A thin film circuit comprises a plurality of thin film transistors, wherein the output response of the circuit is dependent on at least first (36) and second (34, 22) of the transistors. The first thin film transistor (36) is, in use, exposed to a source of illumination (2) and the second thin film transistor (16) is shielded from the source of illumination. The first thin film transistor (36) is operated in use as an analogue switch which provides an analogue output in response to a control input. The illumination of this transistor reduces the effect of dynamic threshold voltage variations, which can be a limitation to circuit performance.