Push–pull output: Difference between revisions

Revision as of 18:10, 23 November 2017

Revision as of 04:36, 2 June 2018

Line 11:

[[Image:MagnavoxFrontcropped.jpg|thumb|A Magnavox stereo tube push–pull amplifier, circa 1960, utilizes two [[EL84|6BQ5]] output tubes per channel]]

Push–pull circuits are widely used inThankuin 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.

== Digital circuits ==

[[File:7400 Circuit.svg|right|thumb|The TTL output stage is a rather complicated push–pull circuit known as a 'totem pole output' (the transistors, diode, and resistor in the right-most slice of this TTL [[logic gate]] circuit). It sinks currents better than it sources current.]]

A digital use of a push–pull configuration is the output of TTL and '''related''' families. The upper transistor is functioning as an active pull-up, in linear mode, while the lower transistor works digitally. For this reason they are not capable of supplying as much current as they can ''sink'' (typically 20 times less). Because of the way these circuits are drawn schematically, with two transistors stacked vertically, normally with a level shifting diode in between, they are called "'''totem pole'''"<!-- [[Totem pole output]] redirects here---> outputs.

A disadvantage of simple push–pull outputs is that two or more of them cannot be connected together, because if one tried to pull while another tried to push, the transistors could be damaged. To avoid this restriction, some push–pull outputs have a third state in which both transistors are switched off. In this state, the output is said to be ''floating'' (or, to use a proprietary term, [[Three-state logic|''tri-stated'']]).