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Collector emmiter open circuit voltage
- Thread starter ham3388
- Start date
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).
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).
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He can't do that until he properly solves for the transistor voltages.
I see many steps here and not sure how you did this. Should it be easier to use Thevenin equivalent right from beginning?
Whatever floats your boat.
Hi,
Hasnt he done that already?
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
You read the datasheet.
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) .
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) .
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.
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?
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?
