Editing Opto-isolator (section)

==Types of opto-isolators==

{| class="wikitable"
|-
! Device type<ref group=note>See Horowitz and Hill, p. 597, for an expanded list of opto-isolator types with their schematic symbols and typical specifications.</ref>
! Source of light<ref name=M100>Mims, p. 100.</ref>
! Sensor type<ref name=M100/>
! Speed
! Current transfer ratio
|-
| rowspan=3 | Resistive opto-isolator<br>(Vactrol)
| [[Incandescent light bulb]]
| rowspan=3 | [[cadmium sulfide|CdS]] or [[cadmium selenide|CdSe]] [[photoresistor]] (LDR)
| Very low
| rowspan=3 | <100%<ref group=note>Current through the photoresistor (output current) is proportional to the voltage applied across it. In theory it can exceed 100% of input current, but in practice dissipation of heat according to [[Joule's first law|Joule's law]] limits current transfer ratio at below 100%.</ref>
|-
| [[Neon lamp]]
| Low
|-
| [[Gallium arsenide|GaAs]] [[infrared]] [[light-emitting diode|LED]]
| Low
|-
| Diode opto-isolator
| GaAs infrared LED
| Silicon [[photodiode]]
| Highest
| 0.1–0.2%<ref name=T5/>
|-
| rowspan=2 | Transistor opto-isolator
| rowspan=2 | GaAs infrared LED
| Bipolar silicon [[phototransistor]]
| Medium
| 2–120%<ref name=T5>Mataré, p. 177, table 5.1.</ref>
|-
| [[Darlington transistor|Darlington]] phototransistor
| Medium
| 100–600%<ref name="T5"/>
<!-- |-
| FET opto-isolator
| GaAs infrared LED
| [[Field-effect transistor#Types of field-effect transistors|Photo FET]]
| Medium
|
-->
|-
| Opto-isolated SCR
| GaAs infrared LED
| [[Silicon-controlled rectifier]]
| Low to medium
| >100%<ref name=M177>Mataré, p. 177</ref>
|-
| Opto-isolated triac
| GaAs infrared LED
| [[TRIAC]]
| Low to medium
| Very high
|-
| [[Solid-state relay]]
| Stack of GaAs infrared LEDs
| Stack of photodiodes driving<br>a pair of [[MOSFET]]s or an [[Insulated-gate bipolar transistor|IGBT]]
| Low to high<ref group=note>Low-cost solid-state relays have switching times of tens of milliseconds. Modern high-speed solid-state relays like Avago ASSR-300 series (see [http://www.avagotech.com/docs/AV02-0452EN datasheet]) attain switching times of less than 70&nbsp;nanoseconds.</ref>
| Practically unlimited
|}

===Resistive opto-isolators===
{{main|Resistive opto-isolator}}
<!-- [[File:OEP series optocouples.jpg|thumb|right|Obsolete photoresistor opto-couples (Russian OEP series pictured) retain a niche in modern [[guitar amplifier]] market.]] -->
The earliest opto-isolators, originally marketed as ''light cells'', emerged in the 1960s. They employed miniature [[incandescent light bulb]]s as sources of light, and [[cadmium sulfide]] (CdS) or [[cadmium selenide]] (CdSe) photoresistors (also called light-dependent resistors, LDRs) as receivers. In applications where control linearity was not important, or where available current was too low for driving an incandescent bulb (as was the case in vacuum tube amplifiers), it was replaced with a [[neon lamp]]. These devices (or just their LDR component) were commonly named ''Vactrols'', after a trademark of Vactec, Inc. The trademark has since been [[genericized trademark|genericized]],<ref group=note>According to the [[United States Patent and Trademark Office]], trademark registered in 1969 for "photocell combined with a light source" is now dead ([http://tess2.uspto.gov/bin/showfield?f=doc&state=4005:pnh880.2.4 USPTO database record serial number 72318344]. Retrieved November 5, 2010). The same trademark, registered in 1993 for "medico-surgical tubing connector sold as a component of suction catheters" is now live and owned by Mallinckrodt Inc. ([http://tess2.uspto.gov/bin/showfield?f=doc&state=4005:pnh880.2.2 USPTO database record serial number 74381130]. Retrieved November 5, 2010).</ref> but the original Vactrols are still being manufactured by [[PerkinElmer]].<ref>Weber, p. 190; PerkinElmer, p. 28; Collins, p. 181.</ref><ref group=note>Vactec was purchased by [[EG&G]] (Edgerton, Germeshausen, and Grier, Inc.), a defense contractor, in 1983. In 1999 EG&G purchased formerly independent PerkinElmer, and changed own name PerkinElmer (see [[reverse takeover]]). An unrelated company, Silonex (a division of [[Carlyle Group]]) brands its photoresistive opto-isolators ''Audiohm Optocouplers''.</ref>

