1. Output Stages and Power
Amplifiers

2. 14 Output Stages and Power Amplifier
Low output resistance
To deal with relatively large signals: small-signal approximation and models
Linearity: total harmonic distortion (THD) – rms value of the harmonic components of the output signal
To deliver the required amount of power to the load in an efficient manner, which implies low power dissipation.

3. 14 Output Stages and Power Amplifier
14.1 Classification of Output Stages
Type A
Conduction Angle: 360
Type B
Conduction Angle: 180

4. 14 Output Stages and Power Amplifier
14.1 Classification of Output Stages (cont.)
Type AB
Conduction Angle: (180, 360)
Type C
Conduction Angle: <180
Type A, B, AB: op amps and audio power amplifier
Type C: RF power amplification (mobile phone, radio, and TV)

5. 14.2 Class A Output Stage
Transfer Characteristics

6. 14.2 Class A Output Stage 14.2.2 Signal Waveforms
vCE1=VCC - vo
Neglecting VCEsat
Neglecting VCEsat
How about an open/short output?

7. 14.2 Class A Output Stage 14.2.2 Signal Waveforms (cont.)
vCE1=VCC - vo
When RL = , iC1 = I, vCE1_max = 2VCC, PD1=2VccI
When RL = 0, iC1  , PD1  , Q1 may burn up.
Short Circuit Protection

8. 14.2 Class A Output Stage 14.2.4 Power-Conversion Efficiency
Typical: 10%~20%

9. 14.3 Class B Output Stage
Circuit Operation
PUSH and PULL

10. 14.3 Class B Output Stage
Transfer Characteristics

11. 14.3 Class B Output Stage 14.3.3 Power-Conversion Efficiency

12. 14.3 Class B Output Stage
Power Dissipation
TRADEOFF?

13. 14.3 Class B Output Stage 14.3.5 Reducing Crossover Distortion
0.7 V dead band is reduced to: 0.7/A0
Another option: Class AB stage

14. 14.3 Class B Output Stage
Single-Supply Operation

15. 14.4 Class AB Output Stage 14.4.1 Circuit Operation For vI = 0, vo = 0
When vI goes positive

16. 14.4 Class AB Output Stage 14.4.1 Circuit Operation (cont.)
For small vI: both transistors conduct
As vI increases, or decrease, one of the two transistors takes over the operation
Crossover distortion will be eliminated

17. Homework:
Exercise: 14.1, 14.2, 14.3, 14.4, 14.5, 14.7, 14.8, 14.9