Power Amplifiers 4

McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved.

ElectronicsElectronics Principles & ApplicationsPrinciples & Applications

Seventh EditionSeventh Edition

Chapter 8Large-Signal Amplifiers

(student version)

Charles A. Schuler



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

INTRODUCTION


Dear Student:

This presentation is arranged in segments. Each segmentis preceded by a Concept Preview slide and is followed by aConcept Review slide. When you reach a Concept Reviewslide, you can return to the beginning of that segment byclicking on the Repeat Segment button. This will allow youto view that segment again, if you want to.


Concept Preview• Efficiency is most important in power amplifiers.• Poor efficiency means that much of the input power is

converted to heat.• A class A amplifier conducts for the entire signal cycle and

has the lowest efficiency.• A class B amplifier conducts for only half of the signal

cycle.• A class C amplifier conducts for less than half of the signal

cycle.• A class D amplifier switches between cutoff and saturation.


PowerAmplifier

PIN

Efficiency =

Input signal

POUT

POUT

PIN

Output signal

HEAT = PIN - POUTHigh efficiency means less heat.


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.


t

IC

t

IC

t

IC

t

IC

ISAT

A B

C D

The major classes of amplifier operation


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


Concept Review• Efficiency is most important in power amplifiers.• Poor efficiency means that much of the input power is

converted to heat.• A class A amplifier conducts for the entire signal cycle and

has the lowest efficiency.• A class B amplifier conducts for only half of the signal

cycle.• A class C amplifier conducts for less than half of the signal

cycle.• A class D amplifier switches between cutoff and saturation.

Repeat Segment


Concept Preview• Class A amplifiers operate at the center of the load

line and have a large quiescent current flow.• Class B amplifiers operate at cutoff and have no

quiescent current flow.• Class B amplifiers are usually operated in push-pull

configurations.• Class B amplifiers have crossover distortion.• Class AB reduces crossover distortion.• Bridge amplifiers provide four times the output power

and eliminate the output coupling capacitor.


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 V1.2 k

= = 15 mA

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


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0.20.40.60.81.01.21.4

VCE in Volts

IC in A

5 mA

0 mA

25 mA20 mAmA

10 mA

ISAT =VCC

RL

18 V12 = = 1.5 A

QThis is a Class A amplifier.

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


0 2 4 6 8 10 12 14 16 18

0.20.40.60.81.01.21.4

VCE in Volts

IC in A

5 mA

0 mA

25 mA20 mAmA

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.


0 2 4 6 8 10 12 14 16

0.20.40.60.81.01.21.4

5 mA

0 mA

25 mA20 mAmA

10 mA

Class B

The collector signal is too distorted forlinear applications.


C

B

C

B E

E

+VCC

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

linearity for some applications.

NPN

PNP


NPN

PNP

Class B


C

B

C

B E

E

+VCC

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

NPN

PNP


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


0 2 4 6 8 10 12 14 16 18

0.20.40.60.81.01.21.4

VCE in Volts

IC in A Q

The quiescent power dissipation is moderate for class AB.

The efficiency is much better than class A.


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.


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


Concept Review• Class A amplifiers operate at the center of the load

line and have a large quiescent current flow.• Class B amplifiers operate at cutoff and have no

quiescent current flow.• Class B amplifiers are usually operated in push-pull

configurations.• Class B amplifiers have crossover distortion.• Class AB reduces crossover distortion.• Bridge amplifiers provide four times the output power

and eliminate the output coupling capacitor.

Repeat Segment


Concept Preview• Class C amplifiers are biased beyond cutoff for a small

conduction angle and high efficiency.• Class C amplifiers used tuned tank circuits to reduce

distortion in RF applications.• Class C amplifiers cannot be used in wideband applications

like audio.• Class D amplifiers switch between cutoff and saturation for

very high efficiency.• Class D amplifiers operate at a relatively high switching

frequency and often use PWM.• Class D can be used in audio applications.


0 2 4 6 8 10 12 14 16 18

0.20.40.60.81.01.21.4

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.


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.


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.


VBB

0.7 V

0 A

VBE waveform

IC waveform

VCE waveform

Class C amplifierwaveforms

(with tank circuit)

Low VCE when IC is flowing


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.


IB >> 0

Three transistor operating modes:

IB = 0 IB > 0

Cutoff Linear(PC > 0)

Saturation

PC = 0 in both of these modes


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

saturation. The efficiency is very high.

C

BERB

They are alsocalled Class D

amplifiers.


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


PWM LPF

The low-pass filter rejects the switching frequency.


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


Concept Review• Class C amplifiers are biased beyond cutoff for a small

conduction angle and high efficiency.• Class C amplifiers used tuned tank circuits to reduce

distortion in RF applications.• Class C amplifiers cannot be used in wideband applications

like audio.• Class D amplifiers switch between cutoff and saturation for

very high efficiency.• Class D amplifiers operate at a relatively high switching

frequency and often use PWM.• Class D can be used in audio applications.

Repeat Segment


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

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Power Amplifiers 4