This is the fifth of a series of articles covering electronics material which is common to A-level physics syllabuses offered by the various examination boards. The articles are aimed at teachers who are not familiar with this area of study. The first two articles dealt with elementary Boolean algebra and logic gates. The third article was written as an introduction to the operational amplifier, while the fourth discussed op. amp. action, negative feedback and the noninverting amplifier system. Copies of these may be obtained from the Editor ofElectronic Systems News. This article derives the working equations for the inverting amplifier system, discusses frequency response, and examines op. amp. action in astable and comparator circuits.