TFT pixel threshold voltage compensation circuit with data voltage applied at light-emitting device

A method of operating a pixel circuit for a display device comprising the steps of:

• providing a pixel circuit comprising:
• a drive transistor configured to control an amount of current to a light-emitting device depending upon a voltage applied to a gate of the drive transistor;
• and
• a light-emitting device that is connected at a first node to a third terminal of the drive transistor and at a second node to a first voltage supply;
• 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 reference signal;
• during a phase preceding the emission phase, the second transistor is diode connected, and the phase preceding the emission phase includes applying a data voltage to the first node of the light-emitting device and the second terminal of the drive transistor, the data voltage being set so that a voltage across the light-emitting device is lower than a threshold voltage of the light emitting device;
• applying the reference signal to the second plate of the capacitor, wherein the reference signal is connected to a reference power supply;
• and
• changing the reference signal to a level for compensation for variations of a threshold voltage of the drive transistor.

A method of operating a pixel circuit for a display device comprising the steps of:

• providing a pixel circuit comprising:
• a drive transistor configured to control an amount of current to a light-emitting device depending upon a voltage applied to a gate of the drive transistor;
• and
• a light-emitting device that is connected at a first node to a third terminal of the drive transistor and at a second node to a first voltage supply;
• during a phase preceding the emission phase, the second transistor is diode connected, and the phase preceding the emission phase includes applying a data voltage to the first node of the light-emitting device and the second terminal of the drive transistor, the data voltage being set so that a voltage across the light-emitting device is lower than a threshold voltage of the light emitting device;
• wherein the pixel circuit is operable in an initialization phase, a combined programming and compensation phase, and in the emission phase;
• wherein the pixel circuit further comprises:
• a second transistor connected to the gate of the drive transistor and a second terminal of the drive transistor, such that when the second transistor is in an on state the drive transistor becomes diode-connected such that the gate and the second terminal of the drive transistor are connected through the second transistor;
• a third transistor connected to the first node of the light-emitting device, which connects a data voltage to the first node of the light-emitting device;
• a fourth transistor that is connected between the second terminal of the drive transistor and a second voltage supply;
• and
• at least one capacitor having a first plate that is connected to the gate of the drive transistor and a second plate that is connectable to a reference signal;
• the operating method comprising the steps of:
• during the initialization phase, reducing memory effects from previous frames by performing the steps of:
• keeping the fourth transistor in the on state and keeping the second and third transistors in an off state;
• inputting the reference signal wherein the reference signal during the initialization phase is set at a level corresponding to the drive transistor being in an off state;
• switching the second and third transistors from the off state to the on state, wherein the drive transistor becomes diode-connected through the second transistor, thereby connecting the gate of the drive transistor to the second voltage supply;
• and
• applying a data voltage VDAT to the first terminal of the light-emitting device, VDAT being set so that the voltage across the light-emitting device is lower than a threshold voltage;
• during the combined programming and compensation phase, at least partially compensating a threshold voltage of the drive transistor by the steps of:
• placing the fourth transistor in an off state;
• changing the reference signal to a level for compensation for variations of a threshold voltage of the drive transistor;
• and
• updating VDAT to a data voltage for a current pixel;
• and during the emission phase, performing the steps of:
• placing the second and third transistors in an off state;
• adjusting the reference signal to turn on the drive transistor;
• placing the fourth transistor in the on state to connect the second power supply to the drive transistor;
• and
• controlling an amount of current to the light-emitting device depending upon a voltage applied to a gate of a drive transistor.