One of the functions of a resistor is in biasing the base of a transistor. A transistor basically needs a small base
voltage (>0.6) to make a large voltage flow through its
collector/emitter terminals. But the base of a transistor is
quite vulnerable to high currents, so a resistor is incorporated here to limit the current and provide a safe biasing voltage.
The value of the base resistor of a transistor may be
calculated through the below given formula:
R = (V – 0.6).Hfe / I,
- V = source voltage to the base resistor
I = thecollector load current
Hfe = forward gain of a transistor(150 nominal) and 0.6 = minimum transistor biasing voltage.
These resistors are generally of low values so that the over all performance of the circuit is not affected.