In electronics, a Q multiplier is a circuit added to a radio receiver to improve its selectivity and sensitivity.
The Q multiplier was a common accessory in shortwave receivers of the vacuum tube era as either a factory installation or an add-on device.
In use, the Q multiplier had to be adjusted to a point just short of oscillation to provide maximum sensitivity and rejection of interfering signals.
This could be used for reception of single sideband or Morse radiotelegraphy, but in that case the circuit no longer provided improved selectivity.
At least one console-model broadcast superheterodyne receiver used positive feedback to improve selectivity in a 1926 design.