Operating a transistor requires that a voltage be applied to its base. However, a resistor must always be connected between the applied voltage and the base, because the current through the baseemitter junction of the transistor (a diode) should not exceed the maximum allowed base current. If it does, the transistor will be destroyed immediately or have a very limited lifespan. Making sure that the base current does not exceed the manufacturer's maximum base current specification requires that you calculate a value for the base resistor. This requires that you know the manufacture's maximum base current specification.

Obtain the base current specification from the transistor data sheet from the manufacturer. For this example use a maximum base current of 7 amperes given in the data sheet for the STMicrolectronics 2N3055 silicon epitaxialbase planar NPN power transistor (see Resources).

Find out the maximum base voltage that will be applied to the base of the transistor and the minimum voltage that will be applied to the emitter of the transistor for your specific transistor circuit configuration. For this example, use a maximum base voltage of 20 volts and a minimum emitter voltage of 6 volts.

Subtract the maximum base voltage from the minimum emitter voltage. Call this voltage the base resistor to emitter voltage. For this example, calculate that this voltage is equal to 14 volts since 20 minus six is 14.

Divide the "base resistor to emitter voltage" by the maximum base current specification. For this example, divide 14 volts by seven amperes to obtain two ohms for the minimum base resistor needed, since voltage divided by current is equal to resistance in ohms.

Double the value of the base resistor calculated to compensate for variations in the transistor specifications that are a result of manufacturing process variations. Use a base resistor of four ohms for this power transistor example, since two times two is four.
Tips & Warnings
 Apply transistor circuit analysis methods for different types of transistor circuit configurations. If your transistor circuit contains a resistor in the emitter, the sum of the base resistor and the emitter resistor should add to the value obtained when you divide the "base resistor to emitter voltage" by the maximum base current specification. This means, that in this case, you can safely use a lower value for the base resistor.
 Improper use of electronic equipment and components can result in fire, serious injury or death. Always work under the supervision of a safety certified electronic technician or electronic engineer. Obtain a electronic safety certificate before you work with electronic equipment and components.
References
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