Jump to content

Push–pull output

From Wikipedia, the free encyclopedia
This is an old revision of this page, as edited by 202.78.155.231 (talk) at 08:35, 4 June 2011 (added a link). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
For other uses of "push–pull", see Push–pull (disambiguation).
A Class B push–pull output driver using bipolar junction transistors configured as emitter followers

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 amplifiers, 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.

Vacuum tubes (valves) are not available in complementary types (as are pnp/npn transistors), so 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.

A Magnavox stereo tube push–pull amplifier, circa 1960, utilizes two 6BQ5 output tubes per channel

Digital circuits

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 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.

In simpler digital circuits, especially in CMOS, 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

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 to be 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 less distortion for the same power as the same devices used in single-ended configuration. Class AB and class B dissipate less power for the same output as class A; distortion can be kept low by negative feedback.

Push-pull transistor output stages

Categories include:

Transformer-output transistor power amplifiers

It is now very rare to use output transformers with transistor amplifiers, although such amplifiers offer the best opportunity for matching output devices (with only PNP or only NPN devices required).

Totem-pole push-pull output stages

Two matched transistors of the same polarity (or, less often, Vacuum tubes) can be arranged to supply opposite halves of each cycle without the need for an output transformer, although in doing so the driver circuit often is asymmetric and one transistor will be used in a Common-emitter configuration while the other is used as an Emitter follower. This arrangement is less used today than during the 1970s; it can be implemented with few transistors (not now so important) but is relatively difficult to balance and so keep to a low distortion (the highly non-linear TTL circuits such as the 7400 use this arrangement).

Symmetrical Push-pull

Each half of the output pair "mirror" the other, in that an NPN (or N-Channel FET) device in one half will be matched by a PNP (or P-Channel FET) in the other. This type of arrangement tends to give lower distortion than quasi-symmetric stages because even harmonics are cancelled more effectively with greater symmetry.

Quasi-symmetrical push-pull

In the past when good quality PNP complements for high power NPN silicon transistors were limited, a workaround was to use identical NPN output devices, but fed from complementary PNP and NPN driver circuits in such a way that the combination was close to being symmetrical (but never as good as having symmetry throughout), and so distortion due to mismatched gain on each half of the cycle could be a significant problem.

Super-symmetric output stages

Employing some duplication in the whole driver circuit, to allow symmetrical drive circuits can improve matching further, although driver asymmetry is a small fraction of the distortion generating process. Using a Bridge-tied load arrangement allows a much greater degree of matching between positive and negative halves, compensating for the inevitable small differences between NPN and PNP devices.

Square-law push-pull

The output devices, usually MOSFETs, are configured so that their square-law transfer characteristics (that generate second harmonic Distortion is used in a single-ended circuit) cancel distortion to a large extent. That is, as the voltage across one transistor's gate-source voltage increases the remaining bias voltage to the complementary device is reduced by that amount and the drain current change in the second device approximately corrects for the non-linearity in the increase of the first. [1]

Push-pull tube (valve) output stages

See article: Valve audio amplifier – technical#The push-pull power amplifier.

These usually involve an output transformer to drop the output impedance to levels suitable for loudspeakers, although Output-transformerless (OTL) tube stages exist (such as for headphones, for 100 Volt line Public address sound systems, or for rare high-impedance loudspeakers).

Ultra-linear push-pull

Pentodes and Tetrodes can have their screen grid fed from a percentage of the primary voltage on the output transformer, giving efficiency and distortion that is a good compromise between triode (or Triode-strapped) power amplifiers circuits and conventional pentode or tetrode output circuits where the screen is fed from a relatively constant voltage source. See article: Ultra-Linear.

  1. ^ Ian Hegglun. "Practical Square-law Class-A Amplifier Design". {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |Journal= ignored (|journal= suggested) (help)

See also

Retrieved from "https://en.wikipedia.org/w/index.php?title=Push–pull_output&oldid=432486741"