Signal amplification using a transistor

[Resistanceisfutile]

so I just had a couple of questions about signal amplification using transistors e.g. CE amplier.
If I apply a positive voltage to the base and the nessecary power to the common, the emitter produces an amplified signal proportional to the base in current.
1. How does this affect voltage?
2. If I apply an AC signal to the base, what happens t the negative component, does this get increased in current too? and if so, do you use a resistor to shift the signal back so that the signal rests at zero?
Feel free to go into a huge amount of detail so I learn more.

 

[LvW]

Feel free to go into a huge amount of detail so I learn more.

The basic principle is as follows:
A DC bias voltage between B and E of app Vbe=0.65...0.7 volts allows a DC current Ic between C and E (provided that there is DC voltage of at least -theoretical- V+= 2 volts between C and E; in practice: V+=5...10 volts).
Now - a small signal voltage Vin applied to the base causes a variation Delta-Ic around the quiescent dc current Ic. This is possible, of course in both directions.
A collector resistance Rc (between C and the pos. supply) allows to transfer the current variation into a voltage variuation Vout. The ratio Vout/Vin is the voltage gain.
In most cases, the dc bias Vbe at the base node is produced using a resistive voltage divider between DC supply V+ and ground.
(The above explantion applies to an npn transistor in common-emitter configuration).