# How to Calculate Rπ (Rpi) of a Transistor?

**Rπ (Rpi)** is the internal resistance of a transistor from the base to the emitter. It is a needed calculation
when doing AC analysis of a
transistor circuit.

Its calculation is fairly easy and straightforward. The formula for calculating rπ is:

The current, IEQ, is obtained by doing DC analysis of the transistor. is the thermal voltage of a transistor and it's equal approximately to 25mV.

**Example**

Let's suppose a transistor has a β of 100 and a IEQ of 4ma. At room temperature, a transistor will have a thermal voltage of around 25mV.

So the calculation of rπ is as follows:

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