Optoelectronics is a branch of electronics that deals with light-emitting and light-detecting devices. Light-emitting devices, such as lamps and light-emitting diodes (LEDs), create electromagnetic energy (e.g., light) by using an electric current to excite electrons into higher energy levels (when an electron changes energy levels, a photon is emitted). Light-detecting devices such as phototransistors and photoresistors, on the other hand, are designed to take incoming electromagnetic energy and convert it into electric currents and voltages. This is usually accomplished using photons to liberate bound electrons within semiconductor materials. Light-emitting devices typically are used for illumination purposes or as indicator lights. Light-detecting devices are used primarily in light-sensing and communication devices, such as dark-activated switches and remote controls. This chapter examines the following optoelectronic devices: lamps, LEDs, photoresistors, photodiodes, solar cells, phototransistors, photothyristors, and optoisolators.
Photons are the elemental units of electromagnetic radiation. White light, for example, is composed of a number of different kinds of photons; some are blue photons, some are red photons, etc. It is important to note that there is no such thing as a white photon. Instead, when the combination of the various colored photons interacts with our eye, our brain perceives what we call white light.
Photons are not limited to visible light alone. There are also radiofrequency photons, infrared photons, microwave photons, and other kinds of photons that our eyes cannot detect.
In terms of the physics, photons are very interesting creatures. They have no rest mass, but they do carry momentum (energy). A photon also has a distant wavelike character within its electromagnetic bundle. The wavelength of a photon (horizontal distance between consecutive electrical or magnetic field peaks) depends on the medium in which it travels and on the source that produced it. It is this wavelength that determines the color of a photon. A photon's frequency is related to its wavelength by λ = ν/f, where ν is the speed of the photon. In free space, ν is equal to the speed of light (c = 3.0 × 108 m/s), but in other media, such as glass, ν becomes smaller than the speed of light. A photon with a large wavelength (or small frequency) is less energetic than a photon with a shorter wavelength (or a higher frequency). The energy of a photon is equal to E = hf, where h is Planck's constant (6.63 × 10-34 J∙s).
The trick to "making" a photon is to accelerate/decelerate a charged particle. For example, an electron that is made to vibrate back and forth within an antenna will produce radiofrequency photons that have very long wavelengths (low energies) when compared with light photons. Visible light, on the other hand, is produced when outer-shell electrons within atoms are forced to make transitions between energy levels, accelerating in the process. Other frequency photons may be created by vibrating or rotating molecules very quickly, while still others, specifically those with very high energy (e.g., gamma rays), can be created by the charge accelerations within the atomic nuclei.
Figure 5.2 shows the breakdown of the electromagnetic spectrum. Radiofrequency photons extend from a few hertz to about 109 Hz (wavelengths from kilometers to about 0.3 m). They are often generated by alternating currents within power lines and electric circuits such as radio and television transmitters.
Microwave photons extend from about 109 up to 3 × 1011 Hz (wavelengths from 30 cm to 1 mm). These photons can penetrate the earth's atmosphere and hence are used in space vehicle communications, radio astronomy, and transmitting telephone conversations to satellites. They are also used for cooking food. Microwaves are often produced by atomic transitions and by electron and nuclear spins.
Infrared photons extend from about 3 × 1011 to 4 × 1014 Hz. Infrared radiation is created by molecular oscillations and is commonly emitted from incandescent sources such as electric heaters, glowing coals, the sun, human bodies (which radiate photons in the range of 3000 to 10,000 nm), and special types of semiconductor devices.
Light photons comprise a narrow frequency band from about 3.84 × 1014 to about 7.69 × 1014 Hz and are generally produced by a rearrangement of outer electrons in atoms and molecules. For example, in the filament of an incandescent light bulb, electrons are randomly accelerated by applied voltages and undergo frequent collision. These collisions result in a wide range of electron acceleration, and as a result, a broad frequency spectrum (within the light band) results, giving rise to white light.
Ultraviolet photons extend from approximately 8 × 1014 to 3.4 × 1016 Hz and are produced when an electron in an atom makes a long jump down from a highly excited state. The frequency of ultraviolet photons—unfortunately for us—tend to react badly with human cell DNA, which in turn can lead to skin cancer. The sun produces a large output of ultraviolet radiation. Fortunately for us, protective ozone molecules in the upper atmosphere can absorb most of this ultraviolet radiation by converting the photon's energy into a vibrating motion within the ozone molecule.
