The invention of the transistor in the 1950’s changes the electronic world for ever. The transistor is a so called ‘semiconductor’ and is capable of amplifying signals. The transistor has a Collector (C), a base (B) and an emitter (E). It can be seen as a current controlled switch. A small current from the base to the emitter creates a bigger current (Ic) from the collector to the emitter. Take a look at this video-page, for a detailed explanation.
Imagine the transistoras the following setting: a large tube connected to a big pool of water. Halfway the tube there is a valve that can be changed position by a small tap on the side. Check figure 1.
If there’s no base current (ib=0) the transistor is closed and there’s no collector-current. If the base current Ib is increasing (we turn the small tap on the side – Ib > 0), the water-flow (=current or Ic) will increase as well. Check figure 2. If the Ib is maximum, the collector current Ic is at its maximum as well. The proportion between Ib and Ic is called the Hfe. This number represents the amount of current amplification.