Assigned by:Engr. USMAN ALI KHAN

PHOTOTRANSISTOR

PREPARED BY:

Muhammad Muazzam (10ES26) Muhammad Bilal Nasir (10ES39) Muhammad Hamir (10ES38) Muhammad Rizwan Saeed (10ES42) Ahmad Hasan (10ES53)

PHOTOTRANSISTOR:

Introduction to phototransistor. Construction. Comparison with BJT. Principle & working of phototransistor. Advantages and disadvantages. Applications of phototransistor.

INTRODUCTION TO PHOTOTRANSISTOR

HistoryDefinition of phototransistorRecap of photodiode (definition , principle with diagram, difference)SymbolMaterial

HISTORY

The invention of the phototransistor was announced in Murray Hill NJ. This was a transistor operated by light rather than electric current, invented by Dr. John Northrup Shive of the Bell Telephone Laboratories at Murray Hill, N.J.

WHAT IS PHOTOTRANSISTOR?

An alternative photo-junction device to the photodiode is the Phototransistor which is basically a photodiode with amplification.

Basics: Two-junctions. Operation. Gain. Sensitivity. BGT to phototransistor.

LET’S DISCUSS RECAP OF PHOTODIODE

Photodiode:A photodiode consists of an active p-n junction which is operated in reverse bias. When light falls on the junction, a reverse current flows which is proportional to the luminance.

PHOTODIODE:

Symbol:

Working and Characteristics:

BASIC DIFFERENCE IN PHOTODIODE AND PHOTOTRANSISTOR:

Junction difference. Frequency Response . Gain. Temperature Response.

DEFINITION OF PHOTOTRANSISTOR

A phototransistor is an electronic switching and current amplification component which relies on exposure to light to operate.

Exposed-base Section. Depend on light to operate it .

SYMBOL OF PHOTOTRANSISTOR:

Configurations of Phototransistor:

MATERIAL:

The first phototransistors used single semiconductor materials such as germanium and silicone in their construction. Modern components use several differing material junctions including gallium and arsenide for higher efficiency levels. The physical structure of the transistor is also optimized to allow for maximum light exposure.

Construction of Phototransistor

A photo transistor is nothing but an ordinary bi-polar transistor in which the base region is exposed to the illumination.

n-type

p-typen

Emitter

Base

Collector

Window

Construction of Phototransistor

Available in both the P-N-P and N-P-N types.Common emitter configuration is generally used.The base terminal is made open.

Different symbols of phototransistor

Phototransistor: Bipolar junction transistor:

Collector base junction very sensitive to light.

Its working condition depends upon intensity of light on base.

Its symbol is with or without the base terminal.

Its collector base junction surface area comparatively greater.

There is a lens to focus the light.

Collector base junction not sensitive to light.

Its working condition depends upon the input current or its input is base current.

Its symbol is always with base terminal.

Its collector base surface area comparatively smaller.

There is no lens.

Difference between:

CIRCUIT DIAGRAM OF P-N JUNCTION

Function of circuit

As a reverse biased collector

base junction

Resistor function

Function of lens

Increasing reverse current by

light

FUNCTION DIAGRAM

Two light isolated devices

Light emitting device

Light sensitive device as

phototransistor.

Function of

phototransistor.

SIMPLE USE OF PHOTOTRANSISTOR

Phototransistor as a AND

gate.

How it works?

Vcc for providing a

biasing

Light as a input

OUTPUT CHARACTERISTICS

Explanation of IC and VCE

Dark current

ABOUT LUX

Lumen per square meter.

Unit of luminance.

Measure the amount of visible light.

Lux which measures apparent brightness (power multiplied by the human eye’s sensitivity).

Phototransistor:1 mA @ 1000 lux

GAIN EQUATION

IC = β IB + (1+ β) ICBO

Where β = Current amplification factor in CE conf

IB = base current

ICBO = Collector-to-base leakage current

Since base terminal open, IB = 0

IC = (1+ β) ICBO

When CB is illuminated by incident

light, I CBO increases

PHOTOTRANSISTOR CIRCUIT CONFIGURATIONS

Common emitter.

Common collector.

Common base.

Common base is seldom used.

Choice on requirement.

COMMON EMITTER PHOTOTRANSISTOR CIRCUIT

Operation.

O/P of circuit before

light.

O/P of circuit after

light.

COMMON COLLECTOR PHOTOTRANSISTOR

Operation.

O/P before light.

O/P after light.

PHOTORESISTOR AND PHOTOTRANSISTOR: Phototransistors work in a similar way to

photo-resistors. Phototransistor able to produce both current

and voltage. Photo-resistor produce only current.

(LDR) is a resistor whose resistance decreases with increasing incident light intensity.

A photo resistor is made of a high resistance semiconductor.   

A PHOTO RESISTOR OR LIGHT DEPENDENT RESISTOR

VIDEO:

PHOTOTRANSISTOR SPECTRAL RESPONCE

ADVANTAGES OF PHOTOTRANSISTOR:

Phototransistors produce a higher current than photodiodes.

Phototransistors produce a voltage, that photo-resistors cannot do so.

Phototransistors are very fast and are capable of providing nearly instantaneous output.

Phototransistors are relatively inexpensive, simple, and small enough to fit several of them onto a single integrated computer chip.

DISADVANTAGES OF PHOTOTRANSISTOR :

Phototransistors that are made of silicon are not capable of handling voltages over 1,000 Volts.

Phototransistors also do not allow electrons to move as freely as other devices do, such as electron tubes.

Phototransistors are also more vulnerable to surges and spikes of electricity as well as electromagnetic energy.

APPLICATIONS OF PHOTOTRANSISTOR

Some of the areas of application for the phototransistor include.

Punch-card readers.

Computer logic circuitry.

Lighting control (highways etc).

APPLICATIONS OF PHOTOTRANSISTOR

Level indication

Relays

Counting systems

IR detectors

TRANSISTOR AS A SWITCHING DEVICE

Cut Off state:

Base Emitter junction is not forward biased.

Open circuit between collector and emitter.

TRANSISTOR AS A SWITCHING DEVICE

Saturation state:

Base Emitter and Base Collector junction forward biased.

Base is current is made high to get Ic to its saturation value.

Collector and Emitter are short.

VCE( saturation) is obtained.

Conditions for Cutoff:

Base Emitter junction is not forward biased.

All currents are equal to zero.

Conditions for Saturation:

IB must be significantly greater to keep the transistor into operation.

AN AND GATE.

An Example of Application in switching circuits and Gates.

The terminology high isolation simply refers to the lack of an electrical connection between the input and output circuits .

A high-isolation AND gate using three phototransistors and three LEDs (light-emitting diodes) .

PHOTO-DARLINGTON:

A photo Darlington consists of a phototransistor connected in a Darlington arrangement with a conventional BJT.

Higher current gain. Higher collector current. Greater light sensitivity.

LIGHT OPERATED RELAY CIRCUIT

DARKNESS OPERATED RELAY CIRCUIT

LIGHT INTERRUPTION ALARM

LIGHT ACTIVATED SILICON CONTROLLED RECTIFIER:

An LASCR is a four layer semiconductor device that conducts current in one direction when activated by a sufficient amount of light and continues to conduct untill light falls below a specified value.

ANY QUESTION !!!