Hello everyone ! How can I solve this problem ? I don't have any idea.
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Yes, it's a homework.hi Tobias,
Welcome to AAC.
As this appears to be Homework, I will move to the Homework Forum.
Please post your attempt at answering the question, we can then help you.
E
Hello,Hello everyone ! How can I solve this problem ? I don't have any idea.
But for active mode Vce> Vbe gives me Ib<5,65/β. I don't know what value choose for ib to find the range of Rb that's my problem.Hello,
Ok then to figure out the values you would assume that in the active mode Vce is greater than Vbe, and in the saturation mode Vce is less than Vbe. You then only have to solve for the base resistor values given your Beta values.
How did you calculate ?For Rb, I am calculating the values between 11.25kΩ and 90kΩ
Anyone agree or disagree?
Hi Tobias12,How did you calculate ?
Hi,But for active mode Vce> Vbe gives me Ib<5,65/β. I don't know what value choose for ib to find the range of Rb that's my problem.
Please do NOT do the latter. Remember, increasingly homework is done via online tools that just want the answer. If you give the answer then the student doesn't need to work out how you do it because you've given them what they need in order to claim credit for solving the problem and move on  which is quite detrimental in the end. Also, intentionally giving inaccurate answers is almost always counterproductive. Consider what you have just stated yourself  your approach is to give them the answer and let them work out solutions to that answer; so if you give them inaccurate answers you can only be expecting them to spend a bunch of time trying to figure out some way to work out a solution that leads to that wrong answer. So even if they work the problem correctly, they will assume that they've done something wrong because they didn't get the solution that you are expecting them to work out a solution to.Hi Tobias12,
Sorry, but I am not going to disclose how I arrived at these results. It is up to you to do the work and discover how I arrived at these results, after all, it is your homework... also the results I gave may not be accurate.
People can give you hints and solutions to work out the results (as most people do) or they can give you the answer and for you to work out the solutions to that answer  I did the latter.
I suspect that there is some sidechannel information you are expected to know and use. For instance, a common practical definition of "saturation" is that the β has dropped to the point where it is no more than 10. This is an arbitrary threshold and is not universally used, but it is extremely common for smallsignal transistors.But for active mode Vce> Vbe gives me Ib<5,65/β. I don't know what value choose for ib to find the range of Rb that's my problem.
You wanna say that Rb = (Vc0  Vbe)/Ib, right ? If i succeed to find Ib i can solve the problem easy, But my problem didn't give me a certain value for the current base Ib. I don't know how to find first the value of base current and then, use the above formula to find Rb.Hi,
Well no, this is not to calculate the Beta the idea is to take the known values 25 and 200 and calculate the values of Rb which satisfies the criteria given. For the active mode i would think these would be the minimum values of Rb, and for the sat mode i would think it would be the maximum values.
To choose Rb for any given current, just calculate the current through Rb knowing the input voltage and the Vbe voltage and the current. So it is the difference divided by the current.
Hi,You wanna say that Rb = (Vc0  Vbe)/Ib, right ? If i succeed to find Ib i can solve the problem easy, But my problem didn't give me a certain value for the current base Ib. I don't know how to find first the value of base current and then, use the above formula to find Rb.
What I have to do is choose Vce= 0.7 V or Vce = 0.8 V and find the current Ic ? Sorry but i don't clearly understand this concept.Hi,
Without given the whole solution, the iB base current is based on the collector current required to bring the Vce down to the target voltage given the value of the collector resistor and the chosen Beta. So if you are using Vbe=0.7v for example then the BC diode gets reverse biased when Vce becomes lower than that, or for active mode just above that as you know. So your target Vce is either 0.7 or 0.8 volts.
Do you understand this concept?
Hi,What I have to do is choose Vce= 0.7 V or Vce = 0.8 V and find the current Ic ? Sorry but i don't clearly understand this concept.
But what beta are you going to use?Hi,
Let's look at one case where we make Vbe=0.7v.
Now when it goes into sat, Vce will be less than Vbe even by a tiny amount here. So we have
Vce<Vbe
but because it is a tiny amount we can solve for:
Vce=Vbe
and go from there.
Now what does it take to get Vce the same as Vbe?
It takes a certain base current which develops a collector current which in turn develops a collector voltage which is the voltage Vce in this case.
So knowing any Beta, find the base current required for that in order to pull the collector voltage down to the same level as Vbe. Once you have the base current, you can calculate the base resistor value.
You know that the Beta acts as a current controlled current source right?
Hi,But what beta are you going to use?
If it is in saturation, then it does NOT act like a currentcontrolled current source. It only behaves that way in the active region. As you approach saturation, the beta starts falling. So what is it when Vce = Vbe?
Even if you model it that way, you haven't answered the question. You told the TS what to do if they known the beta. So, what beta do they "know"? The datasheet gives the range of betas when the device is well into the active region and it gives the Vce at a beta that is well into saturation. How does the TS use the datasheet to determine what beta they should use when it is operated at the point where Vce = Vbe?Hi,
It's not like that. It is always a current controlled current source. The trick is that the BC diode steals base current so the effective Beta looks less. The current controlled current source still pumps out B*ib, but ib falls due to the now forward bias of the BC diode.
Now when Vce=Vbe the BC diode is not conducting yet, although with a real diode it would be. But this simplified model has to be handled in a more idealized way. We have to set some thresholds even though they are not the same with a real diode.
So if the Beta is 25 then we will see one result, and if the Beta is 200 we will see another result. This will happen for both modes of operation 
Hi,Even if you model it that way, you haven't answered the question. You told the TS what to do if they known the beta. So, what beta do they "know"? The datasheet gives the range of betas when the device is well into the active region and it gives the Vce at a beta that is well into saturation. How does the TS use the datasheet to determine what beta they should use when it is operated at the point where Vce = Vbe?
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