converter SMPS

The output o the switching(power supply is regulated by using 43 '4ulse idth

3odulation). &s shown in the circuit above, the switch is driven by the 43

oscillator, such that the power ed to the step(down transormer is controlled

indirectly, and hence, the output is controlled by the 43, as this pulse width signal

and the output voltage are inversely proportional to each other.

I the duty cycle is 7:=, then the maimum amount o power is transerred through

the step(down transormer, and, i duty cycle decreases, then the amount o power

transerred will decrease by decreasing the power dissipation.

#. $! to ! !on"erter SMPS Working Principle

The &% to !% converter 1341 has an &% input. It is converted into !% by

rectifcation process using a rectifer and flter. This unregulated !% voltage is ed to

the large(flter capacitor or 40% '4ower 0actor %orrection) circuits or correction o

power actor as it is a$ected. This is because around voltage pea#s, the rectifer

draws short current pulses having signifcantly high(re/uency energy which a$ects

the power actor to reduce.

AC to DC converter SMPS


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It is almost similar to the above discussed !% to !% converter, but instead o direct

!% power supply, here &% input is used. 1o, the combination o the rectifer and

flter, shown in the bloc# diagram is used or converting the &% into !% and

switching is done by using a power 3;10ET amplifer with which very high gain can

be achieved. The 3;10ET transistor has low on(resistance and can withstand high

currents. The switching re/uency is chosen such that it must be #ept inaudible to

normal human beings 'mostly above >:8"9) and switching action is controlled by a

eedbac# utili9ing the 43 oscillator.

This &% voltage is again ed to the output transormer shown in the fgure to step

down or step up the voltage levels. Then, the output o this transormer is rectifed

and smoothed by using the output rectifer and flter. & eedbac# circuit is used to

control the output voltage by comparing it with the reerence voltage.

%. &ly-'ack !on"erter type SMPS Working Principle

The 1341 circuit with very low output power o less than 6:: 'watts) is usually o

0ly(bac# converter type 1341, and it is very simple and low( cost circuit compared

to other 1341 circuits. "ence, it is re/uently used or low(power applications.

Fly-bac Converter type SMPS

The unregulated input voltage with a constant magnitude is converted into a

desired output voltage by ast switching using a 3;10ET? the switching re/uency is

around 6:: #"9. The isolation o voltage can be achieved by using a transormer.

The switch operation can be controlled by using a 43 control while implementing

a practical @y(bac# converter.

0ly(bac# transormer ehibits di$erent characteristics compared to general

transormer. The two windings o the @y(bac# transormer act as magnetically

coupled inductors. The output o this transormer is passed through a diode and a

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capacitor or rectifcation and fltering. &s shown in the fgure, the voltage across

this flter capacitor is ta#en as the output voltage o the 1341.

(. &orward !on"erter type SMPS Working

0orward converter type 1341 is almost similar to the 0ly(bac# converter type 1341,but in the orward converter type, a control is connected or controlling the switch

and at the output o the secondary winding o the transormer, and the rectifcation

and fltering circuit is complicated as compared to the @y(bac# converter.

It can be called as a !% to !% buc# converter, along with a transormer used or

isolation and scaling. In addition to the diode !6 and capacitor %, a diode !> and an

inductor are connected at the output end. I switch 1 gets switched ;. Ay using the flter inductor, the re/uired

voltage across the diode !> and to maintain the E30 re/uired or maintaining the

continuity o the current at inductive flter.

Even though the current is diminishing against the output voltage, approimately

the constant output voltage is maintained with the presence o the large capacitive

flter. It is re/uently used or switching applications with a power in the range o

6:: to >:: .

http://en.wikipedia.org/wiki/Switched-mode_power_supply_applicationshttp://en.wikipedia.org/wiki/Switched-mode_power_supply_applications


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!i$erent types o topologies are there in which 1341 can be reali9ed such as Auc#

converter, Aoost converter, 1el ;scillating @y(bac# converter, Auc#(boost

converter, Aoost(buc#, %u#, 1epic. Aut only a ew are discussed in this article,

namely !% to !% converter, &% to !% converter, 0ly(bac# converter and 0orward

converter. 0or more inormation regarding the types o switch(mode power supply

and the types o 1341 with their wor#ing principles, eel ree to write your

comments or improving this article technically so that you can help the other

readers to get awareness o 1341.

