40985852 Power Amplifiers

7/29/2019 40985852 Power Amplifiers

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Power Amplifier


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Electronic Amplifier

The termamplifieras used in this article can mean

either a circuit (or stage) using a single active device or a

complete system such as a packaged audio hi-fi amplifier.


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An electronic amplifier is a device for increasing the power of asignal. It does this by taking power from a power supplyandcontrolling the output to match the input signal shape but with alarger amplitude. In this sense, an amplifier may be considered asmodulating the output of the power supply.

Real world amplifiers are not ideal and this control is thus

imperfect. One consequence is that the power supply itself may influence

the output, and must itself be considered when designing theamplifier

The amplifier circuit has an "open loop" performance, that can

be described by various parameters (gain, slew rate, outputimpedance, distortion, bandwidth, signal to noise ratio ...)

Many modern amplifiers use negative feedback techniques tohold the gain at the desired value.
http://en.wikipedia.org/wiki/Power_%28physics%29http://en.wikipedia.org/wiki/Signal_%28information_theory%29http://en.wikipedia.org/wiki/Power_supplyhttp://en.wikipedia.org/wiki/Power_supplyhttp://en.wikipedia.org/wiki/Signal_%28information_theory%29http://en.wikipedia.org/wiki/Power_%28physics%29


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Classification of amplifier stages and

systems

Different designs of amplifiers are used fordifferent types of applications and signals. Wecan broadly divide amplifiers into four

categories:

Small signal amplifiers

Low frequency power amplifiers

Radio frequency (RF) power amplifiers

Microwave power amplifiers

Each of these calls for a slightly different design approach, mainly because of the

physical limitations of the components used to implement the amplifier, and theefficiencies that can be realised.


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Amplifier Classes

Classes

Class A Class AB Class CClass B

Fig. 6.1 Amplifier Classes


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Amplifier classes

Class A 100% of the input signal is used (conduction angle a = 360or 2)

Class AB more than 50% but less than 100% is used. (181 to 359, < a < 2)

Class AB1 applies to tube or transistor amplifiers in class AB where thegrid or base is more negatively biased than it is in class A.

Class AB2applies to tube or transistor amplifiers in class AB where thegrid or base is often more negatively biased than in AB1, and the inputsignal is often larger. When the drive is high enough to make the grid orthe base more positive, the grid or base current will increase. It is possibledepending on the level of the signal input for the amplifier to move fromclass AB1 to AB2.

Class B 50% of the input signal is used (a = 180or )

Class C less than 50% is used (0 to 179, a < )


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Fig. 6.2 Amplifier operating

classes.


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Comparison of Amplifier classes

Class A AB B C D

Operating

cycle

360 180 to

360

180 Less

than

180

Pulse

operation

Power

efficiency

25%

to

50%

Between

25%

(50%)and

78.5%

78.5% * Typically

over 90%

* Class C is usually not used for delivering large amounts of power and

thus the efficiency is not given here.


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Class A

Amplify over the whole ofthe input cycle such that theoutput signal is an exact

scaled-up replica of theinput with no clipping.

Usual means of

implementing small signalamplifiers

Not very efficient.

A theoretical maximum of

50% is obtainable with

inductive output coupling

and 25% with capacitive but

for small signals, this waste

of power is still extremely

small, and can be easily

tolerated.

Fig. 6.3 Class A amplifier
http://en.wikipedia.org/wiki/Image:Electronic_Amplifier_Class_A.png


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The amplifying element is biased such that thedevice is always conducting to some extent, andis operated over the most linear portion of its

characteristic curve (known as its transfercharacteristic or transconductance curve).

Because the device is always conducting, even if

there is not input at all, power is wasted.

This is the reason for its inefficiency.


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Used for audio power amplifiers, though someaudiophiles believe that Class A gives the bestsound quality due to it being operated in aslinear a manner as possible.

