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The answer to this question could be very long - or very short.
I´ll try a short one: There is no principal difference.
The main purpose of a voltage amplifier is to amplify with good signal quality (quasi-linear amplification) - mainly without regard to the amount of current which can be delivered at the output.
The main task of a power amp is to amplify an input voltage and to provide an output which also consists of a voltage - but at the same time with a large current capability (low output resistance). Moreover, signal quality is not as critical as for the voltage amplifier and , therefore, the amplitudes along the characteristic curves are larger resulting in a larger THD. There is a quality figure for each power amp called "efficiency" which is the ratio of the input to the output power. As part of the input power is the bias current-voltage product most power amps consists of two transistors working alternating and with (nearly) zero bias.
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?
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.