1. Chapter 9 output stages and power amplifiers
Introduction
9.1 Classification of output stages
9.2 Class A output stage
9.3 Class B output stage
9.4 Class AB output stage
9.5 Biasing the class AB circuit

2. Power Amplifier
Small-signal approximation and models either are not applicable or must be used with care. THD (total harmonic distortion)
Deliver the power to the load in efficient manner. Ex. To provide the amplifier with a low output resistance so that it can deliver the output signal to the load without loss of gain.
Power dissipation is as low as possible.

3. Classification of Power Amplifier
Power amplifiers are classified according to the collector current waveform that results when an input signal is applied.
Conducting angle.

4. Classification of Power Amplifier
Collector current waveforms for transistors operating in (a) class A, (b) class B

5. Classification of Power Amplifier
class AB class C

6. CE amplifier is not proper for using as a power amplifier
DC supply power: ICQVcc , the area of ABCO
Rc power: ICQURc， the area of QBCD
Collector dissipation power: ICQUCEQ， the area of AQDO
Load power:

7. Class A Output Stage

8. Figure 14. 3 Transfer characteristic of the emitter follower
Figure Transfer characteristic of the emitter follower. This linear characteristic is obtained by neglecting the change in vBE1 with iL. The maximum positive output is determined by the saturation of Q1. In the negative direction, the limit of the linear region is determined either by Q1 turning off or by Q2 saturating, depending on the values of I and RL.
sedr42021_1403.jpg

9. sedr42021_1404a.jpg
Figure Maximum signal waveforms in the class A output stage of Fig under the condition I = VCC /RL or, equivalently, RL = VCC /I.

10. Class A Output Stage

11. Class B Output Stage A class B output stage. Complementary circuits.
Push-pull operation
Maximum power-conversion efficiency is 78.5%

12. Class B Output Stage When Vi=0, QN and Qp are cut off, Vo=0
When Vi>0.5V, QN is on and Qp is cut off, Vo=Vi-VBEN
When Vi>0.5V, Qp is on and QN is cut off, Vo=Vi-VBEP

13. Transfer Characteristic

14. Crossover Distortion

15. Power –conversion efficiency
The load power
Maximum load power

16. Power –conversion efficiency
Total supply power
Maximum total supply power

17. Power –conversion efficiency
Maximum power-conversion efficiency

18. Power Dissipation
Power dissipation
Maximum Power dissipation

19. Power Dissipation
At the point of maximum power dissipation the efficiency can be evaluated:

20. Example: 9.1

21. Reducing crossover distortion

22. Class AB Output Stage

23. A Class AB Output Stage Utilizing Diodes for Biasing
The diode biasing has an
Important advantage:
Thermal stabilization of
Quiescent current

24. A Class AB Output Stage Utilizing A VBE Multiplier for Biasing

25. Homework:
May 25th, 2008
9.12; 9.13; 9.14; 9.15