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Active Matrix Displays
• Multiplexing is limited and not adequate for high
resolutions (slow response, poor viewing angle, no
gray scale)
• A non-linear element is build into the substate at each
pixel, usually a thin-film-transistor
• Being isolated from other pixels by TFT’s, the voltage
remains constant while the other pixels are being
addressed
• Not subject to Alt-Pleshko Formalism
Active Matrix Circuit
Scan Line
Drain
Source
Liquid Crystal
Active Matrix: A Complex Device
Drain
Principle of Operation-Active Matrix
• One line at a time addressing
• A positive voltage pulse (duration T/N, N # rows, T frame time)
is applied to the line turning on all TFT’s
• The TFT’s act as switches allowing electrical changes to the
LC capacitors from the columns (data or source)
• When addressing subsequent rows a negative voltage is
applied to the gate lines thereby turning off the transistors for
one frame time T, until ready to readdress it
• For AC drives schemes (LCD’s) the polarity is alternated on the
data voltage
4 Basic Steps of TFT
1. At time 1, a positive voltage VG of duration T/N is applied to
gate to turn on TFT. The LC pixel (ITO) is changed to VON at
time 2 within T/N, due to the positive source voltage VSD=VON.
2. At time 2, the gate voltage VG becomes negative, turning off
the source voltage VSD from VON to –VON. During the time
period 2 and 3, of duration (N-1)/NT, the pixel voltage VP remains
about >0.9 VON as the LC capacitor is now isolated from data lines.
3. At time 3 (the next addressing time), the TFT is turned on again
by applying a positive gate voltage of duration T/N. The LC
capacitor now sees a negative source-to-drain voltage VSD=-VON.
The pixel electrode is discharged from VP=VON at time 3 to
VP=-VON within the time duration T/N.
4. At time 4, the TFT is turned off by the negative gate voltage, and
simultaneously the source voltage VSD changes from –VON to +VON.
TFT Addressing
VG
T/N
T
T
0
1 2
VSD
0
VON
time
Gate Voltage
time
Source Drain
Voltage
time
Pixel Voltage
3 4
VP
Notice that VP is not constant during the duration (n-1)T/N because
of a slight leakage current of LC cell. LC materials must have a
high voltage holding ratio (VHR) to minimize this.