MULTISTAGE AMPLIFIER - MIT

Spring 2007 Lecture 241 Lecture 24 MULTISTAGE Amplifiers (I) MULTISTAGE multi-stage voltage MULTISTAGE voltage current AMPLIFIER stagesReading Assignment:Howe and Sodini, Chapter 9, Sections Spring 2007 Lecture 2421. Introduction What amplifying stages should be used and in what order? What devices should be used, BJT or MOSFET? How is biasing to be done?Issues:Most often, single stage AMPLIFIER does not accomplish design goals: Need more gain than could be provided by a single stage Need to adapt to specified RSand RLto maximize efficiency MULTISTAGE Spring 2007 Lecture 243 Summary of single stage AMPLIFIER characteristicsCurrent bufferCommon BaseVoltage bufferCommon CollectorTranscon-ductance amplifierCommon EmitterCurrent bufferCommon GateVoltage BufferCommon DrainTranscon-ductance amplifierCommon SourceKey FunctionRoutRinAvo, AioStage)//(ocomvorrgA =ro//rocAvo gmgm+gmb 1gm+gmbAio 1 Avo= gm(ro//roc)mbmgg+1roc//[ro(1+gmRS)] rro//rocAvo 1 Aio 1()LocooRrrr//// +1gm+RS omg1roc//[ro(1+gmr //RS())]Differences between BJT s and MOSFETsr = ogm gmb gmgm=ICVth> gm=2WL CoxIDro=VAIC> ro=1 Spring 2007 Lect

5. Coupling Amplifier Stages • Advantages – Can select bias point for optimum operation – Can select bias point close to the mid-point of the power rails for maximum voltage swing • Disadvantages – To approximate AC short, large capacitors are needed and they consume large area. CAPACITIVE COUPLING

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