The transfer characteristic has the form shown :
The Active Region of the transfer characteristic is used for amplification because in this region, Ic increases almost linearly with increase of V, The circuit diagram of the base biased transistor amplifier, in CE configuration, is shown below.:
Working: The sinusoidal voltage, superimposed on the d.c. base bias, causes the base current to have sinusoidal variations.
As a result the collector current, also has similar sinusoidal variations present in it. The output, between the collector and the ground, is an amplified version of the input sinusoidal voltage. (Also accept 'other forms' for explanation of 'working')