X-rays are highly energetic photons that extend from about 2.4 × 1016 to 5 × 1019 Hz, making their wavelengths often smaller than the diameter of an atom. One way of producing x-rays is to rapidly decelerate a high-speed charged particle. X-rays tend to act like bullets and can be used in x-ray imagery.
Gamma rays are the most energetic of the photons, whose frequency begins around 5 × 1019 Hz. These photons are produced by particles undergoing transitions within the atomic nuclei. The wavelike properties of gamma rays are extremely difficult to observe.
Lamps are devices that convert electric current into light energy. One approach used in the conversion process is to pass a current through a special kind of wire filament. As current collides with the filament's atoms, the filament heats up, and photons are emitted. (As it turns out, this process produces a variety of different wavelength photons, so it appears that the emitted light is white in color.) Another approach used to produce light involves placing a pair of electrodes a small distance apart within a glass gas-enclosed bulb. When a voltage is set across the electrodes, the gas ionizes (electrons are stripped from the gas atoms) emitting photons in the process. Figure 5.3 gives an overview of some of the major kinds of lamps.
A lamp's brightness is measured in what is called the mean spherical candle power (MSCP). Bulb manufacturers place a lamp at the center of an integrating sphere that averages the lamp's light output over its surface. The actual value of the MSCP for a lamp is a function of color temperature of the emitting surface of the lamp's filament. For a given temperature, doubling the filament's surface area doubles the MSCP. Other technical things to consider about lamps include voltage and current ratings, life expectancy, physical geometry of the bulb, and filament type. Figure 5.4 shows a number of different bulb types.
In recent years, incandescent light bulbs have become something of a rarity. In domestic lighting situations, they have almost completely been replaced by compact fluorescent designs, and indeed are not even for sale in some European countries. There is also a trend toward using arrays of LEDs for illumination in place of incandescent bulbs (see the next section). In applications that formerly would have used incandescent indicator lights, LEDs now rule supreme. LEDs have a much longer life, lower power consumption, and much greater tolerance of physical and thermal shock.
Light-emitting diodes (LEDs) are two-lead devices that are similar to pn-junction diodes, except that they are designed to emit visible or infrared light. When a LED's anode lead is made more positive in voltage than its cathode lead (by at least 0.6 to 2.2 V), current flows through the device and light is emitted. However, if the polarities are reversed (anode is made more negative than the cathode), the LED will not conduct, and hence it will not emit light. The symbol for an LED is shown in Fig. 5.5.
LEDs are available in a wide range of colors. Historically, red was the first LED color. Yellow and green and infrared LEDs followed. It was not until the 1990s that blue LEDs became available. These days, LEDs are available in pretty much any color, including white.
There are also high-powered LEDs that are used for illumination and organic LEDs (OLEDs) made from polymers that can be built into displays.
Often, LEDs (especially infrared LEDs) are used as transmitting elements in remote-control circuits (e.g., TV remote control). The receiving element in this case may be a phototransistor that responds to changes in LED light output by altering the current flow within the receiving circuit.
The light-emitting section of an LED is made by joining n-type and p-type semiconductors together to form a pn junction. When this pn junction is forward-biased, electrons in the n side are excited across the pn junction and into the p side, where they combine with holes. As the electrons combine with the holes, photons are emitted. Typically, the pn-junction section of an LED is encased in an epoxy shell that is doped with light-scattering particles to diffuse the light and make the LED appear brighter. Often a reflector placed beneath the semiconductor is used to direct light upward. The cathode and anode leads are made from a heavy-gauge conductor to help wick heat away from the semiconductor.
LEDs emit visible light, infrared radiation, or even ultraviolet radiation when forward-biased. Visible single-tone LEDs emit relatively narrow bands of green, yellow, orange, red, and blue light (with spectrum spread usually less than 40 nm at 90 percent peak intensity). Infrared diodes emit one of several bands beyond red. White LEDs provide a variety of wavelengths to mimic white light and are used in low-level lighting applications, such as backlighting, headlamps, and nightlights.
High-power LEDs (HPLEDs) are now available. These have forward currents of hundreds of mA to more than 1 A. Such LEDs are very bright but also generate a lot of heat. They must be mounted on a heat sink to prevent thermal destruction.
