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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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


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.

Power Amplifier


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

Conducting angle.

Classification of Power Amplifier


Classification of Power Amplifier

Collector current waveforms for transistors operating in (a) class A, (b) class B


Classification of Power Amplifier

class AB class C


CE amplifier is not proper for using as a power amplifier

DC supply power: ICQVcc , the area of ABCO

Rc power: ICQURc, the area ofQBCD

Collector dissipation power:ICQUCEQ, the area ofAQDO

Load power:


Class A Output Stage


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.


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.


Class A Output Stage


Class B Output Stage

  • A class B output stage.

  • Complementary circuits.

  • Push-pull operation

  • Maximum power-conversion efficiency is 78.5%


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


Transfer Characteristic


Crossover Distortion


The load power

Maximum load power

Power –conversion efficiency


Power –conversion efficiency

  • Total supply power

  • Maximum total supply power


Power-conversion efficiency

Maximum power-conversion efficiency

Power –conversion efficiency


Power dissipation

Maximum Power dissipation

Power Dissipation


Power Dissipation

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


Example: 9.1


Reducing crossover distortion


Class AB Output Stage


A Class AB Output Stage Utilizing Diodes for Biasing

The diode biasing has an

Important advantage:

Thermal stabilization of

Quiescent current


A Class AB Output Stage Utilizing A VBE Multiplier for Biasing


May 25th, 2008

9.12; 9.13; 9.14; 9.15

Homework:


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