Valve amplifiers (because it is large, andexpensive) since Class A amplifier uses a singledevice.

long-tailed pair (op-amps-medium power, lowefficiency, and high cost audio amplifiers)

Low efficiency and high heat dissipation


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The dc bias set by Vcc and RB fixes the dc base-bias current at;

With the collector current then being

With the collector-emitter voltage then

0.7CCB

B

V VI

R

C BI I

CE CC C C V V I R


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The power into an amplifier is provided by the

supply.With no input signal, the dc current drawn is the

collector bias current ICQ.

The power then drawn from the supply is,( )i CC CQP dc V I


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The ac power delivered to the load (Rc )may beexpressed by;

Using ac signals;

2

2

( ) ( ) ( )

( ) ( )

( )( )

o CE C

o C C

Co

C

P ac V rms I rms

P ac I rms R

V rmsP ac

R

2

2

( ) ( )( )

2

( )( )2

( )( )

2

CE Co

Co

C

CEo

C

V p I pP ac

I pP acR

V pP ac

R


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Using peak-to-peak signals;

The efficiency of the amplifier;

2

2

( ) ( )

( ) 8

( )( )

8

( )( )

8

CE C

o

Co C

CEo

C

V p p I p p

P ac

I p pP ac R

V p pP ac

R

( )% 100%

( )

o

i

P acX

P ac


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Example 6.1

Calculate the input power, output power, andefficiency of the amplifier circuit in fig. below for aninput voltage that results in a base current of 10mApeak.

Fig. 6.4 Circuit for Ex. 6.1


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Solution:

This bias point is marked on the transistor collectorcharacteristic of fig. b. the ac variation of the output signal canbe obtained graphically using the dc load line drawn on fig. b.by connecting VCE=VCC=20V with IC=VCC/RC=1000mA=1A.

Determine the Q-point;

0.7 20 0.719.3

125(19.3 ) 482.5 0.48

20 (0.48 )(20 ) 10.4

Q

Q

Q

CCB

B

C B

CE CC C C

V V V VI mA

R kI I mA mA A

V V I R V V

2 3 2

When the input ac base current increases from its dc bias level,

the collector current rise by;

( ) ( ) 25(10 peak) 250 peak

( ) (250 10 )( ) (20 ) 0.625

2 2

( ) (20 )(0.48 )Q

c B

co C

i CC C

I p I p mA mA

I p X AP ac R W

P dc V I V A

9.6

( ) 0.625% 100% 100% 6.5%( ) 9.6

o

i

W

P ac WX XP dc W


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Class B and AB

Only amplify half of the inputwave cycle.

As such, they create a large amount

of distortion, but their efficiency isgreatly improved and is muchbetter than Class A.

Has a maximum theoreticalefficiency of 78.5%. (because theamplifying element is switched offaltogether half of the time, and so

cannot dissipate power.

Fig. 6.5 Class B amplifier
http://en.wikipedia.org/wiki/Image:Electronic_Amplifier_Class_B_fixed.png


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A single Class B element is rarely found in practice,

though it can be used in RF power amplifiers where thedistortion is unimportant. A practical circuit using Class B elements is the

complementary pair or push pull arrangement. Here, the complimentary devices are used to each

amplify the opposite halves of the input signal, which isthen recombined at the output.

This arrangement gives excellent efficiency, but cansuffer from the drawback that there is a small glitch at

the joins between the two halves of the signal. This is called crossover distortion. A solution to this is to bias the devices just on, rather

than off altogether when they are not in use and is

called the Class AB operation.


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Class AB sacrifices someefficiency over class B for

linearity, so will always be lessefficient (below 78.5%).

It is typically much more efficientthan Class A.

Class B or AB push-pull circuitsare the most common form ofdesign founf in audio poweramplifiers.

Class AB is considered a good

compromise for audio amplifiers,since much of the time the musicis quiet enough that the signalstays in the Class A region, whereit is reproduced with good

fidelity.

Fig. 6.6 Class B Push-Pull Amplifier

Are used for RF linear

amplifiers (Class B and AB),

also favored in battery-operateddevices, such as transistor

radios (Class B).
http://en.wikipedia.org/wiki/Image:Electronic_Amplifier_Push-pull.png


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The power supplied to the load by an amplifier

is drawn from the power supply that providesthe input or dc power shown in fig. below.