Di!ital Data

In aCD (and any other digital recording technology), the goal is to create a recording with

veryhigh fidelity (very high similarity between the original signal and the reproduced

signal) andperfect reproduction (the recording sounds the same every single time you

play it no matter how many times you play it).

To accomplish these two goals, digital recording converts the analog wave into a stream of

numbers and records the numbers instead of the wave. The conversion is done by a device

called ananalog-to-digital converter (ADC). To play back the music, the stream of

numbers is converted back to an analog wave by adigital-to-analog converter (DAC). The

analog wave produced by the DAC is amplified and fed to thespeakers to produce the

sound.

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The analog wave produced by the DAC will be the same every time, as long as the

numbers are not corrupted. The analog wave produced by the DAC will also be

very similar to the original analog wave if the analog-to-digital converter sampled

at a high rate and produced accurate numbers.

You can understand why CDs have such high fidelity if you understand the analog-

to-digital conversion process better. Lets say you have a sound wave, and you wish

to sample it with an ADC. !ere is a typical wave "assume here that each tic# on the

hori$ontal a%is represents one-thousandth of a second&'

(hen you sample the wave with an analog-to-digital converter, you have control

over two variables'

• The samplin! rate ( %ontrols how many samples areta#en per second

• The samplin! precision ( %ontrols how many di$erent

gradations '/uanti9ation levels) are possible when ta#ing

the sample

)n the following figure, lets assume that the sampling rate is *,+++ per second and

the precision is *+'

The green rectangles represent samples. very one-thousandth of a second, the

ADC loo#s at the wave and pic#s the closest number between + and . The number

chosen is shown along the bottom of the figure. These numbers are a digital


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representation of the original wave. (hen the DAC recreates the wave from these

numbers, you get the blue line shown in the following figure'

You can see that the blue line lost uite a bit of the detail originally found in the

red line, and that means the fidelity of the reproduced wave is not very good. This

is the sampling error. You reduce sampling error by increasing both the sampling

rate and the precision. )n the following figure, both the rate and the precision have

been improved by a factor of / "/+ gradations at a rate of /,+++ samples per

second&'

)n the following figure, the rate and the precision have been doubled again "0+

gradations at 0,+++ samples per second&'


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You can see that as the rate and precision increase, the fidelity "the similarity

between the original wave and the DACs output& improves. )n the case of CD

sound, fidelity is an important goal, so the sampling rate is 00,*++ samples per

second and the number of gradations is 12,231. At this level, the output of the

DAC so closely matches the original waveform that the sound is essentially

4perfect4 to most human ears.

CD Stora!e Capacity

One thing about the CD's sampling rate and precision is that it produces a lot of data. On a

CD, the digital numbers produced by the ADC are stored asbytes, and it takes 2 bytes to

represent 65,536 gradations. There are two sound streams being recorded (one for each of

the speakers on a stereo system). A CD can store up to 74 minutes of music, so the total

amount of digital data that must be stored on a CD is:

44,100 samples/(channel*second) * 2 bytes/sample * 2 channels * 74 minutes * 60

seconds/minute = 783,216,000 bytes

With digital recording) audio engineers con"ert analog wa"es into digital

signals. There are many different kinds of e*uipment that can con"ert

analog to digital. Some audio studios record a performance on an analog

master tape first) then transfer the sound to a digital format. +thers will usespecial e*uipment to record directly to digital.

,arly digital recordings sacrificed fidelity) or sound *uality) in fa"or of

relia'ility. +ne of the draw'acks of an analog format is that analog media

tends to wear down. inyl al'ums can warp or get scratched) which can

dramatically impact sound *uality. Magnetic tape e"entually wears out and

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is "ulnera'le to magnets) which can erase or destroy information stored on

the tape. igital media like compact discs can reproduce sound indefinitely.

