Power Amplifiers

UNIT-III POWER AMPLIFIERS

ANALOG ELECTRONICS 1

ELECTRONICS & COMMUNICATION ENGINEERING DEPARTMENT

Question Bank

1. Compare voltage amplifier with power amplifier. (3)

2. Give classification of the power amplifier. (4)

3. Draw & explain circuit diagram of the series fed class-A power amplifier with its

principle. (4)

4. Explain working of the series fed class-A power amplifier. (3)

5. Give advantages & disadvantages of the series fed class-A power amplifier. (2)

6. Derive efficiency of the series fed class-A power amplifier. (4 to 7)

7. Draw & explain circuit diagram of the transformer coupled class-A power amplifier with

its principle. (4)

8. Explain working of the transformer coupled class-A power amplifier. (4)

9. Give advantages & disadvantages of the transformer coupled class-A power amplifier. (2)

10. Derive efficiency of the transformer coupled class-A power amplifier. (4 to 7)

11. Draw & explain circuit diagram of the series fed class-B power amplifier with its

principle. (3)

12. Explain working of the series fed class-B power amplifier. (4)

13. Give advantages & disadvantages of the series fed class-B power amplifier. (2)

14. Derive efficiency of the series fed class-B power amplifier. (4 to 7)

15. Draw & explain circuit diagram of the class-B push pull power amplifier with its

principle. (3)

16. Explain working of the fed class-B push pull power amplifier. (4)

17. Give advantages & disadvantages of the class-B push pull power amplifier. (2)

18. Explain cross-over distortion. How to eliminate cross-over distortion? (4)

19. Derive efficiency of the class-B push pull power amplifier. (4 to 7)

20. Draw & explain circuit diagram of the class-AB push pull power amplifier with its

principle. (3)

21. Explain working of the class-AB push pull power amplifier. (4)

22. Give advantages & disadvantages of the class-AB push pull power amplifier. (2)

23. Draw & explain working of the class-C power amplifier. (4 to 7)

24. Draw & explain working of the class-D power amplifier. (4 to 7)

25. Draw & explain complementary symmetry power amplifier. (4 to 7)

26. Compare different types of power amplifiers. (4)




Comparison of voltage amplifier with power amplifier:

Sr.

No. Description Voltage Amplifier Power Amplifier

1 Transistor

Small size transistor

with value of β more

than 100 is used

Large size transistor with value of β

between 20 to 50 is used. Heat sink

is used with the transistor.

2 Coupling Normally RC coupling

is used

Normally transformer coupling or

tuned circuit is used

3 Input voltage Low of the order of

few mV High of the order of few Volts

4 Collector

current

Low of the order of

few mA

High of the order of 100 mA or

more

5 Collector circuit

resistance

High of the order of

4k to 10k Low of the order of 5 to 20

6 Output

impedance

High of the order of

few k Low of the order of few ohms

7 Power output Low of the order of

few mW High of the order of few Watts

8 Voltage gain High Low

9 Current gain Low High

10 Power gain Low High

11 Applications Initial stages Intermediate or final stages




Classification of the power amplifier.

Classification based on the mode of operation:

1) Class A power amplifier:

Figure: Class A operation

Q-point is located on the mid-point of the load line in active region. So, transistor can

conduct under zero signal bias condition.

Also, transistor can conduct for the entire cycle (i.e., for 360º) of the ac input signal;

hence we can get output current and voltage of the full cycle as shown in the above

figure.

2) Class B power amplifier:

Figure: Class B operation

Q-point is located on the cut-off point of the load line in cut-off region. So, transistor

cannot conduct under zero signal bias condition.

Also, transistor can conduct for only the half cycle (i.e., for 180º) of the ac input signal;

hence we can get output current and voltage of the half cycle as shown in above figure.




3) Class AB power amplifier:

Figure: Class AB operation

Q-point is located above the cut-off point and below the mid-point on the load line in

active region. So, transistor can conduct under zero signal bias condition.

Also, transistor can conduct more than 180º and less than 360º of the ac input signal;

hence we can get output current and voltage as shown in above figure.

4) Class C power amplifier:

Figure: Class C operation

Q-point is located below the cut-off point of the load line in dip cut-off region. So,

transistor cannot conduct under zero signal bias condition.

Also, transistor can conduct for less than the half cycle (i.e., for < 180º) of the ac input

signal; hence we can get output current and voltage of the half cycle as shown in above

figure.




