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 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 special configuration of push-pull, though in fact an exception, are the outputs 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 protection diode in between, they are called "totem pole" outputs.
Vacuum tubes (valves) are not available in complementary types (as are pnp/npn transistors) so that the tube push-pull amplifier has a pair of identical output tubes or groups of tubes with the control grids driven in antiphase; these tubes drive current through the two halves of the primary winding of a center-tapped output transformer in such a way that the signal currents add, while the distortion signals due to the non-linear characteristic curves of the tubes subtract. These amplifiers were first designed long before the development of solid-state electronic devices; they are still in use by both audiophiles and musicians who consider them to sound better.
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[edit] 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).
[edit] Analog circuits
A conventional amplifier stage which is not push-pull is sometimes called single-ended to distinguish it from a push-pull circuit.
In analog push-pull power amplifiers the two output devices (transistors, tubes, FETs) or sets of devices operate in antiphase (i.e. 180° apart). The two antiphase outputs are connected to the load in a way that causes the signal outputs to be added, but distortion components due to non-linearity in the output devices are subtracted from each other; if the non-linearity of both output devices is similar, distortion is much reduced. Symmetrical push-pull circuits must cancel even order harmonics, like f2, f4, f6 and therefore promote odd order harmonics, like (f1), f3, f5 when driven into the nonlinear range.
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.
[edit] See also
- single-ended output
- Push–pull converter for more details on implementation
- Open drain
[edit] External links
- Push–pull vs. single-ended output in analogue tube amplifiers
