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Lecture 17: Common Source/Gate/Drain Amplifiers EECS 105 Fall 2003, Lecture 17 Lecture 17: Common Source/Gate/Drain Amplifiers Prof. Niknejad Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Lecture Outline MOS Common Source Amp Current Source Active Load Common Gate Amp Common Drain Amp Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Common-Source Amplifier Isolate DC level Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Load-Line Analysis to find Q V −V I = DD out RD RD Q 1 5V slope = I = 10k D 10k 0V I = D 10k Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Small-Signal Analysis =∞ Rin Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port Parameters: Generic Transconductance Amp Rs + vs Rin Gmvin RL vin Rout − Find Rin, Rout, Gm =∞ Rin = = Gm gm RrRout o|| D Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port CS Model Reattach source and load one-ports: Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Maximize Gain of CS Amp =− AgRrv mD|| o Increase the gm (more current) Increase RD (free? Don’t need to dissipate extra power) Limit: Must keep the device in saturation =− > VVIRVDS DD D D DS, 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 to close to saturation Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Current Source Supply Solution: Use a current source! Current independent of voltage for ideal source Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CS Amp with Current Source Supply Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Load Line for DC Biasing Both the I-source and the transistor are idealized for DC bias analysis Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port Parameters From current source supply =∞ Rin = Gm gm = Rrrout o|| oc Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad P-Channel CS Amplifier DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Two-Port Model Parameters Small-signal model for PMOS and for rest of circuit Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Common Gate Amplifier DC bias: == IISUP BIAS I DS Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG as a Current Amplifier: Find Ai ==− ioutii d t =− Ai 1 Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Input Resistance =− vvgs t vv− At input: =− + + t out igvgvtmgsmbt ro Output voltage: =− = voutdocLtocLir( || R ) ir ( || R ) v − ()rRi|| =+ +t oc L t igvgvtmtmbt ro Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Approximations… We have this messy result 1 gg++ i mmbr 1 ==t o Rv r || R in t 1+ oc L ro But we don’t need that much precision. Let’s start approximating: 1 R +>> L ≈ ggmmb r || RR≈ 0 r ocL L o ro 1 R = in + ggm mb Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Output Resistance vvv− sst−−−+ = gvmgs( g mbs v)0 RrSo v 11+++=t vggsmmb RrrS oo Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Output Resistance Substituting vs = itRS 11++ + =vt iRtS g m g mb RrrS oo The output resistance is (vt / it)|| roc r =+++o RrRout oc||1 S grgr m o mb o RS Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Approximating the CG Rout = + + + Rout roc || [ro gmro RS gmbro RS RS ] The exact result is complicated, so let’s try to make it simpler: ≈ µ ≈Ωµ ≈ gm 500 S gmb 50 S ro 200k ≅ + + Rout roc || [ro gmro RS RS ] Assuming the source resistance is less than ro, ≈ + = + Rout roc || [ro gmro RS ] roc || [ro (1 gm RS )] Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CG Two-Port Model Function: a current buffer • Low Input Impedance • High Output Impedance Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Common-Drain Amplifier W 1 IC=−µ ( VV)2 DS oxL 2 GS T 2I VV=+ DS GS T W µC ox L Weak IDS dependence Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Voltage Gain Note vgs = vt –vout vout =− gvm gs g mb v out rroc|| o v out =−−() gvvm t out gv mb out rroc|| o Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Voltage Gain (Cont.) v out =−−() KCL at source node: gvvm t out gv mb out rroc|| o 1 ++ = ggvgvmb m out m t rroc|| o Voltage gain (for vSB not zero): vgout= m 1 vin ++ ggmbm rroc|| o vg out≈≈ m 1 + vggin mb m Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Output Resistance Sum currents at output (source) node: v Rrr= ||||t i =+gv g v out o oc i t mt mbt t 1 R ≈ out + ggm mb Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad CD Output Resistance (Cont.) ro || roc is much larger than the inverses of the transconductances ignore 1 R ≈ out + ggm mb Function: a voltage buffer • High Input Impedance • Low Output Impedance Department of EECS University of California, Berkeley EECS 105 Fall 2003, Lecture 17 Prof. A. Niknejad Department of EECS University of California, Berkeley.
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