Sir ,in the given picture what would be the complete path of current,mean it will start from base to emitter,but then what happen when it'll reach the negative terminal of vcc. .will it not repel,because in npn there is a flow of electronic current?
There was no picture so I am guessing a conventional common emitter amplifier configuration.
In any even, a npn transistor circuit has positive battery feeding the top end of the circuit. Whereas an pnp requires a negative power supply voltage at the top of the circuit.
The basic idea is to have voltage gain from input (at the base) to the output (at collector) wherein the output of an ac signal is 180 degrees out of phase with the input; this because large going input signal causes transistor to conduct or increase current which drives voltage downward with increasing collector current.
The current through the collector is beta times the current being fed into the base; that is why it is an amplifier.
If you need more, let me know.