2011-06-18 Generating a constant current with diodes.

Find the potential V_C and current I_X flowing in 2.7 kΩ resistor when the potential V_X is -5 V and -10 V, respectively. Observe if there is a change in I_X.

Solution

We develop equations that relate V_X with V_C and I_X. 

KVL;  

V_X + 0.3 V + 0.3 V = V_C

V_C = V_X + 0.6 V  

KVL;

V_X + 0.3 V + 0.3 V + 0.7 V + 0.7 V = V_X + 0.3 V + (2.7 kΩ)(I_X)

I_X = (1.7 V) / (2.7 kΩ) = 0.63 mA

For V_X = -5 V;

V_X + 0.6 V = V_C = -4.4 V       Ans

I_X = 0.63 mA                         Ans

For V_X = -10 V;

V_X + 0.6 V = V_C = -9.4 V       Ans

I_X = 0.63 mA                         Ans

We can see that the potential V_C depends on V_X but the current I_X is constant.