Timeline for BJT Constant Current, Resistor Equations
Current License: CC BY-SA 4.0
8 events
| when toggle format | what | by | license | comment | |
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| Jul 9, 2018 at 17:58 | comment | added | Olin Lathrop | It seems what you are missing is that the problem is when the collector is left open. Then there is no gain to do the heavy lifting to provide the current for R1. The load on the digital output is then out of spec. | |
| Jul 7, 2018 at 17:36 | comment | added | vicatcu | To be clear, though, it depends if the transistor is being operated in Saturation or Active regions which set of equations are in play. What I described assumes the transistor is in saturation, which may have been a bad assumption. | |
| Jul 7, 2018 at 9:41 | comment | added | LvW | More than that, as far as I understand the question, it is the control signal via R2 only that is to be used for changing the DC conditions. | |
| Jul 7, 2018 at 9:37 | comment | added | LvW | No doubt about it. The control voltage Vbe depends on R1 as well as R2. But your statement was "I don't think R2 matters at all". | |
| Jul 6, 2018 at 14:16 | comment | added | vicatcu | @LvW ok, lets make this a side-band teachable moment then. My recollection was that Ic = beta * Ib, and that V_be was basically 0.7V fixed (to a first order), but I suppose those relation depends on a circuit topology that I'm forgetting or that is not applicable here? Regarding R2, what I meant was that it doesn't really accomplish anything that simply changing the value of R1 alone wouldn't also accomplish? | |
| Jul 6, 2018 at 8:51 | comment | added | LvW | Of course, R2 matters! The input resistance rin of the BJT at the base node is not infinite . That means: The control voltage CTRL drives a current through a voltage divider R2-rin. And it is the VOLTAGE across the B-E path which determines the collector ciurrent (and NOT the base current). | |
| Jul 6, 2018 at 2:04 | history | edited | vicatcu | CC BY-SA 4.0 | added 55 characters in body |
| Jul 6, 2018 at 1:56 | history | answered | vicatcu | CC BY-SA 4.0 |