$nother ad"antage digital media has o"er analog is that you can make as

many copies of the original sound file as you like without hurting it.,"entually) e"en an analog master recording isnt going to sound as good

as the original performance. $s long as nothing corrupts a digital file) it will

stay the same no matter how much time has passed or how many copies

engineers make.

$nalog audio signals are suscepti'le to noise and distortion) due to the innate characteristics of electronic

circuits and associated de"ices. istur'ances in a digital system do not result in error unless the distur'ance is

so large as to result in a sym'ol 'eing misinterpreted as another sym'ol or distur' the se*uence of sym'ols. /t

is therefore generally possi'le to ha"e an entirely error-free digital audio system in which no noise or distortionis introduced 'etween con"ersion to digital format) and con"ersion 'ack to analog.

$ digital audio signal may 'e encoded for correction of any errors that might occur in the storage or

transmission of the signal) 'ut this is not strictly part of the digital audio process. This techni*ue) known

as 0channel coding0) is essential for 'roadcast or recorded digital systems to maintain 'it accuracy. The

discrete time and le"el of the 'inary signal allow a decoder to recreate the analog signal upon replay. ,ight to

&ourteen it Modulation is a channel code used in the audio !ompact isc 2!3.

Conversion process

The lifecycle of sound from its source) through an $!) digital processing) a $!) and finally as sound again.

$ digital audio system starts with an $! that con"erts an analog signal to a digital signal.4note 15 The $! runs at

a specified sampling rate and con"erts at a known 'it resolution. ! audio) for e6ample) has a sampling rate of

((.1 k782(()199 samples per second3) and has 1:-'it resolution for each stereo channel. $nalog signals thatha"e not already 'een'andlimited must 'e passed through an anti-aliasing filter 'efore con"ersion) to pre"ent

the distortion that is caused 'y audio signals with fre*uencies higher than the ;y*uist fre*uency) which is half

of the systems sampling rate.

$ digital audio signal may 'e stored or transmitted. igital audio can 'e stored on a !) a digital audio player )

a hard dri"e) a


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compression techni*ues) such as MP%) $d"anced $udio !oding) +gg or'is) or &=$!) are commonly

employed to reduce the file si8e. igital audio can 'e streamed to other de"ices.

&or play'ack) digital audio must 'e con"erted 'ack to an analog signal with a $!. $!s run at a specific

sampling rate and 'it resolution) 'ut may useo"ersampling) upsampling or downsampling to con"ert signals

that ha"e 'een encoded with a different sampling rate.
https://en.wikipedia.org/wiki/Audio_compression_(data)https://en.wikipedia.org/wiki/MP3https://en.wikipedia.org/wiki/Advanced_Audio_Codinghttps://en.wikipedia.org/wiki/Advanced_Audio_Codinghttps://en.wikipedia.org/wiki/Vorbishttps://en.wikipedia.org/wiki/Free_Lossless_Audio_Codechttps://en.wikipedia.org/wiki/Free_Lossless_Audio_Codechttps://en.wikipedia.org/wiki/Streaming_mediahttps://en.wikipedia.org/wiki/Oversamplinghttps://en.wikipedia.org/wiki/Upsamplinghttps://en.wikipedia.org/wiki/Upsamplinghttps://en.wikipedia.org/wiki/Downsamplinghttps://en.wikipedia.org/wiki/Downsamplinghttps://en.wikipedia.org/wiki/Audio_compression_(data)https://en.wikipedia.org/wiki/MP3https://en.wikipedia.org/wiki/Advanced_Audio_Codinghttps://en.wikipedia.org/wiki/Vorbishttps://en.wikipedia.org/wiki/Free_Lossless_Audio_Codechttps://en.wikipedia.org/wiki/Streaming_mediahttps://en.wikipedia.org/wiki/Oversamplinghttps://en.wikipedia.org/wiki/Upsamplinghttps://en.wikipedia.org/wiki/Downsampling

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Amplifier Classes from A to H.docx

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