Other unity gain buffer amplifiers include the bipolar junction transistor in common-collector configuration (called an "emitter follower" because the emitter voltage follows the base voltage, or a "voltage follower" because the output voltage follows the input voltage); the field effect transistor in common-drain configuration (called a source follower because the source voltage follows the gate voltage or, again, a "voltage follower" because the output voltage follows the input voltage); or similar configurations using vacuum tubes (cathode follower), or other active devices. All such amplifiers actually have a gain of slightly less than unity, but the difference is usually small and unimportant.