correctsay Vcc=5, and let hfe be between 100 and 200, and max collector current be 100mA (values from datasheet)
correctIf Rc=0, then device will work provided that Rb such that Ib<=0.5mA, that is Ic<=100mA.
correctIf Rb=0 then there's no way to make the device work because the very large Vbe will kill it regardless of the value of Rc.
the emitter current when you use an emitter resistor is (Vb-Ve-0.7)/Re, the collector current will be Iemitter-Ibase.If you put it in common collector, thus remove Rc and add Re on emitter, then:
If Rb=0, device will work with Re>=43ohm so that emitter current<=100mA (or 101mA if that made a difference...)
when Re=0 and Rb=2k the base current is (Vb-Ve-0.7)/Rb , assuming 5v Vb you get about 2.15mA, not 0.5mAIf Re=0, then device will work for such that 200*Ib<=100mA, that is Ib<=0.5mA hence Rb>=2K.
Be careful relying on base current multiplied by hfe to limit collector current, assuming that is what you are trying to do. hfe is very variable and cannot be assumed to be any specific value, simply a range. Also, watch out for power dissipation - that is an equally likely reason to kill the transistor.
Keith
when Re=0 and Rb=2k the base current is (Vb-Ve-0.7)/Rb , assuming 5v Vb you get about 2.15mA, not 0.5mA
thanks keith!
So does my resistor on the emitter limit both collector current and base current?
thanks
Davide
When you have an emitter resistor you don't care about that (assuming that your source can provide the base current), the base current will be as much as needed to give the collector current set by the emitter resistor.
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