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Differential amplifier
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===Historical background=== Modern differential amplifiers are usually implemented with a basic two-transistor circuit called a ''“long-tailed” pair'' or ''differential pair''. This circuit was originally implemented using a pair of [[vacuum tube]]s. The circuit works the same way for all three-terminal devices with current gain. The bias points of “long-tail” resistor circuit are largely determined by [[Ohm's law]] and less so by active-component characteristics. The long-tailed pair was developed from earlier knowledge of push–pull circuit techniques and measurement bridges.<ref>{{cite journal |last1=Eglin |first1=J. M. |title=A Direct-Current Amplifier for Measuring Small Currents |journal=Journal of the Optical Society of America |date=1 May 1929 |volume=18 |issue=5 |pages=393–402 |doi=10.1364/JOSA.18.000393|bibcode=1929JOSA...18..393E }}<!--|access-date=15 February 2016--></ref> An early circuit which closely resembles a long-tailed pair was published by British neurophysiologist Bryan Matthews in 1934,<ref>{{cite journal |last1=Matthews |first1=Bryan H. C. |title=Proceedings of the Physiological Society |journal=The Journal of Physiology |date=1 December 1934 |volume=81 |issue=suppl |pages=28–29 |doi=10.1113/jphysiol.1934.sp003151 |doi-access=free}}</ref> and it seems likely that this was intended to be a true long-tailed pair but was published with a drawing error. The earliest definite long-tailed pair circuit appears in a patent submitted by [[Alan Blumlein]] in 1936.<ref>{{cite web |title=US Patent 2185367 |url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US2185367.pdf |publisher=Freepatensonline.com |access-date=15 February 2016}}</ref> By the end of the 1930s the topology was well established and had been described by various authors, including Frank Offner (1937),<ref>{{cite journal |last1=Offner |first1=Franklin |title=Push-Pull Resistance Coupled Amplifiers |journal=Review of Scientific Instruments |date=1937 |volume=8 |issue=1 |pages=20–21 |doi=10.1063/1.1752180|bibcode=1937RScI....8...20O }}<!--|access-date=15 February 2016--></ref> [[Otto Schmitt]] (1937)<ref>{{cite journal |last1=Schmitt |first1=Otto H. |title=Cathode Phase Inversion |journal=Review of Scientific Instruments |date=1941 |volume=12 |issue=11 |pages=548–551 |doi=10.1063/1.1769796 |url=https://www.aikenamps.com/images/Documents/schmt_a.pdf |access-date=15 February 2016}}</ref> and Jan Friedrich Toennies (1938),<ref>{{cite web |title=US Patent 2147940 |url=https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US2147940.pdf |publisher=Google Inc. |access-date=16 February 2016}}</ref> and it was particularly used for detection and measurement of physiological impulses.<ref>Geddes, L. A. ''Who Invented the Differential Amplifier?''. IEEE Engineering in Medicine and Biology, May/June 1996, p. 116–117.</ref> The long-tailed pair was very successfully used in early British computing, most notably the [[Pilot ACE]] model and descendants,<ref group="nb">Details of the long-tailed pair circuitry used in early computing can be found in ''Alan Turing’s Automatic Computing Engine'' (Oxford University Press, 2005, {{ISBN|0-19-856593-3}}) in Part IV, "ELECTRONICS".</ref> Maurice Wilkes’ [[EDSAC]], and probably others designed by people who worked with Blumlein or his peers. The long-tailed pair has many favorable attributes if used as a switch: largely immune to tube (transistor) variations (of great importance when machines contained 1,000 tubes or more), high gain, gain stability, high input impedance, medium/low output impedance, good clipper (with a not-too-long tail), non-inverting (''EDSAC contained no inverters!'') and large output voltage swings. One disadvantage is that the output voltage swing (typically ±10–20 V) was imposed upon a high DC voltage (200 V or so), requiring care in signal coupling, usually some form of wide-band DC coupling. Many computers of this time tried to avoid this problem by using only AC-coupled pulse logic, which made them very large and overly complex ([[ENIAC]]: 18,000 tubes for a 20-digit calculator) or unreliable. DC-coupled circuitry became the norm after the first generation of vacuum-tube computers.
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