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Npn transistor base collector emitter6/24/2023 ![]() ![]() 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 NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Ic provides the output. The transistor current in an 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. We know 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". 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. So for an NPN transistor to conduct the Collector is always more positive with respect to both the Base and the Emitter. Also the Collector supply voltage is positive with respect to the Emitter ( V CE ). The voltage between the Base and Emitter ( V BE), is positive at the Base and negative at the Emitter because for an NPN transistor, the Base terminal is always positive with respect to the Emitter. ![]() The construction and terminal voltages for an NPN transistor are shown above. (Note: Arrow defines the emitter and conventional current flow, "out" for an NPN transistor.) In this tutorial we will look more closely at the "Common Emitter" configuration using NPN Transistors with an example of the construction of a NPN transistor along with the transistors current flow characteristics is given below. We also learnt that the transistor junctions can be biased in one of three different ways - Common Base, Common Emitterand Common Collector. An NPN ( Negative- Positive- Negative) type and a PNP ( Positive- Negative- Positive) type, with the most commonly used transistor type being the NPN Transistor. In the previous tutorial we saw that the standard Bipolar Transistoror BJT, comes in two basic forms. Round Shell Connector/Aviation Connector. ![]()
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