BJT DC Biasing Question

[frozone45]

For part b of the problem in the attached picture, I have determined that the upper limit for the value of VBB, for the transistor to remain active, is 15 [V]. I can not seem to find the lower limit for the value of VBB, for the transistor to remain active. Does anyone know what I am missing in my understanding of part b of the problem in the attached picture?

My attempt at answering the question is shown in the attached picture.

 

[Ratch]

For part b of the problem in the attached picture, I have determined that the upper limit for the value of VBB, for the transistor to remain active, is 15 [V]. I can not seem to find the lower limit for the value of VBB, for the transistor to remain active. Does anyone know what I am missing in my understanding of part b of the problem in the attached picture?

My attempt at answering the question is shown in the attached picture.View attachment 26317

The book wants you to find the range of Vbb for saturation and cutoff. The value of Vbb for saturation is ((Vbb-0.7)/(100*Rb))*100*Rc=15-1, where 1 is the voltage across the transistor during saturation. Solving the equation gives Vbb=14.7 volts. Just above cutoff occurs at Vbb=0.7 volts. Therefore, the range of Vbb is 0.7<Vbb<14.7 .

Ratch