Question 9.5: Analysis by Assumed Diode States Use the ideal-diode model t...

With D_1 off and D_2 on, the equivalent circuit is shown in Figure 9.16(b).
Solving results in i_{D2} = 0.5 \text{ mA}. Since the current in D_2 is positive, our assumption that D_2 is on seems to be correct. However, continuing the solution of the circuit of Figure 9.16(b), we find that v_{D1} = +7 \text{ V}. This is not consistent with the assumption that D_1 is off. Therefore, we must try another assumption.
This time, we assume that D_1 is on and D_2 is off. The equivalent circuit for these assumptions is shown in Figure 9.16(c). We can solve this circuit to find that i_{D1} = 1 \text{ mA and }v_{D2} = -3 \text{ V}. These values are consistent with the assumptions about the diodes (D_1 on and D_2 off) and, therefore, are correct.