Electronics Circuit Analysis

Unit – I

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Large-Signal Amplifiers

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

INTRODUCTION

Power

Amplifier

P_IN

Efficiency =

P_OUT

P_OUT

P_IN

HEAT = P_IN - P_OUT

High efficiency means less heat.

Efficiency

• The dc power supplied to an amplifier is P_IN = V_CC x I_DC
• Efficiency = P_OUT/P_IN 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

I_C

t

I_C

t

I_C

t

I_C

I_SAT

A

B

C

D

The major classes of amplifier operation

Class and Efficiency Quiz

If P_OUT = 100 W and P_IN = 200 W, the

efficiency is _________.

50%

The efficiency of an ideal amplifier is

__________.

100%

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

heat

An amplifier that conducts for the entire cycle is operating Class _______.

A

An amplifier that conducts for half the cycle is operating Class _______.

B

A large-signal amplifier can also be called a power amplifier.

This class A amplifier has a large quiescent collector current.

C

B

E

V_CC = 18 V

R_L = 12 Ω

R_B = 1.2 kΩ

C_C

β = 60

I_C = β x I_B = 60 x 15 mA = 0.9 A

0

2

4

6

8

10

12

14

16

18

0.2

0.4

0.6

0.8

1.0

1.2

1.4

V_CE in Volts

I_C in A

This is a Class A amplifier.

P_C = V_CE x I_C = 7.2 V x 0.9 A = 6.48 W

0

2

4

6

8

10

12

14

16

18

0.2

0.4

0.6

0.8

1.0

1.2

1.4

V_CE in Volts

I_C in A

This is a Class B amplifier.

P_C = V_CE x I_C = 18 V x 0 A = 0 W

Its quiescent power dissipation is zero.

Class B

The collector signal

is too distorted for

linear applications.

C

B

E

E

+V_CC

The complementary-symmetry Class B

push-pull amplifier has acceptable

linearity for some applications.

NPN

PNP

NPN

PNP

Class B

C

B

E

E

+V_CC

Since the base-emitter junction potential

is 0.7 V, there is some crossover distortion.

NPN

PNP

C

B

E

E

+V_CC

Crossover distortion is eliminated

by applying some forward bias

to the transistors (class AB).

NPN

PNP

1.4 V

0

2

4

6

8

10

12

14

16

18

0.2

0.4

0.6

0.8

1.0

1.2

1.4

V_CE in Volts

I_C in A

The quiescent power dissipation is moderate for class AB.

The efficiency is much better than class A.

Single-ended amplifier

A bridge-tied load provides four times the output

power for a given supply voltage and load resistance.

Bridge amplifier

Class A, B, and AB Quiz

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

center

Class B amplifiers have their Q-points at ____________.

cutoff

The conduction angle for class B is

_________.

180^o

To reduce distortion, two class B transistors

are arranged in _____________.

push-pull

Class AB is a solution for __________

distortion.

crossover

0

2

4

6

8

10

12

14

16

18

0.2

0.4

0.6

0.8

1.0

1.2

1.4

A

B

C

AB

The class of an amplifier

is determined by the bias

which establishes the Q-point.

Class C is established by reverse

biasing the base-emitter junction.

Conduction Angles�& theoretical max. efficiencies:

• Class A = 360^o 50 %*
• Class B = 180^o 78.5 %
• Class AB ≅ 200^o (between A & B)
• Class C ≅ 90^o 100 %

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

C

B

E

V_CC

R_B

C_C

V_BB

Class C Amplifier

Tank circuit

The transistor is

off for most of

the input cycle

and the conduction

angle is small.

V_BB reverse biases the base-emitter junction.

V_BB

0.7 V

0 A

V_BE waveform

I_C waveform

V_CE waveform

Class C amplifier

waveforms

(with tank circuit)

Low V_CE when I_C is flowing

C

B

E

V_CC

R_B

C_C

Class C Amplifier with Signal Bias

The base-emitter

junction rectifies

the input signal

and charges C_C.

Signal bias increases when the input

signal increases in amplitude.

I_B >> 0

Three transistor operating modes:

I_B = 0

I_B > 0

Cutoff

Linear

Saturation

P_C = 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

B

E

R_B

They are also

called Class D

amplifiers.

If the switching frequency is a good deal higher

than the signal frequency, a Class D amplifier is

capable of linear amplification. Pulse-width

modulation and a low-pass filter are often used.

PWM

LPF

The low-pass filter rejects

the switching frequency.

Class C and D Quiz

Class C amplifiers use _______ circuits to restore sinusoidal signals.

tank

The base-emitter junction in a class C

amplifier 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

REVIEW

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