Abstract:
An improved AM OLED pixel circuit and method of wide dynamic range dimming for AM OLED displays are disclosed that maintain color balance throughout the dimming range, and also maintain the uniformity of the luminance and chromaticity of the display at low gray-levels as the display is dimmed to lower luminance values. As such, AM OLED displays can meet the stringent color/dimming specifications required for existing and future avionics, cockpit, and hand-held military device display applications. Essentially, the OLED pixel circuit and method of dimming that are disclosed use Pulse Width Modulation (PWM) of the OLED pixel current to achieve the desired display luminance. Two example circuits are disclosed that externally PW modulate the common cathode voltage or common power supply voltage to modulate the OLED current in order to achieve the desired display luminance. Three example circuits are disclosed that incorporate additional transistor switches in the pixel circuit to modulate the OLED current during the frame time. By PWM of the OLED current, in combination with data voltage (or current) modulation, wide dynamic range dimming can be achieved while maintaining the color balance and the luminance and chromaticity uniformity required over the surface of the display involved.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of Ser. No. 11/043,657 filed on Jan. 26, 2005. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to the field of flat panel displays, and more specifically, but not exclusively, to an improved Active Matrix Organic Light Emitting Diode (AM OLED) display and method of wide dynamic range dimming in such a display for commercial and military applications, such as, for example, cockpit displays, avionics displays, or hand-held military communication device displays. 
     2. Description of Related Art 
     AM OLED displays are an emerging flat panel display technology, which has already produced such new products as passive matrix-addressed displays that can be used for cell-phones and automobile audio systems. AM OLED displays are most likely to replace backlit AM Liquid Crystal Displays (LCDs) because AM OLED displays are more power efficient, rugged, weigh less, cost less, and have much better image quality than existing AM LCDs. As such, the market for AM OLED-based displays is estimated to reach about $1.7 B per year by 2006. 
     Cockpit display applications are relatively demanding for existing display technologies, because of the stringent requirements imposed with respect to image quality and the need for superior operational performance within a broad range of environments, such as high temperature, humidity, and ambient lighting environments. For the better part of the past ten years, AM LCDs have replaced Cathode Ray Tube (CRT) displays in cockpit applications, because of the advantages of AM LCDs over CRT displays in terms of lower weight, flatter form factor, less power consumption, the use of large active areas with relatively small bezels, higher reliability, higher luminance, greater luminance uniformity, wider dimming range, and better sunlight readability. As such, AM LCDs have been the displays of choice for cockpit and avionics display applications for a number of years. 
     A significant problem that exists with AM LCDs for display applications (e.g., cockpit, avionics and hand-held device displays) is that the backlighting of the AM LCDs adds a significant amount of weight and volume to these types of displays. However, an advantage of this backlighting feature of AM LCDs is that it provides a highly controllable function for (independently) dimming the display in order to achieve optimum performance over a range of ambient lighting conditions. Some critical display applications (e.g., avionics and certain military device displays) require wide dynamic ranges of dimming (e.g., &gt;2000:1) for the display to be viewed comfortably in both daytime (bright) and night-time (dark) viewing conditions. Currently, this dimming function can be accomplished with AM LCDs by dimming the display backlight (through a large dynamic range), while maintaining the AM LCD&#39;s optimized driving conditions. 
     The weight and volume problems that exist with AM LCDs for avionics or hand-held device applications, for example, can be alleviated with AM OLED displays. Compared to AM LCDS, AM OLED displays offer such significant advantages as wider viewing angles, lower power consumption, lighter weight, superior response time, superior image quality, and lower cost. However, a drawback of the existing AM OLED displays is that they are not easily dimmable (i.e., their brightness adjusted) to the desired luminance levels, except by changing the driving conditions of the AM OLED displays, or by varying the anode (V DD ) and/or cathode (V K ) voltages. 
     Generally, the existing AM OLED displays&#39; grayscale driving conditions are optimized for “normal” daytime (bright ambient) viewing conditions. However, changing either the grayscale driving conditions or the V DD /V K  voltages of AM OLED displays to achieve lower display luminance levels for night (dark ambient) conditions using a conventional AM OLED display results in luminance and color non-uniformities across the surfaces of these displays. 
