EE105 Fall 2006 Microelectronic Devices and Circuits Prof Jan M Rabaey jan eecs Lecture 12 Amplifiers General Concepts Administrativia Hw 5 posted Midterm 1 graded 2 1 Midterm 1 Total 30 Average 15 65 Median 15 25 StdDev 6 72 Max 29 Min 3 3 Overview Last lecture Voltage and Current Sources This lecture Moving to Amplifiers General 4 2 A Current Source iOUT nIREF ro2 Region of validity vOUT 5 n W L 2 W L 1 Current Sinks and Sources Sink output current goes to ground Source output current comes from voltage supply 6 3 Current Mirrors Idea we only need one reference current to set up all the current sources and sinks needed for a multistage amplifier 7 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 8 4 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 9 Moving to Amplifiers 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 current are defined Source Load one port Amplifier two port 10 5 Two Port Models Amplifiers iin iout Circuit Block v in v out 11 One Port Models EECS 40 A terminal pair across which a voltage and associated current are defined iab vab Only 1 independent variable Circuit Block iab vab iab Rthev vthev vab Rthev ithev 12 6 Small Signal Two Port Models iin iout vin vout General case four variables vin iin vout iout Two independent two dependent Four choices Math 54 iin f vin vout iout f vin vout vin f iin vout iout f iin vout iin f vin iout vout f vin iout vin f iin iout vout f iin iout 13 Small Signal Two Port Models iin iout vin vout We assume that input port is linear and that the amplifier is unilateral Output depends on input but input is independent of output Output port depends linearly on the current and voltage at the input and output ports Unilateral assumption is good as long as overlap capacitance is small MOS 14 7 A Mathematical Perspective Can write linear system of equations for either iout or vout in terms of two of iin vin iout or vout possibilities are iout 1vin 2 vout iout 3iin 4vout vout 5vin 6iout vout 7iin 8iout What is physical meaning of 1 of 6 15 EE Perspective Four amplifier types determined by the output signal and the input signal both of which we select usually obvious Voltage Amp V V Current Amp I I Transconductance Amp V I Transresistance Amp I V Need methods to find the 6 parameters for the four models and equivalent circuits for unilateral two ports 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 Two Port Small Signal Amplifiers Rout Rs vin vs Rin Av vin RL Voltage Amplifier iin is Rs Rin Aii in Current Amplifier Rout RL 18 9 Two Port Small Signal Amplifiers Rs vin vs Rin Gm vin Rout RL Transconductance Amplifier Rout iin is Rs Rin Rmiin RL Transresistance Amplifier 19 Input Resistance Rin Looks like a Thevenin resistance measurement but note that the output port has the load resistance attached Rin vt it RS removed RL attached 20 10 Output Resistance Rout Looks like a Thevenin resistance measurement but note that the input port has the source resistance attached Rout vt it RL removed RS attached 21 Finding the Voltage Gain Av Key idea the output port is open circuited and the source resistance is shorted 22 11 Finding the Current Gain Ai Key idea the output port is shorted and the source resistance is removed Ai iout iin RS RL 0 23 Finding the Transresistance Rm 24 12 Finding the Transconductance Gm 25 Common Source Amplifier again How to isolate DC level 26 13 DC Bias 5V Neglect all AC signals 2 5 V Choose IBIAS W L 27 Load Line Analysis to find Q I RD VDD Vout RD Q ID slope 5V 10k ID 0V 10k 1 10k 28 14 Small Signal Analysis Rin 29 Two Port Parameters Generic Transconductance Amp Find Rin Rout Gm Rs vs vin Rin Gm vin Rout RL Rin Gm g m Rout ro RD 30 15 Two Port CS Model Reattach source and load one ports 31 16
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