This blog post is about determining the value of the base resistor of a MMBT3904L, which is ON Semiconductor's variant of the general purpose 2N3904 NPN Bipolar Junction Transistor (BJT), when used as a switch. The value is determined when the transistor is in the common emitter configuration. Sounds like a simple task and so it should be too. However, as we have commented many a time in our blog posts tasks that appear to be easy never are. Hence, while undertaking this task we decided to follow Lao Tzu's (Who? - Ed) advice, which he elegantly formulated in the following quote:
Do the difficult things while they are easy and do the great things while they are small. A journey of a thousand miles must begin with a single step - Lao Tzu.
So, as he has suggested the sensible approach in determining the value of the resistance of the base resistor of a BJT is to perform the calculation a step at a time, as we demonstrate below.