Abstract:
A shift register without a feedback signal of a post-stage shift register utilizing a latch mechanism and a clock signal to control the voltage of an output of the shift register is provided. The shift register reduces the transistor size and the circuit layout area. The shift register also improves the issue the overlapping between two adjacent shift registers to reduce the after-image of a liquid crystal display.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a shift register and more particularly to a shift register without a feedback signal from an output signal of a post-stage shift register. 
   2. Description of the Related Art 
   Liquid crystal displays (LCD) have become the major product of displays, and minimizing the size and the weight of the LCDs, to dispose the driving circuit on the substrate of the LCD has become a major technology focus. Take the TFT LCD for example, since the amorphous Si process is the main technology, the low electron mobility of amorphous Si limits the size of the element formed by the amorphous Si process, such as a thin film transistor (TFT). If the transistor formed by the amorphous Si process wants to receive a larger current, the width of the channel of the transistor increases, thus, the layout area increases. 
     FIG. 1  is a circuit diagram of a conventional amorphous Si shift register. To ensure that the output of the shift register rapidly charges and discharges, the charging transistor T 4  and the discharging transistors T 5  and T 6  receive larger current, thus, the width of those transistors is about a thousand micro-meters, and thus occupy larger layout areas. Moreover, the conventional shift register utilizes an output signal of a post-stage shift register to pull down the output signal, and if the shift register is the last-stage shift register, the output signal thereof may not completely discharge, such as shown in the dotted frame  20  of the  FIG. 2 .  FIG. 2  is a timing diagram of the amorphous Si shift register of the  FIG. 1 . In  FIG. 2 , curve  21 ,  23 ,  25  and  27  respectively represents the charging and the discharging of the (N−1)th stage shift register, Nth stage shift register, (N+1)th stage shift register, and (N+2)th stage shift register. In  FIG. 2 , the (N+2)th stage shift register is the last-stage shift register, thus, the output signal thereof does not completely discharge, as shown by dotted frame  20 . Take the output signal of the (N−1)th stage shift register for example, the discharge time thereof is about 25 μs and when the output signal of the Nth stage shift register charges, the output signal of the (N−1)th stage shift register does not discharge completely, thus, an overlap occurs, and this causes the incorrect LCD output. 
   Therefore, an amorphous Si shift register circuit capable of reducing the layout area and reducing or eliminating overlapping is desirable. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention provides an amorphous Si shift register without a feedback signal from an output signal of a post-stage shift register. 
   The invention provides a shift register capable of reducing the circuit layout area. 
   The invention provides a shift register circuit having a cascade of shift registers comprising the follow elements: a first transistor having a gate, a first source/drain and a second source/drain, wherein the gate and the first source/drain of the first transistor is coupled to an output signal of a pre-stage shift register; a second transistor having a gate, a first source/drain and a second source/drain, wherein the gate of the second transistor is coupled to the first source/drain of the first transistor, and the second source/drain of the second transistor is coupled to a low voltage level; a third transistor having a gate, a first source/drain and a second source/drain, wherein the gate of the third transistor is coupled to the first source/drain of the second transistor, the first source/drain of the third transistor is coupled to the second source/drain of the first transistor, and the second source/drain of the third transistor is coupled to the low voltage level; a fourth transistor having a gate, a first source/drain and a second source/drain, wherein the gate of the fourth transistor is coupled to the second source/drain of the first transistor, the first source/drain of the fourth transistor is coupled to a first clock signal, and the second source/drain of the fourth transistor is coupled to a first output; a sixth transistor having a gate, a first source/drain and a second source/drain, wherein the gate and the first source/drain of the sixth transistor is coupled to a high voltage level; a seventh transistor having a gate, a first source/drain and a second source/drain, wherein the first source/drain of the seventh transistor is coupled to the second source/drain of the sixth transistor, and the second source/drain of the seventh transistor is coupled to the low voltage level; an eighth transistor having a gate, a first source/drain and a second source/drain, wherein the gate and the first source/drain of the eighth transistor is coupled to the high voltage level; a ninth transistor having a gate, a first source/drain and a second source/drain, wherein the first source/drain of the ninth transistor is coupled to the second source/drain of the eighth transistor, the second source/drain of the ninth transistor is coupled to the low voltage level, and the gates of the seventh and ninth transistors are coupled to the first output. 
   The invention further provides a shift register circuit having a cascade of shift registers, comprising the following elements: a first transistor having a gate, a first source/drain and a second source/drain, wherein the gate and the first source/drain of the first transistor is coupled to an output signal of a pre-stage shift register; a second transistor having a gate, a first source/drain and a second source/drain, wherein the gate of the second transistor is coupled to the first source/drain of the first transistor, and the second source/drain of the second transistor is coupled to a low voltage level; a third transistor having a gate, a first source/drain and a second source/drain, wherein the gate of the third transistor is coupled to the first source/drain of the second transistor, the first source/drain of the third transistor is coupled to the second source/drain of the first transistor, and the second source/drain of the third transistor is coupled to the low voltage level; a fourth transistor having a gate, a first source/drain and a second source/drain, wherein the gate of the fourth transistor is coupled to the second source/drain of the first transistor, the first source/drain of the fourth transistor is coupled to a first clock signal, and the second source/drain of the fourth transistor is coupled to a first output; a fifth transistor having a gate, a first source/drain and a second source/drain, wherein the first source/drain of the fifth transistor is coupled to the first output, and the second source/drain of the fifth transistor is coupled to the low voltage level; a first inverting device having a input and an output, wherein the output of the first inverter is coupled to the gate of the third transistor, and the input of the first inverter is coupled to the first output; a second inverting device having a input and an output, wherein the output of the second inverter is coupled to the gate of the fifth transistor, and the input of the second inverter is coupled to the first output. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  is a circuit diagram of a conventional amorphous Si shift register. 
       FIG. 2  is a timing diagram of the amorphous Si shift register of the  FIG. 1 . 
       FIG. 3  is a circuit diagram of a shift register of an embodiment of the invention. 
       FIG. 4  is a schematic diagram of charging and discharging of the output of the shift register of  FIG. 3 . 
       FIG. 5  is a circuit diagram of an embodiment of the inverting device I 1  of  FIG. 3 . 
       FIG. 6  is a circuit diagram of an embodiment of the inverting device I 2  of  FIG. 3 . 
       FIG. 7  is a circuit diagram of another embodiment of the shift register of the invention. 
       FIG. 8  is a waveform of the circuit of  FIG. 7 . 
       FIG. 9  is a circuit diagram of another embodiment of the shift register of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     FIG. 3  is a circuit diagram of a shift register of an embodiment of the invention. The gate and first source/drain of transistor T 1  is coupled to an output signal N−1 of a pre-stage, (N−1)th stage, shift register and the transistor T 1  turns on when the output signal of the pre-stage shift register is at high voltage level. The second source/drain of transistor T 1  is coupled to the gate of transistor T 4 . The first source/drain of transistor T 4  is coupled to a clock signal CLK, and the second source/drain of transistor T 4  is coupled to an output signal N of the Nth stage shift register. The gate of transistor T 2  is coupled to the first source/drain of transistor T 1 , the second source/drain of transistor T 2  is coupled to a low voltage level VSS, and the first source/drain of transistor T 2  is coupled to the gate of transistor T 3 . The second source/drain of transistor T 3  is coupled to the low voltage level VSS, and the first source/drain of transistor T 3  is coupled to the first source/drain of transistor T 1 . An input of an inverting device I 1  is coupled to the output signal N, and an output of the inverting device I 1  is coupled to the gate of transistor T 3 . An input of an inverting device I 2  is coupled to the output signal N and an output of the inverting device I 2  is coupled to the gate of T 5 . The second source/drain of transistor T 5  is coupled to the low voltage level VSS, and the first source/drain of transistor T 5  is coupled to the output signal N. 
