Digital circuits 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 sourcing 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" 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, tri-stated). An alternative to push–pull output is a single switch that disconnects or connects the load to ground (called an open collector or open drain output), or a single switch that disconnects or connects the load 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 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 2f, 4f, 6f and therefore promote odd order harmonics, like f, 3f, 5f 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. Distortion can occur at the moment the outputs switch: the "hand-off" is not perfect. This is called crossover distortion. Class AB and class B dissipate less power for the same output than class A; general distortion can be kept low by negative feedback, and crossover distortion can be reduced by adding a 'bias current' to smoothen the