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Calculating your Rb Value

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Ic = Vcc/RL = 12v/155Ω = 78mA

Vb = 3.3v - Vbe(sat) = 3.3 - 1.2 = 2.1v

Calculating using Ib=Ic/10

It is common to assume that base current is 1/10 of the collector current.

Ib = Ic/10 = 78/10 = 7.8mA

Rb = Vb/Ib = 2.1/0.0078 = 269 Ω

Tested with 220 Ω and it worked.

Calculating using hFE

To guarantee that the transistor operates in the saturation region, we multiply the base current by a factor of three.

hFE=100 (@100mA)

Ib = Ic/Hfe = 78/100 = 0.78mA

Rb = Vb/Ibx3 = 2.1/(0.00078x3) =  897 Ω

Calculating using the online calculator

See reference

hFE=100

Rb = 960 Ω 

Why are these all so different??? I gues it would work from 200 to 1KΩ.

References

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