A bipolar transistor switch is the simplest RTL gate (inverter or NOT gate) implementing logical negation.[2] It consists of a common-emitter stage with a base resistor connected between the base and the input voltage source. The role of the base resistor is to expand the negligible transistor input voltage range (about 0.7 V) to the logical "1" level (about 3.5 V) by converting the input voltage into current. Its resistance is settled by a compromise: it is chosen low enough to saturate the transistor and high enough to obtain high input resistance.