Help with Transistor analysis

Discussion in 'Homework Help' started by DYLH, Sep 14, 2013.

1. DYLH Thread Starter New Member

Aug 13, 2013
28
1
For the problem in the attached image, I'm able to find the Vo:

-5 + 10000 Ib + Vbe + 200 Ie = 0
Ic = 100 Ib
Ie = Ib + Ic

-5 + 10000 Ib + 0.7 + 200 (Ib + Ic) = 0
10000 Ib + 200 (Ib + Ic) = 4.3
10000 Ib + 200 (Ib + 100 Ib) = 4.3
Ib = 142.38 uA
Ic = 14.238 mA
Ie = 14.38 Ma

Vo = Ie * R
Vo = 14.38 mA * 200
Vo = 2.876 V (matches given solution)

I'm stuck on solving for Vce, I'm comming up with:
Vo - 12 + Ic (500) + Vce = 0
2.876 - 12 + (14.238)(500)(10^-3) = -Vce
Vce = 2.005 (doesn't match solution of 1.984V)

where am I going wrong?

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2. #12 Expert

Nov 30, 2010
16,026
6,540
12 - 500 Ic = Vc (4.881)
200 Ie = Ve (2.876)
Vc-Ve = Vce (2.005)

Ima 'fraid it doesn't match my answer, either.
But then, I'm surprised I could read your math, this being 37 years since I was in a school room and they keep changing how they teach things.

Another helper will be along soon enough and double check our math.

3. The Electrician AAC Fanatic!

Oct 9, 2007
2,255
311
Where you calculated:

Vo - 12 + Ic (500) + Vce = 0

I think they calculated:

Vo - 12 + Ie (500) + Vce = 0

4. WBahn Moderator

Mar 31, 2012
17,446
4,699

Vo = 2.876V
Ie = Vo/200Ω = 14.380mA

Vce = 1.984V

Vc = Ve + Vce = Vo + Vce = 2.876V + 1.984V = 4.860V
Ic = (12V-Vc)/500Ω = (12V-4.860V)/500Ω = 7.14V/500Ω = 14.280mA

Ib = Ie - Ic = 14.380mA - 14.280mA = 100μA

β = Ic/Ib = 14.280mA/100μA = 142.8

It would not appear that the author's solution is not consistent with the problem given.

Now, keep in mind that the difference between their answer and your answer is only about 1%, so it is very possibly due to roundoff error on their part. But since THEY choose to use answers with four sig figs, it was THEIR responsibility to provide an answer that was correct to four sig figs.

5. WBahn Moderator

Mar 31, 2012
17,446
4,699
That sounds very likely -- and since β=100 would throw the answer off by 1%, that matches well. So they took the often used approximation that Ic≈Ie but then felt justified in reporting answers to four sig figs when a β of 100 only means that Ic matches Ie to two sig figs. Sloppy.

But we all make sloppy mistakes, so don't judge the book or the author based on an occasional sloppy mistake.

6. DYLH Thread Starter New Member

Aug 13, 2013
28
1
Thanks, all!

FWIW, this is for Fundamentals of Electric Circuits by Alexander and Sadiku 5th edition, Practice Problem 3.12... in the event someone else searches.