2011-06-20 Finding the potential and current in a two-diode circuit.

Find V_H and the current flowing in the 4 kΩ resistor.

Solution

Since the potential +10 V is higher than +5 V and 0V, the Ge diode and Si diode tend to be forward and reverse biased, respectively. Assuming that the potential V_H is higher than +5 V but lower than +10 V so that the Ge diode is forward biased and Si diode is reverse biased. Therefore, the same current flows through Ge diode and 4 kΩ resistor.

KCL;  

I_Ge + I_Si = I_4kΩ ;  I_Si = 0 A

I_Ge = I_4kΩ

KVL;

0 V + (4 kΩ)(I_4kΩ) + 0.3 V + (2 kΩ)(I_4kΩ) = +10 V 

(6 kΩ)(I_4kΩ) = 9.7 V 

I_4kΩ = 1.62 mA          Ans

KVL;

0 V + (4 kΩ)(I_4kΩ) = V_H 

V_H = (4 kΩ)(1.62 mA) =  6.48 V      Ans

Check assumptions;

+5 V < V_H < +10 V ; OK.

V_H > +5 V ; Si diode is reverse biased; OK.

The assumptions are true.