Lecture 18 ANNOUNCEMENTS HW 10 will be posted tonight OUTLINE Basic MOSFET amplifier MOSFET biasing MOSFET current sources Common source amplifier Reading Chapter 7 1 7 2 EE105 Fall 2007 Lecture 18 Slide 1 Prof Liu UC Berkeley Basic MOSFET Amplifier For large small signal gain the MOSFET should be operated in the saturation region Vout should not fall below Vin by more than VTH EE105 Fall 2007 Lecture 18 Slide 2 Prof Liu UC Berkeley MOSFET Biasing R2 VDD The voltage at node X is determined by VDD R1 and R2 VX R1 R2 Also VX VGS I D RS 1 W 2 I D nCox VGS VTH 2 L R2VDD VGS V1 VTH V 2V1 VTH R1 R2 1 where V1 W nCox RS L 2 1 EE105 Fall 2007 Lecture 18 Slide 3 Prof Liu UC Berkeley Self Biased MOSFET Stage Note that there is no voltage dropped across RG M1 is operating in the saturation region I D RD VGS RS I D VDD EE105 Fall 2007 Lecture 18 Slide 4 Prof Liu UC Berkeley MOSFETs 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 5 Prof Liu UC Berkeley Common Source Stage 0 Amplifier circuit Small signal analysis circuit for determining voltage gain Av W Av g m RD 2 nCox I D RD L Rin Small signal analysis circuit for determining output resistance Rout Rout RD EE105 Fall 2007 Lecture 18 Slide 6 Prof Liu UC Berkeley Common Source Stage 0 Channel length modulation results in reduced small signal voltage gain and amplifier output resistance Small signal analysis circuit for determining voltage gain Av Small signal analysis circuit for determining output resistance Rout Av g m RD rO Rin Rout RD rO EE105 Fall 2007 Lecture 18 Slide 7 Prof Liu UC Berkeley CS 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 W 2 nCox I D 2 nCoxWL L Av g m ro I D ID EE105 Fall 2007 Lecture 18 Slide 8 Prof Liu UC Berkeley CS Stage with Current Source Load Recall that a PMOSFET can be used as a current source from VDD Use a PMOSFET as a load of an NMOSFET CS amplifier Av g m1 rO1 rO 2 Rout rO1 rO 2 EE105 Fall 2007 Lecture 18 Slide 9 Prof Liu UC Berkeley PMOS CS Stage with NMOS Load An NMOSFET can be used as the load for a PMOSFET CS amplifier Av g m 2 rO1 rO 2 Rout rO1 rO 2 EE105 Fall 2007 Lecture 18 Slide 10 Prof Liu UC Berkeley CS Stage with Diode Connected Load Amplifier circuit Small signal analysis circuit including MOSFET output resistances 0 1 Av g m1 rO 2 rO1 gm2 1 Rout rO 2 rO1 gm2 If 0 1 Av g m1 g m2 W L 1 W L 2 Av is lower but it is less dependent on process parameters n and Cox and drain current ID EE105 Fall 2007 Lecture 18 Slide 11 Prof Liu UC Berkeley CS Stage with Diode Connected PMOS Load 0 1 Av g m 2 ro1 ro 2 g m1 1 Rout ro1 ro 2 g m1 EE105 Fall 2007 Lecture 18 Slide 12 Prof Liu UC Berkeley CS Stage with Degeneration Amplifier circuit If 0 Av EE105 Fall 2007 Small signal analysis circuit for determining voltage gain Av RD 1 RS gm Lecture 18 Slide 13 Prof Liu UC Berkeley Example A diode connected device degenerates a CS stage Av EE105 Fall 2007 RD 1 1 g m1 g m 2 Lecture 18 Slide 14 Prof Liu UC Berkeley Rout of CS Stage with Degeneration Degeneration boosts the output impedance Small signal analysis circuit for determining output resistance Rout Current flowing down through ro is i X g m v1 i X g m i X RS i X g mi X RS v1 i X RS rO i X g mi X RS i X RS v X vX rO 1 g m RS RS rO g m rO RS iX EE105 Fall 2007 Lecture 18 Slide 15 Prof Liu UC Berkeley Output Impedance Examples Rout 1 rO1 1 g m1 gm2 EE105 Fall 2007 Lecture 18 Slide 16 Rout g m1rO1rO 2 rO1 Prof Liu UC Berkeley CS 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 17 Prof Liu UC Berkeley CS Core with Biasing R1 R2 RD Av RG R1 R2 1 R S gm EE105 Fall 2007 Lecture 18 Slide 18 R1 R2 Av gm R D RG R1 R2 Prof Liu UC Berkeley
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