Editing Push–pull output (section)

{{Short description|Type of electronic circuit}}
{{Other uses of|push–pull|Push–pull (disambiguation){{!}}Push–pull}}
{{More citations needed|date=November 2017}}

[[File:Pushpull.svg|right|thumb|A Class B push–pull output driver using a pair of complementary PNP and NPN [[bipolar junction transistor]]s configured as [[emitter follower]]s]]

A '''push–pull''' amplifier is a type of [[electronic circuit]] that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load.  This kind of amplifier can enhance both the load capacity and switching speed.

Push–pull outputs are present in [[Transistor-transistor logic|TTL]] and [[CMOS]] digital [[logic circuit]]s and in some types of [[electronic amplifier|amplifiers]], and are usually realized by a complementary pair of [[transistor]]s, one dissipating or ''sinking'' current from the load to ground or a negative power supply, and the other supplying or ''sourcing'' current to the load from a positive power supply.

A push–pull amplifier is more efficient than a single-ended [[Power amplifier classes#Class A|"class-A"]] amplifier. The output power that can be achieved is higher than the continuous dissipation rating of either transistor or tube used alone and increases the power available for a given supply voltage. Symmetrical construction of the two sides of the amplifier means that even-order harmonics are cancelled, which can reduce distortion.<ref>Joe Carr, ''RF Components and Circuits'', Newnes,  page 84</ref> DC current is cancelled in the output, allowing a smaller output transformer to be used than in a single-ended amplifier. However, the push–pull amplifier requires a phase-splitting component that adds complexity and cost to the system; use of center-tapped [[transformer]]s for input and output is a common technique but adds weight and restricts performance. If the two parts of the amplifier do not have identical characteristics, distortion can be introduced as the two halves of the input waveform are amplified unequally.  [[Crossover distortion]] can be created near the zero point of each cycle as one device is cut off and the other device enters its active region.

[[File:Tube push pull poweramplifier.PNG|thumb|alt=Schematic diagram of vacuum-tube amplifier| A vacuum tube amplifier often used a center-tapped output transformer to combine the outputs of tubes connected in push–pull.]]
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| image1   = Vacuum tube push-pull amplifier 1924.jpg
| caption1 = Transformer coupled vacuum tube push-pull audio amplifier from 1924.  The two [[triode]] output tubes are on right.
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| image2   = MagnavoxFrontcropped.jpg
| caption2 = A Magnavox stereo tube push–pull amplifier, circa 1960, utilizes two [[EL84|6BQ5]] output tubes per channel.  The two pairs of push-pull tubes are visible in front of the output transformers.
| width2   = 180
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Push–pull circuits are widely used in many amplifier output stages.  A pair of [[audion]] tubes connected in push–pull is described in [[Edwin H. Colpitts]]' US patent 1137384 granted in 1915, although the patent does not specifically claim the push–pull connection.<ref>Donald Monroe McNicol, ''Radios' Conquest of Space: The Experimental Rise in Radio Communication'' Taylor & Francis, 1946 page 348</ref> The technique was well known at that time <ref>http://www.leagle.com/xmlResult.aspx?page=5&xmldoc=193278360F2d723_1537.xml&docbase=CSLWAR1-1950-1985&SizeDisp=7 WESTERN ELECTRIC CO. v. WALLERSTEIN retrieved 12/12/12</ref> and the principle had been claimed in an 1895 patent predating electronic amplifiers.<ref>US Patent 549,477 ''Local Transmitter Circuit for Telephones.'', W. W. Dean</ref> Possibly the first commercial product using a push–pull amplifier was the [[RCA]] ''Balanced amplifier'' released in 1924 for use with their [[Radiola III]] regenerative broadcast receiver.<ref>[http://web.eecs.umich.edu/~srs/Antiques/templ.php?pid=223&collection=Radios Radios - RCA Radiola Balanced Amplifier 1924]</ref>  By using a pair of low-power vacuum tubes in push–pull configuration, the amplifier allowed the use of a loudspeaker instead of headphones, while providing acceptable battery life with low standby power consumption.<ref>Gregory Malanowski ''The Race for Wireless: How Radio Was Invented (or Discovered?)'', AuthorHouse, 2011 {{ISBN|1463437501}} pages 66-67, page 144</ref> The technique continues to be used in audio, radio frequency, digital and power electronics systems today.