Lecture 18Basic MOSFET AmplifierMOSFET BiasingSelf-Biased MOSFET StageMOSFETs as Current SourcesCommon-Source Stage: = 0Common-Source Stage: 0CS Gain Variation with LCS Stage with Current-Source LoadPMOS CS Stage with NMOS LoadCS Stage with Diode-Connected LoadCS Stage with Diode-Connected PMOS LoadCS Stage with DegenerationExampleRout of CS Stage with DegenerationOutput Impedance ExamplesCS Stage with Gate ResistanceCS Core with BiasingEE105 Fall 2007 Lecture 18, Slide 1 Prof. Liu, UC BerkeleyLecture 18OUTLINE•Basic MOSFET amplifier•MOSFET biasing•MOSFET current sources•Common-source amplifierReading: Chapter 7.1-7.2ANNOUNCEMENTS•HW#10 will be posted tonightEE105 Fall 2007 Lecture 18, Slide 2 Prof. Liu, UC Berkeley•For large small-signal gain, the MOSFET should be operated in the saturation region. Vout should not fall below Vin by more than VTH. Basic MOSFET AmplifierEE105 Fall 2007 Lecture 18, Slide 3 Prof. Liu, UC BerkeleyMOSFET BiasingThe voltage at node X is determined by VDD, R1, and R2:Also, SoxnTHDDTHGSRLWCVVRRVRVVVVV1 where212121211DDXVRRRV212SDGSXRIVV 221THGSoxnDVVLWCI EE105 Fall 2007 Lecture 18, Slide 4 Prof. Liu, UC BerkeleySelf-Biased MOSFET Stage•Note that there is no voltage dropped across RG M1 is operating in the saturation region.DDDSGSDDVIRVRI EE105 Fall 2007 Lecture 18, Slide 5 Prof. Liu, UC BerkeleyMOSFETs as Current Sources•A MOSFET behaves as a current source when it is operating in the saturation region.•An NMOSFET draws current from a point to ground (“sinks current”), whereas a PMOSFET draws current from VDD to a point (“sources current”).EE105 Fall 2007 Lecture 18, Slide 6 Prof. Liu, UC BerkeleyCommon-Source Stage: = 0DoutinDDoxnDmvRRRRILWCRgA2Amplifier circuit Small-signal analysis circuitfor determining voltage gain, AvSmall-signal analysis circuit fordetermining output resistance, RoutEE105 Fall 2007 Lecture 18, Slide 7 Prof. Liu, UC BerkeleyCommon-Source Stage: 0•Channel-length modulation results in reduced small-signal voltage gain and amplifier output resistance. ODoutinODmvrRRRrRgA||||Small-signal analysis circuitfor determining voltage gain, AvSmall-signal analysis circuit fordetermining output resistance, RoutEE105 Fall 2007 Lecture 18, Slide 8 Prof. Liu, UC BerkeleyCS Gain Variation with L•An ideal current source has infinite small-signal resistance.The largest Av is achieved with a current source as the load. •Since is inversely proportional to L, Av increases with L. DoxnDDoxnomvIWLCIILWCrgA22EE105 Fall 2007 Lecture 18, Slide 9 Prof. Liu, UC BerkeleyCS Stage with Current-Source Load 21211||||OOoutOOmvrrRrrgA•Recall that a PMOSFET can be used as a current source from VDD.Use a PMOSFET as a load of an NMOSFET CS amplifier.EE105 Fall 2007 Lecture 18, Slide 10 Prof. Liu, UC BerkeleyPMOS CS Stage with NMOS Load •An NMOSFET can be used as the load for a PMOSFET CS amplifier. 21212||)||(OOoutOOmvrrRrrgAEE105 Fall 2007 Lecture 18, Slide 11 Prof. Liu, UC BerkeleyCS Stage with Diode-Connected LoadAv is lower, but it is less dependent on process parameters (n and Cox and drain current (ID).Amplifier circuit Small-signal analysis circuitincluding MOSFET output resistances 2121//1:0 IfLWLWggAmmv1221221||||1||||1OOmoutOOmmvrrgRrrggA:0EE105 Fall 2007 Lecture 18, Slide 12 Prof. Liu, UC BerkeleyCS Stage with Diode-Connected PMOS Load2112112||||1||||1oomoutoommvrrgRrrggA:0EE105 Fall 2007 Lecture 18, Slide 13 Prof. Liu, UC BerkeleyCS Stage with DegenerationSmDvRgRA1 :0 IfAmplifier circuit Small-signal analysis circuitfor determining voltage gain, AvEE105 Fall 2007 Lecture 18, Slide 14 Prof. Liu, UC BerkeleyExample•A diode-connected device degenerates a CS stage.2111mmDvggRAEE105 Fall 2007 Lecture 18, Slide 15 Prof. Liu, UC BerkeleyRout of CS Stage with Degeneration•Degeneration boosts the output impedance:Small-signal analysis circuit fordetermining output resistance, Rout SOmOSSmOXXXSXSXmXORrgrRRgrivvRiRigir1SXRiv 1Current flowing down through ro is SXmXSXmXmXRigiRigivgi 1EE105 Fall 2007 Lecture 18, Slide 16 Prof. Liu, UC BerkeleyOutput Impedance Examples21111mmOoutggrR1211 OOOmoutrrrgR EE105 Fall 2007 Lecture 18, Slide 17 Prof. Liu, UC BerkeleyCS Stage with Gate Resistance•For low signal frequencies, the gate conducts no current. Gate resistance does not affect the gain or I/O impedances.EE105 Fall 2007 Lecture 18, Slide 18 Prof. Liu, UC BerkeleyCS Core with
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