1. Output Stages and Power Amplifiers Classification of Output Stages Class A,B & AB Biasing AB Power BJT

2. Introduction Low output resistance to deliver power w/o loss, i.e. efficiency Linearity, i.e. total harmonic distortion criteria Junction temperature and high power BJT Waveform shape?

3. Class A Output Stage Collector current waveform

4. Transfer Characteristic of Class A Emitter follower Q 1 biased with constant current I supplied by Q 2

5. Transfer characteristic of the emitter follower

6. Waveforms from Class A I = V CC /R L R L = V CC / I

7. Class B Conducts only half of the cycle

8. Power Dissipation Maximum instantaneous power in Q 1 is V CC I. 1.Emitter follower dissipated most power when v O = 0. 2.If R L =0, short circuit, protection? 3.Power conversion efficiency:

9. Power Efficiency Class B Power dissipation of the class B output stage versus amplitude of the output sinusoid

10. Linearity Transfer characteristic for the class B output stage

12. Op-amp Implementation Op amp connected in a negative-feedback loop to reduce crossover distortion

13. Class B Single Power Supply

14. Class AB Conduction more than half cycle

15. Transfer Function Class AB

16. Determining Output Impedance Class AB

17. Class AB using Diodes

18. Class AB using V_BE multiplier

19. Class AB with V_be multip+pot. The potentiometer is adjusted to yield the desired value of quiescent current in Q N and Q P.

20. Used in Radio frequency applications (mobile phones, radio and TV) Class C

21. Thermal Resistance

22. Thermal Dissipation The seating plane has two holes for screws to bolt it to a heat sink. The collector is electrically connected to the case. Therefore an electrically insulating but thermally conducting spacer is used between the transistor case and the “heat sink.” Diameter 2.2 cm; the outside dimension of the “seating plane” is about 4 cm. TO3 package for power transistors.

23. Thermal Circuit Electrical analog of the thermal conduction process when a heatsink is utilized.

24. T_max: Max temperature

25. Example 14.5

26. BJT Safe Operation

27. Variation on Class AB

28. Darlington Pair

29. Compound PNP

31. Compound + Darlington

32. Thermal Shutdown Protection