Push–pull output: Difference between revisions

Revision as of 17:23, 11 April 2017

Revision as of 05:06, 26 June 2017

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 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 ==

Line 29:

A push–pull amplifier produces less [[distortion]] than a single-ended one. This allows a [[Class-A amplifier|class-A]] or AB push–pull amplifier to have less distortion for the same power as the same devices used in single-ended configuration. [[Class B amplifier#Class B and AB|Class AB and class B]] dissipate less power for the same output than class A; distortion can be kept low by [[negative feedback]] and by biassing the output stage to reduce crossover distortion.

A class - B push–pull amplifier is more efficient than a class-A power amplifier because each output device amplifies only half the output waveform and is cut off during the opposite half. It can be shown that the theoretical full power efficiency (AC power in load compared to DC power consumed) of a push–pull stage is approximately 78.5%. This compares with a class-A amplifier which has efficiency of 25% if directly driving the load and no more than 50% for a transformer coupled output.<ref name=Yunik73>Maurice Yunik ''Design of Modern Transistor Circuits'', Prentice-Hall 1973 {{ISBN |0-13-201285-5}} pp. 340-353</ref> A push–pull amplifier draws little power with zero signal, compared to a class-A amplifier that draws constant power. Power dissipation in the output devices is roughly one-fifth of the output power rating of the amplifier.<ref name=Yunik73/> A class-A amplifier, by contrast, must use a device capable of dissipating several times the output power.

The output of the amplifier may be direct-coupled to the load, coupled by a transformer, or connected through a dc blocking capacitor. Where both positive and negative power supplies are used, the load can be returned to the midpoint (ground) of the power supplies. A transformer allows a single polarity power supply to be used, but limits the low-frequency response of the amplifier. Similarly, with a single power supply, a capacitor can be used to block the DC level at the output of the amplifier.<ref>Donald G. Fink, ed. ''Electronics Engineer's Handbook'', McGraw Hill 1975 {{ISBN |978-0-07-020980-0}} pp. 13-23 through 13-24</ref>

Where bipolar junction transistors are used, the bias network must compensate for the negative temperature coefficient of the transistors' base to emitter voltage. This can be done by including a small value resistor between emitter and output. Also, the driving circuit can have silicon diodes mounted in thermal contact with the output transistors to provide compensation.