     As such, an important requirement imposed on AM OLED displays in such critical applications as cockpit displays, avionics displays, or military hand-held device displays is that such displays have to be capable of adjusting their luminance (brightness) over a wide dynamic range (e.g., &gt;2000:1) without affecting the color balance and/or the uniformity of the luminance and chromaticity across the surface of the display as the display is being dimmed. The drive methods used for existing AM OLED displays achieve the desired luminance by adjusting the grayscale data voltage (or current) or V DD /V K  voltage (s). However, these existing methods of adjusting the luminance of AM OLED displays create numerous problems for wide dynamic range display dimming applications, such as: (1) it is a relatively difficult problem to achieve the desired wide dynamic range dimming requirements with the existing driving methods using 8-bit data (column) drivers currently available for AM OLED displays; (2) when the grayscale data voltages (or currents) or the V DD /V K  voltages, which are optimized for “normal” daylight operation, are changed (e.g., reduced) for night-time (low luminance) operation, typically the display color balance is changed due to the different transfer characteristics (luminance versus voltage) for the Red, Green and Blue (R, G, B) AM OLED display materials used; and (3) operation of the existing AM OLED displays at the low luminance levels associated with night-time viewing conditions results in significant non-uniformities in the luminance and chromaticity across the surface of the displays due to increased variations in the Thin-Film Transistor (TFT) and OLED performance in the low luminance (gray-level) regime. 
     As such, to illustrate these problems with existing AM OLED displays,  FIG. 1  depicts an electrical schematic diagram of a typical AM OLED sub-pixel circuit  100  (labeled “Prior Art”), which is currently used in a conventional method for dimming an AM OLED display. Referring to  FIG. 1 , conventional sub-pixel circuit  100  includes a first TFT  102 , a second TFT  104 , a storage capacitor  106 , and an OLED pixel  108 . As shown, transistor  102  is a scan transistor, and transistor  104  is a drive transistor. The gate terminal  110  of the scan transistor  102  is connected to the row (scan/row enable) address bus of the display involved, and the drain terminal  112  of scan transistor  102  is connected to the column (data) address bus of the display. The source of scan transistor  102  is connected to the node  107  at the storage capacitor  106  and the gate terminal of the drive transistor  104 . During the row addressing time period of the display operation, scan transistor  102  charges the node  107  at the storage capacitor  106  and the gate terminal of the drive transistor  104  to the data voltage (signal), V DATA . After the row addressing time period, scan transistor  102  is switched off, and the OLED pixel  108  is electrically isolated from the data bus. During the remainder of the frame time, the power supply voltage, V DD , which is connected to the drain terminal  114  of the drive transistor  104 , provides the current for driving the OLED pixel  108 . 
     The grayscale from this conventional method in the AM OLED display circuit  100  depicted in  FIG. 1  is achieved by varying the data voltages (signals) on the data bus. In addition, the brightness (maximum luminance) of the display is adjusted (for display dimming) directly by changing the data voltages (signals) or V DD /V K  voltages. However, as discussed earlier, it can be seen from  FIG. 1  that a significant problem with these conventional methods of adjusting the luminance of an AM OLED display is that because the dimming is performed by changing the data voltage (or current), or by changing the power supply (V DD  and/or V K ) voltages to adjust the grayscale, wide dynamic range dimming (e.g., &gt;2000:1) cannot be achieved with suitable uniformity. Nevertheless, as described in detail below, the present invention provides an improved AM OLED display and method of adjusting luminance with superior dimming capability (e.g., wide dynamic range&gt;2000:1) that resolves the problems encountered with existing AM OLED displays and other prior art displays. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved AM OLED pixel circuit and method of wide dynamic range dimming for AM OLED displays that maintains color balance throughout the dimming range, and also maintains the uniformity of the luminance and chromaticity of the display at low gray-levels as the display is dimmed to lower luminance values. As such, the present invention enables AM OLED displays to meet the stringent color/dimming specifications required for existing and future avionics, cockpit, and hand-held military device display applications. Essentially, the present invention provides an improved AM OLED pixel circuit and method of dynamic range dimming that uses Pulse Width Modulation (PWM) of the OLED pixel current to achieve the desired display luminance (brightness). 
