Question

D-17.121 The MOSFET in the circuit of Fig. P7.121 has V, 0.8 V, k,-5 mA/V", and V,-40 V (a) Find the values of Rş, Rp, and Ro so that Ip 0.4 mA, the largest possible value for Rp is used while a maximum signal swing at the drain of ±0.8 V is possible, and the input resistance at the gate is 10 M2. Neglect the Early effect. (b) Find the values of g and r at the bias point. (c) If terminal Z is grounded, terminal X is connected to a signal source having a resistance of 1 ΜΩ, and terminal Y is connected to a load resistance of 10 kS2, find the voltage gain from signal source to load. (d) If terminal Y is grounded, find the voltage gain from X to Z with Z open-circuited. What is the output resistance of the source follower?

Answer 1

(a) I_D = 1/2 k_n V_ov^2 implies 0.4 = 1/2 times 5 time V_ov^2 V_ov = 0.4 V V_as = V_ov + V_t = 0.4 + 0.8 = 1.2 V V_a - 0 implies v_as = v_a - v_s v_s = -v_as = -1.2 V Apply KVL in loop = R_s = v_s - (-s)/I_D = -1.5 + 5/0.4 times 10^-3 = 8.75 k_2 R_a = 10 M_2 Apply KVL in loop we have v_DS = 5 - R_D I_D = 5 - R_D times 0.4 largest possible R_D is a for v_DS min v_DS greaterthanorequalto v_as - v_t v_DS min = v_ov implies v_DS = 1.2 V implies R = 5 - 1.2/0.4 = 9.5 k Ohm (b) g_m = 2 I_p/v_ov = 2 times 0.4 times 10^-3/0.4 = 2 mA/v v_o = v_A/I_D = 40/0.4 times 10^-3 = 100 k Ohm (c) I is grounded then the circuit becomes common source configuration. The voltage gas is \r\n G_v = R_a/R_a + R_sin g_m (r_o // R_D // R_L) = 10 m/10 m + 1 m times 2 times 10^-3 (100 k // 9.5 k // 10 k] = 8.446 v/v