| Electronics > Beginners |
| Transistor Current source |
| (1/2) > >> |
| gogoman:
Hi I'm a bit confused. the current source has a emitter resistor Re, If Rc limits the transistor current Rc~= Re, why is Re required? If Re =1ohm and Rc = 10K, Rc dictates the current Ic. thanks |
| exe:
This doesn't look like current source to me. Anyway, Re is "emmiter degeneration resisor", it is used to provide some negative feedback to stabilize bjt and make the circuit less sensitive to temperature and gain : https://en.wikipedia.org/wiki/Common_emitter#Emitter_degeneration |
| soldar:
Re limits base current and, in fact, fixes collector current. Just analyze the model. |
| rstofer:
Consider a fixed voltage on the base. This will try to hold the emitter at 0.7V (give or take) below the base and the emitter resistor times the current flow creates that emitter voltage. If too much current tries to flow, the base-emitter voltage drops (because the emitter voltage rises and the base voltage is fixed) and the transistor shuts down a little bit. If too little current is flowing, the base-emitter voltage increases and the transistor turns on a little harder. |
| mikerj:
--- Quote from: gogoman on April 18, 2019, 02:27:57 pm ---Hi I'm a bit confused. the current source has a emitter resistor Re, If Rc limits the transistor current Rc~= Re, why is Re required? If Re =1ohm and Rc = 10K, Rc dictates the current Ic. thanks --- End quote --- You are looking at this the wrong way, the transistor is not operating as a saturated switch so Rc does NOT limit the collector current. Collector current is determined by setting the emitter current Ic ~ Ie = (Vb-Vbe)/Re. Since some base current has to flow through the emitter, the collector current is a little lower but for a high gain transistor it's close. |
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