EE105 Fall 2006 Microelectronic Devices and Circuits Prof Jan M Rabaey jan eecs Lecture 13 MOS Single Stage Amplifiers Overview Last lecture The Amplifier as a Two Port This lecture Common Source Read 8 3 8 5 Common Gate Read 8 8 2 Common Drain Read 8 9 2 2 1 Common Source Amplifier again How to isolate DC level 3 DC Bias 5V Neglect all AC signals 2 5 V Choose IBIAS W L 4 2 Load Line Analysis to find Q I RD VDD Vout RD Q ID slope 5V 10k ID 1 10k 0V 10k 5 Input Output Characteristics VDD MOSFET is saturated high slope vO vDS Higher value of RL means higher gain MOSFET is triode low slope VTn 0 negative supply VSS 0 vIN vGS 6 3 Small Signal Analysis Rin 7 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 8 4 Two Port CS Model Reattach source and load one ports L 9 Maximize Gain of CS Amp Av g m RD ro Increase gm more current Increase RD free Don t need to dissipate extra power Limit Must keep the device in saturation VDS VDD I D RD VDS sat For a fixed current the load resistor can only be chosen so large To have good swing we d also like to avoid getting too close to edge of saturation How to enable large current and large resistance at the same time 10 5 Answer Current Source Load Current independent of voltage for ideal source 11 CS Amp with Current Source Supply 12 6 Load Line for DC Biasing Both the I source and the transistor are idealized for DC bias analysis 13 Two Port Parameters From current source supply Rin Gm g m Rout ro roc 14 7 P Channel CS Amplifier DC bias VGS VBIAS VDD sets drain current IDp ISUP 15 Two Port Model Parameters Small signal model for PMOS and for rest of circuit 16 8 Common Gate Amplifier 17 CG as a Current Amplifier Find Ai iout id it Ai 1 18 9 CG Input Resistance vgs vt At input Output voltage v v it g m vgs g mb vt t out ro vout id roc RL it roc RL v roc RL it it g m vt g mb vt t ro 19 Approximations We have this messy result g m g mb 1 ro i 1 t r RL Rin vt 1 oc ro But we don t need that much precision Let s start approximating g m g mb 1 ro roc RL RL Rin RL 0 ro 1 g m g mb 20 10 CG Output Resistance vs v v g m vgs g mb vs s t 0 RS ro 1 1 v vs g m g mb t ro ro RS 21 CG Output Resistance Substituting vs itRS 1 1 v it RS g m g mb t ro ro RS The output resistance is vt it roc r Rout roc RS o g m ro g mb ro 1 RS 22 11 Approximating the CG Rout Rout roc ro g m ro RS g mb ro RS RS The exact result is complicated so let s try to make it simpler g m 500 S g mb 50 S ro 200k Rout roc ro g m ro RS RS Assuming the source resistance is less than ro Rout roc ro g m ro RS roc ro 1 g m RS 23 CG Two Port Model Function a current buffer Low Input Impedance High Output Impedance 24 12 Common Drain Amplifier I DS Cox VGS VT 2 I DS W Cox L W 1 VGS VT 2 L 2 Weak IDS dependence 25 CD Voltage Gain Note vgs vt vout vout g m vgs g mb vout roc ro vout g m vt vout g mb vout roc ro 26 13 CD Voltage Gain Cont vout g m vt vout g mb vout roc ro KCL at source node 1 g mb g m vout g m vt roc ro Voltage gain for vSB not zero vout vin gm 1 g mb g m roc ro vout gm 1 vin g mb g m 27 CD Output Resistance Sum currents at output source node Rout ro roc vt it Rout it g m vt g mb vt 1 g m g mb 28 14 CD Output Resistance Cont ro roc is much larger than the inverses of the transconductances ignore Rout 1 g m g mb Function a voltage buffer High Input Impedance Low Output Impedance 29 30 15
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