why common emitter configurations are used in the single stage amplifier design??? please explain me 
"Why are common emitter configurations used in the single stage amplifier design?"
The common emitter configuration creates an inverter with voltage gain. The npn transistor has current gain, but to get voltage gain, a load resistor forms an inverter. This is better than an emitter follower that is non-inverting.
In particular, the inverting common emitter allows the collector voltage to rise greatly when the base current goes from small to smaller. The base voltage does not constrain the output voltage in the way a non-inverting stage is constrained.
One alternative to the common emitter is the emitter follower configuration. This has current gain, but not voltage gain. The output voltage is attenuated from the input voltage, so the voltage gain is less than one. It is because of the non-inverting shape of the circuit. The input voltage pulls up the output voltage as a follower, so the output voltage is limited by the pn diode drop from the base to the emitter.
Even logic gates and differential amplifiers use these principals. A logic gate must have a gain over one. Even AND gates with positive inputs giving positive outputs use inverting stages, hidden from view. Those are common emitters circuits.
thank you very much!! it really helped me out