# Collector emmiter open circuit voltage

Discussion in 'Homework Help' started by ham3388, Oct 26, 2016.

1. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
Hello dear friends
Please go through my work. I don't know how to find the value of CEO voltage.
I have done the remaining.
Could someone

2. ### crutschow Expert

Mar 14, 2008
20,818
5,917
I see some errors in your calculations starting with the second equation.
It can't be 24V as you are not including any voltage drop across the transistor.

First you calculate the approximate voltage at the base due to R3 and R4 (initially ignoring the small base current), and then go from there to calculate the emitter and collector voltages and current.
After that you can backtrack to include the small base current voltage drop due to the collector current in the equivalent R3||R4 resistance and tweak the base voltage.
Then go back and tweak the resulting emitter and collector voltages.

Here's the results of my LTspice simulation:
See if you can get your calculations to be reasonably close to those values.
(The simulation has about a Vbe of 0.7V rather than the 0.6V you are instructed to use, so that's one difference).

Last edited: Oct 26, 2016

Jun 17, 2014
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4. ### crutschow Expert

Mar 14, 2008
20,818
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He can't do that until he properly solves for the transistor voltages.

5. ### anhnha Well-Known Member

Apr 19, 2012
866
61
I see many steps here and not sure how you did this. Should it be easier to use Thevenin equivalent right from beginning?

Mar 14, 2008
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7. ### MrAl AAC Fanatic!

Jun 17, 2014
5,537
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Hi,

What i was asking is based on what he already calculated, what is his choice for the transistor, as that was the whole point of doing this in the first place. He has three choices and he has to pick one now.

My boat sank a long time ago

8. ### Ramussons Well-Known Member

May 3, 2013
754
133
Since no pricing has been mentioned, irrespective of the voltages calculated, I'd choose BC 107. It has the best ratings among the 3 choices.

9. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
Hi again
If the values below are correct. , please show me how to calculate Vceo.

Here is my work

10. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
I found error in the calculation
Here is the corrected one

11. ### crutschow Expert

Mar 14, 2008
20,818
5,917
I believe your last calculations are correct.
The difference between my simulation and your calculations is that you ignored the effect of the base current on the base voltage from the input bias resistors.

12. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
So how to find the Vceo ?

13. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
Does it mean Vce when the base is open circuit?

Apr 26, 2005
3,972
1,762

15. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
I need to calculate it and then compare it with the value in the data sheet. , but how?

16. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,635
1,298
What you have found is Uceq ( quiescence point, operating point of a device) ≈ 10.7V and Ptotq ≈ 0.22W. And now, based on those calculations you have to choose (select) a proper (suitable) transistor from table B.

And $V_{CE0}$ is the maximum allowable voltage that can be applied from the collector to the emitter (with base terminal left open) of a transistor before it becomes damaged (before magic smoke escape) .

17. ### crutschow Expert

Mar 14, 2008
20,818
5,917
Yes.
Vceo is the worst-case condition for the maximum collector emitter voltage so you would look for a transistor that has a Vceo rating of at least 50% more than the maximum Vce the transistor would see in the circuit for good reliability.

Note that this maximum voltage with an AC input will be greater than the bias value you calculated.

18. ### ham3388 Thread Starter Member

Jul 3, 2012
97
3
If this is the case then
Vceo should be more than
50% × 10.64 =16 v

Both BC 109 and BC 107 satisfy our requirement .
Which one should I select in this case?
Since there is no price limitations I think I should select BC 107 since its rating is greater.
What you suggest?

19. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,158
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Use the collector and emitter resistance values to calculate a max current if transistor shorted.
About 37 mA. This further limits your transistor choices to just one with parameters exceeding in-circuit conditions.

20. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,635
1,298
But 10.64V is not the maximum Vce that the transistor would see in the circuit. Vce_max appear when BJT is on the edge of cur off. In real life we pick BJT with Vce0 > Vcc.