Class b amplifier

Here, I've explained about the Class B amplifier of Analog communication. the working of amplifier.

1. CLASS B AMPLIFIER Gauravsinh Parmar ( 170410117023) Neel Patel (170410117033)

2. Class B Amplifier Class B amplifier is a type of power amplifier where the active device (transistor) conducts only for one half cycle of the input signal. That means the conduction angle is 180° for a Class B amplifier. Since the active device is switched off for half the input cycle, the active device dissipates less power and hence the efficiency is improved.   Theoretical maximum efficiency of Class B power amplifier is 78.5%. The schematic of a single ended Class B amplifier and input , output waveforms are shown 

3. Circuit Diagram

4. Class B The class B amplifier uses two complimentary transistor (NPN-PNP) for each half of the o/p waveform. One transistor conducts for one-half of the signal waveform while the other conducts for the other or opposite half of the signal waveform. This means that each transistor spends half of its time in the active region and half time in cut-off region thereby amplifying only 50%of the I/p signal.

5. Common-collector class B amplifier.

6. Class B push-pull ac operation.

7. Transfer Characteristics

8. Crossover Distortion

9. Transformer coupled push-pull amplifiers. Q 1 conducts during the positive half-cycle; Q 2 conducts during the negative half-cycle. The two halves are combined by the output transformer.

10. Biasing the push-pull amplifier to eliminate crossover distortion.

11. Collector waveform

12. Transfer Characteristic

13. Class B Output Stage Q 1 and Q 2 form two unbiased emitter followers Q 1 only conducts when the input is positive Q 2 only conducts when the input is negative Conduction angle is, therefore, 180° When the input is zero, neither conducts i.e. the quiescent power dissipation is zero

14. Class B Current Waveforms I out I C1 I C2 time time time

15. Class B Efficiency Average power drawn from the positive supply: I C1 Phase, q A/R L p 2p A sin( q )

16. Efficiency / Power Dissipation Peak efficiency of the class B output stage is 78.5 %, much higher than class A. Unlike class A, power dissipation varies with output amplitude. Remember, there are two output devices so the power dissipation is shared between them.

17. 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").

18. ADVANTAGES The Collector efficiency quite high due to class B operation. Distortion free o/p is obtained, they give more ac o/p power. They are light in weight. o/p transformer use to push pull amplifier circuit are light, smaller and less expensive.

19. DISADVANTAGES Two identical transformer required. It required two equal and opposite voltage at the I/p. If the parameter of the two transistor differ, there’ll be unequal amplification of the two halves of the signal which introduce more distortion.