Apparatus for providing a drive transistor control signal to a gate electrode of a drive transistor in an electroluminescent, EL, subpixel, comprising: (a) the electroluminescent, EL, subpixel having an EL emitter with a first and second electrode (207, 208), and having the drive transistor with a first supply electrode, a second supply electrode, and the gate electrode, wherein the second supply electrode of the drive transistor is electrically connected to the first electrode of the EL emitter for applying current to the EL emitter; (b) a first voltage supply electrically connected to the first supply electrode of the drive transistor; (c) a second voltage supply electrically connected to the second electrode of the EL emitter; (d) a test voltage source electrically connected to the gate electrode of the drive transistor; (e) a voltage controller for controlling voltages of the first voltage supply, second voltage supply and test voltage source to operate the drive transistor in a linear region; (f) a measuring circuit for measuring the current passing through the first and second supply electrodes (204, 205) of the drive transistor at different times to provide a status signal representing variations in the characteristics of the drive transistor and EL emitter caused by operation of the drive transistor and EL emitter over time, wherein the current is measured while the drive transistor is operated in the linear region; (g) means for providing a linear code value; (h) a compensator for changing the linear code value in response to the status signal to compensate for the variations in the characteristics of the drive transistor and EL emitter; and (i) a source driver for producing the drive transistor control signal in response to the changed linear code value for driving the gate electrode of the drive transistor, characterized in that the measuring circuit includes a first current mirror for producing a mirrored current which is a function of the drive current passing through the first and second supply electrodes (204, 205) and a second current mirror for applying a bias current to the first current mirror to reduce impedance of the first current mirror. (a) the electroluminescent, EL, subpixel having an EL emitter with a first and second electrode (207, 208), and having the drive transistor with a first supply electrode, a second supply electrode, and the gate electrode, wherein the second supply electrode of the drive transistor is electrically connected to the first electrode of the EL emitter for applying current to the EL emitter; (b) a first voltage supply electrically connected to the first supply electrode of the drive transistor; (c) a second voltage supply electrically connected to the second electrode of the EL emitter; (d) a test voltage source electrically connected to the gate electrode of the drive transistor; (e) a voltage controller for controlling voltages of the first voltage supply, second voltage supply and test voltage source to operate the drive transistor in a linear region; (f) a measuring circuit for measuring the current passing through the first and second supply electrodes (204, 205) of the drive transistor at different times to provide a status signal representing variations in the characteristics of the drive transistor and EL emitter caused by operation of the drive transistor and EL emitter over time, wherein the current is measured while the drive transistor is operated in the linear region; (g) means for providing a linear code value; (h) a compensator for changing the linear code value in response to the status signal to compensate for the variations in the characteristics of the drive transistor and EL emitter; and (i) a source driver for producing the drive transistor control signal in response to the changed linear code value for driving the gate electrode of the drive transistor,