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Copy of ECE109 Presentation-Phototransistor
Transcript of Copy of ECE109 Presentation-Phototransistor
August 15, 2012
The idea of the phototransistor has been known for many years. William Shockley first proposed the idea in 1947, not long after the ordinary transistor had been discovered.
The phototransistor was invented by Dr. John Northrup Shive in the course of development work on Transistor-like devices in the Bell Telephone Laboratories.
The Bell Telephone Laboratories announced the invention of the first transistor in 30th March, 1950.
The whole apparatus is housed in a tiny cylinder about as big as a 22 calibre rifle cartridge.
It is an electronic switching and current amplification component which relies on exposure to light to operate.
What is a Phototransistor?
It is a transistor in which base current is produced when light strikes the photosensitive semiconductor base region.
A phototransistor can either be a two-lead or a three-lead devices.
Two-lead configuration doesn’t have a base lead bought out which it will only use light as its input.
Three-lead configuration usually brought out the base lead so that the device can be used as a conventional BJT or without the additional light-sensitivity feature.
As shown in the figure, the base current is produced when the light strikes the collector-base pn junction.
The relationship between the collector current and the light-generated base current in a phototransistor is usually represented by
light-generated base current produced
by light intensity usually measured in"lux"
Dark current-a small amount of current that flows in the phototransistor even when no light is present. It represents a small number of carriers that are injected into the emitter.
Phototransistor bias circuit and its collector characteristic curves
Visible block: This phototransistor is specially made to not respond to visible light.
Visible: This phototransistor is made to have useful response from infrared through the visible spectrum although the response factor in the visible spectrum is not very high.
Blue enhanced: This phototransistor is specially made to have significantly more response at shorter wavelengths thus making it more useful for visible light applications.
Types of Spectral Response
Low cost visible and near-IR photodetection
Available with gains from 100 to over 100,000
Moderately fast response times
Available in a wide range of packages including epoxy coated, transfer molded, cast, hermetic packages and in chip form
Usable with almost any visible or near infrared light source such as LEDs, neon, fluorescent, incandescent bulbs, laser, flame sources, sunlight, etc....
Can be specially selected to meet the requirements of a particular application
It has a much lower level of noise.
Why Use Phototransistors?
The phototransistor can be used in a variety of different circuit configurations.
The phototransistor can be used in common emitter and common collector circuits. Common base circuits are not normally used because the base connection is often left floating.
The choice of common emitter or common collector phototransistor circuit configuration depends upon the requirements for the circuit.
The two phototransistor circuit configurations have slightly different operating characteristics and these may determine the circuit used.
Phototransistor circuit configurations
The phototransistor circuits can be used on one of two basic modes of operation. They are called active or linear mode and a switch mode.
Phototransistor circuit operation
Operation in the "linear" or active mode provides a response that is very broadly proportional to the light stimulus. In reality the phototransistor does not give a particularly linear output to the input stimulus and it is for this reason that this mode of operation is more correctly termed the active mode.
Active mode: VCC > RL x Ic
Linear or Active Mode
The operation of the phototransistor circuit in the switch mode is more widely used in view of the non-linear response of the phototransistor to light.
The switch mode, has two levels: - "on" and "off" as in a digital or logic system. This type of phototransistor mode is useful for detecting objects, sending data or reading encoders, etc.
Switch mode: VCC < RL x Ic
The photodarlington consist of a phototransistor connected in darlington arrangement with a conventional BJT.
The photodarlington transistor has a higher current gain which provides a much higher degree of sensitivity when compared to other phototransistors, but this is at the expense of response time and frequency response
Current gaintotal = HFE1 x HFE2
The phototransistor is normally on, holding the gate of the SCR low.
When the light is interrupted, the phototransistor turns off.
The high-going transition on the collector triggers the SCR and sets off the alarm mechanism.
The momentary contact switch SW1 provides for resetting the alarm.
Dark Sensor Circuit
These relay circuit can be used in a variety of applications such as automatic door activator, process counters, and various alarm systems.
Dark-operated relay circuit
Light-operated relay circuit
track zero detector - floppy drive
margin controls - printers
read finger position - touch screen
detect holes - computer card
monitor paper position - copiers
LED light source - light pens
encoders - measure speed and direction
position sensors - joysticks
remote controllers - toys, appliances, audio/visual equipment
games - laser tag
provide electrical isolation between patient and equipment
monitor intravenous injection rates
Electronic Devices (Conventional Current Version) (8th Edition) -Thomas L. Floyd
Light Sensitive Counting Circuit
Sound Card Shutter Tester
Phototransistor Line Follower