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