Abstract:
A buffer circuit having high slew rate is provided. The buffer circuit is provided, which includes a plurality of transistors having the same conductivity type and a capacitor and whose gain is determined depending on the gain of all the plurality of transistors. A buffer circuit having high driving capability and high gain of a high-frequency component can be provided. Such a buffer circuit has also high slew rate. The plurality of transistors having the same conductivity type in the buffer circuit may be either p-channel transistors or n-channel transistors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a buffer circuit, a device to which the buffer circuit is applied, an electronic device, and the like. 
     2. Description of the Related Art 
     In recent years, display devices such as liquid crystal display devices and light-emitting devices are becoming widespread. In addition, with the development of the information society, the absolute quantity of information to be handled has been drastically increased; thus, memory devices or the like have been also developed. 
     In such display devices and memory devices, a plurality of transistors are provided in a matrix. The plurality of transistors provided in a matrix are controlled by scanning lines, and are supplied with data by signal lines. Note that such transistors formed over the same substrate preferably have the same conductivity type in order to improve the productivity. 
     The transistors are provided in a matrix over the substrate, and over this substrate, signals to be supplied to the scanning lines are supplied from a shift register circuit (see Patent Document 1, for example) or a buffer circuit. 
     However, in a conventional buffer circuit provided with transistors that have the same conductivity type (shown in  FIG. 2 , for example), the transistor, one of a source and a drain of which is connected to an output portion and the other of the source and the drain of which is connected to a power supply line (a high-potential-side power supply line V dd  in the case of an n-channel transistor, and a low-potential-side power supply line V ss  in the case of a p-channel transistor), acts to reduce the gain of the buffer circuit when the gain of the transistor itself is increased. 
     Note that the size of this transistor cannot be made sufficiently large in order to ensure the high gain of the buffer circuit and the slew rate of voltage of the output portion is determined depending on a value of current flowing from this transistor. Therefore, there is a problem in that the slew rate is difficult to improve. 
     Note that in this specification, the “slew rate” is a value obtained by dividing an output voltage by a rising time (in the case of an n-channel transistor) or a falling time (in the case of a p-channel transistor), and is also called a rising characteristic (in the case of the n-channel transistor) or a falling characteristic (in the case of the p-channel transistor). 
     REFERENCE 
     Patent Document 
     
