Re: How does a CE mode single stage transistor used as an amplifier ?

Excellent question!

The answer lies in the fact that the transistor used in the single-stage 
CE amplifier is *biased*; that is, DC levels are established (using 
resistors) so that the base-emitter junction is always forward biased and 
the collector-base junction is always reverse biased.  The AC signal is 
then capacitively coupled into the base.  This causes a large AC current 
to flow from the collector to the emitter in accordance with basic bipolar 
transistor theory and the output is taken from the collector.  The AC gain 
is given by the formula

                 Av = KRc ; where K is a constant (easily determined)
                            and Rc is the value of the collector resistor.

Hope that this answers your question!