The operational amplifier, or op-amp, is perhaps the most adaptable and important analogue integrated circuit for a student to grasp. Op-amps are the workhorse of the analogue design industry, essentially nothing more than a differential amplifier with a very high voltage gain. A wide range of behaviours can be generated from a single device by cleverly applying feedback from an op-output amp's to one or more of its inputs. Although many other op-amp models are inexpensive, the circuits detailed in this chapter will use only those that are widely available.