The present disclosure relates to high speed and high precision characterization of VTSAT and VTLIN of FET arrays, relates to circuit structures and, more particularly, to circuit structures which detect high speed and high precision characterization of VTsat and VTlin of FET arrays and methods of manufacture and use. The circuit includes a control loop comprised of a differential amplifier, a plurality of FET arrays, and at least one analog switch enabling selection between a calibration mode and an operation mode.