1. Field of the Invention
The invention relates to a driving circuit, more particularly to a driving circuit for driving a load, such as a light emitting diode, and a pixel circuit incorporating the same.
2. Description of the Related Art
Organic light emitting diode (OLED) displays are being increasingly widely used due to the advantages of spontaneous emission of light, high luminance, fast response time, and wide viewing angle.
A conventional OLED display utilizes a plurality of pixel circuits that are arranged in matrices and that can emit light of different colors to achieve the function of displaying images. With reference to FIG. 1, a conventional pixel circuit 1 includes an organic light emitting diode (OLED) 11 and a driving circuit 12. The driving circuit 12 generates a driving current (IDRIVE). The organic light emitting diode 11 is driven by the driving current (IDRIVE) from the driving circuit 12 to emit light with a luminance that corresponds to a magnitude of the driving current (IDRIVE).
The driving circuit 12 includes a first transistor 121, a second transistor 122, and a capacitor 123. Each of the first and second transistors 121, 122 is an N-type thin film transistor (TFT), and has a first terminal, a second terminal, and a control terminal.
The organic light emitting diode 11 has a cathode that is adapted for coupling to a first voltage source (VSS). The control terminal of the first transistor 121 is adapted for receiving a scan signal (SCAN). The first terminal of the first transistor 121 is adapted for receiving a data signal (VDATA). The second terminal of the first transistor 121 is coupled electrically to the control terminal of the second transistor 122. The first terminal of the second transistor 122 is adapted for coupling to a second voltage source (VDD). The second terminal of the second transistor 122 is coupled electrically to the second terminal of the first transistor 121 via the capacitor 123, and is coupled electrically to an anode of the organic light emitting diode 11.
Shown in FIG. 2 are timing sequences of the scan signal (SCAN) and the data signal (VDATA) for the driving circuit 12 of the conventional pixel circuit 1. When the scan signal (SCAN) is at a logic high level, the first transistor 121 is turned on, such that the data signal (VDATA) is transferred to the control terminal of the second transistor 122, and such that the capacitor 123 stores energy from the data signal (VDATA). On the other hand, when the scan signal (SCAN) is at a logic low level, the first transistor 121 is turned off. The second transistor 122 operates in the saturation region, and generates the driving current (IDRIVE) with reference to the energy stored in the capacitor 123 according to the following formula:
      I    DRIVE    =            1      2        ⁢                            k          122                ⁡                  (                                    V                              C                ,                123                                      -                          V                              TH                ,                122                                              )                    2      where (k122) is a device transconductance parameter of the second transistor 122, (VC,123) is the voltage across the capacitor 123, and (VTH,122) is a threshold voltage for the second transistor 122.
Since the threshold voltages of the second transistors 122 for individual pixel circuits 1 are not identical, the driving currents (IDRIVE) generated by the pixel circuits 1 differ from each other even with the same data signal (VDATA), thereby resulting in luminance variations among the light emitted by the organic light emitting diodes 11.
Several techniques have been developed in order to diminish the effect of the threshold voltage differences on driving current (IDRIVE) variations and involve adding more transistors and/or capacitors in the driving circuit. However, as the number of components increases, an aperture ratio (i.e., a ratio of coverage area of effective illuminating display region) of the OLED display utilizing these types of driving circuits is reduced. Consequently, utilization efficiency of the light is diminished. Moreover, in these driving circuits, the transistor that generates the driving current (IDRIVE) operates in the saturation region, thereby increasing power consumption.