Examples of input stages of circuits are configured to reduce both negative-bias temperature instability (NBTI) and positive-bias temperature instability (PBTI) in PMOS transistors therein. Current-switched PMOS source follower transistors and a low-side NMOS differential pair is used to process a lower range of a rail-to-rail input signal range of a circuit. A PMOS source follower is disposed between the positive input of the circuit and the positive input of the low-side NMOS differential pair. Another PMOS source follower is disposed between the negative input of the circuit and the negative input of the low-side NMOS differential pair. Various arrangements are provided for generating and maintaining the bias currents of the two PMOS source followers to be approximately the same through the entire lower input signal range.

A cascode amplifier includes a first common-source amplifier (CSA) having a first MOST (metal oxide semiconductor transistor) of a first type configured to receive a first input signal and output a first current to a first node; a first common-gate amplifier (CGA) having a second MOST of the first type and configured to receive the first current from the first node and output a second current to a second node in accordance with a first bias voltage; a first source-follower (SF) having a third MOST of a second type configured to receive a second input signal and output a first voltage at the first node; and a load configured to establish a third voltage at a third node in response to the second current through a DC (direct current) path between the second node and the third node.