I understand the basic concept. A small change in the base current will lead to large change in the collector current and will lead to amplification. But the question is, the small change in base current is brought by input signal or the biasing? Also the coupling capacitors allow only the ac signal to be amplified, so whats the use of dc biasing? It will be wasted. Also whats the effect of ac changing its polarity on the transistor? Some doubts...But i will love to have them cleared. Thank you :] Edit: Small not large change in base current.