An emitter follower is a transistor configuration where the output voltage follows the input voltage, less one diode drop (0.6 volts). This is useful because the output impedance is much lower than the input impedance, meaning the output signal can be used to drive higher power loads than the input. Due to its configuration, an emitter follower is not symmetric - it can source current through the transistor, but cannot sink current.
svg schematics by matt-chv is licensed under CC BY-SA 4.0
