Active matrix display

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The term active matrix display or AMLCD (short for active matrix liquid crystal display ) is mainly used for liquid crystal screens that contain a matrix of thin film transistors with which LCD pixels are controlled. Each individual pixel has a circuit with active components (mostly transistors) and power supply connections.

Bernard J. Lechner ( RCA laboratories ), T. Peter Brody and Fang-Chen Luo (both at Westinghouse Electric ) were pioneers of this type of matrix control for LCDs . Lechner invented the concept and implemented a prototype with discrete components (FET transistors and capacitors) in 1968. Brody and Luo were able to present the first active matrix LCD with thin-film transistors in an integrated form in 1974.

The sample-and-hold circuit assigned to each pixel has two important functions:

  1. The control voltage of a pixel of a passive matrix decreases with an increase in the number of rows and columns used in a pixel matrix, because the effective voltage value (RMS) of the pulse-shaped control is decisive. Therefore, passive matrices are limited in size. A sample-and-hold circuit can keep this voltage at the value required to control the LC cell.
  2. The constant increase in the number of columns and lines and the reduction in the pixel sizes increase the parasitic capacitances of a pixel. In order to achieve the frame rate required for smooth image reproduction, the capacity must not cause a reduction in the switching speed. A local active circuit with a separate power supply line can guarantee fast switching with short, strong current pulses.

Both effects (arbitrarily low control voltage and compensation of parasitic capacitances) lead to the fact that active matrix displays are the only type of LCD , with the constant desire for smaller pixels ( PDAs , cell phones with color displays) or higher numbers of pixels ( notebook , digital camera display, liquid crystal screen) can meet.

The use of IPS LCD cell and the wide angle of the display opposite is super-twisted nematic (STN) - passive matrix displays improved.

The most important representatives of active matrix displays are displays with thin film transistors (English Thin Film Transistor, TFT ), in which the transistor is vapor-deposited directly onto the glass substrate . OLED displays are also implemented as active matrix displays. As long as high-frequency transistors made of plastic are not possible, an OLED requires a similar amplifier structure based on glass or silicon as a conventional LCD, with the difference that a light-emitting diode is operated with a current and not a voltage. Therefore, low-resistance, narrow supply lines in the display area are a challenge.

Displays with electronic paper also contain a TFT active matrix for controlling the pixels.

Individual evidence

  1. Award ceremony for the three inventors in 2011 ( Memento from September 12, 2013 in the Internet Archive ) by the IEEE .