Common-emitter amplifier produces the largest power gain of all the transistor amplifier configurations. Let us consider a NPN transistor for CB amplifier. The voltage is applied to NPN transistor in such a way that emitter-base PN junction is forward-biased by Vee, and the collector-base diode is reverse-biased by Vcc. The capacitor C3 connects the base to ground, bypassing AC current away from resistor.
Therefore both input (base) and output (emitter) are connected to the base (Common base amplifier).
This kind of arrangement ensures Current gain Ic/Ie is less than 1.
The voltage gain is high as the same current flowing through the resistors Rc and Re.
The input impedance is low (approx. 20 ohms) while the output impedance is high,(approx. 1Megohm).
The output signal is not inverted with respect the input signal.
This circuit is often used to match low impedance devices to high impedance ones.