How to Calculate the Base Current, IB, of a Transistor

The base current, IB, of a transistor is a crucial current of a bipolar junction transistor. Without this base current, the transistor can't turn on.

There are several ways to find the base current, IB, of a transistor. And it all depends on what information is already known about the transistor:

DC Analysis

Doing DC analysis of the transistor circuit is the most common way of finding out the value of IB in the circuit.

The equation to solve for Ie is:

So we must solve for Vbb and RB in order to solve for IB.

Next we compute the value of RB:

Now we can calculate the value of the base current, IB, in the circuit:

Example

Using the example of the values for the resistors and the voltage, Vcc, above, we're going to solve for Ieq.

Using Known Values
If the emitter current, Ie, and β are known for the transistor circuit, IB can be calculated by the formula:

Example
If Ie=6ma and β=99, then

Using Known Values
If the emitter current, Ie, and the collector current, Ic, are known, IB can be calculated by the formula:

Example
If Ie=4ma and Ic=3.96ma, then IB calculates out to be:

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