Berkeley ELENG 105 - Lecture 18 (18 pages)

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Lecture 18



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Lecture 18

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Pages:
18
School:
University of California, Berkeley
Course:
Eleng 105 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

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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



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