Module 2 Bipolar Junction Transistor
Dec 25, 2015
Module 2
Bipolar Junction Transistor
Learning Outcomes
1. The 3 terminals or regions of a BJT.2. Construction and symbol of NPN and
PNP types3. Low power and high power packages4. CE. CB. CC configuration5. The relation between the emitter,
collector and base current6. Regions of operation7. Transistor as a switch8. The applications of a transistor.
Regions of a Transistor
A Bipolar Junction Transistor is a three terminal device
containing 3 regions:
Emitter , Base and Collector .
THE TRANSISTOR There are two types of (bipolar junction) transistors, npn and
pnp
Bipolar transistor symbols
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
A transistor consists of two PN junctions. Thus a transistor is like two PN – junction diodes connected back to back.
2.2 Packages
Examples of transistor packages
Transistor Packages (low power & high power)
2.4 Transistor Configurations
•common emitter (CE)
•common base (CB)•common collector
(CC).
Currents in CE Circuit
•
2.5 Emitter, collector and base currents
•In the CE configuration:
Transistor Operation RegionsOperation Region Bias Application
CUT OFF REGION IB and Ic are 0 (base-emitter junction is reverse biased)
Open Switch (OFF)
SATURATION REGION
Base emitter junction is forward biased; IB flows
Closed Switch (ON)
ACTIVE REGION B-E junction is forward biased but C-E junction is reverse biased;
Amplifier
Transistor Applications: Transistor can act as a Switch
Transistor Applications
• Switch • Amplifier• Oscillator
Transistor as a Switch
• Light operated switch• Darkness operated switch• Digital Logic Gates• Switching power supply
Transistor as a Variable Resistor
• Amplifier• Oscillator• Modulator• Detector
Light Failure Warning System using Transistor as a Switch
•
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.