Solving BJT Base Current Homework Problem

[jaus tail]

Homework Statement

[/B]

Homework Equations

For 1st I think answer is B
For second I(c) = B I(b)
B is beta

The Attempt at a Solution

For 1st I think it's B
But book says answer is D
Is book wrong?

For second I'm getting:
I(c) = 3mA
And beta = 200
So I(c) = B I(b)
3mA = 200 I(b)
This gives I(b) as 15 micro amperes.
But that's not in option.
Is the book wrong or is load current some I(e)?
How to figure out load current is I(c) and not I(e)?

I think book is wrong but i can't be sure so I posted here.

 

[NascentOxygen]

At 15μA the BJT would still be in its linear region of operation. I guess they want you to use 20μA, maybe more. Good practice would see it designed for at least a factor of x2 more, that's 40μA, and giving a "forced beta" of 75. Often you'd design for even more than this.

I don't quite understand what is being asked in Q1. Three of the devices listed offer a means of controlling their operation, the diode lacks this.

 

[jaus tail]

Book says answer is 30 micro ampere. Can a beta value be changed? I thought Beta depends on doping level.
I found this example from google:

http://www.electronics-tutorials.ws/transistor/tran_2.html

NPN Transistor Example No1
A bipolar NPN transistor has a DC current gain, (Beta) value of 200. Calculate the base current Ib required to switch a resistive load of 4mA.

Therefore, β = 200, Ic = 4mA and Ib = 20µA.

B is ratio of Ic and Ib.

 

[LvW]

Attention:
In the above contribution, the linked "tutorial" contains many severe errors! I never would recommend it (just one example: The DC load line as shown in a diagram is wrong).

 

[NascentOxygen]

To use a transistor as a switch, you must overdrive it so that base current > I_C/β

Sure, in the linear region, I_B = I_C/β
but to use a BJT as a switch you must make sure that I_B exceeds this so the transistor is saturated. In the saturated region, V_CE is small and almost independent of I_C, resembling a good mechanical switch when it's ON.

β is assumed to be fixed at the value you are given, in school exercises.

 

[cabraham]

The diode is not externally controllable. In the forward direction it conducts, but not in the reverse direction. There is no ability to switch the diode on & off externally.
The BJT when used as a switch must be overdriven with base current to assure saturation. With 3 ma of collector current Ic, dividing by beta vale of 200 gives 15 uA of base current Ib. But as mentioned before that Ib value should be doubled to assure saturation, or 30 uA. But if beta value of 200 is typical, minimum value is less. Also beta decreases at low temperature. So 40 uA for base current is what I recommend.

Claude. :-)