# Finding Q point in BJT

Discussion in 'Homework Help' started by Batusai, May 20, 2008.

1. ### Batusai Thread Starter New Member

May 20, 2008
2
0
Hi there,

Can anyone pls kindly help me with finding Q point of the BJT in the following attachment? Thanks for your help.

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2. ### Papabravo Expert

Feb 24, 2006
11,072
2,152
The load line goes from Ic = 1.78 mA to Vce = Vcc = 10 V.

Starting at the base and summing voltages around the DC circuit
Code ( (Unknown Language)):
1.
2. Vbe - Vcc + (Ic + Ib)Rc + IbRb = 0
3.
4. sinc Ic >> Ib we can simplify to
5.
6. Vbe - Vcc + IcRc + IbRb = 0
7.
8. because Ib = Ic/beta we can solve for Ic to get
9.
10. Ic = (Vcc - Vbe) / (Rc + Rb/beta)
11.
As you can see the location of the Q point depends on the beta of the trnasistor. That is one reason why Collector Feedback bias is dperecated. If Rb = beta*Rc then the Q point will be at the midpoint of the load line. If that is where the Q point is supposed to be it would imply the beta of the transistor is at about 178 and change.

3. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Did the problem statement include an assumed $\beta$ for the transistor or any other transistor parameters?

As stated, the problem forces the solver to make several assumption about key circuit parameters that can only lead to a large number of solutions on the part of those to whom the problem was assigned. An instructor that would provide such an open-ended problem will incur a significant amount of effort. That is unless he has a simulation tool at his disposal with a plan to use it to confirm virtually as many solutions as there are students undertaking the assignment.

hgmjr

4. ### Batusai Thread Starter New Member

May 20, 2008
2
0
Hi,
thanks all the responses. Sorry that I forget to mentioned that "beta=100". In that case will there be any changes in answer replied by "Papabravo"?

Thanks!!

5. ### Papabravo Expert

Feb 24, 2006
11,072
2,152
You have everything you need to solve for Ic. You have the endpoints of the load line so you can also find Vce. You should be good to go.

6. ### hgmjr Moderator

Jan 28, 2005
9,030
215
If you need further information on the technique that papabravo has described, you can take a look at the ebook information on load lines for BJT devices.

hgmjr

7. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Post your efforts at solving for the problem and we will try to help you with any snags you encounter.

hgmjr