Timeline for Constant Current Source BJT
Current License: CC BY-SA 4.0
16 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Mar 18, 2021 at 6:00 | history | tweeted | twitter.com/StackElectronix/status/1372427563428110336 | ||
| Mar 17, 2021 at 15:16 | history | edited | Null♦ | CC BY-SA 4.0 | added 58 characters in body; edited tags; edited title |
| May 29, 2019 at 22:06 | vote | accept | William Reed | ||
| May 29, 2019 at 22:01 | answer | added | Spehro 'speff' Pefhany | timeline score: 1 | |
| May 29, 2019 at 21:55 | answer | added | jonk | timeline score: 0 | |
| May 29, 2019 at 20:14 | comment | added | William Reed | @jonk TYVM! This has helped a lot. | |
| May 29, 2019 at 20:10 | comment | added | jonk | @WARmachin3 The BJT collector will "act like" a current source that simply passes along the emitter current until \$\left[V_\text{C}=V_\text{CC}-I_\text{L}\cdot R_\text{L}\right]\approx V_\text{IN}\$. At that point the BJT just barely begins to move from active mode gradually into increasingly deeper saturation as the collector voltage declines a little more. It's not until \$V_\text{C}\$ forward-biases by about \$500\:\text{mV}\$ or so, that it nears fuller saturation and then the collector acts like a voltage source, instead, and the load current mostly stops changing. | |
| May 29, 2019 at 20:03 | comment | added | William Reed | @jonk OMG that makes so much since. So when I_L*R_L = V_in is when I start getting the non linear drop in current flow? | |
| May 29, 2019 at 19:58 | comment | added | jonk | @WARmachin3 Basically, this is just an emitter follower circuit. The emitter voltage will follow the base voltage (\$V_\text{IN}\$), less a \$V_\text{BE}\$ drop. That voltage, combined with \$R_\text{E}\$, determines the emitter current. This emitter current, less any base current required for recombination, is the collector current. That collector current causes a voltage drop on \$R_\text{C}\$. You have to subtract that voltage drop from the supply voltage rail. It stays in active mode until the collector voltage reaches the base voltage, \$V_\text{IN}\$. (Or less, if your text allows.) | |
| May 29, 2019 at 19:55 | comment | added | William Reed | The \Beta value given is 200 so, this lab book is seriously vague on some topics | |
| May 29, 2019 at 19:54 | comment | added | William Reed | I know the lab book says 100mV but doesn't say the resistor value. And this is where our text does a horrible job. It asks what the highest value of R_L can be before the basic equation is no longer valid, and I assume this is where the curve in the plot is non-linear. I just don't have any clue how to get there mathematically | |
| May 29, 2019 at 19:53 | comment | added | jonk | When they write "assume \$\beta\$ is a high value" do they want you to assume it is \$\beta=\infty\$? | |
| May 29, 2019 at 19:33 | comment | added | G36 | Do you know what saturation is? And for what Rc value your BJT's will start to enter into saturation region? | |
| May 29, 2019 at 19:31 | history | edited | Voltage Spike♦ | CC BY-SA 4.0 | added 15 characters in body |
| May 29, 2019 at 19:25 | review | First posts | |||
| May 29, 2019 at 19:32 | |||||
| May 29, 2019 at 19:23 | history | asked | William Reed | CC BY-SA 4.0 |