Lecture 20 - MIT

1 Spring 20071 Lecture 20 Transistor Amplifiers (II)Other Amplifier StagesOutline Common-drain amplifier Common-gate amplifierReading Assignment:Howe and Sodini; Chapter 8, Sections Spring 200721. Common-drain amplifier A voltage buffer takes the input voltage which may have a relatively large Thevenin resistance and replicates the voltage at the output port, which has a low output resistance Input signal is applied to the gate Output is taken from the source To first order, voltage gain 1 Input resistance is high Output resistance is low Effective voltage bufferstage vgate iDcannot change vsource Source followerHow does it work?vsVBIASvOUTVDDVSSiSUPRSRL signal source+ Spring 20073 Biasing the Common-drain amplifier Assume device in saturation; neglect RSand RL; neglect CLM ( = 0) Obtain desired output bias voltage Typically set VOUTto halfway between VSSand VDD.

2 Output voltage maximum VDD-VDSsat Output voltage minimum set by voltage requirement across +VOUTVGS=VTn(VSB)+ISUPW2L nCoxvsVBIASvOUTVDDVSSVSSiSUPRSRL signal source+ Spring 20074 Small-signal AnalysisUnloaded small-signal equivalent circuit model:vin=vgs+voutvout=gmvgs(ro//roc)The n:Avo=gmgm+1ro//roc 1 GSD+-vin+-vgs+-voutgmvgsroroc+-vin+- Spring 20075 Input and Output ResistanceInput Impedance : Rin= Output Impedance:Rout=1gm+1ro//roc 1gmSmall!Loaded voltage gain:Av=AvoRLRL+Rout RLRL+1gm 1+-vgs+-vingmvgsro//roc+-vtitgmvtro//roc +-vtitRSeffectively:resistance of value 1/gmvin= 0; vt= Spring 20076 Effect of Back Bias Bias is affected VTdepends on VBS VBS= VSS VOUT 0 Small signal figures of merit affected Signal shows up between B and S vbs= -voutIf MOSFET was not fabricated in an isolated p-well, then body is tied to wafer substrate (connected to VSS)Two consequences:vsVBIASvOUTVDDVSSVSSiSUPRSR L signal source+ Spring 20077 Small-signal Analysis (with back-bias)See text for detailsAlso.

3 Avo=gmgm+gmb+1ro//roc gmgm+gmb<1 Rout=1gm+gmb+1ro//roc 1gm+gmbGBSD+-vin+-vgs+-voutgmvgsroroc+-v in+-voutgmvgsro//rocgmbvbsgmbvoutvbs= Spring 20078 Common-Drain Two-Port Model Open circuit voltage gain ~ 1 Input resistance ~ CS Amplifier We want a large input resistance because the controlled generator is voltage controlled Output resistance << CS Amplifier We want a low output resistance to deliver most of the output voltage to the loadvin+ vin+ vout+ + (gm + gmb)1(gm + gmb) Spring 20079 Relationship between circuit parameters and device parameters:gm=2 IDWL nCoxgmb= 2 2 p VBSgmCircuit Parameters|Avo|RinRoutDevice*Parametersg mgm+gmb 1gm+gmbISUP -- W -- nCox -- L -- * VBIASis adjusted so that none of the other parameters changeCommon Drainamplifier is often used as a voltage bufferto drive small output loads (in multistage amplifiers, other stages provide the voltage gain).

4 Spring 2007102. Common-Gate Amplifier: A current buffer takes the input current which may have a relatively small Norton resistance and replicates the current at the output port, which has a high output resistance Input signal is applied to the source Output is taken from the drain To first order, current gain 1 is -iout.(Current Buffer) Input resistance is low Output resistance is high Effective current bufferstageisiOUTVDDVSSVSSiSUPIBIASRL signal Spring 200711 Biasing the Common-Gate Amplifier:Assume device in saturation; neglect RSand RL; neglect CLM ( = 0)Select bias such that IOUT=0 VOUT= MOSFET in saturation (no channel modulation):But VTdepends on VBS:Must solve these two equations +IOUT+IBIAS=0ID=W2L nCoxVGS VT()2=ISUP= IBIASVT=VTo+ n 2 p VBS 2 p() Spring 200712 Small-signal equivalent circuit(unloaded)it= iout Aio=ioutit= 1 Aiois the short circuit current surprising, since in a MOSFET: ig= 0 GBSD+-isioutgmvgsvgs+-vgsrogmvgsroisgmbv bsvbs= Spring 200713 Input ResistanceDo KCL on input node:it gmvt gmbvt vt itroc//RL()ro=0 Then:Rin=vtit=1+roc//RLrogm+gmb+1ro 1gm+gmb+-gmvgsvgsrogmbvgsrocRL+-vtit+-vt itgmvtrovgs=- Spring 200714 Output ResistanceDo KCL on input node.

5 I t gmvgs gmbvgs v t+vgsro=0 Notice also:vgs= i tRsThen:Rout=roc//ro1+Rsgm+gmb+1ro Rout roc//ro1+gmRs()[] roc//gmro()Rs[]+-gmvgsvgsrogmbvgsroc+-vt itRS+-gmvgsvgsrogmbvgs+-vt'it' Spring 200715 Common-Gate Two-Port Model The output resistance depends on the source resistance The CG current buffer is not unilateral Input resistance << CS Amplifier We want a small input resistance because the controlled generator is current controlled Output resistance >> CS Amplifier We want a large output resistance to deliver most of the output current to the load iinroc (ro + gmroRS) 1gm + Spring 200716 Relationship between circuit figures of merit and device parameters.

6 Gm=2 IDWL nCoxgmb= 2 2 p VBSgmro 1 nIDCircuit Parameters|Aio|RinRoutDevice*Parameters- 11gm+gmb()[]smoocRgrr+1//ISUP - W - nCox - L - * VBIASis adjusted so that none of the other parameters changeCommon Gateamplifier is often used as a current transform a current source with medium source resistance to an equal current with high source resistance (in multistage amplifiers, other stages provide the current gain). Spring 200717 What did we learn today?Summary of Key Concepts Common-source amplifier: good voltage amplifierbetter transconductance amplifier Large voltage gain High input resistance Medium / high output resistance Common-drain amplifier: good voltage buffer Voltage gain 1 High input resistance Low output resistance Common-gate amplifier: good current buffer Current gain 1 Low input resistance High output resistanc