DC bias

Thread Starter

leoss

Joined Jan 17, 2012
14
Hi, could you please explain me these sentences? I mainly dont know what DC bias means.

And I exactly dont understand this part " Ideally, over time, the pixel experiences a net dc bias of zero. This is important because minimizing the dc bias over time reduces the chance of image retention"

Whole article is here: www.en-genius.net/includes/files/avt_092109.pdf

Alternating a high and low VC across the pixel allows the liquid crystal to experience opposite polarities (referenced to Vcom). Ideally, over time, the pixel experiences a net dc bias of zero. This is important because minimizing the dc bias over time reduces the chance of image retention or screen burn-in, which ultimately leads to a diminished life span of both the pixel and panel.


Thanks for advices
 

jimkeith

Joined Oct 26, 2011
540
Liquid crystal display pixels or segments may be activated simply by a DC voltage, but DC is to be avoided as the display degrades over a relatively short period of time loses contrast--this is called image retention as it causes a pattern of damage to the display. Applying a perfect AC potential also activates the pixels or segments and multiplies display life by preventing this degradation. If the AC is not balanced (positive and negative potentials or time period for rectangular waveform), it is said to have a DC bias--this DC voltage may be measured via a DC voltmeter.
 

colinb

Joined Jun 15, 2011
351
DC bias is more than the amplitude difference: it has to do also with the duration of time that a specific voltage is applied. That means the ac voltage driving the LCD segment must have a duty cycle of exactly 50%... if it's instead driven by +9 V for 1.02 ms and -9 V for 0.98 ms, there will be an accumulated dc bias.

I don't know how one would quantify the bias. If you take the integral of the voltage over time, you ideally get 0.0 V s (volt-seconds). But volt-seconds may not be very useful.
 

JMW

Joined Nov 21, 2011
123
Bias is a term leftover from vacuum tubes. In analog circuits different classes of amplifiers were used. Class A for example, gives the best fidelity. In order to do this a clean (undistorted) signal had to be inserted on the grid or (base in transistor circuits). Class A amplifiers must amplify equally both the positive and negative going part of the sine wave. Unfortunately the tube (transistor) would not conduct if the sine wave went below 0 on its negative peak. For example, let's say we want to amplify a 5 P-P VAC sine wave. To keep the signal above 0 volts a DC "bias" of 5 volts is placed on the grid (base). The grid then sees a perfect sine wave that varies from 10 to 0 VAC and will amplify it without distortion.
In order for the bias to remain at 5 volts and not creep up or down due to rectification (there are no perfect circuits), various types of circuits were designed in order to maintain the proper voltage.
The LCD segment in question is just the opposite, it must have 0 VDC, and also faithfully receive the digital image signal and process it.
Outside of engineering, bias is usually not looked on favorably. However, if you think about the usage, they are similar.
 
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