Review Final Questions (Ch. 6)
Febuary 21st, 2005 ECE493 Topic 2 1) Why is active matrix better than passive matrix for large displays (> ~5)? In passive matrix displays, the display is just a grid of pixels that is driven by external circuitry. Typically, a row is addressed and the pictures data for that row is placed on the column lines. Only one row at a time can be addressed. Usually the rows are scanned sequentially, and if its done fast and bright enough it fools the eye into thinking that every row is being driven at the same time, instead of sequentially. Each row will be on for 1/(# of rows) of the time. To compensate for this fact, the more rows there are, the harder the pixel must be driven so it appears brighter. For displays greater than five inches, it becomes more and more difficult to fool the eye and the pixels need to be driven so hard that it reduces their lifetime. In active matrix displays, each pixel has a storage element and a driver built in so that after the data is written to the pixel, it will retain that data until it is addressed again. This means that the pixel is always on and doesnt need to be driven hard. This increases lifetime at the cost of increased complexity. 2) Draw an example of an active matrix display pixel driver circuit and explain its operation.
VData VAddress VDD
This circuit represents one possible implementation of an active matrix pixel. In this circuit, VAddress represents a row line, and VData represents a column line. VDD is the positive voltage that drives the pixel, which is represented as a diode in the circuit. When VAddress is high, the row containing this pixel has been selected. The top transistor turns on. The value of VData will be stored on the capacitor, so that when the row is turned off (VAddress goes low) the value will be retained on the capacitor. Connected to this capacitor is the gate of the lower transistor. When a high voltage has been stored on the capacitor, the gate is also high and the lower transistor is on, connecting VDD to the positive side of the diode (turning the pixel on). When a low voltage has been stored on the capacitor, the gate is also low and the lower transistor is off, disconnecting diode (turning the pixel off). 3) Explain the operation of an LCD pixel in a short paragraph. An LCD pixel uses the twisted nematic effect of liquid crystals. It relies on both the liquid crystals ability to bend the polarization of light, and the fact that the liquid crystal changes arrangement under the influence of an E-field. There is a cell full of nematic phase liquid crystal sandwiched between two polarizers, either aligned or offset by 90 degrees. The surfaces of the polarizers that are in contact with the liquid crystal are designed to force a 90 degree twist in the plane of polarization of the nematic fluid from the back of the cell to the front. This has the effect of twisting the plane of polarization of the light coming through the back by 90 degrees. Depending on the orientation of the front polarizer, this either causes a normally white or normally black cell. When an E-field is applied, the liquid crystal is forced out of the nematic phase and the plane of polarization no longer bends. Thus the cell can either twist the plane of polarization, or not twist it depending on or whether not there is an externally applied E-field.
4) How can we combine the benefits of both reflective and transmissive displays? Are there any drawbacks? Tansreflective displays combine features of both transmissive and reflective LCDs. They are reflective for high ambient lighting conditions (like in the sunlight), and they are transmissive for low ambient lighting (in the dark). Typically, they use less power than a fully transmissive LCD because they rely more heavily on ambient lighting to provide the light energy. Each pixel is divided into a reflective part and a transmissive part, so the only drawback is increased manufacturing complexity. 5) What are two methods of depositing OLEDs materials? OLED materials can be deposited using vapour-phase and liquid-phase deposition techniques. Which technique is used depends on the chemistry and physical properties of the organic material being deposited. If liquid-phase deposition is possible, this can enable ink-jet printing of substrates using organic inks in the print head. 6) How can field emission current be used to make a pixel? Either in a cathode ray tube (CRT) or in a field emission display (FED), the electrons that have ejected from the electron emitter (field emission current) strike a phosphor coating on the screen and are turned into visible photons. In a CRT, the electrons are steered using magnetic fields across a screen with a grid of pixels on the surface. The beam is either turned on or off to indicate whether a pixel is on or off. In a FED, each pixel has its own field emission tips built into the substrate behind the screen. This makes addressing easier since you dont have to steer a beam of electrons. However, it uses much more advanced processes than CRTs. 7) What are four ultimate qualities of the ideal electronic paper? Here are eight qualities: Ultimate goal of display technologies: Emulate printed paper Ultra-low power Perfectly bistable (keeps image with no power) Flexi...