The Common Emitter Configuration.Īs well as being used as a semiconductor switch to turn load currents “ON” or “OFF” by controlling the Base signal to the transistor in either its saturation or cut-off regions, Bipolar NPN Transistors can also be used in its active region to produce a circuit which will amplify any small AC signal applied to its Base terminal with the Emitter grounded. Note that Beta has no units as it is a ratio.Īlso, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons diffusing across the junction). The value of β can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the bipolar NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Ic provides the output. The current in a bipolar NPN transistor is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, ( β ). 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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