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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 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 W / L 1 Av g m1 1 W / L 2 gm2 Rout || rO 2 || rO1 gm2 Av is lower, but it is less dependent on process parameters (n and Cox and drain current (ID). If 0 : 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 Rout g m1rO1rO 2 rO1 Lecture 18, Slide 16 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 R1 || R2 Av gm R D RG R1 || R2 Lecture 18, Slide 18 Prof. Liu, UC Berkeley