5) Class D power amplifier:

Class D power amplifiers are designed to operate with digital or pulse type signals. Using

digital techniques makes it possible to have a signal that varies over the entire cycle to

recreate the output from many pieces of input signal.

Classification based on how output is derived:

1) Single ended type power amplifier:

Single ended power amplifiers uses single transistor and derives output power w.r.t. one

end permanently grounded.

2) Double ended type power amplifier:

Double ended or push-pull power amplifier uses two transistors in a single stage. It

consists of two loops in which the transistor collector current flow in opposite directions

but add in the load.

3) Complementary symmetry type power amplifier:

Complementary symmetry or push-pull power amplifier uses two transistors having

complementary symmetry. The term complementary arises from the fact that one

transistor is the NPN type and the other is PNP type.

They have symmetry as they are made with the same material and technology is of same

maximum ratings.




Series fed class A power amplifier:

Principle: To transfer power of DC supply (+VCC) to the weak input signal (AC signal)

to raise its power level.

Circuit Diagram:

Figure: Series fed Class A power amplifier

Description:

The input capacitor Cin couples ac signal voltage to the base of the transistor.

RB provide a fixed biasing to make transistor Q1-ON under zero signal condition.

RC is a resistive load.

Working of the series fed class A power amplifier:

Under zero signal condition a dc current ICQ (collector current) flow in the circuit and a dc

voltage VCEQ (collector-emitter voltage) available at the output.

When ac input signal Vin is applied, during the positive half cycle the base current IBQ is

increased. As IC = IB , the collector current ICQ is also increased.

According to VCE = VCC – IC RC output voltage VCEQ is decreased.

During the negative half cycle the base current IBQ is decreased. As IC = IB , the collector

current ICQ is also decreased.

According to VCE = VCC – IC RC output voltage VCEQ is increased.

So, the operating point Q shifts up and down on the load-line as shown in the figure.

The output collector current increases to IC,max and falls to IC,min. Similarly, the collector-

emitter voltage increases to VCE,max and falls to VCE,min.




Advantages, disadvantages & application of the series fed class A power amplifier:

Advantages:

Simple circuit & easy to design.

Distortion is very less. So, it provides high fidelity output.

No need of transformer. So, circuit becomes cheap.

Disadvantages:

Operation is restricted only over a small central region of the load line, so, it can amplify

signal of small amplitudes.

Output impedance of the transistor is more. So, the circuit cannot use for low impedance

load.

Power is wasted in collector resistor RC . So, AC power output per transistor is small.

Power wastage is more due to ‘transistor ON power loss’ in absence of AC input signal.

Efficiency is only up to 25%.

Applications:

Class A power amplifiers are used where freedom from distortion is prime aim.

Efficiency of the Class A power amplifiers:

Input power is given by

CQCCin(dc)IVP

--- --- --- --- --- [1]

Power dissipated at the collector resistor is given by

C

2

CQRC(dc)RIP

--- --- --- --- --- [2]

Total power getting by transistor is given by

RC(dc)in(dc)tr(dc)PPP

C

2

CQCQCCtr(dc)RIIVP

--- --- --- --- --- [3]

Now if we apply ac input signal to the amplifier then the ac output will be defined as

rmsrmsout(ac)VIP

22

V

22

Ip)CE(pp)C(p

8

VI

Pp)CE(pp)C(p

out(ac)

--- --- --- --- --- [3]




(1) Collector efficiency:

Collector efficiency is defined as ratio of the output ac power to the input dc power

getting by the transistor.

tr(dc)P

P

ηout(ac)

collector

)RII8(V

VI

C2

CQCQCC

p)CE(ppC(p

collector

)

(2) Overall efficiency:

Overall efficiency is defined as the ratio of the output ac power to input dc power given to

the amplifier.

in(dc)

out(ac)

overall P

Pη

CQCC

p)CE(p

p)C(p

overall I8V

VIη

Maximum Power and Efficiency:

Maximum power and efficiency is considered for the full signal on the dc load line as

shown in the figure.




