Lecture 24 Multistage Amplifiers (I) MULTISTAGE AMPLIFIER1. IntroductionSummary of single stage amplifier characteristics2. CMOS Multistage Voltage AmplifierCMOS Multistage Voltage Amplifier (contd.)3. BiCMOS multistage voltage amplifier BiCMOS multistage voltage amplifier (contd.)BiCMOS multistage voltage amplifier (contd.)4. BiCMOS current buffer BiCMOS current buffer (contd).BiCMOS current buffer (contd).5. Coupling Amplifier StagesCoupling Amplifier Stages (contd.)Coupling Amplifier Stages (contd.)Coupling Amplifier Stages (contd.) Summary of DC shifts through amplifier stages:What did we learn today?6.012 Spring 2009 Lecture 24 1Lecture 24Multistage Amplifiers (I)MULTISTAGE AMPLIFIEROutline1. Introduction2. CMOS multi-stage voltage amplifier3. BiCMOS multistage voltage amplifier4. BiCMOS current buffer5. Coupling amplifier stagesReading Assignment:Howe and Sodini, Chapter 9, Sections 9-1-9.36.012 Spring 2009 Lecture 24 21. 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 amplifierVBIAS6.012 Spring 2009 Lecture 24 3Summary of single stage amplifier characteristicsStage Avo, AioRinRoutKey FunctionCommon SourceTranscon-ductance amplifierCommon DrainVoltage BufferCommon GateCurrent bufferCommon EmitterTranscon-ductance amplifierCommon CollectorVoltage bufferCommon BaseCurrent buffer)//(ocomvorrgA −=ro//rocAvo≈gmgm+ gmb∞1gm+ gmbAio≈−1∞Avo=−gm(ro//roc)mbmgg +1roc//[ro(1+gmRS)]πrro//rocAvo≈1Aio≈−1()LocooRrrr ////βπ+1gm+RSβomg1roc//[ro(1+gmrπ// RS())]Differences between BJT’s and MOSFETsrπ=βogm gmb∝ gmgm=ICVth> gm= 2WLμCoxIDro=VAIC> ro=1λIDBJT MOSFET6.012 Spring 2009 Lecture 24 42. CMOS Multistage Voltage AmplifierGoals:• High voltage gain, Avo• High input resistance, Rin• Low output resistance, RoutGood starting point: Common-Source stage:• Rin=∞• Avo=-gm(ro//roc), probably insufficient• Rout= (ro//roc), too high6.012 Spring 2009 Lecture 24 5Add second CS stage to get more gain:CMOS Multistage Voltage Amplifier (contd.)• Rin=∞• Avo=gm1(ro1//roc1) gm2(ro2//roc2)• Rout= (ro2//roc2), still too highAdd CD stage at output (to reduce Rout):• Rin=∞••Avo= gm1ro1|| roc1()gm2ro2|| roc2()gm3gm3+ gmb3Rout=1gm3+ gmb36.012 Spring 2009 Lecture 24 63. BiCMOS multistage voltage amplifier Rin2Rout1+ Rin2≈Rin2Rout1≈rπ2ro1|| roc1<<1How about adding a CE stage following the CS stage?Avo(CE) > Avo(CS) because gm(BJT) > gm(MOSFET), but..Rout1=ro1||roc1>>Rin2=rπ2CS stage is best first stage, since Rin=∞.However, inter-stage loading degrades gain:There is a voltage divider between stagesAdditional gain provided by the CE stage is mostly lost to inter-stage loading.vsRLRSCS CEvin1r2+−vout+−(ro1 ⎢⎢roc1) Av1vin1Av2vin2(ro2 ⎢⎢roc2)+−+−+−+−vin26.012 Spring 2009 Lecture 24 7BiCMOS multistage voltage amplifier (contd.)Use two CS stages, but add CC stage at output:Since, in general gm(BJT) > gm(MOSFET), Routcould be better than CD output