Unformatted text preview: gm speed / voltage gain
• Capacitances CGS, CGD, … speed
• Output impedance ro voltage gain
EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 6 Low Frequency Model
• A Taylor series expansion of small signal current gives
(neglect higher order derivatives) • Square law model: EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 7 Transconductance
• Using the square law model we have three equivalent
forms for gm in saturation EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 8 Weak Invesion gm
• In weak inversion we have bipolar behavior EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 9 Transconductance weak
inversion EECS 240 Lecture 2: CMOS  passive devices strong inversion © 2006 A. M. Niknejad and B. Boser 10 Transconductance (cont)
• The transconductance increases linearly with Vgs – VT but
only as the square root of Ids. Compare this to a BJT that
has transconductance proportional to current.
• In fact, we have very similar forms for gm • Since Vdsat >> Vt, the BJT has larger transconductance for
equal current.
• Why can’t we make Vdsat ~ Vt ?
EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 11 Subthreshold Again…
• In fact, we can make Vgs – Vt very small and
operate in the subthreshold region. Then
the transconductance is the same as a BJT
(except the nonideality n factor).
• But as we shall see, the transistor fT drops
dramatically if we operate in this region.
Thus we typically prefer moderate or strong
inversion for highspeed applications.
EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 12 µCox • Square law: • Extracted values strong
function of ID
– Low ID
weak inversion
– Large ID
mobility reduction • Do not use µCox for
design! EECS 240 Lecture 2: CMOS  passive devices © 2006 A. M. Niknejad and B. Boser 13 Transconductor Efficienc...
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 Spring '04
 Boser
 Computer Science, Integrated Circuit, Transistor, B. Boser, Bernhard E. Boser, A. M. Niknejad

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