The turn-on and turn-off lag of an incandescent bulb lies in hundreds of [[milliseconds]] range, which makes the bulb an effective [[low-pass filter]] and [[rectifier]] but limits the practical modulation frequency range to a few [[Hertz]]. With the introduction of [[light-emitting diode]]s (LEDs) in 1968–1970,<ref>Schubert, pp. 8–9.</ref> the manufacturers replaced incandescent and neon lamps with LEDs and achieved response times of 5&nbsp;milliseconds and modulation frequencies up to 250&nbsp;Hz.<ref>PerkinElmer, pp. 6–7: "at 1 [[Foot-candle|fc]] of illumination the response times are typically in the range of 5&nbsp;ms to 100&nbsp;ms."</ref> The name ''Vactrol'' was carried over on LED-based devices which are, as of 2010, still produced in small quantities.<ref>Weber, p. 190; PerkinElmer, pp. 2,7,28; Collins, p. 181.</ref>

Photoresistors used in opto-isolators rely on bulk effects in a uniform film of [[semiconductor]]; there are no [[p-n junction]]s.<ref name=P3/> Uniquely among photosensors, photoresistors are non-polar devices suited for either AC or DC circuits.<ref name=P3/> Their resistance drops in reverse proportion to the intensity of incoming light, from virtually infinity to a residual floor that may be as low as less than a hundred [[Ohm]]s.<ref name=P3/> These properties made the original Vactrol a convenient and cheap [[automatic gain control]] and [[Dynamic range compression|compressor]] for telephone networks. The photoresistors easily withstood voltages up to 400&nbsp;volts,<ref name=P3>PerkinElmer, p. 3</ref> which made them ideal for driving [[vacuum fluorescent display]]s. Other industrial applications included [[photocopier]]s, industrial [[automation]], professional light measurement instruments and [[Exposure (photography)#Automatic exposure|auto-exposure meters]].<ref name=P3/> Most of these applications are now obsolete, but resistive opto-isolators retained a niche in audio, in particular [[guitar amplifier]], markets.

American guitar and organ manufacturers of the 1960s embraced the resistive opto-isolator as a convenient and cheap [[tremolo]] modulator. [[Fender Musical Instruments Corporation|Fender]]'s early tremolo effects used two [[vacuum tubes]]; after 1964 one of these tubes was replaced by an optocoupler made of a LDR and a neon lamp.<ref>Fliegler and Eiche, p. 28; Teagle and Sprung, p. 225.</ref> To date, Vactrols activated by pressing the [[Effects unit#Stompboxes|stompbox pedal]] are ubiquitous in the music industry.<ref>Weber, p. 190.</ref> Shortages of genuine PerkinElmer Vactrols forced the [[Do it yourself|DIY]] guitar community to "roll their own" resistive opto-isolators.<ref name=C181>Collins, p. 181.</ref> Guitarists to date prefer opto-isolated effects because their superior [[Ground (electricity)#Separating low signal ground from a noisy ground|separation of audio and control grounds]] results in "inherently high quality of the sound".<ref name=C181/> However, the [[distortion]] introduced by a photoresistor at [[line level]] signal is higher than that of a professional electrically-coupled [[Variable-gain amplifier|voltage-controlled amplifier]].<ref>PerkinElmer, pp. 35–36; Silonex, p. 1 (see also distortion charts on subsequent pages).</ref> Performance is further compromised by slow fluctuations of resistance owing to [[light history]], a [[memory effect]] inherent in [[cadmium]] compounds. Such fluctuations take hours to settle and can be only partially offset with [[Feedback#Electronic engineering|feedback]] in the control circuit.<ref>PerkinElmer, pp. 7, 29, 38; Silonex, p. 8.</ref>

===Photodiode opto-isolators===

[[File:Optically isolated.jpg|class=skin-invert-image|thumb|right|A fast photodiode opto-isolator with an output-side amplifier circuit]]

Diode opto-isolators employ LEDs as sources of light and silicon [[photodiode]]s as sensors. When the photodiode is reverse-biased with an external voltage source, incoming light increases the reverse current flowing through the diode. The diode itself does not generate energy; it modulates the flow of energy from an external source. This mode of operation is called [[photodiode#Photoconductive mode|photoconductive mode]]. Alternatively, in the absence of external bias the diode converts the energy of light into [[Electric potential energy|electric energy]] by charging its terminals to a voltage of up to 0.7&nbsp;V. The rate of charge is proportional to the intensity of incoming light. The energy is harvested by draining the charge through an external high-impedance path; the ratio of current transfer can reach 0.2%.<ref name=T5/> This mode of operation is called [[Photodiode#Photovoltaic mode|photovoltaic mode]].