         
         [Patent Document 1] Japanese Published Patent Application No. 2004-260788 
       
    
     SUMMARY OF THE INVENTION 
     An object of one embodiment of the present invention is to provide a buffer circuit having high slew rate. 
     Another object is to form such a buffer circuit having high slew rate with the use of transistors having the same conductivity type. When the transistors in the buffer circuit have the same conductivity type (n-type or p-type), the buffer circuit can be manufactured through a simple process; thus, productivity can be increased. 
     Another object is to simplify the structure of the buffer circuit that can solve the above problem, and make the area occupied by the buffer circuit small. 
     In one embodiment of the present invention, in order to increase the slew rate of a buffer circuit, the driving capability is improved and the gain of a high-frequency component is improved. 
     A buffer circuit according to one embodiment of the present invention includes at least a plurality of transistors and a capacitor. The gain of the buffer circuit is determined depending on the gain of all the plurality of transistors. 
     A buffer circuit according to one embodiment of the present invention includes a first transistor; a second transistor; a third transistor; a fourth transistor; a fifth transistor; a sixth transistor; a capacitor; an input portion; and an output portion. A first terminal of the first transistor and a third terminal of the first transistor are connected to a high-potential-side power supply line. A second terminal of the first transistor is connected to a first terminal of the second transistor and a third terminal of the third transistor. A second terminal of the second transistor is connected to a low-potential-side power supply line. A third terminal of the second transistor is connected to the input portion of the buffer circuit. A first terminal of the third transistor is connected to the high-potential-side power supply line. A second terminal of the third transistor is connected to a first terminal of the fourth transistor and a third terminal of the fifth transistor. A second terminal of the fourth transistor is connected to the low-potential-side power supply line. A third terminal of the fourth transistor is connected to the input portion of the buffer circuit. A first terminal of the fifth transistor is connected to the high-potential-side power supply line. A second terminal of the fifth transistor is connected to a first terminal of the sixth transistor and the output portion of the buffer circuit. A second terminal of the sixth transistor is connected to the low-potential-side power supply line. A third terminal of the sixth transistor is connected to the input portion of the buffer circuit. The second terminal of the first transistor is connected to the output portion of the buffer circuit through the capacitor. The first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor, and the sixth transistor are each an n-channel transistor. 
     In the above buffer circuit, the first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor, and the sixth transistor each preferably include an oxide semiconductor. 
     A buffer circuit according to another embodiment of the present invention includes a first transistor; a second transistor; a third transistor; a fourth transistor; a fifth transistor; a sixth transistor; a capacitor; an input portion; and an output portion. A first terminal of the first transistor is connected to a high-potential-side power supply line. A second terminal of the first transistor is connected to a first terminal of the second transistor and a third terminal of the fourth transistor. A third terminal of the first transistor is connected to the input portion of the buffer circuit. A second terminal of the second transistor and a third terminal of the second transistor are connected to a low-potential-side power supply line. A first terminal of the third transistor is connected to the high-potential-side power supply line. A second terminal of the third transistor is connected to a first terminal of the fourth transistor and a third terminal of the sixth transistor. A third terminal of the third transistor is connected to the input portion of the buffer circuit. A second terminal of the fourth transistor is connected to the low-potential-side power supply line. A first terminal of the fifth transistor is connected to the high-potential-side power supply line. A second terminal of the fifth transistor is connected to a first terminal of the sixth transistor and the output portion of the buffer circuit. A third terminal of the fifth transistor is connected to the input portion of the buffer circuit. A second terminal of the sixth transistor is connected to the low-potential-side power supply line. The second terminal of the first transistor is connected to the output portion of the buffer circuit through the capacitor. The first transistor, the second transistor, the third transistor, the fourth transistor, the fifth transistor, and the sixth transistor are each a p-channel transistor. 
     Note that in this specification, a “first terminal” means one of a source and a drain, and a “second terminal” means the other thereof. In addition, a “third terminal” means a gate. 
     Note that in this specification, the “gain” means a ratio of an output voltage to an input voltage. 
     Note that in this specification, the “driving capability” means the capability to output current to an output load. 
     Note that in this specification, an “n-channel transistor” means all transistors which are turned on when a gate voltage (a gate potential relative to a source potential) is higher than the threshold voltage. A “p-channel transistor” means all transistors which are turned on when a gate voltage (a gate potential relative to a source potential) is lower than the threshold voltage. Therefore, the terms “n-channel” and “p-channel” do not limit matters other than those defined as above. 
     Note that here, the “slew rate” is determined depending on the gain and the driving capability. In the case where the driving capability is low, it takes time to charge an output load (a capacitor) even when the gain is high. On the other hand, in the case where the driving capability is high and the gain is low, it takes time to sufficiently increase V gs  of a transistor of the last stage; accordingly, it takes time to utilize the high driving capability. Therefore, in order to improve the slew rate, the gain is increased and the driving capability is made sufficiently high. 
     A buffer circuit having high driving capability and high gain of a high-frequency component can be provided. Such a buffer circuit has also high slew rate. 
     Further, since such a buffer circuit having high slew rate can be formed using transistors having the same conductivity type, it can be manufactured through a simple process. In other words, the slew rate of a buffer circuit provided with transistors that have the same conductivity type can be improved, which has been conventionally difficult. 
     Furthermore, the structure of such a buffer circuit having the above effect is simplified as much as possible, whereby the area occupied by the buffer circuit can be made small. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a buffer circuit of Embodiment 1, which is one embodiment of the present invention. 
         FIG. 2  illustrates an example of a buffer circuit. 
         FIG. 3  illustrates an example of a buffer circuit. 
         FIG. 4  illustrates a buffer circuit of Embodiment 2, which is one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be hereinafter described in detail with reference to the accompanying drawings. Note that the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention is not construed as being limited to description of the embodiments below. 
     [Embodiment 1] 
     In this embodiment, a buffer circuit provided with n-channel transistors, which is one embodiment of the present invention, will be described. 
       FIG. 1  illustrates a buffer circuit  100  which is one embodiment of the present invention. 
     The buffer circuit  100  illustrated in  FIG. 1  includes first to sixth transistors, a capacitor, an input portion, and an output portion. A first terminal and a third terminal of a first transistor  101  are connected to a high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of a second transistor  102  and a third terminal of a third transistor  103 . A second terminal of the second transistor  102  is connected to a low-potential-side power supply line V ss , and a third terminal thereof is connected to an input portion V in  of the buffer circuit  100 . A first terminal of the third transistor  103  is connected to the high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of a fourth transistor  104  and a third terminal of a fifth transistor  105 . A second terminal of the fourth transistor  104  is connected to the low-potential-side power supply line V ss , and a third terminal thereof is connected to the input portion V in  of the buffer circuit  100 . A first terminal of the fifth transistor  105  is connected to the high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of a sixth transistor  106  and an output portion V out  of the buffer circuit  100 . A second terminal of the sixth transistor  106  is connected to the low-potential-side power supply line V ss , and a third terminal thereof is connected to the input portion V in  of the buffer circuit  100 . The second terminal of the first transistor  101  is connected to the output portion V out  through a capacitor  107 . 
     The buffer circuit  100  illustrated in  FIG. 1  has high driving capability and high gain of a high-frequency component. Further, the buffer circuit illustrated in  FIG. 1  has high slew rate. Since the buffer circuit  100  illustrated in  FIG. 1  can be formed using transistors having the same conductivity type, it can be manufactured through a simple process. In other words, the slew rate of a buffer circuit provided with transistors that have the same conductivity type can be improved, which has been conventionally difficult. In addition, the buffer circuit can be formed with six transistors and one capacitor, whereby the area occupied by the buffer circuit is small. 
     Here, when the gain of the first transistor  101  is expressed by g 1 , the gain of the second transistor  102  is expressed by g 2 , the gain of the third transistor  103  is expressed by g 3 , the gain of the fourth transistor  104  is expressed by g 4 , the gain of the fifth transistor  105  is expressed by g 5 , the gain of the sixth transistor  106  is expressed by g 6 , the angular frequency of an input signal is expressed by s (=2πf (f represents frequency)), and the electrostatic capacitance of the capacitor  107  is expressed by C, the gain of the buffer circuit  100  illustrated in  FIG. 1  can be expressed by the following mathematical formula (1). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   G 
                   = 
                   
