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 MOSFETs or vacuum tubes, are configured so that their 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. Push–pull tube (valve) output stages 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. 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. Vacuum tube push–pull amplifiers usually use an output transformer, although Output-transformerless (OTL) tube stages exist (such as the SEPP/SRPP and the White Cathode