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Bipolar transistors are amplifying devices and can also be used as switches. There are two types: NPN and PNP. See the picture below for a typical circuit.
A bipolar transistor has three terminals: Base, Collector and Emitter. In case of a NPN transistor, a small current flows from B to E (IB) causes a larger current flow from C to E (IC). The ratio IC/IB is called the current gain, symbol hFE.
Inside a transistor, there's a diode between B and E and between B and C, so VBE,max and VBC,max are about 0.6V to 0.7V.
Let's assume for example RB = 1M, RL = 1k, VS = 9V, hFE = 300 and VBE = 0.6V.
The voltage across RB will be VS-VBE=8.4V, so IB=8.4/1M=8.4μA. IC=IB∙hFE=8.4μA∙300=2.52mA. So the voltage across RL will be 2.52V.