Push–pull output: Difference between revisions

Revision as of 18:19, 14 January 2020

Revision as of 01:57, 19 March 2020

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[[File:Pushpull (English).png|right|thumb|A Class B push–pull output driver using a pair of complementary PNP and NPN [[bipolar junction transistor]]s configured as [[emitter follower]]s]]

A '''push–pull''' amplifier is a type of [[electronic circuit]] that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load. This kind of amplifier can enhance both the load capacity and switching speed.

Push–pull outputs are present in [[Transistor-transistor logic|TTL]] and [[CMOS]] digital [[logic circuit]]s and in some types of [[electronic amplifier|amplifiers]], and are usually realized as a complementary pair of [[transistor]]s, 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 push–pull amplifier is more efficient than a single-ended [[Power_amplifier_classes#Class_A|"class-A"]] amplifier. The output power that can be achieved is higher than the continuous dissipation rating of either transistor or tube used alone and increases the power available for a given supply voltage. Symmetrical construction of the two sides of the amplifier means that even-order harmonics are cancelled, which can reduce distortion.<ref>Joe Carr, ''RF Components and Circuits'', Newnes, page 84</ref> DC current is cancelled in the output, allowing a smaller output transformer to be used than in a single-ended amplifier. However, the push–pull amplifier requires a phase-splitting component that adds complexity and cost to the system; use of center-tapped [[transformer]]s for input and output is a common technique but adds weight and restricts performance. If the two parts of the amplifier do not have identical characteristics, distortion can be introduced as the two halves of the input waveform are amplified unequally. [[Crossover distortion]] can be created near the zero point of each cycle as one device is cut off and the other device enters its active region.

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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 are 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'']]).