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== Digital circuits ==
[[File:7400 Circuit.svg|right|thumb|Circuit of [[Transistor–transistor logic|TTL]] [[NAND gate]] has a 'totem pole output' stage ''(right)'' consisting of two NPN transistors in push pull. When at least one of the inputs is low, transistor ''V''<sub>1</sub> is turned on, ''V''<sub>2</sub> is turned off, ''V''<sub>3</sub> is turned on and ''V''<sub>4</sub> off, pulling output voltage high. When both inputs are high, ''V''<sub>2</sub> is on, ''V''<sub>3</sub> is off and ''V''<sub>4</sub> is turned on, pulling output low.]]
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.
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The transformer-less circuit with two tetrode tubes dates back to 1933: "THE USE OF A VACUUM TUBE AS A PLATE-FEED IMPEDANCE." by J.W.Horton in the Journal of the Franklin Institute 1933 volume 216 Issue 6
The '''White Cathode Follower''' (Patent 2,358,428, Sep. 1944 by E. L. C. White) is similar to the SEPP design above, but the signal input is to the ''top'' tube, acting as a cathode follower, but one where the bottom tube (in common cathode configuration)
Transistor versions of the SEPP and White follower do exist, but are rare.
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