Push-pull output
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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 realised as a complementary pair of transistors, one supplying 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.
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 transistor 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).
Analogue circuits
In analogue circuits such as Class B push-pull amplifiers, the two output transistors operate in antiphase (i.e. 180° apart).
External links
- Push-push vs. single-ended output (http://www.aikenamps.com/SingleEnded.htm) in analogue tube amplifiers