SARVAJANIK COLLEGE OF ENGINEERING & TECHNOLOGY ANALOG ELECTRONICS Presentation Subject:-Class B & Class AB Power amplifiers

Electrical (shift 1) 2 nd year(sem 3) Group no.4 Ravi Bhatt Manthan Bhavsar

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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