     Two example embodiments of the invention are provided for externally (e.g., outside an AM OLED glass display) PW modulating the common cathode voltage (V K ) or common power supply voltage (V DD ) so as to modulate the OLED current in order to achieve the desired display luminance. Three additional example embodiments of the invention are provided that incorporate additional transistor switches in the pixel circuit in order to modulate the OLED current during the frame time. Unlike the conventional methods, the three additional (internal) example embodiments allow modulation of each row of pixels sequentially during the frame time, which eliminates any propensity for display flicker. Thus, by PW modulating the OLED current, in combination with data voltage (or current) modulation, the present invention achieves wide dynamic range dimming while maintaining the color balance and the luminance and chromaticity uniformity required over the surface of the display involved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  depicts an electrical schematic diagram of a prior art AM OLED sub-pixel circuit, which is currently used in a conventional method for dimming an AM OLED display; 
         FIG. 2A  depicts a pictorial representation of an example cockpit or avionics display environment, which may be used as an environment to implement one or more embodiments of the present invention; 
         FIG. 2B  depicts a pictorial representation of an example cockpit or avionics display, in which one or more embodiments of the present invention may be implemented; 
         FIG. 3  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit, which can be used to implement a first embodiment of the present invention; 
         FIG. 4  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit, which can be used to implement a second embodiment of the present invention; 
         FIG. 5  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit, which can be used to implement a third embodiment of the present invention; 
         FIG. 6  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit, which can be used to implement a fourth embodiment of the present invention; and 
         FIG. 7  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit, which can be used to implement a fifth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures,  FIG. 2A  depicts a pictorial representation of an example cockpit or avionics display environment  200 A, which may be used as an environment to implement one or more embodiments of the present invention.  FIG. 2B  depicts a pictorial representation of an example cockpit or avionics display  200 B (e.g., from within the example environment  200 A) including an example display  202 B, in which one or more embodiments of the present invention may be implemented. As such, although  FIGS. 2A and 2B  depict an exemplary environment and avionics or cockpit display, the present invention is not intended to be so limited and can be implemented in any suitable display requiring, for example, wide dynamic range dimming (e.g., military or commercial hand-held device with flat panel display, etc.). 
       FIG. 3  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit  300 , which can be used to implement a first embodiment of the present invention. As such, AM OLED sub-pixel circuit  300  can be used in a preferred method for dynamically dimming an AM OLED display using, for example, an external (to the display) PWM scheme. Referring now to  FIG. 3 , AM OLED sub-pixel circuit  300  includes a first TFT  302 , a second TFT  304 , a storage capacitor  306 , an OLED pixel  308 , and a transistor  310 , represented here by a Field Effect Transistor (FET). As shown, transistor  302  is a scan transistor, and transistor  304  is a drive transistor. The gate terminal  312  of the scan transistor  302  is connected to the row (scan/row enable) address bus of the display involved, and the drain terminal  314  of scan transistor  302  is connected to the column (data) address bus of the display. The source of scan transistor  302  is connected to the node  307  at the storage capacitor  306  and the gate terminal of the drive transistor  304 . The source of drive transistor  304  is connected to a terminal of OLED pixel  308 . The second terminal  318  of OLED pixel  308  is connected to one (e.g. drain) terminal of transistor  310 . The other (e.g. source) terminal of transistor  310  is connected to a common cathode terminal, V K    320 . 
     For this exemplary embodiment, an AM OLED display incorporating AM OLED pixel circuit  300  can include a plurality of (e.g., two or more) common cathode terminals, V K    320 . One such common cathode terminal, V K    320 , can be used to cover a top half of the display rows on the display involved, and another common cathode terminal, V K    320 , can be used to cover a bottom half of the display rows on the display involved. For example, a display can include 480 rows and 640 columns. Each of the common cathode terminals, V K    320 , in such an AM OLED display can be switched to the cathode voltage through the transistor  310  controlled by a PWM signal generator  322 . An example frequency for a PWM signal from generator  322  is 60 Hz. 
     During the row addressing time period of the display operation, scan transistor  302  charges the node  307  at the storage capacitor  306  and the gate terminal of the drive transistor  304  to the data voltage (signal), V DATA . After the row addressing time period, scan transistor  302  is switched off, and the OLED pixel  308  is electrically isolated from the data bus. 
     For this exemplary embodiment, the common cathode voltage, V K    320 , is PW modulated by the signal applied from PWM signal generator  322 , which functions to apply a reverse bias across the row(s) of OLED pixels (e.g., OLED pixel  308 ) associated with this common cathode terminal, V K    320 , which in turn, switches “off” the OLED pixels (e.g., OLED pixel  308 ) associated with this common cathode terminal, V K    320 , in order to control the brightness or luminance during the frame time of the display involved. Thus, in accordance with this embodiment of the present invention, an AM OLED pixel circuit and method are provided for achieving wide dynamic range dimming while maintaining the color balance and the luminance and chromaticity uniformity required over the surface of the display involved. In this case, an external transistor  310  can be used to modulate the cathode power supply, V K    320 , of the OLED pixel  308  in order to dynamically dim the display. Thus, by PW modulating the common cathode voltage, V K    320 , the luminance or brightness of the display is averaged over a suitable period of time. Therefore, using the PWM method of the present invention allows significantly more uniform dimming of OLED displays than currently provided for the existing OLED displays. 
