Output Stages and Power Amplifiers

1
Output Stages and Power Amplifiers

• Chapter 14
• Classification of Output Stages
• Class A,B AB
• Biasing AB
• Power BJT

2
Schedule

• Due Friday, Nov 9
• 13.4,13.14,13.23,13.27
• Due Friday Nov. 16
• 14.8,14.11,14.27,14.37
• Review Nov. 26
• Quiz 3 Nov 30
• Project Presentation Dec 3 and Dec 7

3
Introduction

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

4
Class AOutput Stage

• Collector current waveform

5
Transfer Characteristic of Class A

• Emitter follower Q1 biased with constant current
  I supplied by Q2

6
Transfer characteristic of the emitter follower
7
Waveforms from Class A
I VCC /RL RL VCC /I
8
Class B

• Conducts only half of the cycle

9
Power Dissipation

• Maximum instantaneous power in Q1 is VCCI.
• Emitter follower dissipated most power when vO
  0.
• If RL0, short circuit, protection?
• Power conversion efficiency

10
Power Efficiency Class B
Power dissipation of the class B output stage
versus amplitude of the output sinusoid
11
Linearity

• Transfer characteristic for the class B output
  stage

12
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13
Opamp Implementation
Op amp connected in a negative-feedback loop to
reduce crossover distortion
14
Class B Single Power Supply
15
Class AB

• Conduction more than half cycle

16
Class AB

• Conduction more than half cycle

17
Transfer Function Class AB
18
Determining Output ImpedanceClass AB
19
Class AB using Diodes
20
Class AB using V_BE multiplier
21
Class AB with V_be multippot.
The potentiometer is adjusted to yield the
desired value of quiescent current in QN and QP.
22
Class C

• Used in Radio frequency applications (mobile
  phones, radio and TV)