# Transistor,help me to check the answer

Discussion in 'Homework Help' started by xx123321, May 10, 2011.

Apr 27, 2011
15
0

2. ### xx123321 Thread Starter New Member

Apr 27, 2011
15
0
can any one help? urgent...

3. ### jegues Well-Known Member

Sep 13, 2010
735
43

I will continue to look at the others and edit this post accordingly.

EDIT:

The answers on the 2nd page are also correct.

EDIT:

For question number 3,

After using the thevenin equivalent we can write a KVL in the base-emitter loop,

$-V_{th} + I_{b}R_{in(base)} + 0.7 + I_{e}R_{e} = 0$

$\rightarrow I_{c} = \frac{V_{th} - 0.7}{\frac{R_{in(base)}}{\beta} + \frac{R_{e}}{\alpha}}$ where $\alpha = \frac{\beta}{\beta + 1}$

This gives me,

$I_{c} = 1.37 mA$

The rest follows from here.

I haven't studied MOSFETS yet, but perhaps I'll take a look and get back to you.

EDIT: For question 4,

This is the typical arrangement for biasing using a drain-to-gate feedback resistor. The large feedback resistance forces the DC voltage at the gate to be equal to that at the drain (Ig = 0).

Thus,

$V_{gs} = V_{ds} = V_{dd} - R_{d}I_{d}$

If V1 is non zero(as you've indicated) then certainly Id is nonzero as well.

Hope this helps!

Last edited: May 10, 2011
4. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,963
1,098
Yours answers look good except question number 3. And jegues has done the good job to show you the right solution.

5. ### xx123321 Thread Starter New Member

Apr 27, 2011
15
0
first,thanks jegues and Jony130 very much for helping me !
I have some questions about Rin(base) in question 3.
1-Does Rin(base) mean R(th) ?
2-In my study book,it shows me that if the following condition be ture,it will take the approximation.
"If βRE>>RTH, then IE=(VTH-VBE)/RE"
so, does my answer still be wrong?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,963
1,098
Well, i don't know, maybe yes or maybe not.
Maybe they define Rin(base) as
Rin(base) = R(th)|| (β+1)*Re = 103.83KΩ

Rin(base) = R(th) = R1||R2 = 2.83KΩ
But which value for Ie your Professor will be preferred.
The exact one, or approximate value ?

Last edited: May 12, 2011
xx123321 likes this.
7. ### jegues Well-Known Member

Sep 13, 2010
735
43

Rin(base) means the input resistance looking into the base.

I believe it turns out to be what you have called Rth.

8. ### jegues Well-Known Member

Sep 13, 2010
735
43
I'm not sure where your book is getting that, I would lean more towards my solution.

KVL has no if/ands/or/buts, in this situation atleast!

Last edited: May 11, 2011
xx123321 likes this.
9. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,963
1,098
Maybe I'm pick on but if you looking into the base.
Then for sure you will see R1||R2 || (β+1)Re.

Last edited: May 12, 2011
10. ### jegues Well-Known Member

Sep 13, 2010
735
43
Hmmm... I'm not sure what you would call it then, but I'm fairly sure the intention was for it to be R1//R2.

This resistance value is commonly denoted as RBB in certain textbooks, and likewise the resulting thevenin source, VBB.

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
Are you sure you don't mean ....??

R1||R2||(β+1)Re

12. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,963
1,098
Yes, my mistake.
And thanks for correct my stupid mistake