An input stage for an integrated circuit device provides constant gain in selectable modes of either differential or single-ended operation. In one embodiment, transconductance devices are arranged so that the input impedance of the input stage matches the signal source impedance, regardless whether the input stage is selected to operate in a differential mode or in a single-ended mode. In accordance with an alternative embodiment, constant-gain and constant-impedance conditions are maintained in a configuration that comprises and input signal attenuator. In one application, the input stage may serve as a low-noise amplifier (LNA) for an integrated transceiver.