LEDs have very fast response times, excellent efficiency, and long lifetimes. They are current-dependent devices whose light output is directly proportional to the forward current.
To light an LED, apply a voltage greater than the LED's forward voltage VLED, and limit current flow via a series resistor to a level below the LED's maximum rating, usually to ILED—the manufacturer's recommended value. The following equation is used to select the series resistor:
If you want brightness control, throw in a 1-K potentiometer in series, as shown in Fig. 5.10.
VLED varies with LED color. Typical VLED values are 1.7 V for non-high-brightness red, 1.9 V for high-brightness high-efficiency low-current red, 2 V for orange and yellow, 2.1 V for green, 3.4 to 3.6 V for bright white and most blue types, and 6 V for 430-nm blue. Given the preceding voltage drops, it's a good idea to use at least a 3-V supply voltage for lower-voltage LEDs, 4.5 V for 3.4-V types, and 6 V for 430-nm blue. If you don't know the recommended ILED value of a given LED, it's usually safe to assume it will be around 20 mA. Table 5.1 shows the range of LED types and corresponding characteristic values.
WAVELENGTH |
COLOR NAME |
FWD VOLTAGE (VF AT 20 MA) |
INTENSITY (5-MM LEDS) |
LED DYE MATERIAL |
940 |
Infrared |
1.5 |
16 mW @ 50 mA |
GaAlAs/GaAs |
880 |
Infrared |
1.7 |
18 mW @ 50 mA |
GaAlAs/GaAs |
850 |
Infrared |
1.7 |
26 mW @ 50 mA |
GaAlAs/GaAs |
660 |
Ultra Red |
1.5–1.8 |
200 mcd @ 50 mA |
GaAlAs/GaAs |
635 |
High Eff. Red |
2.0 |
200 mcd @ 20 mA |
GaAsP/GaP |
633 |
Super Red |
2.2 |
3500 mcd @ 20 mA |
InGaAlP |
620 |
Super Orange |
2.2 |
4500 mcd @ 20 mA |
InGaAlP |
612 |
Super Orange |
2.2 |
6500 mcd @ 20 mA |
InGaAlP |
605 |
Orange |
2.1 |
160 mcd @ 20 mA |
GaAsP/GaP |
595 |
Super Yellow |
2.2 |
5500 mcd @ 20 mA |
InGaAlP |
592 |
Super Pure Yellow |
2.1 |
7000 mcd @ 20 mA |
InGaAlP |
585 |
Yellow |
2.1 |
100 mcd @ 20 mA |
GaAsP/GaP |
574 |
Super Lime Yellow |
2.4 |
1000 mcd @ 20 mA |
InGaAlP |
570 |
Super Lime Green |
2.0 |
1000 mcd @ 20 mA |
InGaAlP |
565 |
High Efficiency Green |
2.1 |
200 mcd @ 20 mA |
GaP/GaP |
560 |
Super Pure Green |
2.1 |
350 mcd @ 20 mA |
InGaAlP |
555 |
Pure Green |
2.1 |
80 mcd @ 20 mA |
GaP/GaP |
525 |
Aqua Green |
3.5 |
10,000 mcd @ 20 mA |
SiC/GaN |
505 |
Blue Green |
3.5 |
2000 mcd @ 20 mA |
SiC/GaN |
470 |
Super Blue |
3.6 |
3000 mcd @ 20 mA |
SiC/GaN |
430 |
Ultra Blue |
3.8 |
100 mcd @ 20 mA |
SiC/GaN |
370–400 |
UV LED |
3.9 |
NA |
GaN |
4500K |
"Incandescent White" |
3.6 |
2000 mcd @ 20 mA |
SiC/GaN |
6500K |
Pale White |
3.6 |
4000 mcd @ 20 mA |
SiC/GaN |
8000K |
Cool White |
3.6 |
6000 mcd @ 20 mA |
SiC/GaN |
Some other specifications to consider include power dissipation (100 mW typical), reverse voltage rating, operating temperature (-40 to +85°C typical), pulse current (100 mA typical), luminous intensity (given in millicandles, mcd), viewing angle (given in degrees), peak emission wavelength, and spectral width (20 to 40 nm typical).
In Chap. 13, we will look at how you drive LED displays from a microcontroller, including techniques such as multiplexing, Charlieplexing, and driving RGB LEDs with PWM signals to mix colors.