stage if ro2||roc2is not too large. Otherwise, CD output stage is better.Rout2= ro2||roc2, Rin3=rπ3+βo3ro3|| roc3|| RL()Inter-stage loading:Then, inter-stage loss:Rin3Rout 2+ Rin3=rπ3+βo3ro3||roc3||RL()ro2|| roc2+ rπ3+βo3ro3|| roc3|| RL()Better than trying to use a CE stage, but still pretty bad.The good thing is that Routhas improved:Rout= Rout3=1gm3+Rout2βo3=1gm3+ro2|| roc2βo3vsRLRSCS − CS CCvinr3 + βo3(ro3 ⎢⎢roc3 ⎢⎢RL)+−vout+−vin3(ro2 ⎢⎢roc2) Av1Av2vinro2 ⎢⎢roc2+−+−+−gm3 βo3 +1+−vin36.012 Spring 2009 Lecture 24 8BiCMOS multistage voltage amplifier (contd.)What is the best order?Since Rin(CD)=∞, best to place CD first:Better voltage buffer: cascade CC and CD output stagesInter-stage loading:Rin3Rin3+ Rout2= 1Rin4Rin4+ Rout3=rπ4+βo4ro4|| roc4|| RL()1gm3+ gmb3+ rπ4+βo4ro4|| roc4|| RL()≈ 1The output resistance is excellent:Rout= Rout4=1gm4+Rout3βo4=1gm4+1βo4gm3+ gmb3()Av1Av2vin(ro2 ⎢⎢roc2)vinvin3vin3vin4vin4voutvsCS − CS CD − CCr4 + βo4(RL ⎢⎢ro ⎢⎢roc)(gm3 + gmb3)βo4(gm3 + gmb3)1RLRS1gm41++−+−+−++−+−+−+−−6.012 Spring 2009 Lecture 24 94. BiCMOS current bufferGoals:• Unity current gain, Ai=1• Very low input resistance, Rin• Very high output resistance, RoutStart with a common-base stage:• Aio=−1• Rin=1gm• Rout= roc|| ro1+ gmrπ|| RS()[]{}Note that if RSis “high enough”, Rout≈ roc||(βoro).Can we increase Routfurther by adding a second CB stage?6.012 Spring 2009 Lecture 24 10BiCMOS current buffer (contd).CB-CB Current BufferNowRout= Rout2= roc2|| ro21+ gm2rπ2|| Rout1()[]{}Plugging in Rout1≈ roc1|| (βo1roc1).Rout= roc2|| ro21+ gm2rπ2|| roc1||βo1ro1()[]{}But, since rπ2<< roc1|| (β1roc1), thenRout= roc2|| ro21+gm2rπ2()[]≈roc2||βo2ro2()Did not improve anything! The base current limits the number of CB stages that can improve the output resistance to just one.Since the CG stage has no gate current, cascade it with the CB stageisRLRSiout−iin1−iin2iin2iin1[ gm2ro2(r2 ⎢⎢βo1ro1 ⎢⎢roc1)] ⎢⎢roc2CB CBβo1ro1 ⎢⎢roc1gm11gm216.012 Spring 2009 Lecture 24 11BiCMOS current buffer (contd).CB-CG Current BufferRout= Rout2= roc2|| ro21+gm2Rout1()[]Plugging in Rout1≈ roc1|| (βo1roc1).Rout= roc2|| ro2gm2roc1||βo1ro1()[]Now Routhas improved by about gm2ro2, but only to the extent that roc2is high enough…isRLRSiout−iin1−iin2iin2iin1[gm2ro2(βo1ro1 ⎢⎢roc1)] ⎢⎢roc2CB CGβo1ro1 ⎢⎢roc1gm11gm216.012 Spring 2009 Lecture 24 125. 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 COUPLINGCapacitors that have large enough value behave as AC short, so the signal goes through but bias is independent for each stage.Example, CD-CC voltage buffer:4.0 V5.0 V2.5 V3.2 V2.5 VAssumes VBE = 0.7 VVGS = 1.5 VISUP1ISUP25.0 V6.012 Spring 2009 Lecture 24 13Coupling Amplifier Stages (contd.)DIRECT COUPLING: share bias points across stages.Example, CD-CC voltage buffer:• Advantages– No capacitors needed– compact•
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