Subthreshold Operation and gm/Id design - CppSim

PerrottAnalysis and design of Analog Integrated CircuitsLecture 16 Subthreshold Operation and gm/IdDesignMichael H. PerrottApril 1, 2012 Copyright 2012 by Michael H. PerrottAll rights PerrottA Closer Look at Transconductance Assuming device is in strong inversion and in saturation:2 IdVgsId_op VVTHVgs_opVds > VM1 IdVgsNMOS gsdgm = Vgs IdVgs_opID= nCox2WL(Vgs VTH)2(1 + Vds) gm= Id Vgs nCoxWL(Vgs VTH) s2 nCoxWLId gm Idq2 nCoxW/L Id 2Id(Vgs VTH) PerrottUnity Gain Frequency for Current Gain, ft Under fairly general conditions, we calculate: ftis a key parameter for characterizing the achievable gain bandwidth product with circuits that use the device3|Id/Iin| PerrottCurrent Density as a Key Parameter Current density is defined as the ratio Id/W:-We ll assume that current density is altered by keeping Idfixed such that only W varies Maintains constant power ro( , 1/gds= 1/( Id)) will remain somewhat PerrottInvestigating Impact of Current Density For simplicity, let us assume that the CMOS device follows the square law relationship-This will lead to the formulations:-These formulations are only accurate over a narrow region of strong inversion (with the device in saturation)-However.

M.H. Perrott A Closer Look at Transconductance Assuming device is in strong inversion and in saturation: 2 Id Vgs Id_op ΔV VTH Vgs_op Vds > ΔV M1 Id Vgs NMOS g s d gm = ΔV gs ΔId Vgs_op

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