The fastest opto-isolators employ [[PIN diode]]s in photoconductive mode. The response times of PIN diodes lie in the [[nanosecond|subnanosecond]] range; overall system speed is limited by delays in LED output and in biasing circuitry. To minimize these delays, fast digital opto-isolators contain their own LED drivers and output amplifiers optimized for speed. These devices are called ''full logic opto-isolators'': their LEDs and sensors are fully encapsulated within a digital logic circuit.<ref>Horowitz and Hill, pp. 596–597.</ref> The [[Hewlett-Packard]] 6N137/HPCL2601 family of devices equipped with internal output amplifiers was introduced in the late 1970s and attained 10&nbsp;[[Baud|MBd]] data transfer speeds.<ref>Porat and Barna, p. 464. See also full specifications of currently produced devices: ''[http://www.avagotech.com/docs/AV02-0940EN 6N137 / HCPL-2601 datasheet]''. [[Avago Technologies]]. March 2010. Retrieved November 2, 2010.</ref> It remained an industry standard until the introduction of the 50&nbsp;MBd [[Agilent Technologies]]<ref group=note>The former semiconductor division of Agilent Technologies operates as an independent company, [[Avago Technologies]], since 2005.</ref> 7723/0723 family in 2002.<ref name=AVA2002>''[http://www.thefreelibrary.com/TRADE+NEWS%3A+Agilent+Technologies+Introduces+Industry's+Fastest...-a094761414 Agilent Technologies Introduces Industry's Fastest Optocouplers]''. Business Wire. December 2, 2002.</ref> The 7723/0723 series opto-isolators contain [[CMOS]] LED drivers and a CMOS [[buffer amplifier|buffered amplifier]]s, which require two independent external power supplies of 5&nbsp;V each.<ref>[[Agilent Technologies]] (2005). ''[http://www.datasheetcatalog.org/datasheet2/1/03rgplhxdo8wqdacrplq8kjq29fy.pdf Agilent HCPL-7723 & HCPL-0723 50 MBd 2 ns PWD High Speed CMOS Optocoupler (Datasheet)]''. Retrieved November 2, 2010.</ref>

Photodiode opto-isolators can be used for interfacing analog signals, although their [[Diode#Current–voltage characteristic|non-linearity]] invariably [[Amplitude distortion|distorts the signal]]. A special class of analog opto-isolators introduced by [[Burr-Brown Corporation|Burr-Brown]] uses ''two'' photodiodes and an input-side [[operational amplifier]] to compensate for diode non-linearity. One of two identical diodes is wired into the [[feedback|feedback loop]] of the amplifier, which maintains overall current transfer ratio at a constant level regardless of the non-linearity in the second (output) diode.<ref name=HH598/>

A novel idea of a particular optical analog signal isolator was submitted on 3, June 2011. The proposed configuration consist of two different parts. One of them transfers the signal, and the other establishes a negative feedback to ensure that the output signal has the same features as the input signal. This proposed analog isolator is linear over a wide range of input voltage and frequency.<ref>Modern Applied Science Vol 5, No 3 (2011). ''[http://www.ccsenet.org/journal/index.php/mas/article/view/9543/7725  A Novel Approach to Analog Signal Isolation through Digital Opto-coupler (YOUTAB)]''.</ref> However linear opto couplers using this principle have been available for many years, for example the IL300.<ref>Vishay website, IL300 data (accessed 10-20-2015), ''http://www.vishay.com/optocouplers/list/product-83622/'' {{Webarchive|url=https://web.archive.org/web/20161227174819/http://www.vishay.com/optocouplers/list/product-83622/ |date=2016-12-27 }}.</ref>

[[Solid-state relays]] built around [[MOSFET]] switches usually employ a photodiode opto-isolator to drive the switch. The gate of a MOSFET requires relatively small total [[electric charge|charge]] to turn on and its leakage current in steady state is very low. A photodiode in photovoltaic mode can generate turn-on ''charge'' in a reasonably short time but its output ''voltage'' is many times less than the MOSFET's [[threshold voltage]]. To reach the required threshold, solid-state relays contain stacks of up to thirty photodiodes wired in series.<ref name=VI>Vishay Semiconductor.</ref>