                     - 
                     
                       ( 
                       
                         
                           
                             
                               g 
                               4 
                             
                             ⁢ 
                             
                               
                                 g 
                                 5 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     g 
                                     1 
                                   
                                   + 
                                   sC 
                                 
                                 ) 
                               
                             
                           
                           
                             
                               g 
                               1 
                             
                             ⁢ 
                             
                               
                                 g 
                                 3 
                               
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     g 
                                     5 
                                   
                                   + 
                                   sC 
                                 
                                 ) 
                               
                             
                           
                         
                         + 
                         
                           
                             
                               g 
                               6 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   g 
                                   1 
                                 
                                 + 
                                 sC 
                               
                               ) 
                             
                           
                           
                             
                               g 
                               1 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   g 
                                   5 
                                 
                                 + 
                                 sC 
                               
                               ) 
                             
                           
                         
                         + 
                         
                           
                             g 
                             2 
                           
                           
                             g 
                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     That is, s is significantly large when the frequency is high (g 1 , g 5 &lt;&lt;sC), so that the gain of the high-frequency component is expressed by the following mathematical formula (2) when the equation 1/s=0 is satisfied. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     H 
                   
                   = 
                   
                     - 
                     
                       ( 
                       
                         
                           
                             
                               g 
                               4 
                             
                             ⁢ 
                             
                               g 
                               5 
                             
                           
                           
                             
                               g 
                               1 
                             
                             ⁢ 
                             
                               g 
                               3 
                             
                           
                         
                         + 
                         
                           
                             g 
                             6 
                           
                           
                             g 
                             1 
                           
                         
                         + 
                         
                           
                             g 
                             2 
                           
                           
                             g 
                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In addition, s is significantly small when the frequency is low (g 1 , g 5 &gt;&gt;sC), so that the gain of the low-frequency component is expressed by the following mathematical formula (3) when the equation s=0 is satisfied. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     L 
                   
                   = 
                   
                     - 
                     
                       ( 
                       
                         
                           
                             g 
                             4 
                           
                           
                             g 
                             3 
                           
                         
                         + 
                         
                           
                             g 
                             6 
                           
                           
                             g 
                             5 
                           
                         
                         + 
                         
                           
                             g 
                             2 
                           
                           
                             g 
                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Here, the above mathematical formula (1) is obtained in the following manner. When a current flowing between the first terminal and the second terminal of the first transistor  101  is expressed by I 1 , a current flowing between the first terminal and the second terminal of the third transistor  103  is expressed by I 3 , a current flowing between the first terminal and the second terminal of the fifth transistor  105  is expressed by I 5 , and a current flowing between two electrodes of the capacitor  107  is expressed by I 7 , the following mathematical formulae (4) to (7) are obtained. 
     [Mathematical Formula 4]
 
 I   1   =g   1 (0 −V   a )= g   2 ( V   in −0)+ I   7   (4)
 
[Mathematical Formula 5]
 
 I   3   =g   3 ( V   a   −V   b )= g   4 ( V   in −0)  (5)
 
