Chapter 12 Power Amplifier - الصفحات الشخصية | الجامعة ...site.iugaza.edu.ps/ahdrouss/files/2010/02/lect-7m-class... · · 2014-11-16Chapter 12 Power Amplifier

Chapter 12

Power Amplifier

Copyright ©2009 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458 • All rights reserved.

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

Class B Amplifier

In class B, the transistor is biased

just off. The AC signal turns the

transistor on.

The transistor only conducts when it

is turned on by one half of the AC

cycle.

In order to get a full AC cycle out of

a class B amplifier, you need two

transistors:

• An npn transistor that provides the

negative half of the AC cycle

• A pnp transistor that provides the

positive half.

2




Input Power

3

• The power supplied to the load by an amplifier is

drawn from the power supply

• The amount of this DC power is calculated using

• The average value for the current is given by

• The input power can be written as

dcCCdci IVP )(

pdc II

2

pCCdci IVP

2)(




Class B Amplifier: Efficiency

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

4

For maximum power, VL=VCC

L

pL

L

ppL

acoR

V

R

VP

28

)()(

)(

For output power,




Example 1

5

Example 1: For class B amplifier providing a 20-V peak

signal to a 16-Ω speaker and a power supply of VCC=30

V, determine the input power , output power and the

efficiency

Solution:

The input power is given by

The peak collector load current can be found from

pCCdci IVP

2)(




CB: Example 1

6

Solution:

The input power is

The output power is given by

The efficiency is

WP dci 9.23)25.1(302

)(




CB: Example 2

7

Example 2: For class B amplifier using a supply of

VCC=30 V and driving a load of 16-Ω, determine the

input power , output power and the efficiency

Solution:

The maximum output power is given by

The maximum input power drawn from the supply is




CB: Example 2

8

Solution:

The efficiency is given by

The maximum power dissipated by each transistor is




Complementary symmetry circuits

9

• This circuit uses both npn and pnp transistor to construct class B amplifier as shown to the left

• One disadvantage of this circuit is the need for two separate voltage supplies




Complementary symmetry circuits

10

• Another disadvantage of this circuit is the resulting cross over distortion

• Cross over distortion can be eliminated the by biasing the transistors in class AB operation where the transistors are biased to be on for slightly more than half a cycle




Transformer Coupled Pull-Push Class B

The center-tapped

transformer on the input

produces opposite

polarity signals to the

two transistor inputs.

The center-tapped

transformer on the

output combines the two

halves of the AC

waveform together.

11




Transformer Coupled Pull-Push Operation

During the positive half-

cycle of the AC input,

transistor Q1 (npn) is

conducting and Q2 (pnp) is

off.

• During the negative half-

cycle of the AC input,

transistor Q2

(pnp) is conducting and Q1

(npn) is off.

12

Each transistor produces one-half of an AC cycle. The transformer

combines the two outputs to form a full AC cycle




Crossover Distortion

If the transistors Q1 and Q2

do not turn on and off at

exactly the same time, then

there is a gap in the output

voltage..

13




Quasi-Complementary Push-Pull Amplifier

A Darlington pair and a

feedback pair combination

perform the push-pull

operation. This increases

the output power capability.

14




CB: Example 3

15

Example: For the circuit shown, calculate the input power, output power and the power handled by each transistor and the efficiency if the input signal is 12 Vrms

Solution:

The peak input voltage is

The output power is




CB: Example 3

16

Solution:

The peak load current is

The dc current can be found from the peak as

The input power is given by

The power dissipated by each transistor is given by




Amplifier Distortion

If the output of an amplifier is not a complete AC sine wave, then it

is distorting the output. The amplifier is non-linear.

This distortion can be analyzed using Fourier analysis. In Fourier

analysis, any distorted periodic waveform can be broken down into

frequency components.

These components are harmonics of the fundamental frequency

17




Harmonics

Harmonics are integer multiples of a fundamental frequency.

If the fundamental frequency is 5kHz:

18

Note that the 1st and 3rd harmonics are called odd harmonics and

the 2nd and 4th are called even harmonics.




Harmonics Distortion

19

According to Fourier

analysis, if a signal is not

purely sinusoidal, then it

contains harmonics.




Harmonics Distortion Calculations

20

Harmonic distortion (D) can be calculated:

where

A1 is the amplitude of the fundamental frequency

An is the amplitude of the highest harmonic

The total harmonic distortion (THD) is determined by:




21




Power Transistor Derating Curve

22

Power transistors dissipate a

lot of power in heat. This can

be destructive to the amplifier

as well as to surrounding

components.




Class C Amplifiers

23

A class C amplifier conducts for less

than 180°.

In order to produce a full sine wave

output, the class C uses a tuned circuit

(LC tank) to provide the full AC sine

wave.

Class C amplifiers are used extensively

in radio communications circuits..




Class D Amplifiers

24

A class D amplifier amplifies

pulses, and requires a pulsed

input.

There are many circuits that can

convert a sinusoidal waveform

to a pulse, as wellas circuits that

convert a pulse to a sine wave.

This circuit has applications in

digital circuitry.

Documents

Chapter 12 Power Amplifier - الصفحات الشخصية | الجامعة ...site.iugaza.edu.ps/ahdrouss/files/2010/02/lect-7m-class... · · 2014-11-16Chapter 12 Power Amplifier