Laser diodes are light-emitting diodes with two "mirrors" on the surface of the diode to create a laser cavity. When the diode is forward-biased, charges are injected into the active area of the junction, while electrons and holes recombine in the junction, creating spontaneous emission of photons. These photons can cause other electron-hole pairs to recombine by stimulated emission. When the current is high enough, the device lases. Laser diodes are driven by low-voltage power and usually incorporate optical feedback from a monitor photodiode (commonly built into the same laser diode package) to regulate the laser diode current.
Compared to LEDs, laser diodes have quicker response times and very narrow spectrum spread (around 1 nm), and can focus radiation to a spot as small as 1 µm in diameter—even for a cheap laser diode using simple optics found in a CD player. Unlike gas lasers, however, a laser diode's output beam is divergent, typically elliptical or wedge-shaped, and astigmatic, requiring refocusing.
The output wavelength of a laser diode is usually fixed to a single mode: for example red (635 nm, 670 nm), infrared (780 nm, 800 nm, 900 nm, 1,550 nm, etc.), and green, blue, violet. However, multimode laser diodes also exist where the emission spectrum consists of several individual spectral lines with a dominant line (line with greatest intensity) occurring at the nominal wavelength of the device. Multimode laser diodes are often desirable because problems with mode hops are suppressed—consequently, they generally have a better signal-to-noise ratio. Mode hops are slight changes in the wavelength caused by thermal expansion of the laser cavity.
Typical optical output for low-powered laser diodes range from around 1 mW to 5 mW, while high-power laser diodes can reach 100 W or more. The highest-power units consist of arrays of laser diodes—not a single device alone.
Laser diodes are used in CD players, CD-ROM drives, and DVD and Blu-ray players. They are also used in laser printers, laser fax machines, laser pointers, sighting and alignment scopes, measurement equipment, high-speed fiber-optic and free-space communication systems; as pump source for other lasers; in bar-code and UPC scanners; and in high-performance imagers. In those applications requiring high-speed modulation or pulse rates (into the gigahertz range), special integrated driver chips are needed to control the laser diode drive current (more on that in a bit).
A variety of small laser diodes are found in CD players, CD-ROM drives, laser printers, and bar-code scanners. The most common laser diodes around are those used in CD players and CD-ROM drives. These produce a mostly invisible beam in the near infrared spectrum at 780 nm. The optical output from the actual laser diode itself may be up to 5 mW, but once it passes through the optics leading to the CD, the power drops to 0.3 to 1 mW. Higher-power IR laser diodes found in read-write drives have more power output—up to 30 mW or so. Even higher-power blue laser diodes can be found in Blu-ray players.
Visible laser diodes have replaced the helium-neon lasers used in bar-code scanners, laser pointers, positioning devices in medicine (e.g., CT and MRI scanners), and many other applications. The first visible-light laser diode emitted with a wavelength around 670 nm in the deep red spectrum. More recently, 650- and 635-nm red laser diodes have dropped in price. 635 to 650-nm laser diodes are used in DVD technology. The shorter wavelength compared to 780 nm is one of the several improvements that enable DVDs to store about eight times the amount of information as CDs (4 to 5 GB per layer and up to two layers on each side of the disc, as compared to a typical CD that stores only around 650 MB). Like the IR laser diodes, the visible laser diodes have a typical maximum power around 3 to 5 mW, and cost about $10 to $50 for the basic laser diode device—more with optics and drive electronics. Higher-power types are also available but can cost upward of several hundred dollars for something like a 20-mW module. Very high-power diode lasers using arrays of laser diodes or laser diode bars with power output of watts or greater may cost thousands of dollars.
It's important to note that you should never look into a laser beam or any specular reflection of the laser beam—"you might poke your eye out." Also, laser diodes are extremely sensitive to electrostatic discharge (ESD), so it's important to use grounding straps and grounded equipment when working with them, as well as following manufacturer's suggested handling precautions.
You should never drive a laser diode without the proper drive circuitry. Without the proper drive circuitry, you can run into all sorts of problems stemming from swings in operating temperature with unstable injection current. The results can lead to a fried laser diode, or one whose life is short. For this reason, it's important to have a driver circuit that can provide stable current without the possibility of supply transients screwing things up. Two basic techniques used to achieve stable optical output from a laser diode are described here.
