An amplifier operating with the grid bias adjusted so plate current flows for the entire 360 degrees of the input waveform, by biassing the tube halfway between cutoff and saturation, in the most linear portion of the operating curves. The distortion is lowest in class A operation, but the efficiency is also very low. With the exception of single-ended amplifiers, the amplifiers most manufacturers call "class A" are actually just cathode-biased class AB amplifiers.