Tft compensation circuit for display device using reference current

A method of operating a pixel circuit for a display device that is operable in a combined programming and compensation phase, and operable in an emission phase;

• the operating method comprising the steps of:
• providing a pixel circuit comprising:
• a drive transistor configured to control an amount of current to the light-emitting device during the emission phase depending upon a voltage applied to a gate of the drive transistor;
• a second transistor connected to the gate of the drive transistor;
• a third transistor connected to a second terminal of the drive transistor;
• and
• a capacitor having a first plate that is connected to the gate of the drive transistor and a second plate that is connectable to a data voltage VDAT and a voltage supply;
• during the combined programming and compensation phase, at least partially compensating a threshold voltage and/or a carrier mobility of the drive transistor by application of a reference current by the steps of:
• placing the second transistor in an on state, wherein the drive transistor becomes diode-connected such that the gate and the second terminal of the drive transistor are connected through the second transistor;
• placing the third transistor in an on state to permit the reference current to be applied through the drive transistor;
• and
• applying a data voltage VDAT to the second plate of the capacitor;
• and during the emission phase, performing the steps of:
• placing the second transistor in an off state;
• placing the third transistor in an off state to remove the reference current;
• isolating the second plate of the capacitor from VDAT and connecting the voltage supply to the second plate of the capacitor;
• and
• controlling an amount of current to the light-emitting device depending upon a voltage applied to a gate of a drive transistor.