Question

How is Va obtained? I already know the steps to find I(f) but not sure about Va

Q3. A logic gate circuit is designed as follows, where R1-5kΩ and R2-4kQ, A is input and Y is output. If A-0, find Y. Calculate the values of Va, and l; If A-1, find Y. Calculate the values of Va, and . (a) (b) Vcc-5V If R2 21 ï,

Answer 1

Case (1) If A = 0 V_cc = 5 V R_1 = 5 k ohm R_2 = 4 k ohm A = 0 the diode D_1 is having more forward voltage due to that it comes into conduction first V_a = 0 V (If diode is ideal) I_f = V_cc - V_A/R_1 = 5 - 0/5 k = 1 m Amp Case (2) A = 1 is applied D_1 - Reverse bias D_2 & D_3 in forward bias V_BE = 0.7 V Due to D_2 & D_3 are ideal V_a = V_BE = 0.7 V I_f = V_cc - V_a/R_1 = 5 - 0.7/5 k = 0.86 m Amp If diodes are practical Case (1) V_a = 0.7 V due to forward voltage drop I_f = 5 - 0.7/5 k = 0.86 m Amp Case (2) V_a = V_D_2 + V_D_3 + V_BE = 0.7 + 0.7 + 0.7 = 2.1 V I_f = 5 - 2.1/5

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