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# Chapter 9 output stages and power amplifiers - PowerPoint PPT Presentation

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

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 current waveform that results when an input signal is applied.

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

Classification of Power Amplifier current waveform that results when an input signal is applied.

class AB class C

CE amplifier is not proper for using as a power amplifier current waveform that results when an input signal is applied.

DC supply power: ICQVcc , the area of ABCO

Rc power: ICQURc， the area ofQBCD

Collector dissipation power:ICQUCEQ， the area ofAQDO

Class A Output Stage current waveform that results when an input signal is applied.

Figure 14.3 current waveform that results when an input signal is applied.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 current waveform that results when an input signal is applied.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 current waveform that results when an input signal is applied.

Class B Output Stage current waveform that results when an input signal is applied.

• A class B output stage.

• Complementary circuits.

• Push-pull operation

• Maximum power-conversion efficiency is 78.5%

Class B Output Stage current waveform that results when an input signal is applied.

• 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 current waveform that results when an input signal is applied.

Crossover Distortion current waveform that results when an input signal is applied.

The load power current waveform that results when an input signal is applied.

Power –conversion efficiency

Power current waveform that results when an input signal is applied.–conversion efficiency

• Total supply power

• Maximum total supply power

Power-conversion efficiency current waveform that results when an input signal is applied.

Maximum power-conversion efficiency

Power –conversion efficiency

Power dissipation current waveform that results when an input signal is applied.

Maximum Power dissipation

Power Dissipation

Power Dissipation current waveform that results when an input signal is applied.

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

Example: 9.1 current waveform that results when an input signal is applied.

Reducing crossover distortion current waveform that results when an input signal is applied.

Class AB Output Stage current waveform that results when an input signal is applied.

A Class AB Output Stage Utilizing Diodes for Biasing current waveform that results when an input signal is applied.

The diode biasing has an

Thermal stabilization of

Quiescent current

A Class AB Output Stage Utilizing A current waveform that results when an input signal is applied.VBE Multiplier for Biasing

May 25th, 2008 current waveform that results when an input signal is applied.

9.12; 9.13; 9.14; 9.15

Homework: