# Pixel Circuit Designs for Active Matrix Displays
## 3 OLED Pixel Circuits  
### OLED Typical Pixel Circuits  
OLEDs are current-driven devices with display brightness proportional to the current flowing through them. This driving characteristic places stringent requirements on the driving capability and stability of the TFT. Designing and fabricating pixel circuits for OLEDs is more challenging than for TFT-LCDs [56]. 2T1C is the basic pixel circuit [40] shown in Figure 6a. When \(V_{\text{SCAN}}\) is at high voltage, the data signal \(V_{\text{DATA}}\) enters the internal pixel circuit through T1 and is stored in the capacitor Cl. When the Cl storage voltage exceeds the threshold voltage of T2, T2 provides the current for the continuous OLED lighting. During this process, the switching TFT requires a low leakage current to prevent the capacitor discharge. The driving TFT remains on throughout the frame cycle, continuously passing current to the OLED, requiring a high current transfer capability [56].  
LTPS TFTs and MO TFTs are currently the mainstream TFT technologies for OLED displays because of their good electrical characteristics. OLEDs operate by converting \(V_{\text{DATA}}\) to pixel current. However, both LTPS TFTs and IGZO TFTs exhibit unequal field effect mobility and \(V_{\text{TH}}\) when manufactured over a broad area due to manufacturing issues [65; 66]. Their gates are biased at a fixed voltage for long periods, causing \(V_{\text{TH}}\) shift [66; 67]. These shifts result in the uneven display. To address the aforementioned \(V_{\text{TH}}\) shift issue and increase the uniformity of OLED current, AM-OLED panels must employ compensatory mechanisms. There are two types of compensating methods: external compensation and internal compensation [34; 68].  
Figure 6: Typical 2T1C pixel circuit (**a**) and external compensation (**b**) [69].  
External compensation circuits (Figure 6b) facilitate simple pixel circuit structures. This method not only reduces the effects of TFT defects and simplifies drive signals but also improves aperture ratios and enables narrow-bezel designs. LGD successfully developed an 88-inch 8K OLED by using an external compensation method to detect and compensate for mobility and \(V_{\text{TH}}\) in real time [70]. Its pixel circuit is a simple 3T1C circuit and has no compensation function. Its external circuit requires an analogue-to-digital converter, a sensing data memory, a compensation algorithm unit, etc., making it more complex than the internal compensation method. The external compensation method requires an external IC, increasing the cost compared to the internal compensation method.  
Internal compensation circuits can be categorised as voltage programming circuits and current programming circuits according to the type of compensation. In the voltage programming method, the \(V_{\text{DATA}}\) is written to the gate of the drive TFT through a selector switch and stored in a storage capacitor [27]. Voltage programming circuits have been extensively studied and mass-produced due to their high compensability and ease of manufacture. Typical examples include the 4T2C circuit reported by [PERSON] [71], the 6T1C circuit of Samsung [72], etc. For current programming pixel circuits, mainstream compensation structures include the non-mirrored current source method [73] and the mirrored current source method [74]. Current current-programmed pixel circuits can effectively cope with the \(V_{\text{TH}}\) shift problem of driving TFTs, but this is not the main reason for adopting current programming. However, such circuits have limitations in high-frequency display applications due to the large RC delay of the data lines at low currents, which results in long signal build-up times [75].