Disclosed herein are signal amplifiers that have an input impedance that varies over different bias currents. The signal amplifier includes a gain stage with a plurality of switchable amplification branches that are each capable of being activated such that one or more of the activated amplification branches provides a targeted adjustment to the input impedance. In addition, disclosed herein are signal amplifiers that have a variable-gain stage configured to provide a plurality of gain levels that result in different input impedance values presented to a respective signal by the variable-gain stage. The variable-gain stage can include a plurality of switchable amplification branches that provide a targeted adjustment to the respective input impedance values. The variable-gain stage can include a plurality of switchable inductive elements that are configured to be activated to provide a targeted adjustment to the respective input impedance values.