Fig. 6.7 Connection of pushpull amplifier to load: (a) using two voltage

supplies; (b) using one voltage supply.


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The amount of input power can be calculated by;

Where Idc is the average or dc current drawn from thepower supplies.

The value of the average current;

Where I(p) is the peak value of the output currentwaveform.

Using Eq. (6.1) and (6.2) results;

( )i CC dcP dc V I

2( )dcI I p

(6.1)

(6.2)

2( ) ( )i CCP dc V I p

(6.3)


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The power delivered to the load (usually referred to asa resistance RL) can be calculated using any one of a

number of equations. If one is using an rms meter to measure the voltage

across the load, the output power can be calculated as,

If one is using an oscilloscope, the measured peak or

peak-to-peak output voltage can be used;

2 ( )( ) LoL

V rmsP acR

(6.4)

2 2( ) ( )( )

8 2

L Lo

L L

V p p V pP ac

R R

(6.5)


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The efficiency of the class B amplifier can be calculatedusing the basic equation;

The power dissipated (as heat) by the output powertransistors is the difference between the input powerdelivered by the supplies and the output power delivered

to the load,

Where P2Q is the power dissipated by the two output

power transistors. The dissipated power handled by each transistor is then

( )

% 100%( )

o

i

P ac

XP dc

(6.6)

2 ( ) ( )Q i oP P dc P ac (6.7)

2

2

Q

Q

PP (6.8)


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Example 6.2

For a class B amplifier providing a 20-V peak signalto a 16- load (speaker) and a power supply of

VCC=30V, determine the input power, outputpower and circuit efficiency.


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A 20-V peak signal across a 16 load provides a peak load current of

The dc value of the current drawn from the power supply is then

And the input power delivered by the supply voltage is

The output power delivered to the load is

For a resulting efficiency of

( ) 20( ) 1.25

16

LL

L

V p VI p A

R

2 2( ) (1.25 ) 0.796dc LI I p A A

( ) (30 )(0.796 ) 23.9i CC dcP dc V I V A W

2 ( ) (20 )2

( ) 12.52 2(16 )L

o

L

V p V

P ac WR

( ) 12.5% 100% 100% 52.3%

( ) 23.9

o

i

P ac WX X

P dc W


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Class C

Conduct less than 50% of theinput signal and the distortionat the output is high, but

efficiencies of up tp 90% canbe reached.

Application-megaphones, RF

transmitters

Fig. 6.8 Class C Amplifier
http://en.wikipedia.org/wiki/Image:Electronic_Amplifier_Class_C.png


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As shown in fig., Class C amplifiers is

biased to operate for less than 180 ofthe input signal cycle.

The tuned circuit in the output, howeverwill provide a full cycle of output signalfor the fundamental or resonantfrequency of the tuned circuit (L and C

tank circuit) of the output. This type of operation is limited to use at

one fixed frequency.

Fig. 6.9 Class C amplifier circuit.

Operation of a class C

circuit is not intended

primarily for large-signal

or power amplifier.


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Class D a power amplifier where all power devices are operated in on/off

mode. Application-pulse generators

use pulse width modulation, pulse density modulation(sometimes referred to as pulse frequency modulation) or more

advanced form of modulation such as Delta-sigma modulation The main advantage of a class D amplifier is power efficiency

very little power is dissipated by the transistors except during thevery short interval between the on and off states.

can be controlled by either analog or digital circuits. A digitalcontroller introduces additional distortion called quantization errorcaused by its conversion of the input signal to a digital value.

widely used to control motors, and almost exclusively for smallDC motors, but they are now also used as audio amplifiers, with

some extra circuitry to allow analogue to be converted to a muchhi her fre uenc ulse width modulated si nal.


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Fig. 6.10 Block diagram of class D

amplifier.


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40985852 Power Amplifiers