Definitions and Amplifier Types, Lecture notes of Electronics

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C

hapter 12

C aptePower Amplifiers

DefinitionsDefinitions

In small-signal amplifiers the main factors are:

Amplification

Linearity

Gain

Since large-signal, or power, amplifiers handle relatively largevoltage signals and current levels, the main factors are:

Efficiency

Maximum power capabilityI

d

t hi

t

th

t

t d

i

I

mpedance matching to the output device

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

22

Amplifier TypesAmplifier Types

Class CClass CClass CClass C

The amplifier conducts less than 180 of the input. The Q-point is locatedbelow the cutoff level.

Class DClass D

This is an amplifier that is biased especially for digital signals

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44

Class A AmplifierClass A Amplifier

The output of a class A amplifierconducts for the full 360

of the

cycle.

The Q-point is set at the middle ofth

l

d li

th t th

AC

i

l

the load line so that the AC signalcan swing a full cycle.

Remember that the DC load lineindicates the maximum and minimumlimits set by the DC power supply.

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Class AB AmplifierClass AB Amplifier

This amplifier is a compromise between theclass A and class B amplifier—the Q-pointis above that of the Class B but below theis above that of the Class B but below theclass A.The output conducts between 180

and

f th

AC i

t

i

l

of the AC input signal.

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

77

Class CClass C

The output of the class Cconducts for less than 180

of the

conducts

for less than 180

of the

AC cycle. The Q-point is belowcutoff.

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

88

SeriesSeries-

-Fed Class A Amplifier

Fed Class A Amplifier

This is similar to thesmall-signal amplifier except that it will handlehigher voltages. Thetransistor used is a high-

i

power transistor.

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1010

SeriesSeries-

-Fed Class A Amplifier

Fed Class A Amplifier

A small input signalcauses the output voltageto swing to a maximum ofto swing to a maximum ofV

cc

and a minimum of 0V.

The current can alsoswing from 0mA to I

CSAT

g

CSAT

(V

CC

/R

C

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1111

TransformerTransformer-

-Coupled Class A Amplifier

Coupled Class A Amplifier

This circuit uses a transformer to couple tothe load. This improvesthe efficiency of the ClassA

A to 50%.

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1313

Transformer ActionTransformer Action

A transformer improves the efficiency because it is able to transformthe voltage, current, and impedance

Voltage RatioVoltage Ratio

2 1

2 1

N

N

V

V

Current RatioCurrent Ratio

1

1

1

2

N

I

Impedance RatioImpedance Ratio

1 2

2 1

N

N

I

I

Impedance RatioImpedance Ratio

2

2

1 2

2

1

L L

a

N

N

R R

R R

=

⎞⎟⎟⎠

⎛⎜⎜⎝

=

=

′

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1414

TransformerTransformer-

-Coupled Class A Amplifier

Coupled Class A Amplifier

Signal Swing and Output AC PowerSignal Swing and Output AC Power

The voltage swing:

min

CE

max

CE

)

p

p

(

CE

V

V

V

−

=

−

The current swing:

min

C

max

C

I

I

−

8

)

I

)(I

V

(V

P

Cmin

Cmax

CEmin

CEmax

o(ac)

−

−

=

The AC power:

8

(

)

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1616

TransformerTransformer-

-Coupled Class A Amplifier

Coupled Class A Amplifier

EfficiencyEfficiency

Power input from the DC source:Power input from the DC source:

CQ

CC

i(dc)

I

V

P

=

o(ac)

i(dc)

Q

P

P

P

−

=

Power dissipated as heat across the transistor:Power dissipated as heat across the transistor:

Note: The larger the input and output signal, thelower the heat dissipation.

2

V

V

⎞

⎛

Maximum efficiencyMaximum efficiency:

Note: The larger V

and smaller V

the

CEmin

CEmax

CEmin

CEmax

V

V

V

V

50

%

η

⎞⎟⎟⎠

⎛⎜⎜⎝

− +

=

Note:

The

larger V

CEmax

and smaller V

CEmin

, the

closer the efficiency approaches the theoreticalmaximum of 50%.

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1717

Class B Amplifier: EfficiencyClass B Amplifier: Efficiency

The maximum efficiency of a class B is 78.5%..

100

P

)

P

%

)

dc

(

i

ac

(

o

×

=

η

i

L

2

CC

o(dc)

V 2R

P

maximum

=

For maximum power, V

L

=V

CC

CC

2

CC

CC

dc

CC

i(dc)

2V

2V

V

I

(maximum

V

P

maximum

L

L

CC

dc

CC

i(dc)

R π R π ) (

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

1919

TransformerTransformer-

-Coupled Push

Coupled Push-

-Pull

Pull

Cl

B A

lifi

Cl

B A

lifi

Cl

ass B Amplifier

Class B Amplifier

The center

-tapped

The center tappedtransformer on the inputproduces oppositepolarity signals to the two transistor inputs.The center-tapped transformer on theoutput combines the twohalves of the AC

f

t

th

waveform together.

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

2020