Question

5. (a) Find the output resistance in each of the current sources shown in Fig. 4(a) and (b) and verify that the roul of the MOS current source is much smaller than the rout of the B.JT current source using the following device parameters: BJT V 100 MOS : λ=0.2 : k'=60 × 10 (b) Find the W/L of the four identical transistors that make up the cascode MOS current source shown in Fig. 4(c) so that its roul is the same as that of the BJT current mirror in Fig. 4(a).

Please answer both part(a) and part(b). This is my last question for this month's subscription. Please answer them all. Thank you!

Answer 1

a) R_out = 1/lambda I_D = 1/(0.2) (100 mu) = 50 k ohm R_out (BJT) = V_A/I_C = 100/100 mu = 1 M ohm Therefore R_out (MOS) is less than that of R_out (BJT) b) R_out of MOS car code connection (C) is R_out = (r_01 g_m2 r_02) = 1 M ohm 1/lambda I_D times 2 I_D/V_ov times 1/lambda I_D = 1 M ohm (50 k) 200 mu/V_ov (50 K) = 1000 K V_ov = 0.5 V equation (1) I_D = k^�/2 (w/L) (V_ov)^2 (w/L) = 2 times 100 mu/60 times 10^-6 times (0.5)^2 (from (1)) W/L = 13.33 with this (w/L) value, R_out of both MOSFET & BJT will be same