However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input. Then we can see that the transistor is a current operated device (Beta model) and that a large current ( Ic ) flows freely through the device between the collector and the emitter terminals when the transistor is switched “fully-ON”. This link between the input and output circuits is the main feature of transistor action because the transistors amplifying properties come from the consequent control which the Base exerts upon the Collector to Emitter current.
So in a NPN Transistor it is the movement of negative current carriers (electrons) through the Base region that constitutes transistor action, since these mobile electrons provide the link between the Collector and Emitter circuits. The Base supply voltage V B is connected to the Base resistor R B, which again is used to limit the maximum Base current. The Collector is connected to the supply voltage V CC via the load resistor, RL which also acts to limit the maximum current flowing through the device.
Then the voltage sources are connected to an NPN transistor as shown.
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