Maximum CE(min)CE(max)P)CE(P

VVV

0VCC

CCV

Maximum C(min)C(max)P)C(P

III

0R

V

C

CC

C

CC

R

V

Maximum power:

(1) CQCCmaxin(dc),

IVP

--- --- --- --- --- [1]

C

CCCC 2R

VV

C

2

CC

2R

V

(2) C

2

CQCQCCmaxtr(dc),RIIVP

--- --- --- --- --- [2]

C

2

C

CC

C

CC

CCR

2R

V

2R

VV

C

2

CC

C

2

CC

4R

V

2R

V

C

2

CC

4R

V

(3) 8

VIP

P)CE(PP)C(P

maxout(ac),

--- --- --- --- --- [3]

8

1V

R

V

CCC

CC

=

C

2

CC

8R

V




Efficiency:

(1) maxtr(dc),

maxout(ac),

maxcollector, P

Pη

X100%/4RV

/8RV

C2

CC

C2

CC

X100%2

1

50%

(2) max)in(dc,

max)out(ac,

maxoverall, P

Pη

= 100% X /2RV

/8RV

L

2CC

L

2CC

= 100% X 4

1

= 25%

Transformer coupled class A power amplifier:



Circuit Diagram:

Figure: Transformer Coupled Class A Power Amplifier

Description:

R1 and R2 provide potential divider biasing & RE is used for bias stabilization.

CE is used to prevent ac voltage flowing into the RE.

Cin couples ac signal voltage to the base.

A step-down transformer of suitable turn ratio is provided to couple the high

impedance collector circuit to low impedance load.




Working of the transformer coupled class A power amplifier:

When ac input signal Vin is applied, during the positive half cycle the base current IBQ

increased. As IC = IB , the collector current ICQ also increased in the primary winding of

the transformer. So, magnetic flux is increased in the primary winding and hence emf is

increased at output.

During the negative half cycle the base current IBQ decreased. As IC = IB , the collector

current ICQ also decreased in the primary winding of the transformer. So, magnetic flux is

decreased in the P.W. and hence emf is decreased at output.

Use of the transformer:

Figure: Transformer Impedance Matching

According to the maximum power transfer theorem the power transferred from the power

amplifier to the load (loudspeaker) will be maximum only if the amplifier output imped-

ance equals the load impedance RL.

Hence for transfer of maximum power from amplifier to the output load, impedance

matching of amplifier output impedance with the impedance of output load is necessary.

This is accomplished by using a step-down transformer of suitable turn-ratio.

The transformer impedance matching circuit is shown separately, where RL is the

resistance looking into the primary of the transformer and is given as

2

2

1

1

2

2

1

2

2

1

1

L

L

N

N =

I

I

V

V =

IV

IV

= R

R

Where, 2

1

2

1

N

N

V

V and

1

2

2

1

N

N

I

I

Thus the ratio of the transformer input and an output resistance varies directly as the

square of the transformer turn ratio:

2

2

2

1

L

L aN

N =

R

R

or L

2

L RaR




Advantages & disadvantages of transformer coupled class A power amplifier:

Advantages:

No need of centre tapping transformer.

Maximum power can transfer at output because of impedance matching between collector

circuit and the load.

Efficiency is more than the series fed class A power amplifier.

Disadvantages:

Harmonic distortion is more.

Power wastage is more due to ‘transistor ON power loss’ in absence of AC input signal.

Maximum efficiency is of 50% only.

Efficiency of the transformer coupled class A power amplifier:

Collector Efficiency:

For ideal transformer, there is no power drop; hence all the power supplied by dc supply

VCC is delivered to transistor.

0)(

dcRLP and

)()( dctrdcinPP

)(

)(

)(

)(

dctr

acout

dcin

acout

P

P

P

P

collectoroverall

(1) CQCCin(dc)

IVP

--- --- --- --- --- [1]

(2) rmsrmsout(ac)

VIP

22

V

22

IP)CE(PP)C(P

8

VI

PP)CE(PP)C(P

out(ac)

--- --- --- --- --- [2]

in(dc)

out(ac)

collector P

Pη

CQCC

P)CE(PP)C(P

collector I8V

VIη




Maximum Collector Efficiency:

As primary winding of transformer has very small resistance, it is assumed to be zero.

CCPCE(P2V

)V

CQP)C(P2II

L

P)CE(P

R

V

L

CC

R

2V

Where, L

2

LRnR and

2

2

12

N

Nn

(1) L

CC

CCmaxin(dc), R

VVP

L

2CC

R

V

(2) 8

VIP

P)CE(PP)C(P

maxout(ac),

CC'

L

CC 2V8R

2V

'

L

2

CC

2R

V




100%P

Pη

maxin(dc),

maxout(ac),

max 100%

R

V

2R

V

'

L

2CC

'

L

2CC

100%2

1

50%ηmax

Series fed class B power amplifier.