   When the output signal N−1 is at high voltage level,transistors T 1  and T 2  turn on and, then, the gate of transistor T 3  is at low voltage level due to the switch-on of transistor T 2 . The gate of transistor T 4  receiving a high voltage signal of the output signal N−1 turns on due to the switch-on of transistor T 1 . When the clock signal CLK is at high voltage level, the output signal N is at high voltage level, thus, the voltage level of transistor T 4  increases due to a coupling capacitor between the first source/drain and the gate of transistor T 4 . When the clock signal CLK is at the low voltage level, the output signal N is at the low voltage level. Transistors T 3  and T 5  turn on via the inverting devices I 1  and I 2  and the voltage of the gate of transistor T 4  discharges via transistor T 3  to the low voltage level, thus, transistor T 4  turns off. The output signal N stays at the low voltage level due to the switch-on of transistor T 5 . 
   Please refer to  FIG. 4 .  FIG. 4  is a schematic diagram of charging and discharging of the output of the shift register of  FIG. 3 . Curve  41  is a charging and discharging curve of the output signal N−1 of the pre-stage shift register, and curve  43  is a charging and discharging curve of the output signal N. In  FIG. 4 , it is found that the shift register of the invention rapidly discharges and reduces an issue of uncompleted discharging, such as shown in the dotted frame  20  of  FIG. 2 . Moreover, the shift register of the invention has a shorter charging and discharging time to eliminate the issue of incomplete charging of the LCD. 
   The inverting devices I 1  and I 2  of  FIG. 3  preferably are inverters. To further illustrate, the embodiments of the inverting devices I 1  and I 2  are provided.  FIG. 5  is a circuit diagram of an embodiment of the inverting device I 1  of  FIG. 3 . Node  51  coupling to the gate of transistor T 3  is the output terminal of the inverting device I 1 . Node  52  coupling to the output signal N is the input terminal of the inverting device I 2 . The gate and the first source/drain of transistor T 8  are coupled to the high voltage source VDD, and the second source/drain of transistor T 8  is coupled to the node  51  and the first source/drain of transistor T 9 . The gate of transistor T 9  is coupled to the node  52 , and the second source/drain of transistor T 9  is coupled to a low voltage source VSS. When the output signal N is at high voltage level, the transistor T 9  turns on, thus, the node  51  is at low voltage level. When the output signal N is at low voltage level, the transistor T 9  turns off, and the node is at high voltage level due to the switch-on of the transistor T 8 . 
     FIG. 6  is a circuit diagram of an embodiment of the inverting device I 2  of  FIG. 3 . Node  61  coupling to the gate of transistor T 5  is the output terminal of the inverting device I 2 . Node  62  coupling to the output signal N is the input terminal of the inverting device I 2 . The gate and first source/drain of transistor T 6  are coupled to VDD, and the second source/drain of transistor T 6  is coupled to the first source/drain of transistor T 7 . The gate of transistor T 7  is coupled to the node  62 , and the second source/drain of transistor T 7  is coupled to VSS. When the output signal N is at high voltage level, the transistor T 7  turns on, thus, the node  61  is at low voltage level. When the output signal N is at low voltage level, the transistor T 7  turns off, and the node is at high voltage level due to the switch-on of the transistor T 6 . 
