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A push–pull output 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. Push–pull outputs are present in TTL and CMOS digital logic circuits and in some types of amplifiers, and are usually realized as a complementary pair of transistors, 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 "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.〔Joe Carr, ''RF Components and Circuits'', Newnes, page 84〕 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 transformers 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. 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.〔Donald Monroe McNicol, ''Radios' Conquest of Space: The Experimental Rise in Radio Communication'' Taylor & Francis, 1946 page 348〕 The technique was well-known at that time 〔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〕 and the principle had been claimed in an 1895 patent predating electronic amplifiers.〔US Patent 549,477 ''Local Transmitter Circuit for Telephones.'', W. W. Dean〕 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.〔(Radios - RCA Radiola Balanced Amplifier 1924 )〕 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.〔Gregory Malanowski ''The Race for Wireless: How Radio Was Invented (or Discovered?)'', AuthorHouse, 2011 ISBN 1463437501 pages 66-67, page 144〕 The technique continues to be used in audio, radio frequency, digital and power electronics systems today. == Digital circuits == 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 aren't 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" 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, ''tri-stated''). The alternative to a push–pull output is a single switch that connects the load either to ground (called an open collector or open drain output) or to the power supply (called an open-emitter or open-source output). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Push–pull output」の詳細全文を読む スポンサード リンク
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