Page "Differential amplifier" Paragraph 15

The long-tailed pair was very successfully used in early British computing, most notably the Pilot ACE Model and descendants, Wilkes ' EDSAC, and probably others designed by people who worked with Blumlein or his peers.

The long-tailed pair has many attributes as a switch: largely immune to tube ( transistor ) variations ( of great importance when machines contained 1, 000 or more tubes ), high gain, gain stability, high input impedance, medium / low output impedance, good clipper ( with not-too-long tail ), non-inverting ( EDSAC contained no inverters!

) and large output voltage swings.

One disadvantage is that the output voltage swing ( typically ± 10 – 20 V ) was imposed upon a high DC voltage ( 200 V or so ), requiring care in signal coupling, usually some form of wide-band DC coupling.

Many computers of this time tried to avoid this problem by using only AC-coupled pulse logic, which made them very large and overly complex ( ENIAC: 18, 000 tubes for a 20 digit calculator ) or unreliable.

DC-coupled circuitry became the norm after the first generation of vacuum tube computers.