A.C. performance of f.e.t. analogue switch

The a.c. performance of a series f.e.t. analogue switch is investigated, and it is shown that this choice of switch is not always ideal; since, in the `off¿ state, signal feedthrough occurs, while, in the `on¿ state, turnoff of the device can occur past a critical frequency (i.e. frequency pinchoff). Signal feedthrough is easily reduced to negligible proportions by using shunt/series switching; however, frequency pinchoff is shown to be very difficult to eliminate completely. This is especially true when analogue-signal levels are large, and fast switching is required. Therefore, in such circumstances, the technique of elimination is shown to be the restriction of operation to frequencies below the pinchoff frequency. Graphical data are presented which allow this frequency to be determined, and experimental work has confirmed its accuracy.