Download Fundamentals of Linear Electronics Integrated & Discrete

CHAPTER 20
Optoelectronics
Objectives
Describe and Analyze:
• Cathode Ray Tubes (CRT)
• Liquid Crystal Displays (LCD)
• Light Sensors
• Optoisolators
• Lasers
• Fiber Optics
Introduction
Optoelectronics includes:
• Display devices, such as CRT or LCD
computer screens.
• Light sensors, including infrared security
systems.
• Digital data transmission over fiber-optic
cables.
A Tube?
The original “active device”.
What’s a Tube?
• On February 18, 1908, Lee DeForest was granted U.S.
Patent No. 879,532 for a vacuum tube triode he called
the Audion. It was the first electronic device that could
amplify a signal.
– An incandescent filament (like in a light bulb) gets the
cathode hot enough to boil electrons off it.
– Positive voltage applied to the plate causes current to
flow.
– A small negative voltage applied to the grid (a metal
screen) controls the flow of electrons.
Cathode Ray Tubes (CRTs)
Phosphors emit light when hit by electrons.
CRTs
Color CRTs have three guns, one for each color.
CRTs
• The electron beam in a CRT is deflected back
and forth by the horizontal sweep circuit.
• The beam is deflected up and down by the
vertical deflection circuit.
• Oscilloscope CRTs use electrostatic deflection.
• Television CRTs use magnetic deflection. It is
more efficient for large deflection angles.
• CRTs are bulky and consume a lot of power but,
so far, nothing has replaced them.
Liquid Crystal Display (LCDs)
Very low power, but relatively small and slow.
LCDs
Compared to CRTs:
• Color LCD displays are expensive, and
bigger ones are very expensive.
• LCDs are not as bright.
• LCDs cannot update the screen as fast.
• LCD displays are thin and lightweight.
Light Sensors
Photoresistors are sensitive, but respond slowly.
CdS used in light meters; responds like human eye.
Light Sensors
Light energy knocks electrons loose, generates voltage.
Phototransistors
Light causes E-H pairs, which produce base current.
Optoisolators
Transfer signals across a high-voltage barrier.
Packages
Some interrupters and reflective sensors.
LASERS
LASERs produce coherent light.
LASERS
• Coherent means all the light waves are in phase.
• Coherent light is one color (monochromatic).
• Coherent light rays are parallel (collimated). They do
not “spread out” like flashlight beams.
• A LASER beam is very narrow; it forms a very small
“spot”.
• The spot of a LASER beam has a very high Watts/ in2.
• Fiber-optic cables require a laser as a light source.
Fiber-Optic Cable
Core has a higher refraction index than cladding.
Fiber-Optic Cable
Light is “trapped” by reflection off core-clad boundary.
Fiber-Optic Cable
• The claim to fame of fiber-optic cable is its bandwidth.
The speed of a communications channel in bits/sec is
directly proportional to bandwidth: MAX bps = k  BW.
• Multimode cable is easier to splice and “connectorize”
than single-mode cable.
• Multimode cable suffers from dispersion over longer
distances; that limits its speed.
• Single-mode cable is used for high-speed long-haul
cables.