Push–pull output

Push–pull output

A push–pull output is a type of electronic circuit that can drive either a positive or a negative current into a load. Push–pull outputs are present in TTL and CMOS digital 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 transformer supplying phase reversal. These amplifiers preceded solid-state versions, and are still in limited use by audiophiles..

Digital circuits

Each transistor is 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 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).

Analog circuits

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 distortion than 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.

See also

* single-ended output
* Push–pull converter for more details on implementation
* Open drain

External links

* [http://www.aikenamps.com/SingleEnded.htm Push–pull vs. single-ended output] in analogue tube amplifiers

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