# calculate values from BJT I/O curve

Discussion in 'Homework Help' started by M0GLI, Oct 5, 2013.

1. ### M0GLI Thread Starter New Member

Sep 22, 2013
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how to solve this.image is attached with the post.plz help

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2. ### WBahn Moderator

Mar 31, 2012
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4,920
You need to show YOUR best attempt to solve YOUR homework problem.

We can then use that as a starting point to help guide you from where you are to where you need to end up.

3. ### M0GLI Thread Starter New Member

Sep 22, 2013
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no idea how to start

4. ### LvW Active Member

Jun 13, 2013
674
100
A good starting point would be to remember the definition of alpha and beta.

5. ### M0GLI Thread Starter New Member

Sep 22, 2013
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I know about alpha beta.I know about the I/O characteristics curve and BJT models. but how to draw load line and calculate those from curves, give me hints.I dont want you to solve this for me.just give me some clue

6. ### LvW Active Member

Jun 13, 2013
674
100
Alpha and beta are transistor parameters - independent on any application. Thus - there is no "load line" to be considered.

7. ### LvW Active Member

Jun 13, 2013
674
100
As another hint:
Are you able to determine the DC current gain B?
Do you know if there is a (considerable) difference between the dc current gain B and the differential current gain beta?

8. ### WBahn Moderator

Mar 31, 2012
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Then perhaps the best place to start would be to retake the course where these are taught. Or to go back to the textbook where this is taught and reteach yourself these concepts.

9. ### WBahn Moderator

Mar 31, 2012
18,095
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It's debatable that you know about alpha and beta and characteristic curves and BJT models if you don't have a clue about where to start or think that there is any kind of a load line involved in this problem at all.

So let's take it from the basics.

Q1) What is the definition of the DC beta?

Q2) For this transistor, what is Ic under the conditions specified in part i)?

Q3) Given Q1 and Q2, what is the answer to part i)?

Q4) What is the definition of alpha?

Q5) What is the relationship between Ib, Ic, and Ie?

Q5) Given Q2, Q4, and Q5, what is the answer to part ii)?

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10. ### M0GLI Thread Starter New Member

Sep 22, 2013
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1) answer: base to collector dc curent gain
2) answer: Ic= 1.4mA (from the curve)
3) answer: beta dc= Ic/Ib = (1.4mA/10uA)= 140
5) answer: Ie= Ib+Ic= (1.4mA+10uA)= 1.41mA
alpha= Ic/Ie= 1.4mA/1.41mA= 0.992

Is it correct brother? I understand all the equations and procedure but I doubt about the ques 2 answer. How to determine Ic from the curve ?

11. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,519
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I am reading my text book. I think this is a problem where you use Load Line and Q-point.

Do you have any additional info? Vcc? Rc?

12. ### WBahn Moderator

Mar 31, 2012
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Correct, but arguably not clearly stated. Simpler and clearer might be that β=Ic/Ib under DC conditions.

These are all correct and you are reading the data off the graph just fine. Note that you didn't need the right hand curve at all. It's useful information, just not relevant to these particular questions.

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13. ### WBahn Moderator

Mar 31, 2012
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But a load line involves two different components or circuits that each have to satisfy particular voltage-current characteristics. There's no second component here. You have a transistor with specified operating conditions, thus you only need the voltage-current characteristic for those operating conditions.