       FIG. 4  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit  400 , which can be used to implement a second embodiment of the present invention. As such, AM OLED sub-pixel circuit  400  can be used in a preferred method for dynamically dimming an AM OLED display using, for example, an external (to the display) PWM scheme. Referring now to  FIG. 4 , AM OLED sub-pixel circuit  400  includes a first TFT  402 , a storage capacitor  404 , a second TFT  408 , an OLED pixel  410 , and a transistor  406  represented here by a P-channel FET. In this case, an external (to the display involved) transistor  406  can be used to PW modulate the positive power supply, V DD    418 , of the OLED pixel  410 , in order to turn “off” the voltage across the OLED pixels (e.g., OLED pixel  410 ) associated with the common power supply voltage, V DD    418 , and thus to control the brightness of the display. Also, in this case, the reference voltage, V SC    416 , for storage capacitor  404 , can be removed from the V DD  line to prevent coupling the PW modulated V DD  to the gate voltage, V GS2 , at the node  426  between the gate terminal of transistor  408  and storage capacitor  404 . 
     As shown, for this example embodiment, transistor  402  is a scan transistor, and transistor  408  is a drive transistor. The gate terminal  412  of the scan transistor  402  is connected to the row (scan/row enable) address bus of the display involved, and the drain terminal  414  of scan transistor  402  is connected to the column (data) address bus of the display. The source of scan transistor  402  is connected to the node  426  at the storage capacitor  404  and the gate terminal of the drive transistor  408 . The source of drive transistor  408  is connected to a terminal of OLED pixel  410 . The drain of drive transistor  408  is connected to one (e.g. the drain) terminal  422  of the transistor  406 , and the other (e.g. the source) terminal of transistor  406  is connected to the common power supply voltage, V DD    418 . The second terminal of OLED pixel  410  is connected to a common cathode terminal, V K    424 . 
     For this exemplary embodiment, an AM OLED display incorporating AM OLED sub-pixel circuit  400  can include a plurality of (e.g., two or more) common power supply voltage terminals, V DD    418 . Each one of the common power supply voltages (e.g., V DD    418  in  FIG. 4 ) provides the positive power supply voltage for the particular OLED sub-pixel involved (e.g., OLED  410 ) within the overall display. The control (e.g. gate) terminal of transistor  406  in such a display is connected to a PWM signal generator  420 . 
     During the row addressing time period of the display operation, scan transistor  412  charges the node  426  at the storage capacitor  404  and the gate terminal of the drive transistor  408  to the data voltage (signal), V DATA . After the row addressing time period, scan transistor  412  is switched off, and the OLED pixel  410  is electrically isolated from the data bus. Then, in order to adjust the luminance (e.g., brightness) of the display (e.g., OLED pixel  410 ), the PW modulated signal from PWM signal generator  420  is applied to the gate of the switch transistor  406 , which PW modulates the common power supply voltage, V DD    418 , to turn “off” the voltage across the plurality of OLED pixels (e.g., OLED pixel  410 ) associated with the common power supply voltage, V DD    418 , and thus control the brightness of the overall display. Again, using the PWM method of the present invention, the dimming of the display can be achieved with optimum uniformity. 
       FIG. 5  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit  500 , which can be used to implement a third embodiment of the present invention. As such, AM OLED sub-pixel circuit  500  can be used in a preferred method for dynamically dimming an AM OLED display using, for example, an internal (to the display) PWM scheme. Referring now to  FIG. 5 , AM OLED sub-pixel circuit  500  includes a first TFT  502 , a storage capacitor  504 , a second TFT  506 , a third TFT  508 , and an OLED pixel  510 . In this case, a third TFT  508  (internal to the display involved) can be used at each sub-pixel in the display to PW modulate the current, I OLED    518 , of the OLED pixel  510 , in order to turn “off” the OLED pixel (e.g., OLED pixel  510 ) so that it does not emit light, and thus control the brightness of the overall display. 
