Editing Opto-isolator (section)

===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/>