EE105 Fall 2005 Microelectronic Devices and Circuits Lecture 11 MOS Transistor Small Signal Model Current Sources Announcements Homework 5 due today Homework 6 due next Tuesday Lab 4 this week Reading Chapter 4 4 5 4 6 8 3 Midterm 1 in nine days October 13 6 30 8pm Sibley 2 1 Lecture Material Last lecture MOS amplifier example MOSFET small signal model This lecture Finish MOSFET small signal model MOS current sources 3 Voltage References First topic voltage and current references 9 4 Generating a voltage use a current source to set VGS 15 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 16 Small Signal Source Resistance di RS OUT dvOUT 1 vt it I OUT 0 Equivalent Circuit RS iOUT VS vOUT 17 3 Creating a Current Source 18 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 19 4 Small Signal Resistance of I Source 20 Current Sinks and Sources Sink output current goes to ground Source output current comes from voltage supply 21 5 Current Mirrors Idea we only need one reference current to set up all the current sources and sinks needed for a multistage amplifier 22 6
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