Editing Push–pull output (section)

=== Push–pull transistor output stages ===
[[File:Aura VA 100 Evolution 2 (4061759992) - closeup of output stage.jpg|thumb|upright=1.5|Typical transistor output stage of one channel of a 65 watt stereo amplifier from 1993. The 2 MOSFET push-pull output transistors (''FET2, FET4'') are bolted to the black [[heat sink]].  They are driven by transistors ''Q2, Q5, Q6,'' and ''Q7'']]
{{more citations needed section|date=November 2012}}
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 the output devices (with only PNP or only NPN devices required).

==== Totem pole push–pull output stages ====
Two matched transistors of the same polarity 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 so important today) but is relatively difficult to balance and to keep a low distortion.

==== 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). 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 [[MOSFET]]s or [[vacuum tube]]s, are configured so that their [[Power-law#square-law|square-law]] transfer characteristics (that generate second-harmonic [[distortion]] if used in a single-ended circuit) cancel distortion to a large extent. That is, as one transistor's gate-source voltage increases, the drive to the other device is reduced by the same amount and the drain (or plate) current change in the second device approximately corrects for the non-linearity in the increase of the first.<ref>{{cite journal | author=Ian Hegglun | title=Practical Square-law Class-A Amplifier Design | journal=Linear Audio  |volume=1}}</ref>