No Vbe is not a constant. The Vbe varies with Ib current.Hi Jony,
As i understand, you are trying to find Vbe of a transistor by solving the equation using iterative method. I thought that Vbe for a trans is always constant regardless of β value.
The first method is to simply assume that the Vbe is in range between 0.6V...0.7V.Jony, i don't see the second method that you mentioned. Can you show the other method to figure out Vbe?
Yeah, this was confusing me.As you can see the base current change 45mA/13.8μA = 3260 times. But Vbe change only by 375mV. This very small change in Vbe compared with the huge changes in the base current can be ignored in some cases. And this is why we use Vbe= 0.6...0.7V in hand calculations.
Does this mean that we also can't find exactly the value of Ic?The first method is to simply assume that the Vbe is in range between 0.6V...0.7V.
And next find the base current
Ib = (Vcc - Vbe)/Rb and Ic = β * Ib (current control method)
You need to get out more.
https://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=895330&queryText=lambert+w
Yes, you're right.Does this mean that we also can't find exactly the value of Ic?
Hello there,
Well actually i think they should get out more. Ive done this before year 2000 im sure, and that's the date of publication.
And i cant believe it took them until then to figure it out.
Thank you. To me, it is good engineering practice to have a fair interplay between question and answers. This helps to solve problems and avoids misunderstandings.Hello Winterstone,
I'll try to answer your question since you seem so intent on this answer.
Yes - I agree. The BJT does not act like a car (surprise?). However, in the technical world the BJT is not the only device that can be controlled.The car pedal is not the same as the equation we are looking at. The equation we have looked at has clear inputs and outputs and there's no integration required. It's a simple formula.
I do not understand this comment. Please, read again my replies. I cannot remember that I have used the terms "wrong" or "mistake" in connection with the usage of Shockley`s formula or the corresponding graph in the data sheet.Eric used the data sheet and that seemed like a good idea. So what is wrong with that? Clearly they put that graph(s) into the data sheet so that we could see them right? If you dont agree, then show me what you use that graph for, or tell my why it was a mistake for them to put that data sheet graph into the data sheet.
Is it really necessary to give you an answer to this question?But because you asked this of the equation i then thought that i would ask you if you dont or cant use it for that then what would you use that formula for? Certainly it is not a wasteful formula is it?
So simply show me how you would use this formula and what you would use it for.
Thank you. To me, it is good engineering practice to have a fair interplay between question and answers. This helps to solve problems and avoids misunderstandings.
Yes - I agree. The BJT does not act like a car (surprise?). However, in the technical world the BJT is not the only device that can be controlled.
And because you wanted me to explain the difference between either to "control" a quantity or to "determine" its value I have used a simple mechanical example to explain the difference.
And from your reply I can derive that you still are not clearly aware of the difference, are you?
I do not understand this comment. Please, read again my replies. I cannot remember that I have used the terms "wrong" or "mistake" in connection with the usage of Shockley`s formula or the corresponding graph in the data sheet.
I simply have mentioned that it cannot be used to calculate a "good" value (lets say with an error of 10...20 %) because of large technological uncertainties.
And - if you remember your own comments - you are not very far from me (see your reply#19)
Is it really necessary to give you an answer to this question?
You are asking how and where this formula is used? Well, here is my answer:
Shockley`s famous formula describes how the current through a pn junction is determined (determined!) by the three quantities Vbe, Vt and Is.
And, obviously, if not all of these quantities are known (with sufficient accuracy) you cannot determine the exact value of Ic - however you clearly can see how the current Ic can be externally controlled by the voltage Vbe. To "control" something means to be able to change/modify the value (perhaps without knowing the absolute values; see the example with the car).
And this knowledge (how to control based on Shockley) is very important because of the following:
1.) It clearly shows how we can apply negative feedback (by feeding back a stabilizing voltage to the emitter node in order to influence Vbe)
2.) More than that the formula contains the most important property of the BJT: It`s ability to amplify!
The transconductance which describes the relationship between input (voltage) and output (current) is identical to the slope of the graph from Shockley´s formula.
______________________
I am sure, most of these information is not new for you - however, perhaps some parts are interesting or helpful for some beginners.
Winterstone
Hello Winterstone,
Ok so what are you saying here...are you saying that we can 'determine' the collector current with Vbe but we can not 'control' it, or are you saying that we can 'control' the collector current with Vbe but we can not 'determine' it (using Shockey's equation)?
Hello,
I wont argue over this, but im sure i've used it circa 1980, but i think it was originally written before the 1900's, at least the w function anyway. What i am surprised about is that it took them until 2000 to publish this. If you look at a write up of the function you'll see it screams "Diode"
I appreciate the link anyway.
No, that´s NOT what I am saying. Please read again the end of my reply#48 - I repeat again:So you are basically saying that we cant do anything with Shockley's formula then because we can never know the variables exactly.
Yes, that`s correct (as mentioned already several times in this and a related foregoing thread).So you are saying that we can not 'determine' the Ic with Shockley's formula because we dont know Is exactly?
Hello MrAl, here comes my answer.
No, that´s NOT what I am saying. Please read again the end of my reply#48 - I repeat again:
1.) The formula shows that we can control Ic via Vbe. With other words: From the formula we can derive the information that we can apply
negative voltage feedback (by feeding back a stabilizing voltage to the emitter node in order to influence Vbe)
2.) More than that, the formula reveals the most important property of the BJT: It`s ability to amplify!
The transconductance g which describes the relationship between input (voltage) and output (current) is identical to the slope of the graph from Shockley´s formula.
Yes, that`s correct (as mentioned already several times in this and a related foregoing thread).
Well i ask because people have already done this. The results are not perfect, but they are still usable. And since Vbe does not depend on the emitter resistor being there or not we still see it working to some degree without an emitter resistor. So i am not sure what degree of accuracy you want to say is the criterion for 'works' or 'doesnt work'.
Sorry - I don`t understand what you mean because you always speak about "it". What is better than what?My only point was that it was not any better than using Beta. But you seem to be dismissing it entirely for 'determining' the Ic.
It's unfortunate that Ratchit is not here to reply because he has always shown himself to be a strong advocate of the voltage control method.
Sorry - I don`t understand what you mean because you always speak about "it". What is better than what?
We have a two equation that we can use to determine the Ic. First equations is
Ic = Hfe *Ib and the second one is Ic = Is*(e^(Vbe/Vt) - 1).
And I think that MrAl want to compare this two equation from engineering point of view.
Which of this two equations gives "better" result in determining Ic current.
We have been given this circuit and 2n2222 datasheets
RB = 510kΩ; Rc = 1KΩ
And our task is to find Ib and Ic current. Next we build this circuit and we check our calculation on the bench.
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