Why We Used Power Amplifier

8/7/2019 Why We Used Power Amplifier

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Introduction to power amplfier

Presented By:

Rajen KumarB.Tech ECERoll no B63



Contentsy Need of power amplifiery Power amplifier Vs voltage amplifiery Why voltage amplifier can not work as power amplifier

y Features of power amplifiery Performance quantities of power amplifiery Practical stages of power amplifiery Classification of power amplifiery Class A power amplifier

y Class B power amplifier.y Class C power amplifiery Class AB power amplifiery Harmonic Distortiony Push pull amplifier



1.Why we used power amplifierIn almost all electronics systems, the last stage has to be power

amplifier. For exampel, in a public address system, it is thepower amplifier that drives the loudspeakrs. When a person

speaks into the microphone, The sound waves are converted byit into electrical signal. This electrical signal is of very lowvoltage (a few mV). This signal if fed directly, cannot drive theloudspeakrs, to give sound (audio) output. The voltage level of

this signal is first raised to sufficiently high values by passing itthrough a multistage voltage amplifier .This voltage is then usedto drive the power amplifier. The loudspeakrs finally convertthe electrical energy into sound energy. Thus, a large audiencecan hear the speech (or music from the orchestra, tape recorder,

record player, or any other such gadget).



2.Diff erence between voltage

amplifier and power amplifierVoltage amplifier Power amplifier

y It is an amplifier which

amplify voltage.y It is also called as small

signal amplifier

y

Small size transistorare used.

y Distortion is notpresent at output.

y it is an amplifier which

amplify power.y It is also called as large

signal amplifier.

y

Power transistors areused.

y Distortion is present atoutput.



Diff erence between voltage

amplifier and power amplifierVoltage amplifier Power amplifier

y The main parameter

under consideration isgain.

y RC coupling is used.

y Output voltage in mv

y The main parameter

under consideration isefficiency.

y Transformer couplingis used.

y Output voltage in volt.



3.Why voltage amplifier can not work as power

amplifierThe transistor used in a voltage amplifier used nothave a large dissipation rating. It is not required tohandle large power. However, the transistor musthave large power dissipation rating, if it is to work ina power amplifier circuit. As a practical rule, smallsignal transistor has a power dissipation less than 0.5W; a power transistor more than 0.5 W.



4.Features of power amplifiery Impedance matching is necessary.

y Power amplifiers are bulky.

y Power transistors are used in power amplifier.y Distortion is present at output.

y Power dissipation is large.



5.Performance quantities of power

amplifierEfficiency or collector efficiency ± The ratio of AC

power output to the DC power input is called asefficiency. It is denoted by n= Pac/Pdcdc

Distortion ± The change in output wave shapes frominput wave shape is called as distortion or when theoutput signal is not exact same as input signal. It iscalled as distortion.

Power dissipation capability ± The ability of apower transistor to dissipate heat in the surroundingis called as power dissipation capability of power amplifier.



6.Application of power amplifier

y it is used in radio receiver.

y It is used in CD and caste player.

y It is used in TV receiver .



7.Practical power amplifier stages



Power amplifiers are meant to amplify a weak signal untilsufficient power is available to operate an output device

such as a loudspeaker, a solenoid or a relay. Power amplifier, to provide the desired power amplification, hasgenerally 3 stages (voltage amplification stage, driver stageand output stage), as shown in the block diagram given

below.

Voltage Amplification Stage ± The signal devel-oped bythe input transducer is very small and needs sufficient

amplification so as to operate the output device. Therefore,for raising the level of the weak input signal, it is amplifiedin two or more stages, for which R-C coupling is usuallyemployed.



Driver Stage ± The stage that precedes the output stage is called thedriver stage. The output from the last voltage amplification stage is fedto the driver stage and output from the driver stage is fed to the output

stage. The driver stage renders power amplifica-tion as usual. Here,concentrated effort is made to provide maximum power gain and sotransformer-coupled class. A power amplifier is employed in this stage.The driver trans-former is usually a step-down transformer andfacilitates impedance matching.

Output Stage ± The output stage essentially consists of a power amplifier and is meant for transferring maximum power to the outputdevice. In order to transfer maximum power at high efficiency, push-pull arrangement is employed in the output stage (if a single transistor is used as a class A amplifier in the output stage to provide faithfulamplification, the operation efficiency will be quite low). In thisarrangement two transistors are used in class B operation and are fedfrom the centre-tapped secondary of the transformer whose primary

forms the collector load for the driver stage.



