put resistor at collector or emitter.
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Posted by Boki on April 19, 2006, 2:05 am
Please log in for more thread options Hi All, What's the different to put a resistor at collector or emitter? I am going to drive a LED, there is no RF issue, thanks. Best regards, Boki. | ||||||||||||||||
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Posted by KodKodKod Learning Consulting on April 19, 2006, 3:07 am
Please log in for more thread options http://www.kodkodkod.com/database.htm Hi, If you put resistor at emitter, the collector current will decrease tremendously because V_applied = V_BE + I_E * R since I_E is large, V_BE will be much smaller than V_applied and I_C is proportional to exp(V_BE) If it is at collector, it can hardly hurt your current because the transistor is in linaer mode already and only have the effect due to early voltage. KodKodKod Learning Consulting Visit our database and submit your questions! http://www.kodkodkod.com/database.htm | ||||||||||||||||
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Posted by Boki on April 19, 2006, 3:17 am
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KodKodKod Learning Consulting =E5=AF=AB=E9=81=93=EF=BC=9A so...should we drive LED in linear mode or saturation mode? does linear mode stable enough? | ||||||||||||||||
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Posted by Brian on April 19, 2006, 10:54 am
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> Hi All,
> > What's the different to put a resistor at collector or emitter? > > I am going to drive a LED, there is no RF issue, thanks. > > Best regards, > Boki. > If you have a lot of voltage to work with, it usually doesn't matter. If you don't have much voltage to work with, then put the resistor and LED in the collector side. Brian | ||||||||||||||||
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Posted by colin on April 19, 2006, 11:28 am
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> Hi All,
> > What's the different to put a resistor at collector or emitter? > > I am going to drive a LED, there is no RF issue, thanks. > > Best regards, > Boki. > well both can be used, but assuming the led is in the collector circuit :- with a collector resistor the transistor is driven hard (saturated) and the voltage accros the transistor is ignored, the voltage accros the resistor (wich determines the LED current) is the supply voltage minus the LED voltage. this is probably the easiest and most common way. with an emiter resistor the LED current can be controlled by the base voltage this is usfeul if you want to control it but otherwise the transistor can dispate quite a bit of power, also you need a larger base voltage. the voltage accros the emiter resistor is the base voltage -.7 wich determines the LED current. of course the max voltage available acros the emitter resistor is limited as above. in both cases if the transistor saturates you must limit the base current. Colin. | ||||||||||||||||
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>
> Hi,
>
> If you put resistor at emitter, the collector current will decrease
> tremendously because
>
> V_applied =3D V_BE + I_E * R
>
> since I_E is large, V_BE will be much smaller than V_applied and I_C is
> proportional to exp(V_BE)
>
> If it is at collector, it can hardly hurt your current because the
> transistor is in linaer mode already and only have the effect due to
> early voltage.
>
> KodKodKod Learning Consulting
> Visit our database and submit your questions!
> http://www.kodkodkod.com/database.htm