What Is an Active Matrix Liquid Crystal Display?

Mike Howells

An active matrix liquid crystal display (AMLCD) is a digital display technology often used in electronic devices such as mobile phones and laptop computers. AMLCD and passive matrix are the two types of liquid crystal displays (LCDs). Active matrix designs are typically of higher quality than passive ones, which is why they see use in more advanced electronics.

A desktop computer with an active matrix LCD monitor.
A desktop computer with an active matrix LCD monitor.

A basic, passive matrix LCD, such as the face of a digital watch or alarm clock, has a variety of conductive segments each attached to a separate circuit, generally placed in front of a backlight. Each of the intersections constitute a single pixel. To display different images, different combinations of these circuits are activated at once. When current runs through a particular column and row, the liquid crystal, which is normally twisted to opaque the backlight, unfurls to let light through.

Early laptop computers used this technology, but they suffered from low resolution and poor overall quality. The nature of passive matrix technology lends imprecision to the twisting and untwisting of single pixels in a dense matrix. This leads to a generally blurry quality of passive matrix displays because pixels surrounding the pixel intended to be lit up often partially untwist, letting light leak through.

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An active matrix liquid crystal display is much more complex and employs many small transistors and capacitors known as thin film transistors (TFT) to generate images. TFTs are arranged in a large grid, or matrix, that makes up the whole screen. Each intersection similarly represents a single pixel, but bleeding of light over into adjacent pixels is greatly reduced.

A pixel on an active matrix liquid crystal display is activated by turning on a particular column and sending an electric current through a particular row. While a charge remains in its capacitor, a TFT remains open to allow a backlight to shine through. The speed at which the active matrix can reset and redisplay a new charge is known as refresh rate, and faster rates allow for clearer depiction of fast video sequences and minimizes an effect known as ghosting. The lowest refresh rates on an active matrix liquid crystal display are still many times faster than the best passive matrix LCD.

Thanks to the use of capacitors, TFT pixels are capable of untwisting only partway. This means they can display a large degree of shades for any given color. Unlike passive pixels, an active matrix liquid crystal display can offer much more nuanced images with far better contrast.

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