     FIG. 7  is a circuit diagram of another embodiment of the shift register of the invention. The gate and the first source/drain of the first transistor T 1  is coupled to the output signal N−1 of the pre-stage shift register. The gate of the second transistor T 2  is coupled to the second source/drain of the first transistor T 1 , and the second source/drain of the second transistor T 2  is coupled to a low voltage source VSS. The gate of the third transistor is coupled to the second source/drain of the second transistor T 2 , the first source/drain of the third transistor T 3  is coupled to the second source/drain of the first transistor T 1 , and the second source/drain of the third transistor T 3  is coupled to VSS. The gate of the fourth transistor T 4  is coupled to the second source/drain of the first transistor T 1 , the first source/drain of the fourth transistor T 4  is coupled to a first clock signal CLK, and the second source/drain of the fourth transistor T 4  is coupled to the output signal N. The first source/drain of the fifth transistor T 5  is coupled to the output signal N, the second source/drain of the fifth transistor T 5  is coupled to VSS, and the gate of the fifth transistor T 5  is coupled to the first source/drain of the seventh transistor T 7 . The first source/drain and the gate of the sixth transistor T 6  are coupled to a high voltage source VDD. The first source/drain of the seventh transistor T 7  is coupled to the second source/drain of the sixth transistor T 6 , and the second source/drain of the seventh transistor T 7  is coupled to a low voltage source VSS. The gate and the first source/drain of the eighth transistor T 8  is coupled to VDD. The first source/drain of the ninth transistor T 9  is coupled to the second source/drain of the eighth transistor T 8  and the first source/drain of the second transistor T 2 , the second source/drain of the ninth transistor T 9  is coupled to VSS, and the gates of the ninth transistor T 9  and the seventh transistor are coupled to the output signal N. 
   To further illustrate the operation of the circuit of  FIG. 7 , please refer to  FIG. 8 .  FIG. 8  is a waveform of the circuit of  FIG. 7 . At time t 1 , the output signal N−1 of the pre-stage shift register is at high voltage level, thus, the first transistor T 1  turns on and the node N 1  is at high voltage level. The second transistor T 2  turns on due to the high voltage level of the node N 1 , thus, the node N 2  is coupled to the low voltage source VSS. At time t 2 , the output signal N−1 is at low voltage level, thus, the first transistor T 1  and the ninth transistor T 9  turn off. The voltage level of node N 1  maintains in the high voltage level because of no discharging path, thus, the fourth transistor T 4  is still on. Although the ninth transistor T 9  turns, the node N 2  is still at low voltage level. When the first clock signal CLK is at the high voltage level in time t 2 , the output signal N is at the high voltage level and this causes a charging of a coupling capacitor formed between the first source/drain and the gate of the fourth transistor T 4 , thus, the voltage level of node N 1  increases. At time t 3 , the first clock signal CLK is at the low voltage level, thus, the output signal N is coupled to the low voltage VSS due to the switch-on of the transistor T 4 . The seventh transistor T 7  turns off due to the low voltage level of the output signal N, and the sixth transistor T 6  turns on due to the high voltage level of the node N 2  to turn on the eighth transistor T 8 , thus, the voltage level of the node N 1  discharges through the eighth transistor T 8  and the fourth transistor T 4  turns off. 
   It is found that the shift register of  FIG. 7  pulls down the voltage level of the output signal N to the low voltage level without the output signal of the next-stage shift register according to the waveform of  FIG. 8 . To ensure that the output signal N is pulled down to the low voltage level VSS, a fifth transistor T 5  is added to the circuit of  FIG. 7 . Please refer to the  FIG. 9 .  FIG. 9  is a circuit diagram of another embodiment of the invention. When the fourth transistor T 4  turns on and the first clock signal is at the low voltage level, the output signal N is at the low voltage level, and the seventh transistor T 7  turns off, thus, the fifth transistor T 5  turns on and then the output signal N is coupled to the low voltage level VSS through the fifth transistor T 5 . 
   Moreover, the shift register of the invention reduces the size of the layout areas. In a conventional shift register, such as the shift register of the  FIG. 1 , transistors T 1 , T 4 , T 5  and T 6  are the uses of charging and discharging, thus, the W/L values are at the range between hundreds of micro-meters and thousands of micro-meters, and the W/L values of transistors T 2  and T 3  are about hundreds of micro meters. In the shift register of the  FIG. 7 , the layout areas are reduced even though the numbers of transistors is increased, and the layout areas of the shift register of  FIG. 7  are smaller than the layout areas of the shift register of  FIG. 2 . 
   While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.