Chapter 9 output stages and power amplifiers Introduction

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 Microelectronic Circuits SJTU Yang Hua

Power Amplifier l l l 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. Microelectronic Circuits SJTU Yang Hua

Classification of Power Amplifier l l Power amplifiers are classified according to the collector current waveform that results when an input signal is applied. Conducting angle. Microelectronic Circuits SJTU Yang Hua

Classification of Power Amplifier Collector current waveforms for transistors operating in (a) class A, (b) class B Microelectronic Circuits SJTU Yang Hua

Classification of Power Amplifier class AB Microelectronic Circuits class C SJTU Yang Hua

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: Microelectronic Circuits SJTU Yang Hua

Class A Output Stage Microelectronic Circuits SJTU Yang Hua

Figure 14. 3 Transfer characteristic of the emitter follower. This linear characteristic is obtained by neglecting the change in v. BE 1 with i. L. The maximum positive output is determined by the saturation of Q 1. In the negative direction, the limit of the linear region is determined either by Q 1 turning off or by Q 2 saturating, depending on the values of I and RL. Microelectronic Circuits SJTU Yang Hua

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. Microelectronic Circuits SJTU Yang Hua

Class A Output Stage Microelectronic Circuits SJTU Yang Hua

Class B Output Stage ØA class B output stage. ØComplementary circuits. ØPush-pull operation ØMaximum powerconversion efficiency is 78. 5% Microelectronic Circuits SJTU Yang Hua

Class B Output Stage ØWhen Vi=0, QN and Qp are cut off, Vo=0 ØWhen Vi>0. 5 V, QN is on and Qp is cut off, Vo=Vi. VBEN ØWhen Vi>0. 5 V, Qp is on and QN is cut off, Vo=Vi. VBEP Microelectronic Circuits SJTU Yang Hua

Transfer Characteristic Microelectronic Circuits SJTU Yang Hua

Crossover Distortion Microelectronic Circuits SJTU Yang Hua

Power –conversion efficiency l The load power l Maximum load power Microelectronic Circuits SJTU Yang Hua

Power –conversion efficiency l Total supply power l Maximum total supply power Microelectronic Circuits SJTU Yang Hua

Power –conversion efficiency l Power-conversion efficiency l Maximum power-conversion efficiency Microelectronic Circuits SJTU Yang Hua

Power Dissipation l Power dissipation l Maximum Power dissipation Microelectronic Circuits SJTU Yang Hua

Power Dissipation l At the point of maximum power dissipation the efficiency can be evaluated: Microelectronic Circuits SJTU Yang Hua

Example: 9. 1 Microelectronic Circuits SJTU Yang Hua

Reducing crossover distortion Microelectronic Circuits SJTU Yang Hua

Class AB Output Stage Microelectronic Circuits SJTU Yang Hua

A Class AB Output Stage Utilizing Diodes for Biasing The diode biasing has an Important advantage: Thermal stabilization of Quiescent current Microelectronic Circuits SJTU Yang Hua

A Class AB Output Stage Utilizing A VBE Multiplier for Biasing Microelectronic Circuits SJTU Yang Hua

Homework: l May 25 th, 2008 9. 12; 9. 13; 9. 14; 9. 15 Microelectronic Circuits SJTU Yang Hua