An apparatus for determining an adjustment to a signal voltage for compensating for changes in the threshold voltage (Vth) for a drive transistor in a pixel drive circuit in an active matrix OLED display having at least one OLED light-emitting pixel, comprising: the pixel drive circuit having a data line, a power supply line, a drive transistor; the drive transistor being electrically connected to the power supply line and to the OLED light-emitting pixel; the switch transistor being electrically connected to the gate electrode of the drive transistor and to the data line; first means for applying a first voltage to the power supply line; second means for applying a second voltage to the power supply line opposite in polarity to the first voltage; third means for producing a threshold-voltage-related signal on the data line; and fourth means responsive to the threshold-voltage-related signal for calculating the adjustment to the signal voltage.