Lecture5sankary

Lecture5sankary - Page 1 ECED3201 Introduction to...

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Unformatted text preview: Page 1 ECED3201- Introduction to Electronics Prof. Kamal El-Sankary, Ph.D [email protected] Introduction to Electronics Page 2 Part II: Bipolar Junction Transistors (BJTs) ECED3201- Introduction to Electronics Page 3 Lecture Outline ECED3201- Introduction to Electronics Introduction I. The npn BJT Structure and Physical Operation II. Small Signal Operation and Models III. The pnp Transistor Page 4 Lecture Outline ECED3201- Introduction to Electronics Introduction I. The npn BJT Structure and Physical Operation II. Small Signal Operation and Models III. The pnp Transistor Page 5 Introduction Three-terminal devices, such as transistors, are far more useful than two-terminal one, such as diodes, because they can be used in multitude of applications, ranging from signal amplification to the design of digital logic and memory circuits. The basic principal involved is the use of the voltage between two terminals to control the current flowing in the third terminal. This lecture looks at the bipolar junction transistor (BJT). We will spend some time understanding how the BJT works based on what we know about pn junctions. Once we understand how the BJT device operates, we will take a look at the different circuits (amplifiers) we can build with them. Page 6 Introduction (cnt’d) • A three-terminal device voltage-dependent current source can act as an amplifier. • If KR L is greater than 1, then the signal is amplified. L in out V KR V V A Page 7 Lecture Outline ECED3201- Introduction to Electronics Introduction I. The npn BJT Structure and Physical Operation II. Small Signal Operation and Models III. The pnp Transistor Page 8 I.1. npn Bipolar Junction Transistor BJT has 3 terminals: emitter, base, and collector Voltage between two terminals to control current flowing in third terminal : Use as Amplifier or Switch 2 junctions: emitter-base junction (EBJ) and collector-base junction (CBJ) Depending on the biasing across each of the junctions, different modes of operation are obtained – cutoff, active, and saturation A simplified structure of the pnp transistor. Page 9 1. 2. BJT in Active Mode Two external voltage sources set the bias conditions for active mode : – emitter-base junction (EBJ) is forward biased and collector base junction (CBJ) is reverse biased: V BE > 0, V BC < 0. Operation – Forward bias of EBJ injects electrons from emitter into base (small number of holes injected from base into emitter) – Most electrons shoot through the base into the collector across the reverse bias junction – Some electrons recombine with majority carrier in (P-type) base region Current flow in an npn transistor biased to operate in the active mode Page 10 1.3. BJT in Active Mode (cnt’d) 1– EBJ is forward biased built-in potential barrier reduced and so electrons diffuse into the base (a) 2- Since the base region is thin , most of the injected electrons from the emitter reach the edge of the CBJ and begin to be swept by CBJ built-in electrical field (b)...
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This note was uploaded on 02/12/2011 for the course ECED 3201 taught by Professor Al-sankary during the Spring '11 term at Dalhousie.

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Lecture5sankary - Page 1 ECED3201 Introduction to...

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