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The series resistor between our control source and the base limits current into the base. The base-emitter node can get its happy voltage drop of 0.6V, and the resistor can drop the remaining voltage. The value of the resistor, and voltage across it, will set the current.
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The resistor needs to be large enough to effectively limit the current, but small enough to feed the base enough current. 1mA to 10mA will usually be enough, but check your transistor’s datasheet to make sure.
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