The npn transistor is said to function as an amplifier only in its active state (emitter base is forward biased and collector base is reverse biased). It is considered that majority of the electrons reaching the base layer is dragged into the collector layer. So the emitter current (ec) is approximately equal to the collector current (cc). The reason for amplification is given by the equation P(input) = I^2 (ec) * R(emitter base) P(output) = I^2(cc) * R(collector base) Since R(collector base) >>> R(emitter base) & I(ec) = I(cc), P(output) > P(input)... [because the collector base junction is reverse biased and it offers high resistance to the flow of majority charge carriers across the junction] My question:- The resistance offered by the collector base junction is only to the flow of electron holes from base (p type) to collector (n type) or the flow of electrons from collector (n type) to base (p type). So why are we considering the resistance in output power to be R(collector base)? This resistance does not oppose the motion of electrons from the base to the collector right??