8.Classification of power amplifieron the basis of operating point selection power amplifiers

are divided into four categories

y Class A power amplifier y Class B power amplifier

y Class C power amplifier

y Class AB power amplifier



Efficiency Ratingsy The maximum theoretical efficiency ratings of

class-A, B, and C amplifiers are:

Ref:080327HKN EE3110 Power Amplifier (Class A) 14

Amplifier Maximum TheoreticalEfficiency, Lmax

Class A 25%

Class B 78.5%Class C 99%



9. Class A power amplifier

A class A power amplifier is defined as a power amplifier inwhich output current flows for the full-cycle (360°) of the

input signal. In other words, the transistor remains forwardbiased throughout the input cycle.



Graphical representation of class A power

amplifier



10.Class B power amplifier



Operation of Class B power amplifierIn class B operation the transistor is so biased that zero-

signal collector current is zero. Hence class B operationdoes not need any biasing system. The operating point isset at cut-off. It remains forward biased for only half cycleof the input signal.i.e its conduction angle is 180 degree.

As illustrated in figure, during the positive half cycle of theinput ac signal, the circuit is forward biased and, therefore,collector current flows. On the other hand, during negative

half cycle of the input ac ³signal, the circuit is reversebiased and no collector current flows.



Power and EfficiencyCalculations of class B operation.

Input dc power, P in (dc) = VCC ICCwhere l dc

is the average or direct current taken from the collector supply.

If Ic max is the maximum or peak value of collector or output current, then

l dc = l c max / �

RMS value of collector current, Ic rms

= Ic max

/¥2RMS value of output voltage, Vrms = VCC/¥2

Hence output power during half cycle, Pout (ac) = (Ic max VCC)/ 4

In above expression factor 1/2 is used because power is developed during one half cycle

only.

DC power loss in load, PRc(dc) = (I2dc R c = Ic max/�) R c . ..

DC power loss in collector region or transistor, = Pin dc - PRc(dc) ± Pout (ac)

Overall efficiency, noverall = Pout (ac)/ Pin dc = 0.785 or 78.5% Power and Efficiency



11.Class c power amplifier



A class C power amplifier is biased to operate for less than

180° of the input signal cycle, as shown in circuit.

The tuned circuit in the output, however, will provide a fullcycle of output signal for the fundamental or resonant

frequency of the tuned circuit (L and C tank circuit) of theoutput.

The use of such amplifiers is, therefore, limited for a

fixed frequency, as occurs in communication circuits, for example. < Operation of a class C circuit is not intendedprimarily for large signal or power amplifiers.



12.Class AB power amplifier



In class AB power amplifiers, the biasing circuit is soadjusted that the operating point Q lies near the cut-

off voltage. During a small portion of negative half cycle and for complete positive half cycle of thesignal, the input circuit remains forward biased-andhence collector current flows.

But during a small portion (less than half cycle) of thenegative cycle´µ the input circuit is reverse biasedand, therefore, no collector current flows during thisperiod.

Class AB operation needs a push-pull connection toachieve a full output cycle.



13. Harmonic Distortion



The quality of the sound given by the loudspeaker depends very muchupon the nature of the output signal. If the output is distorted , thequality of the sound will be poor. Consideration of Harmonic or nonlinear distortion is therefore important in power amplifiers.Because they handle large signals, Distortion is always present. If theoutput is not sinusoidal input. We say that Harmonic distortion is

introduced. This type of Distortion is present because the transistor does not work as a linear amplifier when the signal is large. Becauseof this non-linearity, the signal is distorted in its wave shape. If wesupply a signalV sint to the input of a power amplifier the waveform of the

output signal can be mathematically represented asio = I0 + I1 Sin(t) + I2 Sin(2t) + I3 Sin(3t) + ...........



14. Push pill amplifier



W hy is called push pull amplifier?

All the same time one transistor is pushing into

conduction and other transistor is pulling out of conduction that¶s why it is called as push pullamplifier.



Advantagesy Efficiency is high.

y Impedance matching is obtained by using output

transformer.y Harmonics are automatically get cancelled out



T

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Why We Used Power Amplifier