# BJT DC Biasing Question

#### frozone45

Joined Apr 16, 2016
17
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.

#### Jony130

Joined Feb 17, 2009
5,475
What is the base current if Vbb < 0.7V ?
Also Ie = Ib + Ic = Ib + β*Ic = (β + 1)*Ib

#### atferrari

Joined Jan 6, 2004
4,728
What is the base current if Vbb < 0.7V ?
Also Ie = Ib + Ic = Ib + β*Ic = (β + 1)*Ib
Ib + Ic = Ib + β*Ib

• Jony130

#### frozone45

Joined Apr 16, 2016
17
What is the base current if Vbb < 0.7V ?
Also Ie = Ib + Ic = Ib + β*Ic = (β + 1)*Ib
(Negative value [V])/RB

#### MrAl

Joined Jun 17, 2014
10,877
Hello there,

Are you allowed to use the 0.7v constant base emitter voltage rule or not?
Also, do you have to consider collector to base leakage current?
Transistor and diode circuits are a little different than other circuits because the allowed assumptions have to be known beforehand and can vary widely from course to course.

In any case, the point where the transistor becomes inactive is when the base current times the Beta results in an output voltage Vc that is equal to the supply voltage Vcc. When that happens the output can no longer rise.

So you have to come up with an equation that relates Vbb to Vc and see what you can figure out.

Part of this depends on the above allowed assumptions however. For example, if you must consider leakage current then the max output Vc will be LESS than Vcc because there will be an ever present voltage drop in Rc.