2011-06-17 Finding the potential at point B in a three-diode circuit using KVL and KCL.

Find V_B and the current flowing in the Ge diode.

Solution

Assuming that the potential V_B is higher than 0 V but lower than +5 V so that the current I_Si flows from +5 V to point B and I_Ge flows from point B to 0 V.
From this assumption, V_B is higher than -5 V. Therefore, the ideal diode is reverse biased and the current -I_ideal = I_ideal = 0 A.

KCL;  

I_Ge = I_ideal + I_Si = I_Si

KVL;

0 V + 0.3 V + (1 kΩ)(I_Ge) + 0.7 V + (1 kΩ)(I_Si)  = +5 V 

(2 kΩ)(I_Ge) = 4 V 

I_Ge = 2 mA          Ans

KVL;

0 V + 0.3 V + (1 kΩ)(2 mA) =  V_B 

V_B = 2.3 V       Ans

Check assumptions;

0 V < V_B < +5 V ; OK.

V_B > -5 V ; ideal diode is reverse biased; OK.

The assumptions are true.