Timeline for Why is a BJT considered "current-controlled"?
Current License: CC BY-SA 3.0
21 events
| when toggle format | what | by | license | comment | |
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| Aug 22, 2016 at 4:40 | comment | added | jbord39 | @AnalogArsonist: That is a good point :) | |
| Aug 22, 2016 at 4:07 | comment | added | Analog Arsonist | @jbord39 I agree with your points but I do use them practically still.. its hard to find a 400V PMOS. | |
| Aug 21, 2016 at 17:11 | comment | added | jbord39 | @LvW: Yes, I do frequently. Do you ever doubt? And, you are missing my argument, time and time again. I do not misunderstand. I see your side of the argument, and can agree with most of it. I don't think repeating mine again will make a difference, but here it goes. The question is WHY not SHOULD. And the reason WHY is because in contrast to the MOSFET, which requires no dc gate bias current for conduction, the BJT does. My level of involvement with BJTS these days (again, who uses BJT's practically? All companies have switched to MOS) has nothing to do with that. | |
| Aug 21, 2016 at 16:31 | comment | added | LvW | jbord39, it ia absolutely no problem if you are engaged primarily on the MOS side. But - when this means (and as it seems - it is so) that you are not informed about the fact that two alternative explanations can be found in the literature, I would not take part in this discussion. May I quote R. Feynman: "Religion is a culture of faith; science is a culture of doubt.". Do you never doubt? | |
| Aug 21, 2016 at 15:41 | comment | added | jbord39 | @LvW: As I have said, and you have repeated back a few times, I do understand your point. But the irrefutable fact that a BJT without base current will not work, in comparison to a MOSFET which will. And again, the question is "why is it considered" not "should it be considered". How is this a hard distinction to see? You are arguing for nothing; the question is not related to your answers. WHY, not SHOULD. And I am not new in this business either, in fact I am still in it (admittedly on the MOS side since there is not much BJT work going on these days.) | |
| Aug 21, 2016 at 9:09 | comment | added | LvW | @jbord39, ...."as I see it". Correct - I cannot follow you. Can you follow me? No - it is not necessary (who am I?) but perhaps you will become thoughtful after gathering some more reliable information. For this purpose, I could recommend to you some knowledge sources: W. Shockleys patent, Horowitz/Hill (Art of Electronics), Stanford Univ., Berkeley Univ., Mass, Inst. of Techn., Barrie Gilbert - one of the leading semiconductor specialists (Analog Dev.) ,.... As I have told you already - I am not new in this business. | |
| Aug 20, 2016 at 17:40 | comment | added | jbord39 | @LvW: your argument could be more succinctly put: if alpha was ideal (1), then no base current would be required. But, since this is impossible (i have never seen this as anything more than a hopeful approximation during simple derivations), I don't think it really works when clarifying the distinction. Ignoring the base current, then, would hide many other BJT effects. I apologize for my ruder words. It is just frustrating that you cannot acknowledge the reality, as I see it (especially when comparing bjt to mosfet) | |
| Aug 20, 2016 at 17:27 | comment | added | jbord39 | @LvW: because your proofs come after initially stating what everyone here cares about. Current goes in the base. You said this yourself. IN a MOSFET this is not the case. Your "proofs" are meaningless, and just prove irrelevant things (which we all know!!). You spew techno-speak (basically all 1st+2nd order BJT effects) as if it is making your point; it is not. We all know base current is required. You are just making yourself look either stubborn, foolish, or (giving benefit of the doubt) misinterpreting what we are saying. | |
| Aug 20, 2016 at 16:27 | comment | added | LvW | My final comment: I am involved in this question (and I am forced to give answers) since more than 20 years. Each year some students are confused because of the two different explanations which exist for explaining how the BJT works. You can be sure, I have consulted many, many documents, I have evaluated a lot of observable effects and circuit properties - and I have MADE UP MY OWN MIND ! You can believe me I know what I am talking about. Surprisingly, nobody has asked for the proofs I have offered. | |
| Aug 20, 2016 at 16:12 | comment | added | jbord39 | @LvW: if you were somehow able to forward bias the b-e junction without any base current, the situation would quickly collapse. Whatever was creating the voltage would not be able to sustain itself, because such a voltage is necessarily created by additional charge carriers. As the current flowed, the charge carriers would eventually discharge until the the base-emitter junction found a balance. The only way this balance can be in a conductive state is with a constant flow of base current to replenish the carriers lost forward biasing the b-e junction. | |
| Aug 18, 2016 at 17:21 | comment | added | LvW | At first, I never have denied the existence of Ib - how could I? Secondly, it is not "required" to forward bias the pn junction - it is simply the result of voltage biasing. Only a VOLTAGE can work against the diffusion barrier. By the way - are you aware that you produce just assertions without any technical/physical justification? | |
| Aug 18, 2016 at 17:09 | comment | added | jbord39 | @LvW: Try using a BJT without base current. I find your argument ridiculous, because even you yourself acknowledge that Ib is required to forward bias the b-e diode. Without this input current, the transistor won't work, period. | |
| Aug 18, 2016 at 16:06 | comment | added | LvW | Quote: "Actually it is current controlled because physically the base current matters." I cannot resist to quote this sentence. Curd - seriously, is this really your argument for current-control? With other words: It matters because it matters! With all respect - perhaps you try to reconsider your understanding of transistors. | |
| Aug 18, 2016 at 5:25 | comment | added | wbeaty | > ...will characterize beta. Yes, they guarantee that the value for hfe falls somewhere between 80 and 300! | |
| Aug 17, 2016 at 20:51 | comment | added | vicatcu | ... pedandic ... | |
| Aug 17, 2016 at 19:29 | comment | added | Curd | @Kaz: I think it's wrong to say a BJT is not current controlled just because the base current can be expressed as a function of base-emitter voltage. Actually it is current controlled because physically the base current matters. Otherwise you could also say a BJT is temperature controlled instead of current controlled... | |
| Aug 17, 2016 at 18:13 | comment | added | Kaz | @vicatcu Devices can be characterized in any number of ways, including parameters that are fictitious, or functions of other, more primary parameters. | |
| Aug 17, 2016 at 17:42 | comment | added | horta | @vicatcu Beta is not only frequency dependent but bias and current dependent. It's a wild approximation to give component users a place to start from. | |
| Aug 17, 2016 at 17:27 | comment | added | LvW | Yes - beta is specified. So what ? From this fact, do you really derive that the BJT would controlled by the base current ? Or do you have some other arguments? I doubt. | |
| Aug 17, 2016 at 16:40 | comment | added | vicatcu | It's so useful an approximation that any BJT datasheet you will ever look at will characterize beta. | |
| Aug 17, 2016 at 16:33 | history | answered | Kaz | CC BY-SA 3.0 |