Regardless of the type of drive circuit used, the key is to prevent the drive current from overshooting the maximum operating level. Exceeding the maximum optical output, even for a nanosecond, will damage the mirror coatings on the laser diode end facets. Your typical laboratory power supply should not be used to directly drive a laser diode—it simply doesn't provide enough protection. Typical drive circuits incorporate slow-start circuitry, capacitive filtering, and other provisions to eliminate supply spikes, surges, and other switching transients.
Figure 5.12 shows a few do-it-yourself laser diode supplies. Though these drive circuits will work with many low-powered laser diodes that don't require modulation, it's worth your while to check out the laser data manufacturer's data sheets for recommended drive circuits. Creating your own drive circuits can be very tricky, and you'll probably end up frying a few expensive laser diodes in the process. Of course, you can also buy a laser diode drive chip, which may be the best bet if you're doing something a bit more complex than creating a laser pointer. These drive chips may support high-bit-rate modulation in addition to providing the constant current needed for optically stable power. Other types of chips can be adapted to linear and switching regulators. Some companies worth checking out include MAXIM, Linear Technology, Sharp, Toshiba, Mitsubishi, Analog Devices, and Burr-Brown. Often, these manufacturers provide free samples.
Even with a suitable drive circuit, watch out for intermittent or unreliable connections between the laser diode and the drive circuit. An intermittent contact in the photodiode feedback circuit will usually destroy a laser diode. Even if a power-control potentiometer's wiper breaks contact with the resistive element, there can be problems. Also, never use a switch or relay to make or break the connection between the drive circuit and the laser diode. The following are some other laser diode precautions:
If all you really want is a visible laser to shoot around, a commercial diode laser module or some brands of laser pointers (those that include optical feedback based on laser power regulation) may be the ticket. Both the modules and laser pointers include a driver circuit capable of operating reliably on unregulated low-voltage dc input and a collimating lens matched to the laser diode. Many of the modules will permit fine adjustment of the lens position to optimize the collimation or permit focusing to a point at a particular distance. However, neither the module nor the pointer is designed to be modulated at any more than a few hertz, due to heavy internal filtering designed to protect the laser diode from power spikes. Therefore, they are generally not suited for laser communication applications. In general, it's a whole lot easier starting out with the module or pointer rather than a laser diode and homemade power supply, or even a commercial driver, if it isn't explicitly designed for your particular laser diode. There is no way to know how reliable or robust an inexpensive laser pointer will be, or if the beam quality will be acceptable. Diode laser modules are generally more expensive and of higher quality than the pointers, so they may be better for serious applications. Also, consider a helium-neon laser, since even the cheapest type is likely to generate a beam with better beam quality than the typical diode laser module or laser pointer.
Photoresistors are light-controlled variable resistors. These are also known as light-dependent resistors (LDRs). In terms of operation, a photoresistor is usually very resistive (in the megaohms) when placed in the dark. However, when it is illuminated, its resistance decreases significantly; it may drop as low as a few hundred ohms, depending on the light intensity. In terms of applications, photoresistors are used in light- and dark-activated switching circuits and in light-sensitive detector circuits. Figure 5.13 shows the symbol for a photoresistor.
Photoresistors may require a few milliseconds or more to fully respond to changes in light intensity and may require a few seconds to return to their normal dark resistance once light is removed. In general, photoresistors pretty much all function in a similar manner. However, the sensitivity and resistance range of a photoresistor may vary greatly from one device to the next. Also, certain photoresistors may respond better to light that contains photons within a particular wavelength of the spectrum. For example, cadmium sulfide photoresistors respond best to light within the 400- to 800-nm range, whereas lead sulfide photoresistors respond best to infrared photons.
Photodiodes are two-lead devices that convert light energy (photon energy) directly into electric current. If the anode and cathode leads of a photodiode are joined together by a wire and then the photodiode is placed in the dark, no current will flow through the wire. However, when the photodiode is illuminated, it suddenly becomes a small current source that pumps conventional current from the anode through the wire and into the cathode. Figure 5.16 depicts the symbol for a photo diode.
Photodiodes are used most commonly to detect fast pulses of near-infrared light used in wireless communications. They are often found in light-meter circuits (e.g., camera light meters, intrusion alarms, etc.) because they have very linear light/current responses.