     As shown, for this example embodiment, transistor  502  is a scan transistor, and transistor  506  is a drive transistor. The gate terminal  512  of the scan transistor  502  is connected to the row (scan/row enable) address bus of the display involved, and the drain terminal  514  of scan transistor  502  is connected to the column (data) address bus of the display. The source of scan transistor  502  is connected to the node  507  at the storage capacitor  504  and the gate terminal of the drive transistor  506 . The source of drive transistor  506  is connected to the drain of the third TFT  508 , and the source of third TFT  508  is connected to a terminal of OLED pixel  510 . The drain of drive transistor  506  is connected to the common power supply voltage, V DD    516 . The second terminal of OLED pixel  510  is connected to a common cathode terminal, V K    522 . 
     For this exemplary embodiment, an AM OLED display incorporating AM OLED sub-pixel circuit  500  can include a plurality of (e.g., two or more) PWM voltage signal generators, V PWM    520 . Thus, by pixel switching or PWM of the third TFT  508 , the third TFT  508  controls the OLED current I OLED    518  and switches “off” the OLED pixel involved (e.g., OLED pixel  510  in  FIG. 5 ) so that the OLED pixel involved does not emit light. 
     Specifically, the gate terminal of the switching TFT  508 , in each of the pixels in a given row in the display, is connected to a row bus that is addressable from outside the display, as is the row-enable bus. The PW modulated signal, V PWM , from the PWM voltage signal generator  520 , is applied to each row in order to switch “off” the current flow to the OLED pixel  510  and turn the pixel “off”. The “on” time of each of the rows is modulated to control the brightness of the display. A significant amount of modulation (e.g., dimming) can be achieved using such an internal modulation scheme. 
     For example, in a 1000 line (rows) display, the brightness of the display can be modulated (dimmed) by a factor of 1000:1 by the preset PWM method alone, and allowing the desired wide dynamic range dimming (e.g., &gt;2000:1) to be accomplished using gray-levels with higher luminance values. Thus, the present invention significantly improves the uniformity of the luminance and chromaticity across the surface of the display as it is being dimmed, as compared to the conventional dimming methods used for AM OLED displays. 
     As such, the PWM voltage signal generator  520  can be commonly connected to all of the pixels in the display, or each row of pixels can be provided with an independent PWM signal generator (e.g., such as PWM voltage signal generator  520 ). Incidentally, an advantage of providing each row of pixels with a separate PWM voltage (e.g., V PWM    520 ), is that the display flicker can be significantly minimized in comparison to other approaches. 
     During the row addressing time period of the display operation, scan transistor  502  charges the node  507  at the storage capacitor  504  and the gate terminal of the drive transistor  506  to the data voltage (signal), V DATA . After the row addressing time period, scan transistor  502  is switched off, and the OLED pixel  510  is electrically isolated from the data bus. Then, in order to adjust the luminance (e.g., brightness) of the display (e.g., OLED pixel  510 ), the PW modulated signal, V PWM , from PWM voltage signal generator  520  is applied to the gate of the third TFT  508 , which PW modulates the OLED current, I OLED    518 , to turn “off” the subject OLED pixels (e.g., OLED pixel  510 ), and thus control the brightness of the overall display. Again, using the PWM method of the present invention, the dimming of the display can be achieved with optimum uniformity. 
       FIG. 6  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit  600 , which can be used to implement a fourth embodiment of the present invention. As such, AM OLED sub-pixel circuit  600  can be used in a preferred method for dynamically dimming an AM OLED display using, for example, an internal (to the display) PWM scheme. Referring now to  FIG. 6 , AM OLED sub-pixel circuit  600  includes a first TFT  602 , a storage capacitor  604 , a second TFT  606 , a third TFT  608 , and an OLED pixel  610 . In this case, a third TFT  608  (internal to the display involved) can be used at each sub-pixel in the display to PW modulate the current through the OLED pixel involved in order to turn “off” that OLED pixel (e.g., OLED pixel  610 ) so that it does not emit light, and thus control the brightness of the overall display. 
     As shown, for this example embodiment, transistor  602  is a scan transistor, and transistor  606  is a drive transistor. The gate terminal  612  of the scan transistor  602  is connected to the row (scan/row enable) address bus of the display involved, and the drain terminal  614  of scan transistor  602  is connected to the column (data) address bus of the display. The source of scan transistor  602  is connected to the node  620  at the storage capacitor  604 , the drain of third TFT  608 , and the gate terminal of the drive transistor  606 . The source of the drive transistor  606  is connected to the source of the third TFT  608  and one terminal of OLED pixel  610 . The drain terminal of drive transistor  606  is connected to the common power supply voltage, V DD    618 . The second terminal of OLED pixel  610  is connected to a common cathode terminal, V K    622 . 
