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.