A photodiode is built by sandwiching a very thin n-type semiconductor together with a thicker p-type semiconductor. (The n side has an abundance of electrons; the p side has an abundance of holes.) The n side of the combination is considered the cathode, while the p side is considered the anode. Now, if you shine light on this device, a number of photons will pass through the n-semiconductor and into the p-semiconductor. Some of the photons that make it into the p side will then collide with bound electrons within the p-semiconductor, ejecting them and creating holes in the process. If these collisions are close enough to the pn interface, the ejected electrons will then cross the junction. What you get in the end is extra electrons on the n side and extra holes on the p side. This segregation of positive and negative charges leads to a potential difference being formed across the p-n junction. Now if you connect a wire from the cathode (n side) to the anode (p side), electrons will flow through the wire, from the electron-abundant cathode end to the hole-abundant anode end (or if you like, a conventional positive current will flow from the anode to cathode). A commercial photodiode typically places the p-n semiconductor in a plastic or metal case that contains a window. The window may contain a magnifying lens and a filter.
Photodiodes come in all shapes and sizes. Some come with built-in lenses, some come with optical filters, some are designed for high-speed responses, some have large surface areas for high sensitivity, and some have small surface areas. When the surface area of a photodiode increases, the response time tends to slow down. Table 5.2 presents a sample portion of a specifications table for a photodiode.
MNFR # |
DESCRIPTION |
REVERSE VOLTAGE (V) VR |
MAX. DARK CURRENT (NA) ID |
MIN. LIGHT CURRENT (μA) IL |
POWER DISSIPATION (MW) PD |
RISE TIME (ns) tR |
TYPICAL DETECTION ANGLE (º) |
TYPICAL PEAK EMISSION WAVELENGTH (NM) λP |
NTE3033 |
Infrared |
30 |
50 |
35 |
100 |
50 |
65 |
900 |
Solar cells are photodiodes with very large surface areas. The large surface area of a solar cell makes the device more sensitive to incoming light, as well as more powerful (larger currents and voltages) than photodiodes. For example, a single silicon solar cell may be capable of producing a 0.5-V potential that can supply up to 0.1 A when exposed to bright light.
Solar cells can be used to power small devices such as solar-powered calculators or can be added in series to recharge nickel cadmium batteries. Often solar cells are used as light-sensitive elements in detectors of visible and near-infrared light (e.g., light meters, light-sensitive triggering mechanism for relays). Like photodiodes, solar cells have a positive and negative lead that must be connected to the more positive and more negative voltage regions within a circuit. The typical response time for a solar cell is around 20 ms.
Phototransistors are light-sensitive transistors. A common type of phototransistor resembles a bipolar transistor with its base lead removed and replaced with a light-sensitive surface area. When this surface area is kept dark, the device is off (practically no current flows through the collector-to-emitter region). However, when the light-sensitive region is exposed to light, a small base current is generated that controls a much larger collector-to-emitter current. Field-effect phototransistors (photoFETs) are light-sensitive field-effect transistors. Unlike photobipolar transistors, photoFETs use light to generate a gate voltage that is used to control a drain-source current. PhotoFETs are extremely sensitive to variations in light but are more fragile (electrically speaking) than bipolar phototransistors.
Figure 5.23 shows a simple model of a two-lead bipolar phototransistor. The details of how this device works are given below.
The bipolar phototransistor resembles a bipolar transistor (with no base lead) that has an extra large p-type semiconductor region that is open for light exposure. When photons from a light source collide with electrons within the p-type semiconductor, they gain enough energy to jump across the pn-junction energy barrier—provided the photons are of the right frequency/energy. As electrons jump from the p region into the lower n region, holes are created in the p-type semiconductor. The extra electrons injected into the lower n-type slab are drawn toward the positive terminal of the battery, while electrons from the negative terminal of the battery are drawn into the upper n-type semiconductor and across the n-p junction, where they combine with the holes. The net result is an electron current that flows from the emitter to the collector. In terms of conventional currents, everything is backward. That is, you would say that when the base region is exposed to light, a positive current I flows from the collector to the emitter. Commercial phototransistors often place the pnp semiconductor in an epoxy case that also acts as a magnifying lens. Other phototransistors use a metal container and a plastic window to encase the chip.
Like ordinary transistors, phototransistors have maximum breakdown voltages and current and power dissipation ratings. The collector current IC through a phototransistor depends directly on the input radiation density, the dc current gain of the device, and the external base current (for three-lead phototransistors). When a phototransistor is used to control a collector-to-emitter current, a small amount of leakage current, called the dark current ID, will flow through the device even when the device is kept in the dark. This current is usually insignificant (within the nA range). Table 5.3 presents a portion of a typical data sheet for phototransistors.
