M niknejad and b boser 6 low frequency model a taylor

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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 sub-threshold region. Then the transconductance is the same as a BJT (except the non-ideality 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 high-speed 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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This note was uploaded on 03/18/2014 for the course EECS 240 taught by Professor Boser during the Spring '04 term at University of California, Berkeley.

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