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/RL = 12v/155Ω = 78mA
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 = 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
This would be the max value for Rb.
Calculating using the online calculator
See reference
hFE=100
Rb = 960 Ω
This would be the max value for Rb.
Why are these all so different??? I gues it would work from 200 to 1KΩ.