A voltage-controlled resistance with wide dynamic range and low distortion

Field-effect transistors (FET) can be used as voltage-controlled resistors, provided operation in the saturation region is avoided. However, to keep distortion low, the magnitude of the drain-to-source voltage must be kept low. This is true even when drain-to-gate feedback is applied to linearize the drain-current versus drain-to-source voltage characteristics. It is shown here that by employing a dependent source and a FET, a voltage-controlled resistance can be realized which has a linear range considerably greater (an order of magnitude or more) than other FET voltage-controlled resistances. The resulting circuit is stable and easy to implement.