University of Rhode Island Department of Electrical and Computer Engineering ELE541 Electronic Testing II

Bipolar Parameters   npn pnp Units Is 6E-17 7E-16 Amps VAF 325 90 Volts beta 100 75

MOS Parameters   NMOS PMOS Units KP 30E-6 10E-6 Amps/V2 Vth 2 -1.25 Volts Lambda 0.02 0.02 V-1

Vt = KT/q = 0.0257 Volts at room temperature.

1.     Design a 50uA Widlar current source.

For Widlar current mirrors

RIo = Vt ln[Iref/Io]

Given

Io = 50 uA

Pick Iref

Solve for R = (Vt/Io) ln[Iref/Io]

For example if Iref = 10 Io

R = (0.026/50x10^-6) ln[10]

R = 1.2K

Note that the voltage across R is 1.2K*50 uA = 60 mV.
60 mV is 2.3*Vt

Output resistance, R_out = ro[1 + gm RE]
where

ro = VAF/Ic

gm = Vt/Ic

R_out = (VAF/Ic)[1+IcRE/Vt] = (325/50x10^-6)3.3 = 21 MEG