Transcript What is a power amplifier

SARVAJANIK COLLEGE OF
ENGINEERING & TECHNOLOGY
ANALOG ELECTRONICS
Presentation Subject:-Class B & Class AB Power
amplifiers
Electrical (shift 1)
 2nd year(sem 3)
 Group no.4

Ravi Bhatt
 Manthan Bhavsar 
130420109007
130420109008

What is a power amplifier?

A power amplifier is an electronic device that
receives an electrical signal and reprocesses it
to amplify, or increase, its power. The boost in
power is achieved by significantly increasing the
input signal’s voltage. A power amplifier is used
to power an output source, such as a stereo
speaker, a relay or a motor.
Types Of Power Amplifier
Class B Amplifier

Class-B amplifiers only amplify half of the input wave
cycle, thus creating a large amount of distortion, but
their efficiency is greatly improved and is much better
than class A. Class-B amplifiers are also favoured in
battery-operated devices, such as transistor radios .
Class B has a maximum theoretical efficiency of
78.5%This is because the amplifying element is switched
off altogether half of the time, and so cannot dissipate
power.
Class B Amplifier
Class B Amplifier using 2
transistors..
 1.Class B-push pull
amplifier
 2.Class B-complementary
symmetry amplifier
 With 2 transistors we can
obtain full 360 degree
conduction(with some
cross over distortion)

Class B push pull Amplifier
Output of Push pull Amplifier
Complementary Symmetry Class B
Cross Over Distortion in Class B
The main problem with class B push
pull output stages is that each
transistor conducts for NOT QUITE
half a cycle. As shown in Fig. distortion
occurs on each cycle of the signal
waveform as the input signal waveform
passes through zero volts. Because the
transistors have no base bias, they do
not actually begin to conduct until their
base/emitter voltage has risen to about
0.6V. As a result, there is a ‘Dead Zone’
of about 1.2V around the zero volts line
(between −0.6V and +0.6V) where the
signal waveform is not amplified, causing
a "missing" section from the output
signal, resulting in unwanted distortion
during the "crossover" from one
transistor to the other.

To eliminate the cross over distortion the
“Class AB” Operations are used..
Class AB Push pull Amplifier
•In class AB amplifier the voltage
across bases of two transistors is
created in such a way that it over
comes the cross over situation of
class B amplifier.
•1.4 V appears at bases of two
transistors. Both of them obtains
0.7 V cut in voltage , so that it over
comes the cross over situation and
makes the Class AB amplifier.
Complementary Symmetry Class
AB
Complementary Symmetry Class
AB Using Diodes
Summary of CLASS B
A class B output stage can be far more
efficient than a class A stage (78.5 %
maximum efficiency compared with 25 %).
 It also requires twice as many output
transistors…
 …and it isn’t very linear; cross-over
distortion can be significant.

Summary of CLASS B

Class B amplifiers are used in low cost
designs or designs where sound quality is
not that important.

Class B amplifiers are significantly more
efficient than class A amps.

They suffer from bad distortion when the
signal level is low (the distortion in this
region of operation is called "crossover
distortion").
Summary of CLASS B

Class B is used most often where
economy of design is needed.

Before the advent of IC amplifiers, class B
amplifiers were common in clock radio
circuits, pocket transistor radios, or other
applications where quality of sound is not
that critical.
Summary of CLASS AB

Class AB is probably the most common
amplifier class currently used in home
stereo and similar amplifiers.

Class AB amps combine the good points
of class A and B amps.

They have the improved efficiency of class
B amps and distortion performance that
is a lot closer to that of a class A amp.
Summary of CLASS AB

With such amplifiers, distortion is worst
when the signal is low, and generally
lowest when the signal is just reaching the
point of clipping.

Class AB amps use pairs of transistors,
both of them being biased slightly ON so
that the crossover distortion (associated
with Class B amps) is largely eliminated
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