Push–pull output: Difference between revisions

Revision as of 17:15, 11 April 2017

Revision as of 17:17, 11 April 2017

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[[File:7400 Circuit.svg|right|thumb|The TTL output stage is a rather complicated push–pull circuit known as a 'totem pole output' (the transistors, diode, and resistor in the right-most slice of this TTL [[logic gate]] circuit). It sinks currents better than it sources current.]]

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'tare not 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'''"<!-- [[Totem pole output]] redirects here---> 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, [[Three-state logic|''tri-stated'']]).