Hi,
First you should realize the signal and power amplifiers. there after you can analyze the operation of varities of amplifiers.
there after you are going to consider several parameters such as 3db response/Frequency response?
For example:
In PA system mic connected to amplifier and the output connected to very high power rating speakers.
From mic you can get the signal potential upto few mv and few micro amp current. Now we are going to use signal amplifier to amplify such a week signal and increase its potential(V) enormously not power(P=V*I). its not able to drive the speakers due to inadequate power.
Now we are in need of power amplifier to increase the signal power. actually its not greately change the voltage level but very much increase the driving current.thats why its power amplifier.
Technically the hfe for signal amplifier is very high comparing with power amplifier.
Bye.