Push–pull output: Difference between revisions

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{{Short description|Type of electronic circuit}}

{{Other uses of|push–pull|Push–pull (disambiguation){{!}}Push–pull}}

{{More citations needed|date=November 2017}}

[[File:Pushpull (English).pngsvg|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.

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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.

== Digital circuits ==

[[File:7400 Circuit.svg|right|thumb|Circuit of [[Transistor–transistor logic|TTL]] [[NAND gate]] has a 'totem pole output' stage ''(right)'' consisting of two NPN transistors in push pull. When at least one of the inputs is low, transistor ''V''₁ is turned on, ''V''₂ is turned off, ''V''₃ is turned on and ''V''₄ off, pulling output voltage high. When both inputs are high, ''V''₂ is on, ''V''₃ is off and ''V''₄ is turned on, pulling output low.]]

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 sourcing 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'''" outputs.

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==== Totem pole push–pull output stages ====

Two matched transistors of the same polarity can be arranged to supply opposite halves of each cycle without the need for an output transformer, although in doing so the driver circuit often is asymmetric and one transistor will be used in a [[common-emitter]] configuration while the other is used as an [[emitter follower]]. This arrangement is less used today than during the 1970's1970s; it can be implemented with few transistors (not so important today) but is relatively difficult to balance and to keep a low distortion.

==== Symmetrical push–pull ====

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=== Push–pull tube (valve) output stages ===

{{See also|Valve audio amplifier – technical#The push–pull power amplifier}}

[[Vacuum tube]]s (valves) are not available in complementary types (as are pnpPNP/npnNPN transistors), so the tube push–pull amplifier has a pair of identical output tubes or groups of tubes with the [[control grid]]s driven in antiphase. These tubes drive current through the two halves of the primary winding of a center-tapped output transformer. Signal currents add, while the distortion signals due to the non-linear [[Current–voltage characteristic|characteristic curve]]s of the tubes subtract. These amplifiers were first designed long before the development of solid-state electronic devices; they are still in use by both [[audiophile]]s and musicians who consider them to sound better.

Vacuum tube push–pull amplifiers usually use an output transformer, although [[Output transformerless|Output-transformerless (OTL)]] tube stages exist (such as the SEPP/SRPP and the White Cathode Follower below).{{citation needed|date=December 2012}} The phase-splitter stage is usually another vacuum tube but a transformer with a center-tapped secondary winding was occasionally used in some designs. Because these are essentially square-law devices, the comments regarding [[Distortion#Cancellation of even-order harmonic distortion|distortion cancellation]] mentioned [[Push–pull output#Square-law push–pull|above]] apply to most push–pull tube designs when operated in [[Power amplifier classes#Class A|class A]] (i.e. neither device is driven to its non-conducting state).

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The transformer-less circuit with two tetrode tubes dates back to 1933: "THE USE OF A VACUUM TUBE AS A PLATE-FEED IMPEDANCE." by J.W.Horton in the Journal of the Franklin Institute 1933 volume 216 Issue 6

The '''White Cathode Follower''' (Patent 2,358,428, Sep. 1944 by E. L. C. White) is similar to the SEPP design above, but the signal input is to the ''top'' tube, acting as a cathode follower, but one where the bottom tube (in common cathode configuration) ifis fed (usually via a step-up transformer) from the current in the plate (anode) of the top device. It essentially reverses the roles of the two devices in SEPP. The bottom tube acts part way between a constant current sink and an equal partner in the push–pull workload. Again, the drive to each tube therefore might not be equal.

Transistor versions of the SEPP and White follower do exist, but are rare.