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Module 2 Bipolar Junction Transistor
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Learning Outcomes 1.The 3 terminals or regions of a BJT. 2.Construction and symbol of NPN and PNP types 3.Low power and high power packages 4. CE. CB. CC configuration 5.The relation between the emitter, collector and base current 6.Regions of operation 7.Transistor as a switch 8. The applications of a transistor.
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Regions of a Transistor
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A Bipolar Junction Transistor is a three terminal device containing 3 regions: Emitter, Base and Collector.
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THE TRANSISTOR There are two types of (bipolar junction) transistors, npn and pnp Bipolar transistor symbols
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NPN vs PNP NPN and PNP function the same way Power supply polarities are reversed For NPN, positive supply is used NPN has higher frequency response as electrons are the charge carriers. NPN is more widely used
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A transistor consists of two PN junctions. Thus a transistor is like two PN – junction diodes connected back to back.
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2.2 Packages Examples of transistor packages
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Transistor Packages (low power & high power)
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2.4 Transistor Configurations common emitter (CE) common base (CB) common collector (CC).
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Currents in CE Circuit
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2.5 Emitter, collector and base currents
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In the CE configuration:
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Transistor Operation Regions Operation Region BiasApplication CUT OFF REGIONI B and I c are 0 (base-emitter junction is reverse biased) Open Switch (OFF) SATURATION REGION Base emitter junction is forward biased; I B flows Closed Switch (ON) ACTIVE REGIONB-E junction is forward biased but C-E junction is reverse biased; Amplifier
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Transistor Applications: Transistor can act as a Switch
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Transistor Applications Switch Amplifier Oscillator
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Transistor as a Switch Light operated switch Darkness operated switch Digital Logic Gates Switching power supply
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Transistor as a Variable Resistor Amplifier Oscillator Modulator Detector
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Light Failure Warning System using Transistor as a Switch
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Light Failure Warning System In this, the transistor is used as a switch LDR is a light dependant resistor. It is a light sensor. When it is dark, LDR resistance increases; base current flows; the transistor becomes ON and relay switches. So the bell rings.
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