Bjt as amplifier

Take a look at that: http://everycircuit.com/circuit/5403841273528320

Plz help..why amplified signal is not going below zero

The type of amplifier you have here is for amplifying AC signals rather than for amplifying to obtain a switch (where you would want to amplfy a DC input). The AC signal that you have put in at the transistor base could be seen nicely if you get rid of all other oscilloscope traces which would mask that one. See there that the input signal is small as expected. Now get rid of that trace and look again at a trace only of your collector output.....the signal has indeed been amplified just as you wanted. The reason that those traces do not go below zero is because they are there without the DC component due to the capacitors. If you had not used capacitors the DC component would have been left in... which you do not want.

You might want to connect that collector V meter to the transistor emitter and drop the resistor value there down from 2k to 1k. That shoud help your oscilloscope view at zero line. A real oscilloscope has a knob for vertical offset.