Solution 3 :

(a) Input low current,

I(IL) = (Vcc - Vbe(hard) - Vce(hard)) / Rb

I(IL) = (5 -0.8 - 0.5) / 2.8

= 1.32 mA

(b) Fan-out, N

Fan-out = I(OL) / I(IL)

Output low current =I(OL) = Ico = B * Ibo

Ibo = Ies- Icd

Icd = (0.8 - 0.5) / 0.25 = 1.2 mA

Irb = (5 - 0.3- 0.8- 0.8) / 2.8 = 1.11 mA

Irc = (5 - 0.8 - 0.5) / 0.9 = 4.111 mA

Ies = Irb+ Irc = 4.11 + 1.11 = 5.22 mA

Ibo = 5.22 - 1.2 = 4.02 mA

Bf * Ibo = 49 * 4.02 = 196.98 / 1.32 = 149

Fan-out, N = 149