Circuit and method for switching active loads of operational amplifier input stage

Abstract

An operational amplifier having a wide input common mode voltage range includes first () and second () differential input transistor pairs coupled to first () and second () tail current transistors. At least one of the first and second tail current transistor pairs is controlled by a common mode control circuit (). A gate of the first tail current transistor () is coupled to the common mode control circuit () to turn the first tail current transistor on and to turn the second tail current transistor off when the common mode input voltage is below a common mode threshold voltage (CMTHR). A folded cascode stage () is driven by the first and second differential input transistor pairs. Switched active load transistors are coupled to active load transistors of the folded cascode stage and are operable in response to the common mode control circuit to divert part of a current produced by one of the first and second differential input pairs from the folded cascode circuit, depending on whether the common mode input voltage is above or below the common mode threshold voltage.