Transistor and circuit applications

Transistor and circuit applications - Transistors and...

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Transistors and Circuit Applications Sattar Hussain sattar.hussain@ryerson.ca 1
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1- Transistor Transistor is a 3-terminal device mainly used in signal amplification and switching circuits. Transistors are broadly classified to Bipolar Junction Transistors (BJT) and Field Effect transistors (FET) 2- Bipolar Junction transistor (BJT) BJT consists of three semiconductor sections denoted as emitter (E) , base (B) , and collector (C) . There are two types of BJT, npn and pnp 2
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n p n C B E p n p B E C C B E = C B E = The arrow in the symbol identifies the emitter and shows the direction of the current flow The transistor may be considered as two back-to-back pn junctions In the following the npn type will be considered for circuit analysis The base-emitter (BE) junction acts as a diode when it is forward-biased . With the collector open, the charge carriers give rise to a net flow from the base to the emitter. 3
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When the base-collector (BC) junction is reverse- biased , the electrons emitted by the emitter when the BE junction forward-biased reach the narrow base region and after a few lost to recombination process in the base, most of the electrons are collected by the collector. The results is a net flow of current from the collector to the emitter beside the hole current from the base to the emitter. The electron current flowing into the collector through the base is much larger than that flows into the base from the external circuit. Applying KCL: B C E I I I + = 4
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The most important property of BJT is that the small base current controls the amounts of the much larger collector current, or Where β is a current amplification factor (current gain) depends on BJT physical properties. 20 β ≤ 200 3- npn Characteristics The BJT has three different terminal currents, i E , i C , and i B , as well as three voltage drops, v BE , v CE , and v CB . C B V CC V BB R B i C i B i E v CE v BE + - + - E + - + - B C I I β = 5
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There are 6 sets of characteristics that describes the interdependencies of terminal currents and voltages drops, but the characteristics that describe i B vs. v BE and i C vs. v CE are of particular interest for BJT applications.
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This note was uploaded on 02/20/2011 for the course EES 612 taught by Professor Karim during the Fall '10 term at Ryerson.

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Transistor and circuit applications - Transistors and...

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