Circuit Diagram:

Figure: Series fed class B power amplifier

Description:

inV is the input signal to the power amplifier.

inC couples input signal inV to the base of transistor.

BR is base resistance connected between +Vcc and base of transistor.

CR is load resistance where output is developed.

Q is NPN power transistor connected with heat-sink.




Working of the series fed class-B power amplifier:

Class-B power amplifier is fixed bias transistor amplifier.

When input signal inV is zero no current flows through

BR and transistor remains OFF.

This is because we have to set Q - point at the cut-off point.

In positive half cycle the input signal inV is increasing. So, base current

BI is increasing,

collector current CI is increasing and output voltage CEV is decreasing.

In negative half cycle the input signal inV is decreasing. So, base current

BI is decreasing

and transistor is being OFF because Q-point is located at the cut-off point and decrease in

the input signal makes transistor OFF.

Thus we can say that for only 180° of input cycle (in positive half cycle) transistor ON

and we can get half output current CI and half output voltage CEV .

Advantages & disadvantages of the series fed class B power amplifier:

Advantages:

No power loss in absence of AC input signal.

Simple circuit & easy to design.

No need of transformer, so, circuit becomes cheap.

Efficiency is more than the class A power amplifier.

Disadvantages:

Distortion is more as we can get half cycle output current & voltage.

Output impedance of the transistor is more, so, the circuit cannot use for low impedance

load.

Power is wasted in collector resistor RC .

So, AC power output per transistor is small.




Efficiency of the series fed class B power amplifiers.

When ac signal is applied the transistor Q1 is conducting only for the half cycle and we

can get the current as shown in the figure.

Average dc current can be found from the following equation

2π

π

0

dθi

IC

dc

π

0

dθ θ sinI2π

1

C(max)

π

0

dθ θ sin2π

IC(max)

π 0

cosθ2π

IC(max)

cos0cosπ2π

IC(max)

112π

IC(max)

π

II

C(max)

dc

Where, C(max)

I = Peak value of collector current

Input DC Power dcCC

IV

π

IVP

C(max)CC

in(dc)

--- --- --- --- --- [1]




RMS value for CCE

I&V considered as they are only for half cycle.

2

II

C(max)

C(rms)

& 2

VV

CE(max)

CE(rms)

2

VCC

Output AC power considered by peak value,

CE(rms)C(rms)out(ac)VI

2

1P

2

V

2

I

2

1P

CE(rms)C(max)

out(ac)

2

V

2

I

2

1P CCC(max)

out(ac)

4

IVP

C(max)CC

out(ac)

--- --- --- --- --- [2]

DC Power loss in Load =RL(dc)

P

L

2

dcRL(dc)RIP

L

2

C(max)

RL(dc)R

π

I

P

--- --- --- --- --- [3]

DC Power losses in Collector region of transistor is given by:

out(ac)RL(dc)in(dc)C(dc)PPPP

Efficiency:

100%P

P

η

in(dc)

out(ac)

overall

100%

π

IV4

IV

C(max)CC

C(max)CC

100%4

π

% 78.5 =overall

η




Class B push pull power amplifier:



Circuit Diagram:

Figure: Circuit diagram of Class B push-pull power amplifier

Description:

Q1 and Q2 are NPN transistors and both are identical. Their emitters are joined together

and connected to the ground.

There is a centre tapping in the primary winding of output transformer which is connected

to +Vcc supply. Two outer leads of secondary winding of transformer are connected to

the collectors of transistors.

Load LR is connected to the secondary winding of the output transformer.

Input transformer is called driver stage transformer. There is centre tapping in secondary

winding of this transformer generally connected with ground. Two leads of secondary

winding is connected with base of both transistors.




Working:

When input signal is given to the primary winding of input transformer, 180° out of phase

voltages are induced in the windings A0 and B0.

During the first half cycle a positive half cycle is available at A0 and a negative half cycle

is available at B0. So, Q1 is ON and Q2 is OFF.

While Q1 is ON a current flow from its collector & output voltage is induced at the

primary winding of T2.

During the second half cycle a positive half cycle is available at B0 and a negative half

cycle is available at A0. So, Q2 is ON and Q1 is OFF.

While Q2 is ON a current flow from its collector & output voltage is induced at the

primary winding of T2.

At the secondary winding of T2 we can get a full cycle output.

