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Chapter 8

Output stages &. Chapter 8. Output Stages of power amplifiers. Output stage is designed to provide low output resistance and relatively high current drive capability. Followers: Class-A amplifiers- transistors conduct during full 360 0 of signal waveform, conduction angle =360 0 .

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Chapter 8

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  1. Output stages & Chapter 8

  2. Output Stages of power amplifiers • Output stage is designed to provide low output resistance and relatively high current drive capability. • Followers: Class-A amplifiers- transistors conduct during full 3600 of signal waveform, conduction angle =3600. • Push-pull: Class-B- each of the two transistors conducts during 1800of signal wavefrom, conduction angle =1800. • Class-AB: Characteristics of Class-A and Class-B are combined, most commonly used as output stage in op amps.

  3. Power dissipation

  4. Conversion efficiency of Class A amplifier

  5. Conversion efficiency of Class A amplifier

  6. Transformer coupled class A amplifier (improved version) • The main reason for the poor efficiency of a direct coupled class a amplifier is large amount of Dc power that the resistive load in collector must dissipate. This problem can be solved by using a suitable transformer for coupling the load (say a speaker). Since the load is not directly connected to the collector terminal, the dc collector current does not pass thorough it. An ideal transformer primary winding resistance is zero.

  7. Transformer coupled class A amplifier (improved version)

  8. : (emitter follower)

  9. (cont.)

  10. (cont.)

  11. It has high efficiency, It is primarily due to the fact that there is no power drawn by circuit under zero signal conditions. Since in push pull arrangement 180 phase difference exist between even-order harmonics produced by each transistor, they cancel out thereby giving an almost distortion free output. This automatics cancellation of all even order harmonics from the output current makes class push pull amplifier highly desirable for communication sound equipments. Uses: Class B push pull amplifier are extensively used for audio work in portable recorder players, as stereo amplifiers, and in radio receiver.

  12. It, in fact means locating the Q-point of each transistor slightly above the cutoff. So that each one operate more than one half cycle.

  13. Advantage and disadvantage of class AB amplifier

  14. In such amplifiers, transistor is biased beyond cut-off.

  15. Output current flows only during a part of the positive half cycle of the input signal. There is no output current flow during any part of the negative half cycle of the input signal. Class C amplifier have high circuit efficiency of more than 78%. They are used as a high frequency power switches in radio and TV transmitters rather than as a amplifier.

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