Timeline for Why is a BJT considered "current-controlled"?
Current License: CC BY-SA 4.0
22 events
| when toggle format | what | by | license | comment | |
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| Feb 25, 2023 at 2:27 | history | edited | Sredni Vashtar | CC BY-SA 4.0 |
formatting
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| Feb 24, 2023 at 23:31 | history | edited | Sredni Vashtar | CC BY-SA 4.0 |
added 14 characters in body
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| Feb 24, 2023 at 23:13 | history | edited | Sredni Vashtar | CC BY-SA 4.0 |
Reorganized text, added actual simulated results, solar cell example, links to other answers
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| Aug 21, 2016 at 17:07 | comment | added | Sredni Vashtar | @LvW what you did is technically called "mutatio controversiae". It is a well known technique. I suggest you re-read my post with more attention, especially the quote from Munari. BTW, regarding the circuit in the question (not another one, the one in the question), you still have not said what values of vbe you would set to produce 10 e 15 mA in collector current (and how do you plan to set them). Why is that? | |
| Aug 21, 2016 at 16:44 | comment | added | LvW | @Sredni Vashtar, I have asked you two simple technical questions (about VB and RE). Unfortunately, you did not give any answer. And the reader will ask himself - why not? | |
| Aug 21, 2016 at 14:03 | comment | added | Sredni Vashtar | @LvW the above circuit is what is used to exemplify transistor action. I can use the mean value between the extreme values of ib I want to set. The point is that in case of current control (the meaning of which you have yet to grasp) this error becomes irrelevant. And that is why you choose to set the current in base. | |
| Aug 21, 2016 at 13:59 | comment | added | LvW | @Sredni Vashtar, may I ask you one single question? In case you ever have designed BJT based amplifier stages, did you consider an emitter resistor RE? In case the answer is "yes" - can you explain why? | |
| Aug 21, 2016 at 13:54 | comment | added | LvW | @Sredni Vashtar, so what would YOU do with the given circuit? Did you not recognize that I never would use this circuit? But what shall I answer if you force me to use (or to design) the given bad circuit? Again: Which value for VB would YOU use for finding the value of the resistor R=(Vin-VB)/IB ? | |
| Aug 21, 2016 at 13:41 | comment | added | Sredni Vashtar | @LvW "using a guess value around 0.7 volts" is no way to set a control variable. With current control (I hope you now realize what that means: setting the values of ib so that you can get desired values of ic) you can use a guessed value of 0.7 since the error in ib (and then in ic) will be negligible due to the steep characteristic. With voltage control the guess in vbe would translate in a guess in ib, and then a guess in ic. Intolerable, see the numbers. | |
| Aug 21, 2016 at 13:22 | comment | added | LvW | @Sredni Vashtar, perhaps there was a misunderstanding. It was not clear to me if you are speaking about the shown circuit only (because you earlier have mentioned the Ebers-Moll equation and corresponding invers functions). I suppose, there is nobody with some practical experiences who would use the shown circuit as an amplifier. In this circuit, I have no other alternative to use a guess value around 0.7 volts. At the same time, we must guess the value for Ib (very large tolerance values). So - both variables (guesses) are used for finding the R value. What does this proove? Nothing!. | |
| Aug 21, 2016 at 13:11 | comment | added | Sredni Vashtar | @LvW the circuit shown is NOT using emitter degeneration. I repeat my question: how do you propose to set the correct values of vbe (your controlling variable)? | |
| Aug 21, 2016 at 8:57 | comment | added | LvW | @Sredni Vashtar, my answer to your question: It is common practice to use emitter degeneration, which allows us to use guess value of app. Vbe=0.65...0.7V. More than that, in my answer to another question (electronics.stackexchange.com/questions/253115/…) I have shown that - for a proper design - the actual value for Ib plays no important role (we can assume Ib=0). The core of your long contribution is: From the practical point of view, it is easier to rely on current control. But physical laws do not depend on our desires. | |
| Aug 21, 2016 at 1:07 | history | edited | Sredni Vashtar | CC BY-SA 3.0 |
added reference to previously used values for vbe
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| Aug 20, 2016 at 23:59 | history | edited | Sredni Vashtar | CC BY-SA 3.0 |
added 7 characters in body
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| Aug 20, 2016 at 23:50 | comment | added | Sredni Vashtar | @LvW the input circuit of the devices comprises vbe and ib. I chose to set the value of current ib (my controlling variable). How do you propose to set the correct values of vbe (your controlling variable)? | |
| Aug 20, 2016 at 23:41 | history | edited | Sredni Vashtar | CC BY-SA 3.0 |
Put some nice little pictures. Some have even color.
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| Aug 20, 2016 at 17:16 | comment | added | jbord39 | I like this comment: The dependence on Vbe shown in the Ebers-Moll model is not implying a cause-effect relationship. It's just simpler to write the equations in that way. Nobody forbids you from using inverse functions | |
| Aug 19, 2016 at 7:09 | comment | added | LvW | It seems you have overlooked my mentioning of emitter degeneration. More than that, did I spoke about supplying a "pure voltage" to the base? You should try to be fair. As you have mentioned the Early effect. Are you aware that the explanation of this effect prooves voltage-control? Have you ever heard about the tempco -2mV/K ? Have you ever thought about the meaning of this value? | |
| Aug 18, 2016 at 20:34 | comment | added | Sredni Vashtar | I made up those values to exemplify the difference between trying to set veeeeeery veeeeeeery close values of Vbe and discernible values of Ib (I also added in the edit comment that I wanted to make those values more extreme). I did not want to waste time to find plausible values, but later for those who do not have enough mental flexibility, I will add a picture or two. As I wrote above: try to control the BJT by removing Rb and by supplying a pure voltage to Vbe. Good luck. (Oh, by the way: the simplified model cannot be used to explain the Early voltage, too.). | |
| Aug 18, 2016 at 14:59 | comment | added | LvW | In your Note, the first sentence is simply false! The Ebers-Moll model does not "imply" something - instead, it is in fact a cause-effect relationship. Please consult W. Shockleys patent document. You are right, you can always create inverse functions (on paper) - so what? Do you think you can interchange cause and effect on paper? By the way: Did you ever design transistor stages (because you are mentioning some funny Vbe voltages). Are you familiar with emitter degeneration (current-controlled VOLTAGE feedback) ? | |
| Aug 18, 2016 at 4:03 | history | edited | Sredni Vashtar | CC BY-SA 3.0 |
expanded the concept. made the values more extreme
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| Aug 17, 2016 at 22:14 | history | answered | Sredni Vashtar | CC BY-SA 3.0 |