1、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,Power Amplifier,Small-signal approximation and models either are not applicable or must be used w
2、ith 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.,Classification of Power Amplifier
3、,Power amplifiers are classified according to the collector current waveform that results when an input signal is applied. Conducting angle.,Classification of Power Amplifier,Collector current waveforms for transistors operating in (a) class A, (b) class B,Classification of Power Amplifier,class AB
4、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 of QBCD Collector dissipation power: ICQUCEQ， the area of AQDO Load power:,Class A Output Stage,Figure 14.3 Transfer characteristic of the emitter follower. This li
5、near 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.,Figu
6、re 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
7、Output Stage,When Vi=0, QN and Qp are cut off, Vo=0 When Vi0.5V, QN is on and Qp is cut off, Vo=Vi-VBEN When Vi0.5V, Qp is on and QN is cut off, Vo=Vi-VBEP,Transfer Characteristic,Crossover Distortion,Power conversion efficiency,The load powerMaximum load power,Power conversion efficiency,Total supp
8、ly powerMaximum total supply power,Power conversion efficiency,Power-conversion efficiencyMaximum power-conversion efficiency,Power Dissipation,Power dissipationMaximum 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,Homework:,May 25th, 20089.12; 9.13; 9.14; 9.15,