===Phototransistor opto-isolators===

Phototransistors are inherently slower than photodiodes.<ref name=B61>Ball, p. 61.</ref> The earliest and the slowest but still common 4N35 opto-isolator, for example, has rise and fall times of 5 [[microsecond|μs]] into a 100 Ohm load<ref>Horowitz and Hill, p. 596. Ball p. 68, provides rise and fall time of 10 μs but does not specify load impedance.</ref> and its bandwidth is limited at around 10 kilohertz - sufficient for applications like [[electroencephalography]]<ref name=ANA/> or [[Pulse-width modulation#Power delivery|pulse-width motor control]].<ref name=B68>Ball, p. 68.</ref> Devices like PC-900 or 6N138 recommended in the original 1983 [[Musical Instrument Digital Interface]] specification<ref>''[http://www.midi.org/techspecs/electrispec.php MIDI Electrical Specification Diagram & Proper Design of Joystick/MIDI Adapter]''. MIDI Manufacturers Association. 1985. Retrieved November 2, 2010.</ref> allow digital data transfer speeds of tens of kiloBauds.<ref>Ball, p. 67.</ref> Phototransistors must be properly [[biasing|biased]] and loaded to achieve their maximum speeds, for example, the 4N28 operates at up to 50&nbsp;kHz with optimum bias and less than 4&nbsp;kHz without it.<ref name=P73>Pease, p. 73.</ref>

Design with transistor opto-isolators requires generous allowances for wide fluctuations of parameters found in commercially available devices.<ref name=P73/> Such fluctuations may be destructive, for example, when an opto-isolator in the [[Feedback loop#Electronic engineering|feedback loop]] of a [[DC-to-DC converter]] changes its [[transfer function]] and causes spurious oscillations,<ref name=Basso>Basso.</ref> or when unexpected delays in opto-isolators cause a [[short circuit]] through one side of an [[H-bridge]].<ref>Ball, pp. 181–182. Shorting one side of an H-bridge is called ''shoot-through''.</ref> Manufacturers' [[datasheet]]s typically list only worst-case values for critical parameters; actual devices surpass these worst-case estimates in an unpredictable fashion.<ref name=P73/> [[Bob Pease]] observed that current transfer ratio in a batch of 4N28's can vary from 15% to more than 100%; the datasheet specified only a minimum of 10%. Transistor [[Bipolar junction transistor#Transistor .27alpha.27 and .27beta.27|beta]] in the same batch can vary from 300 to 3000, resulting in 10:1 variance in [[Bandwidth (signal processing)|bandwidth]].<ref name=P73/>

Opto-isolators using [[field-effect transistor]]s (FETs) as sensors are rare and, like vactrols, can be used as remote-controlled analog potentiometers provided that the voltage across the FET's output terminal does not exceed a few hundred mV.<ref name=HH598>Horowitz and Hill, p. 598.</ref> Opto-FETs turn on without injecting switching charge in the output circuit, which is particularly useful in [[sample and hold]] circuits.<ref name=HH595/>

===Bidirectional opto-isolators===

All opto-isolators described so far are uni-directional. Optical channel always works one way, from the source (LED) to the sensor. The sensors, be they photoresistors, photodiodes or phototransistors, cannot emit light.<ref group=note>Exception: Ternary and quaternary [[gallium arsenide phosphide|GaAsP]] photodiodes can generate light. - Mims, p. 102.</ref> But LEDs, like all semiconductor diodes,<ref group=note>"Even the garden variety signal diodes you use in circuits have a small photovoltaic effect. There are amusing stories of bizarre circuit behavior finally traced to this." - Horowitz and Hill McCoulny, p. 184.</ref> are capable of detecting incoming light, which makes possible construction of a two-way opto-isolator from a pair of LEDs. The simplest bidirectional opto-isolator is merely a pair of LEDs placed face to face and held together with [[heat-shrink tubing]]. If necessary, the gap between two LEDs can be extended with a [[optical fiber|glass fiber insert]].<ref name=M102>Mims vol. 2, p. 102.</ref>

[[Visible spectrum]] LEDs have relatively poor transfer efficiency, thus [[infrared#Regions within the infrared|near infrared spectrum]] [[gallium arsenide|GaAs]], [[gallium arsenide|GaAs:Si]] and [[aluminium gallium arsenide|AlGaAs:Si]] LEDs are the preferred choice for bidirectional devices. Bidirectional opto-isolators built around pairs of GaAs:Si LEDs have current transfer ratio of around 0.06% in either [[photodiode#Photovoltaic mode|photovoltaic]] or [[photodiode#Photoconductive mode|photoconductive]] mode — less than photodiode-based isolators,<ref>Photodiode opto-isolators have current transfer ratios of up to 0.2% - Mataré, p. 177, table 5.1.</ref> but sufficiently practical for real-world applications.<ref name=M102/>