Bipolar Junction Transistor |
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GENERAL CONCEPTS The BJT are electronic components with three pins used for the elaboration and amplification of the electrical signals. Externally they appear in different forms and with metal, plastic package. The internal composition is based on the same physical principle: three peace of P or N semiconductor assembled together. The sequence can be P-N-P or N-P-N. In the left part of the figure 1 is shown the PNP transistor.
PNP
NPN
c
b
e
b c e c
b
e c- collector Fig. 1 - The schematic composition of the PNP and NPN transistor as the assemble of three peace of crystal. The semiconductor is silicon doped with special elements. The NPN transistor is made as it shown in the right part of the figure 1. The pin is a metallic conductor linked to the semiconductor parts of the transistor. These conductors are sighed with three alphabetic letters "e", "b", "c", which are the initial letters of the words "emitter", "base", "collector", with the following correspondence:
The internal crystalline composition of the transistor is called "chip". The transistor can be considered as particular diode where there are three alternative semiconductor parts. The symbols used for the PNP or NPN transistor are shown in the left part of the figure 2. In the right part of the figure 21 the packages, used for the most use transistor, are shown. .
Fig. 2 -The electrical symbols of the PNP and NPN . The difference is the position of the arrow in the emitter pin. The in the right part the packages of the transistor, The transistor NPN are more used than the PNP. |
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