A push–pull output is a type of
electronic circuitthat can drive either a positive or a negative current into a load. Push–pull outputs are present in TTL and CMOSdigital logic circuits and in some types of amplifier, 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. Because of the way these circuits are drawn schematically, with two transistors stacked vertically, they are sometimes called " totem pole" outputs. Vacuum tubes (valves) are not available in complementary types (as are pnp/npn transistors), so tube push-pull amplifiers are symmetric, with an output transformersupplying phase reversal. These amplifiers preceded solid-state versions, and are still in limited use by audiophiles..
transistoris switched on only when its complement is switched off. 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
loadeither to ground (called an open collectoror open drainoutput) or to the power supply (called an open-emitter or open-source output).
In analog push-pull amplifiers the two output devices (transistors, tubes, FETs) operate in
antiphase(i.e. 180° apart).
A push-pull amplifier produces less
distortionthan a single-ended one. This allows a class A or AB push-pull amplifier to have much less distortion for the same power than the same devices used in single-ended configuration. Class AB and class B dissipate much less power for the same output than class A; distortion can be kept low by large amounts of negative feedback.
Push–pull converterfor more details on implementation
* [http://www.aikenamps.com/SingleEnded.htm Push–pull vs. single-ended output] in analogue tube amplifiers
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