Transcript Flow Control in Packet Networks

Output Stages and Power Amplifiers
• A power amplifier is an amplifier with a highpower output stage.
• An important function of the output stage is to
provide the amplifier with a low output resistance
so that it can deliver the output signal to the load
without loss of gain.
• Output stages are classified according to the
collector current waveform that results when an
input signal is applied as shown in the following
figure.
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Collector Current Waveforms for Transistors
Operating in Different Classes.
Class A
Class B
Class AB
Class C
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vo  vI  vBE
vo max  VCC  VCE1sat
I
vo min   IRL
Class A
vo min  VCC  VCE 2 sat
 VCC  VCE 2 sat
RL
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Power Dissipation
• Maximum instantaneous power dissipation in Q1 = VCCI.
This means that the emitter-follower transistor dissipates
the largest amount of power when vo= 0.
• Power dissipation in Q1 depends on the value of RL. If RL
is , the maximum power dissipation in Q1 will depend on
the value of vo. The maximum power dissipation will
occur when vo=-VCC, for which vCE1 is 2VCC and
pD1=2VCCI, but this situation will not last for long. If RL is
small then a very large current may pass through RL
causing Q1 to burn. Output stages are usually equiped with
short-circuit protection.
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Power Conversion Efficiency
PL
load power


supply power PS
Regardingthe emitter followershownin thepreviousfigure
2
1 Vo  peak
PL 
2
RL
Total Averagesupplypower
PS  2VCC I
 Vo 


V 
 CC 
The maximum efficiency is 25%, thusclass A is not widely
used in high- powerapplications (or more than1 W).
1  Vo
  
4  IRL
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