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 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.

Classification of 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 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 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 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. sedr42021_1403.jpg

sedr42021_1404a.jpg 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

Power –conversion efficiency The load power Maximum load power

Power –conversion efficiency Total supply power Maximum total supply power

Power –conversion efficiency Maximum power-conversion efficiency

Power Dissipation Power dissipation Maximum 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, 2008 9.12; 9.13; 9.14; 9.15