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the main function of transistor as an amplifier , this be clear when you applied ac vlotage as an input of transistor .
To get this be clear pls refer to "Electronice Devices and Circuit Theory" for "Boylestad" in chapter 5.
The transistor used as an amplifier( and in the CE configuration), allows to change the "resistance" in two points (Colector - Emitter), in function to a small signal injected in the Base. In a way, this changes in the signal are "copied" to the resistance variations, and then we get a large signal with the same variations than the small (The name Transistor comes from Tranfer Resistor, because of this effect)