MNFR # |
DESCRIPTION |
COLLECTOR TO BASE VOLTAGE (V) BVCBO |
MAX. COLLECTOR CURRENT (mA) IC |
MAX. COLLECTOR DARK CURRENT (nA) ID |
MIN. LIGHT CURRENT (mA) IL |
MAX. POWER DISSIPATION (mW) PD |
TYPICAL RESPONSE TIME (μS) |
NTE3031 |
npn, Si, visible and IR |
30 (VCEO) |
40 |
100 at 10 V VCE |
1 |
150 |
6 |
NTE3036 |
npn, Si, Darlington, visible and near IR |
50 |
250 |
100 |
12 |
250 |
151 |
Photothyristors are light-activated thyristors. Two common photothyristors include the light-activated SCR (LASCR) and the light-activated triac. A LASCR acts like a switch that changes states whenever it is exposed to a pulse of light. Even when the light is removed, the LASCR remains on until the anode and cathode polarities are reversed or the power is removed. A light-active triac is similar to a LASCR but is designed to handle ac currents. The symbol for a LASCR is shown below.
Optoisolators/optocouplers are devices that interconnect two circuits by means of an optical interface. For example, a typical optoisolator may consist of an LED and a phototransistor enclosed in a light-tight container. The LED portion of the optoisolator is connected to the source circuit, whereas the phototransistor portion is connected to the detector circuit. Whenever the LED is supplied current, it emits photons that are detected by the phototransistor. There are many other kinds of source-sensor combinations, such as LED-photodiode, LED-LASCR, and lamp-photoresistor pairs.
In terms of applications, optoisolators are used frequently to provide electrical isolation between two separate circuits. This means that one circuit can be used to control another circuit without undesirable changes in voltage and current that might occur if the two circuits were connected electrically. Isolation couplers typically are enclosed in a dark container, with both source and sensor facing each other. In such an arrangement, the optoisolator is referred to as a closed pair (see Fig. 5.30a). Besides being used for electrical isolation applications, closed pairs are also used for level conversions and solid-state relaying. A slotted coupler/interrupter is a device that contains an open slot between the source and sensor through which a blocker can be placed to interrupt light signals (see Fig. 5.30b). These devices are frequently used for object detection, bounce-free switching, and vibration detection. A reflective pair is another kind of optoisolator configuration that uses a source to emit light and a sensor to detect that light once it has reflected off an object. Reflective pairs are used as object detectors, reflectance monitors, tachometers, and movement detectors (see Fig. 5.30c).
Closed-pair optoisolators usually come in integrated packages. Figure 5.31 shows two sample optoisolator ICs.
While strictly speaking, optical fibers are "optical" rather than "optoelectrical," they are often used with photodiodes and LEDs/laser diodes as a medium through which to carry data encoded on beams of light. Figure 5.34 shows how an optical fiber works.
The boundary between the two layers, with an outer layer of higher reflective index, ensures that the fiber as a whole acts as a wave guide, with minimal losses of light as it bounces down the fiber. Therefore, optical fibers are much better than wires for transmitting data over long distances. They can also achieve staggering bandwidths, being immune to the induction and capacitive problems associated with wire. As such, they are heavily employed in the telecom industry, especially for high-bandwidth "pipes" between cities and also as submarine cables. It is becoming more and more common for fiber to replace copper in the final mile of telecom connections, actually terminating in homes to provide very high-speed Internet and cable TV.
As Fig. 5.34 implies, light will follow various paths through the fiber, and these paths will be of different lengths, limiting the effective bandwidth. This effect will reduce as the diameter of the internal core reduces. Taken to its limit, a single-mode fiber has a core of between 8 µm and 10.5 µm, and a cladding material diameter of 125 µm. This can be thought of as allowing only the straight-through path of light shown in Fig. 5.34. These fibers can achieve bandwidths in the order of 50 Gb/s over hundreds of miles.
Data are sent using a light source at the sending end (LED or laser diode) and a photodiode or transistor at the other end. Laser diodes are used in telecom systems because the coherent light that they produce travels better, but LEDs are also used in low-cost systems, such as fiber-based audio links in consumer digital audio systems.
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