Editing Opto-isolator (section)

===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>