Chapter 9 output stages and power amplifiers

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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
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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.

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Classification of Power Amplifier

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

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Classification of Power Amplifier
Collector current waveforms for transistors
operating in (a) class A, (b) class B
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Classification of Power Amplifier
class AB class C
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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
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Class A Output Stage
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Figure 14.3 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.
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Figure 14.4 Maximum signal waveforms in the
class A output stage of Fig. 14.2 under the
condition I VCC /RL or, equivalently, RL VCC
/I.
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Class A Output Stage
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Class B Output Stage

• A class B output stage.
• Complementary circuits.
• Push-pull operation
• Maximum power-conversion efficiency is 78.5

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Class B Output Stage

• When Vi0, QN and Qp are cut off, Vo0
• When Vigt0.5V, QN is on and Qp is cut off,
  VoVi-VBEN
• When Vigt0.5V, Qp is on and QN is cut off,
  VoVi-VBEP

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Transfer Characteristic
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Crossover Distortion
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Power conversion efficiency

• The load power
• Maximum load power

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Power conversion efficiency

• Total supply power
• Maximum total supply power

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Power conversion efficiency

• Power-conversion efficiency
• Maximum power-conversion efficiency

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Power Dissipation

• Power dissipation
• Maximum Power dissipation

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Power Dissipation

• At the point of maximum power dissipation the
  efficiency can be evaluated

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Example 9.1
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Reducing crossover distortion
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Class AB Output Stage
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A Class AB Output Stage Utilizing Diodes for
Biasing
The diode biasing has an Important advantage
Thermal stabilization of Quiescent current
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A Class AB Output Stage Utilizing A VBE
Multiplier for Biasing
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Homework

• May 25th, 2008
• 9.12 9.13 9.14 9.15