Advantages & disadvantages of the class B push pull power amplifier:

Advantages:

DC components of the collector currents of both the transistors flow in opposite direction

through the two halves of the primary winding of the output transformer. So, there is no

DC saturation of the core. So, size and cost of the transformer decreased. Due to this the

linear distortion is reduced.

Distortion due to even harmonics is less because even harmonics are cancelled.

Ripple of the power supply get cancelled. So, hum is reduced and cost of the filter is

reduced.

Efficiency is more than the class A power amplifier.

Disadvantages:

Driver and output transformer are required. So, amplifier becomes bulky and costly.

Two transistors must exactly match; otherwise two halves of the input signals are not

amplified equally.

Cross-over distortion is present.




Cross-over distortion:

Figure: Cross-over distortion

In class-B push pull power amplifier both the transistors are biased at the cut-off point.

So, under zero signal condition both the transistors are OFF and no output voltage and

current available at the output.

When AC signal is applied: During the positive half cycle of Vin , the transistor Q1 will

ON when Vin increasing more than VBE,sat (i.e., > 0.7V for Si NPN transistor) and it will

remain ON till Vin decreasing less than VBE,sat (i.e., < 0.7V). After Vin < VBE,sat Q1 is OFF.

Similarly, during the negative half cycle of Vin , the transistor Q2 will ON when Vin

increasing more than VBE,sat and it will remain ON till Vin decreasing less than VBE,sat .

After Vin < VBE,sat Q1 is OFF.

So, we can get distorted output as shown in the figure.

This type of distortion is known as the cross-over distortion.




Efficiency of class B Push pull Amplifier:

When ac signal is applied at the input, transistor Q1 is conducting for the positive half

cycle and Q2 is conducting for the negative half cycle and we can get the current as

shown in the figure.

Average dc current can be found from the following equation

2π

π

0

dθi

2IC

dc

(i.e., for two half cycles)

π

0

dθ θ sinIπ

1

C(max)

π

0

dθ θ sinπ

IC(max)

π 0

cosθπ

IC(max)

cos0cosππ

IC(max)

11π

IC(max)

π

2II

C(max)

dc

where,

dcI = Average current taken from DC supply

Input dc power is given by dcCCin(dc)

IVP

π

2IVP

C(max)CC

in(dc)

--- --- --- --- --- [1]

Assume at output transistor turn ratio (211

N:NN )

So, for each transistor turn ratio is 1N:N21




CE(rms)C(rms)out(ac)VIp

2

V

2

ICCC(max)

2

.IVp

C(max)CC

out(ac)

--- --- --- --- --- [2]

Efficiency:

100%

p

pη

dc)in

out(ac)overall

100%

π

.I2V2

.IV

c(max)cc

c(max)cc

%1004

78.5%ηoverall

Total Collector dissipation for two transistors,

out(ac)in(dc)d(max)PPP

2

IV

π

2.IVP

C(max)CCC(max)CC

d(max)

=

4

V

π

V2I CCCC

C(max)

4π

π4.V2.ICCC(max)

2

.VI0.2

CCC(max)

out(ac)d(max)0.2PP




Class AB push pull power amplifier:



Circuit Diagram:

Figure: Class AB push-pull power amplifier

Purpose:

In class-B push-pull amplifier Q-point is located at cut-off point and the cross over

distortion occurs due to cut-in voltage.

To eliminate this problem we have to set Q-point below active region & near to cut-off

region. We can set above arrangement by biasing resistors R1 & R2 in class-B push-pull

amplifier.

Description:

This arrangement is class-AB push-pull power amplifier where Q-point is located below

the centre point of the load line and above the cut-off point of the load line.

R1 & R2 are biasing resistors.

Q1 & Q2 - are power transistors.

T1 - drive transformer.

T2 - output transformer.




Working of the class AB push pull power amplifier:

The operation of class-AB push-pull power amplifier is same as class-B push-pull power

amplifier.

Here because of R1 & R2 which are voltage divider type biasing resistors Q-point is

adjusted below the centre point of the load line and above the cut-off point of the load

line.

Because of this under zero signal condition the base current flows and both transistors

ON.

In positive half cycle point ‘a’ of the S.W. of T1 is positive w.r.t. point ‘b’. So, Q1-ON

and Q2-OFF.

In negative half cycle point ‘b’ of the S.W. of T1 is positive w.r.t. point ‘a’. So, Q1-OFF

and Q2-ON.

