It helps to understand the meaning of the terms you are working with here. Anything that operates as an amplifier means it takes the current coming in and steps it up in some form or fashion, so that the outgoing current is higher than what came in. Some transistors have wires wrapped a coil with a certain number of turns, and then the outgoing wire has twice the number of turns, which produces a current approximately twice the power of the incoming current. Sometimes there is a need to step up the power supply or step down the power supply. Anything that serves as an amplifier increases the amplitude of the waves of the frequency that is being transmitted, giving the frequency more power, more energy. So transistors provide a link in this needs category, to step up the power being supplied to the device from the standard voltage coming out of the electic outlets in the home.