Transistor Maths
As for a formula that can be used to calculate a transistor general parameters without knowing Hfe/β, I don't think such a thing exists. As amplifying devices, β is the most important factor, this is the gain of the transistor, it's the reason for having a transistor. To suggest that the β of a transistor can be canceled out or ignored in such an equation is quite preposterous. It's like trying not to use the capacity of a capacitor, or the resistance of a resistor in a circuit calculation.
That's ok i guess, but what confused me is you quote the Shockley equation and then you use a different method to arrive at the bias solution. If you quote the Shockley equation then you should show how you used that equation.
So you dont want to actually use the Shockley equation then?
Hello again Winterestone,
Ok that's fine, but you still have not shown any example where you USE the Shockley equation for anything at all. I'd like to see how you use the Shockley equation next because you have quoted that and after all that is what part of my goal was for this thread. Do you understand this now? .
Winterstone said:1.) Nobody applied Vbe to a bjt, it would be destroyed.
(What does this mean ?)
audioguru: post#6 said:If you apply 0.65V [V[SUB]be[/SUB]] to the base-emitter of a transistor then some transistors will have a high base and collector current and maybe will burn out but other transistors will have a low base and collector current because each transistor is different even if they have the same part number. Also, a transistor conducts more when it is warm and conducts less when it is cool.
I will concede that some circuits have no need to use β directly. However, everything I have read says that a transistors transconductance is derived from beta/β/H[SUB]fe[/SUB]. In the words of Wikipedia...Winterstone said:I have some doubts if beta really is "the most important factor" and that beta would be "the gain of the transistor" and that this is the "reason for having a transistor". Regarding the gain properties I think the transconductance g=delta_Ic/delta_Vbe is the relevant parameter (remember the basic expression gain=g*Rc).
Wikipedia said:By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled. Collector current is approximately β (common-emitter current gain) times the base current. [...] Also, as the base–emitter voltage (V[SUB]be[/SUB]) is increased the base–emitter current [I[SUB]be[/SUB]] and hence the collector–emitter current (I[SUB]ce[/SUB]) increase exponentially according to the Shockley diode model and the Ebers-Moll model.
The specific words you are calling out in that paper are more than likely an inexact placeholder for the sake of simplicity, and not intended to be taken so literally. It is convenient to say "apply Vbe" in that document, even if that is not actually what they mean.Winterstone said:It would be interesting to hear what you[()blivion] think about the statements in [the berkeley] text regarding the importance of Ib and beta.
Winterstone said:I have designed a circuit in common emitter configuration - including the bias resistors. And in this calculation, I intentionally have ignored the so-called current gain value.
crutschow said:You can ignore β in your calculations but that doesn't mean it[β] has no effect on the bias point.
Winterstone said:Please. would you tell me why I should show you "any example" where I am going to "USE the Shockley equation for anything at all" ?
I will concede that some circuits have no need to use β directly. However, everything I have read says that a transistors transconductance is derived from beta/β/H[SUB]fe[/SUB]. In the words of Wikipedia...
The specific words you are calling out in that paper are more than likely an inexact placeholder for the sake of simplicity, and not intended to be taken so literally. It is convenient to say "apply Vbe" in that document, even if that is not actually what they mean.
Do YOU understand the meaning of this part of your reply? It just "appears" that I have ignored the current gain value?I can't speak for MrAl, but from what I read he seems to doubt that you have in fact done exactly what you claim hear. I believe his position is that it just appears to you that you have ignored the current gain value. Again, I really can't speak for him.
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I am afraid, you didn`t understand the purpose of my example. What I have done is simply an approximate calculation - without using the parameter beta. It was my intention to show that - in contrast to your claims regarding beta ("most inportant", "gain of the transistor") - a design with good results is possible if dc feedback is applied. At the same time I have mentioned that any rough guess regarding beta would improve the exactness of the design (calculation of a loaded base divider). Didn`t you understand this approach?Even if you have come up with an equation that does not involve β, it does not mean you have an equation that can parameterize a transistor without ever using β.
...."to show your work? In contrary: Several times he has asked me to show how I would use Shockley`s equation to establish a certain collector current.Winterstone... I think MrAl is simply asking you to show your work. Shockley equation or not.
Which equation do you want me to provide? How to calculate a voltage divider? Are you joking? There is no secret equation. I have done a simple calculation that can be found in each beginners textbook and I was of the opinion that I discuss with people who know basic calculations.If what you have actually done is created/used a mathematical equation for voltage controlling a transistor that does not involve beta/β... then you should be able to provide this equation and an example solution (or two or three) for any circuits. It's as simple as that.
As far as I see, you are the one making the extrordinary claims, therfore The burden of proof lies with you.
Winterstone:
As i said above, this is just a question that anyone might ask anyone else that is knowledgeable in the field. You stated that the Shockley equation did SOMETHING or proved something or somehow made something better, so i asked that you show WHAT it is that the Shockley equation could do for us. If you say it can not to compute the Ic that's fine, but you did say it can control, so i'd like to see what it will control for us in a circuit of your choosing. That's not too much to ask is it?
If you knew how to use the Shockley equation then you should know how to find Is somewhere, or else the equation is useless to anybody anywhere any time.
I just wanted to see how you would use it, that's all. Use it for anything at all that you care to show.
I was hoping you could show some work so that we could take it farther, to investigate what we can use for Is and the emission constant and whatever else comes up here.
But we rarely see this Shockley equation except when someone happens to mention it, and then we almost never see any calculation examples to go with it.
So i guess we have to come up with some reasonable values for Is (and later the emission coefficient) and that would get us on our way i think.
Hello MrAl,
your reply contains - for my opinion - a mixture of two items.
1.) I cannot help you in finding a reasonable value for Is - you must ask the BJT manufacturer. Of course, I could start a search in books and internet sites - but do you really expect this from me?
2.) However, as far as the second point is concerned - usage of Shockley`s equation - I can give you some more information (in addition to that I gave you already earlier):
(a) You certainly know the tanh characteristic of the transistor-based differential amplifier. This has been derived based on Shockley`s equation and using the relation exp(x)=sh(x)+ch(x).
May I add a comment? Several times in the past (in other threads) I have asked the "current-control" defenders how they would explain this characteristic based on the current-control principle.
Surprisingly, I never got an answer!
(b) Here is another example that works only for the voltage-control principle using Shockley`s equation: The famous Barry Gilbert (by the way: of course on the "voltage-control side")
has invented the principle of translinear loops, which leads to the very promising signal processing methods in the "log domain".
These are two examples, where Shockley`s equation must be used because that is the only way to solve the particular problem.
I hope I could help you.
Regards
W.
PS: I have started a new thread about BJT current control and I expect comments/corrections/critics from forum members in favour of BJT current-control.
At the end of the day, this thread, and the answers to all the problems in it boil down to two things...
1) Ignoring practical concerns, when you apply a voltage (V[SUB]be[/SUB]) to the gate of a BJT , it develops a current (I[SUB]be[/SUB]) through it. If you apply a current (I[SUB]be[/SUB]) through the base of a BJT, it will develop a voltage (V[SUB]be[/SUB]) across it. This is basic Ohm's law. From this, we can clearly know that there is really no such thing as "voltage control" or "current control" as they simply break down into each other in the end anyway. They are the same thing, and since they are, then obviously there can be no such thing as one being better than the other.
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