Thus the cycle of operation repeats.

Advantages & disadvantages of class AB push pull power amplifier:

Advantages:

Cross over distortion is not present.

Efficiency is more than class A power amplifier.

We can get distortion free output.

Disadvantages:

Driver and output transformer are required. So, amplifier becomes bulky and costly.

Two transistors must exactly match; otherwise two halves of the input signals are not

amplified equally.

Efficiency is less than class B push-pull power amplifier.




Class C power amplifier:

Circuit diagram:

Figure: Class C Power Amplifier

Description:

Q1 transistor is biased with (–VBB) beyond the cut-off.

RFC (Radio Frequency Choke) prevents passing of high frequency input to DC supply. It

allowed passing DC voltage provided by (–VBB).

LC makes a tuned circuit to generate full cycle at the output.

Working operation:

Class-C power amplifier is biased to operate for less than 180° of the input signal cycle.

Tuned circuit will provide a full cycle output signal at the output for the fundamental or

resonant frequency of the tuned circuit (LC tank circuit).

These amplifiers are not used for the purpose of the large power amplification.

Application:

Because of high distortion class-C power amplifiers are not used for audio frequency.

They are used for high power output at radio frequencies, where harmonic distortion can

be removed by simple circuits.

These power amplifiers are used in the fixed frequency communication circuits.




Class D power amplifier:

Description:

Class D amplifiers can attain efficiencies of 90 %, and with careful component selection

it can exceed 95 % even.

Block diagram:

Figure: Block diagram of Class-D power amplifier

Working:

A class D power amplifier is designed to operate with digital or pulse-type signals.

In class D power amplifier, any input signal is converted into a pulse-type waveform and

then it is used to drive a large power load. It is converted back to a sinusoidal signal to

recover the original signal.

As shown in the block diagram, in comparator circuit a saw tooth waveform is applied

with a sinusoidal input signal. So, a sinusoidal signal can be converted into a pulse-type

signal.

Amplifier amplifies the pulse-type signal.

Low pass filter converts a pulse-type signal back to the sinusoidal-type signal.

Transistor devices of the amplifier provide current only when they are turned ON, with

little power loss due to their low on-voltage.

Most of the power supplied to the amplifier is transferred to the load. So efficiency of

class-D power amplifier is very high.

A power MOSFET is used in the class D power amplifier.




Waveforms:

Complementary symmetry push-pull amplifier:



Circuit diagram:

Figure: Complementary Symmetry Push-pull Power Amplifier




Description:

Complementary means one transistor is NPN (Q1) and the other transistor is PNP (Q2).

Both transistors are symmetrical (i.e., they are made from the same materials and same

technology with identical characteristics).

R1 and R2 provide voltage divider bias to forward bias the EB junction of Q1 transistor.

R3 and R4 provide voltage divider bias to forward bias the EB junction of Q2 transistor.

These resistors are so selected that under zero signal condition, the operating point is at

cut-off and so Q1, Q2 does not conduct.

RL is the load resistor.

VCC1 and VCC2 are two separate power supplies.

CC1 and CC2 are coupling capacitors.

Working operation:

During positive half cycle of input signal Vin , Q1 transistor conducts and Q2 transistor

does not conduct. At this time Q1 transistor provides a positive half cycle output across

the load resistor RL .

During negative half cycle of input signal Vin , Q2 transistor conducts and Q1 transistor

does not conduct. At this time Q2 transistor provides a negative half cycle output across

the load resistor RL .

Thus the cycle of operation repeats.

Advantages:

It does not require driver and output transformers. So, size, weight and cost are reduced.

It eliminates 180 two out of phase input signals.

We can use this circuit in the integrated form.

Disadvantages:

Problem in impedance matching.

Two separate power supplies are needed.

If transistors will not match then even harmonics will not be cancelled and harmonic

distortion will be introduced.

Cross-over distortion is present.




Comparison of the different power amplifiers:

Class A B AB C

Conduction

Angle 360

o 180

o 180 to 360

o Less than 90

o

Position of

the

Q-point

Mid-point of the

load line

Exactly on the

cut-off point

In between the

mid-point of load

line and the cut-

off point

Dip cut-off

Overall

Efficiency Poor, 25 to 30% Better, 70 to 80%

Better than A but

less than B 50 to

70%

Higher than 80%

Signal

Distortion

None if correctly

biased

Cross-over

distortion Small amounts Large amounts

Documents

Power Amplifiers