EE105 Fall 2005 Microelectronic Devices and Circuits Lecture 12 MOS Current Sources Two port Models Announcements Homework 6 due next Tuesday Lab 4 this week Reading Chapters 9 4 8 MOS only Midterm 1 next week October 13 6 30 8pm Sibley Review session on Tuesday October 11 6 30 8pm 277 Cory 2 1 Lecture Material Last lecture MOSFET small signal model This lecture MOS current sources Generalized two port models 3 Voltage References First topic voltage and current references 9 4 Generating a voltage use a current source to set VGS 4 2 Modeling the Voltage Source Find iOUT versus vOUT MOSFET is off or saturated why W 2 i OUT iD SAT I REF nCox vGS VTn 1 n vDS I REF 2L iOUT vOUT Typical operating point iOUT 0 A 5 Small Signal Source Resistance di RS OUT dvOUT 1 vt it I OUT 0 Equivalent Circuit RS iOUT VS vOUT 6 3 Creating a Current Source 7 8 4 Equivalent Circuit for I Source Find the DC current for gray circle equivalent circuit I OUT nCox W 2 VREF VTn L 2 2 Substitute for VREF 9 Small Signal Resistance of I Source 10 5 Current Sinks and Sources Sink output current goes to ground Source output current comes from voltage supply 11 Current Mirrors Idea we only need one reference current to set up all the current sources and sinks needed for a multistage amplifier 12 6 13 Application of Current Mirrors Digital to Analog Converter VDD RL vOUT RREF D0 D2 D1 iDo MREF M0 iD1 M1 D4 iD2 M2 iD3 M3 Digital input word D0D1D2D3 voltages are either VDD or 0 V Transistors M0 M1 M2 M3 have binary weighted W L ratios 14 7 15 Example Input word D0D1D2D3 0101 IREF 100 A W L REF W L 0 W L 1 2 W L 0 W L 2 2 W L 1 W L 3 2 W L 2 Output voltage is vOUT VDD RL iD0 iD2 VDD RL IREF 4 IREF Sources of error fab imprecision channel length modulation 16 8 Generalized Amplifier VDD RS vS VBIAS vs vs I DD VBIAS iD f vin iL I DD iD vo RL vin is I BIAS VSS Active Device 17 Amplifier Terminology Sources Signal its source resistance and bias voltage or current Load Use resistor current source or in Chap 8 but could be a general impedance Port A pair of terminals across which a voltage and an associated current are defined Source Load one port Amplifier two port 18 9 One Port Models EECS 40 A terminal pair across which a voltage and associated current are defined iab vab Circuit Block iab vab iab Rthev vthev vab Rthev ithev 19 10
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