Overview
We will calculate the value of Rb based on the following circuit:
Calculating the Rb Value
Precondition | Value |
---|---|
Vcc | 12v |
Vi | 3.3v |
RL | 155Ω |
NPN BC337 | |
Vbe(sat) | 1.2v |
Vce(sat) | 0.7v |
hFE (@100mA) | 100 |
Calculations for Ic:
Ic = (Vcc-Vce)/RL = 11.3v/155Ω = 73 mA
Calculation for Vb
Vb = Vi - Vbe(sat) = 3.3 - 1.2 = 2.1v
Calculating using Ib=Ic/10 (*** Best Approach)
It is common to assume that base current is 1/10 of the collector current.
Ib = Ic/10 = 73/10 = 7.3mA
Rb = Vb/Ib = 2.1/0.0073 = 288 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 = 73/100 = 0.73mA
Rb = Vb/Ibx3 = 2.1/(0.00073x3) = 959 Ω
This would be the max value for Rb.
Calculating using the online calculator
hFE=100
Rb = 960 Ω
This would be the max value for Rb.
Formulas
Ic=(V-Vce)/RL
Ib=Ic/hFE
Rb=(Vi-Vbe)/(3*Ib)