# Pixel Circuit Designs for Active Matrix Displays
## 2 TFT-LCD Pixel Circuits
### TFT-LCD Novel Pixel Circuits
By leveraging these two sets of inverter circuits, the circuit can reach a higher operating voltage, which in turn allows the BPLCD to attain enhanced transmittance. The high-frequency effect is suppressed by generating a current that continuously charges the BPLCD, thus preventing the BPLCD from storing insufficient charge during the hold period and reducing the greyscale voltage. This circuit eliminates the need for a conventional common signal (COM) and improves the pixel opening ratio. It is worth noting that inverter topologies are commonly used in LCDs to provide high voltage power to the backlight and are not common in pixel circuits. LITPO TFT LCD pixel circuits  
The low leakage current of LTPO TFT reduces the power consumption of the display, and the pixel voltage can be maintained for a long time without refreshing. Therefore, LTPO TFT can be used in automotive displays that require long display times. To further reduce power consumption, some researchers have combined LTPO technology with low refresh rate memory-in-pixel (MIP) circuits for liquid crystal displays [63; 64] and achieved the desired results. Based on a previous study, [PERSON], [PERSON] et al. at Kyung Hee University proposed a self-refreshing multiphase MIP circuit with a simple structure for low-power LCDs [60]. The circuit consists of driver transistors (T1 and T2), programming transistors (T3 and T4 are oxide-TFTs with low leakage current), and storage capacitors (C1 and C2). The operating process consists of three phases: the programming phase, the refresh phase, and the emission phase. During the programming phase, the scanning signal turns on the programming transistors T3 and T4 in turn, through which \(V_{\text{DATA},\text{P}}\) and \(V_{\text{DATA},\text{N}}\) are programmed at the gate nodes of the driver transistors T1 and T2, respectively. After the programming phase, the row and column drivers are turned off and therefore do not consume power. During the negative polarity refresh phase, \(V_{\text{PXL},\text{N}}\) and \(V_{\text{RF},\text{N}}\) act on all pixels simultaneously. All pixels sequentially follow \(V_{\text{PXL},\text{N}}\) until a pixel-specific voltage corresponding to the programmed data voltage in each pixel is reached. When a still image is displayed, all pixels are refreshed at the same time instead of the traditional 1T1C line-by-line refresh, which consumes much more power. Compared to SRAM-type MIP [63], this circuit uses a simple multilevel memory cell to achieve multilevel greyscale adjustment with higher aperture ratios and pixel densities. Currently, LTPO is less used in TFT-LCD products. As the technology develops, we may see more LCD technology combined with LTPO, especially in terms of energy savings and refresh rate optimisation. Double-gate TFT LCD pixel circuits  
Oxide-TFTs have the advantage of low leakage current, their carrier mobility can be increased, and subthreshold swing can be reduced using a double gate structure to achieve better device performance. [PERSON], [PERSON] et al. proposed a self-refreshing multilevel memory-in-pixel circuit for reflective low-power LCD including oxide-TFTs [61]; the circuit consists only of two DG IGZO-TFTs and one storage capacitor (Figure 5b). The two gates of this TFT are connected for driving, and \(V_{\text{RF}}\) and \(V_{\text{PXL}}\) are global signal lines connected to the gate and source of the \(\text{T}_{\text{Memory}}\) (composed of T2 and \(\text{C}_{\text{ST}}\)), respectively. Its working process consists of three phases: the programming phase (\(\text{T}_{\text{PRG}}\)), the refresh phase (\(\text{T}_{\text{RF}}\)), and the display phase (\(\text{T}_{\text{DISP}}\)). Programming phase: the data voltage is programmed in the pixels. Refresh phase: all pixels are simultaneously refreshed according to the data voltages programmed in the programming phase, the refresh process of which is not depicted in detail here. In the display phase, the pixel voltages are retained.