We already know that transistors are made for this job. Take a look at the picture below.

RL is an 8Ω loudspeaker. V_BE does not depend on the AC input voltage; it's always about 0.6V. This means that for AC voltages v_E = v_B => v_BE = 0 => v_RL = v_IN. Since the AC emitter voltage 'follows' the base voltage, this kind of amplifier is also called an 'emitter follower'. The picture next to the schematic above shows the output signal. As you can see, this amplifier works only for the positive half period of the input signal. During the negative half period, the base voltage drops below 0V, and T1 can be considered as an open switch. If we replace T1 with a PNP transistor (and connect its collector to -15V), the amplifier works only for the negative half period of the input signal. So we must combine these two.