     For this exemplary embodiment, an AM OLED display incorporating AM OLED sub-pixel circuit  600  can include a plurality of (e.g., two or more) PWM voltage signal generators, V PWM    624 . Thus, by PWM of the gate voltage, V GS2    620 , at the gate of the drive transistor  606 , the third TFT  608  can control the current through the OLED pixel involved (e.g., OLED pixel  610 ) by turning “off” the drive transistor  606  and, therefore, turning “off” the OLED pixel involved (e.g., OLED pixel  610  in  FIG. 6 ) so that the OLED pixel involved does not emit light. As such, the PWM voltage signal generator  624  can be common to all of the pixels in the display, or each row of pixels can be provided with an independent PWM signal generator (e.g., such as PWM voltage signal generator  624 ). Once again, an advantage of providing each row of pixels with a separate PWM voltage (e.g., V PWM    624 ), is that the present method can significantly reduce the display&#39;s propensity for flicker in comparison with other existing approaches. 
     During the row addressing time period of the display operation, scan transistor  602  charges the node  620  at the storage capacitor  604  and the gate terminal of the drive transistor  606  to the data voltage (signal), V DATA . After the row addressing time period, scan transistor  602  is switched off, and the OLED pixel  610  is electrically isolated from the data bus. Then, in order to adjust the luminance (e.g., brightness) of the display (e.g., OLED pixel  610 ), the PW modulated signal, V PWM , from PWM voltage signal generator  624  is applied to the gate of the third TFT  608 , which PW modulates the gate voltage, V GS2    620 , and turns “off” the drive transistor  606 . In response, PW modulation of the drive transistor  606  controls the current through the OLED pixel involved, and turns “off” the subject OLED pixel (e.g., OLED pixel  610 ) to control the brightness of the overall display. Again, using the PWM method of the present invention, the dimming of the display can be achieved with optimum uniformity. 
       FIG. 7  depicts an electrical schematic diagram of an example AM OLED sub-pixel circuit  700 , which can be used to implement a fifth embodiment of the present invention. As such, AM OLED sub-pixel circuit  700  can be used in a preferred method for dynamically dimming an AM OLED display using, for example, an internal (to the display) PWM scheme. Referring now to  FIG. 7 , AM OLED sub-pixel circuit  700  includes a first TFT  702 , a storage capacitor  706 , a second TFT  710 , a third TFT  704 , a fourth TFT  712 , and an OLED pixel  714 . In this case, two additional transistors (e.g., third TFT  704  and fourth TFT  712 ), which are both internal to the display involved, can be used at each sub-pixel in the display to enable PWM of the current through the OLED pixel involved (e.g., I OLED    718 ), in order to turn “off” that OLED pixel (e.g., OLED pixel  714 ) so that it does not emit light, by changing the gate voltage, V GS2    716 , from a pre-selected value to “off”. At a selected time after the storage capacitor  706  is charged to the pre-selected value, the PWM voltage, V PWM    730 , goes high, which shuts “off” third TFT  704  and (e.g., disconnecting V C    706  from V GS2    716 ) and turns “on” fourth TFT  712 , which in turn, shuts “off” drive transistor  710 . This PWM method of the present invention thus controls the current through the OLED pixel  714  involved (e.g., I OLED    718 ), which controls the brightness of the overall display. 
     As mentioned earlier, a significant advantage of providing each row of pixels with a separate PWM voltage (e.g., V PWM    730 ), is that the present method can significantly reduce the display&#39;s propensity for flicker in comparison with other existing approaches. Also, using the PWM method of the present invention, the dimming of the AM OLED display can be achieved with optimum uniformity. 
     It is important to note that while the present invention has been described in the context of a fully functioning AM OLED display, those of ordinary skill in the art will appreciate that the processes of the present invention are capable of being distributed in the form of a computer readable medium of instructions and a variety of forms and that the present invention applies equally regardless of the particular type of signal bearing media actually used to carry out the distribution. Examples of computer readable media include recordable-type media, such as a floppy disk, a hard disk drive, a RAM, CD-ROMs, DVD-ROMs, and transmission-type media, such as digital and analog communications links, wired or wireless communications links using transmission forms, such as, for example, radio frequency and light wave transmissions. The computer readable media may take the form of coded formats that are decoded for actual use in a particular AM OLED display. 
     The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. These embodiments were chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.