Editing Sallen–Key topology (section)

==History and implementation==
In 1955, Sallen and Key used [[vacuum tube]] [[cathode follower]] amplifiers; the cathode follower is a reasonable approximation to an amplifier with unity voltage gain. Modern analog filter implementations may use [[operational amplifier]]s (also called ''op amps''). Because of its high input impedance and easily selectable gain, an operational amplifier in a conventional [[Operational amplifier applications#Non-inverting amplifier|non-inverting configuration]] is often used in VCVS implementations.{{Citation needed|date=January 2009}} Implementations of Sallen–Key filters often use an op amp configured as a [[Operational amplifier applications#Voltage follower|voltage follower]]; however, [[common collector|emitter]] or [[common drain|source]] followers are other common choices for the buffer amplifier.