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ElectronicsElectronics

Principles & ApplicationsPrinciples & ApplicationsEighth EditionEighth Edition

Chapter 8Large-Signal Amplifiers

Charles A. Schuler

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• Amplifier Class• Class A• Class B• Class AB• Class C• Class D

INTRODUCTION

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PowerAmplifier

PIN

Efficiency =

Input signal

POUT

POUT

PIN

Output signal

HEAT = PIN - POUTHigh efficiency means less heat.

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Efficiency

• The dc power supplied to an amplifier is PIN = VCC x IDC

• Efficiency = POUT/PIN x 100%

• The maximum efficiency for Class A amplifiers with a dc collector resistance and a separate load resistance is 25%.

• Class A is usually not acceptable when watts of power are required.

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t

IC

t

IC

t

IC

t

IC

ISAT

A B

C D

The major classes of amplifier operation

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Class and efficiency quiz

If POUT = 100 W and PIN = 200 W, theefficiency is _________. 50%

The efficiency of an ideal amplifier is__________. 100%

When efficiency is poor, too much of the inputis converted to ________. heat

An amplifier that conducts for the entire cycleis operating Class _______. A

An amplifier that conducts for half the cycleis operating Class _______. B

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A large-signal amplifier can also be called a power amplifier.This class A amplifier has a large quiescent collector current.

C

BE

VCC = 18 VRL = 12 RB = 1.2 k

CC

= 60

=VCC

RB

18 V

1.2 k= = 15 mA

IC = x IB = 60 x 15 mA = 0.9 A

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VCE in Volts

IC in A

5 mA

0 mA

25 mA

20 mA

mA

10 mA

ISAT =VCC

RL

18 V12 = = 1.5 A

Q

This is a Class A amplifier.

PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W

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VCE in Volts

IC in A

5 mA

0 mA

25 mA

20 mA

mA

10 mAQ

This is a Class B amplifier.

PC = VCE x IC = 18 V x 0 A = 0 W

Its quiescent power dissipation is zero.

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5 mA

0 mA

25 mA

20 mA

mA

10 mA

Cla

ss B

The collector signal is too distorted forlinear applications.

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C

B

C

B E

E

+VCC

The complementary-symmetry Class B push-pull amplifier has acceptable

linearity for some applications.

NPN

PNP

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NP

N

PN

P

Class B

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C

B

C

B E

E

+VCC

Since the base-emitter junction potentialis 0.7 V, there is some crossover distortion.

NPN

PNP

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C

B

C

B E

E

+VCC

Crossover distortion is eliminatedby applying some forward bias

to the transistors (class AB).

NPN

PNP

1.4 V

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VCE in Volts

IC in AQ

The quiescent power dissipation is moderate for class AB.

The efficiency is much better than class A.

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Cap. required

+VCC

RL

RL

+VCC

Single-ended amplifier

A bridge-tied load provides four times the outputpower for a given supply voltage and load resistance.

+VCC

2

Max.

Max. = VCC

Bridge amplifier

Max. = 2 x VCC

Max.

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Class A, B, and AB quiz

Class A amplifiers are biased to operate nearthe ________ of the load line. center

Class B amplifiers have their Q-points at____________. cutoff

The conduction angle for class B is_________. 180o

To reduce distortion, two class B transistorsare arranged in _____________. push-pull

Class AB is a solution for __________distortion. crossover

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A

B

C

AB

The class of an amplifieris determined by the bias

which establishes the Q-point.

Class C is established by reversebiasing the base-emitter junction.

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Conduction Angles& theoretical max. efficiencies:

• Class A = 360o 50 %*

• Class B = 180o 78.5 %

• Class AB 200o (between A & B)

• Class C 90o 100 %

*Class A amplifiers are seldom driven to maximum output and typically provide much less efficiency.

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C

BE

VCC

RB

CC

VBB

Class C amplifier

VBB reverse biases the base-emitter junction.

Tank circuit

The transistor isoff for most ofthe input cycle

and the conductionangle is small.

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VBB

0.7 V

0 A

VBE waveform

IC waveform

VCE waveform

Class C amplifierwaveforms

(with tank circuit)

Low VCE when IC is flowing

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C

BE

VCC

RB

CC

Class C amplifier with signal bias

The base-emitterjunction rectifiesthe input signaland charges CC.

Signal bias increases when the input signal increases in amplitude.

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IB >> 0

Three transistor operating modes:

IB = 0 IB > 0

Cutoff Linear Saturation

Ideally, PC = 0 in both of these modes

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A switch-mode amplifier uses a rectangular input signal to drive power transistors rapidly between cutoff and

saturation. The efficiency is high, allowing large power outputs from relatively small boards and heat sinks.

C

BE

RB

They are alsocalled Class D

amplifiers.

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8-25Two, 250-W outputs into 4-loads, or one 500-W bridged

output into an 8-load

classdaudio.com

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Power Out

Note that Class D offers high efficiency when the amplifier output is modest or low!

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If the switching frequency is a good deal higherthan the signal frequency, a Class D amplifier is

capable of linear amplification. Pulse-widthmodulation and a low-pass filter are often used.

PWM Signal

Input Signal

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Low-pass filterComparator Class D amplifier

Triangular wave

Input signal Output signal

Max.pulsewidth

Min.pulsewidth

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PWM LPF

The low-pass filter rejects the switching frequency.

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Class C and D quiz

Class C amplifiers use _______ circuits torestore sinusoidal signals. tank

The base-emitter junction in a class Camplifier is ________ biased. reverse

The theoretical maximum efficiency for class C is ___________. 100%

Class D amplifiers are also known as__________ amplifiers. switch-mode

Class D amplifiers employ a varying duty-cycle known as _________. PWM

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REVIEW

• Amplifier Class• Class A• Class B• Class AB• Class C• Class D