[Mathematical Formula 6]
 
 I   5   =g   5 ( V   b   −V   out )= g   6 ( V   in −0−i I 7   (6)
 
[Mathematical Formula 7]
 
 I   7   =sC ( V   a   −V   out )  (7)
 
     Note that here, V a  represents the potential of a node connected to the third terminal of the third transistor  103 , and V b  represents the potential of a node connected to the third terminal of the fifth transistor  105 . The solution of the above mathematical formulae (4) to (7) which includes neither V a  nor V b  leads to the above mathematical formula (1). 
     With the use of the buffer circuit  100  illustrated in  FIG. 1 , the gain of the high-frequency component can be increased. This is because, by increasing the gain of the fifth transistor  105 , the gain of the buffer circuit  100  is increased when the frequency is high. 
     In order to explain technical characteristics of the buffer circuit illustrated in  FIG. 1 , which is one embodiment of the present invention, another buffer circuit is described. 
       FIG. 2  illustrates a buffer circuit  200 . 
     The buffer circuit  200  illustrated in  FIG. 2  includes first to fourth transistors, a capacitor, an input portion, and an output portion. A first terminal and a third terminal of a first transistor  201  are connected to a high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of a second transistor  202  and a third terminal of a third transistor  203 . A second terminal of the second transistor  202  is connected to a low-potential-side power supply line V ss , and a third terminal thereof is connected to an input portion V in  of the buffer circuit  200 . A first terminal of the third transistor  203  is connected to the high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of a fourth transistor  204  and an output portion V out  of the buffer circuit  200 . A second terminal of the fourth transistor  204  is connected to the low-potential-side power supply line V ss , and a third terminal thereof is connected to the input portion V in  of the buffer circuit  200 . The second terminal of the first transistor  201  is connected to the output portion V out  through a capacitor  207 . 
     Here, when the gain of the first transistor  201  is expressed by g 1 , the gain of the second transistor  202  is expressed by g 2 , the gain of the third transistor  203  is expressed by g 3 , the gain of the fourth transistor  204  is expressed by g 4 , the angular frequency of an input signal is expressed by s (=2πf (f represents frequency)), and the electrostatic capacitance of the capacitor  207  is expressed by C, the gain of the buffer circuit  200  illustrated in  FIG. 2  can be expressed by the following mathematical formula (8). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   G 
                   = 
                   
                     
                       
                         
                           g 
                           4 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               g 
                               1 
                             
                             + 
                             sC 
                           
                           ) 
                         
                       
                       
                         
                           g 
                           1 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               g 
                               3 
                             
                             + 
                             sC 
                           
                           ) 
                         
                       
                     
                     + 
                     
                       
                         g 
                         2 
                       
                       
                         g 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     That is, s is significantly large when the frequency is high (g 1 , g 3 &lt;&lt;sC), so that the gain of the high-frequency component is expressed by the following mathematical formula (9) when the equation 1/s=0 is satisfied. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     H 
                   
                   = 
                   
                     - 
                     
                       
                         
                           g 
                           2 
                         
                         + 
                         
                           g 
                           4 
                         
                       
                       
                         g 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     In addition, s is significantly small when the frequency is low (g 1 , g 3 &gt;&gt;sC), so that the gain of the low-frequency component is expressed by the following mathematical formula (10) when the equation s=0 is satisfied. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     L 
                   
                   = 
                   
                     - 
                     
                       ( 
                       
                         
                           
                             g 
                             4 
                           
                           
                             g 
                             3 
                           
                         
                         + 
                         
                           
                             g 
                             2 
                           
                           
                             g 
                             1 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     The gain of the third transistor  203  is not included in the above mathematical formula (9); thus, in the buffer circuit  200  illustrated in  FIG. 2 , the gain of the third transistor  203  does not contribute to an increase in gain of the high-frequency component. That is, the transistor of the last stage, which is connected to the high-potential-side power supply line, does not contribute to an increase in gain. Further, as can be seen from the above mathematical formula (10), when the gain of the third transistor  203  is increased, the transistor of the last stage acts to reduce the gain of the buffer circuit when the frequency is low. 
     On the other hand, in the buffer circuit  100  illustrated in  FIG. 1 , the transistor of the last stage, which is connected to the high-potential-side power supply line V dd , contributes to an increase in gain; thus, the gain of the transistor of the last stage is increased by, for example, making the channel length of the transistor large. In such a manner, the gain of the buffer circuit can be increased. 
       FIG. 3  illustrates a buffer circuit  300 . 
     The buffer circuit  300  illustrated in  FIG. 3  includes first to sixth transistors, a capacitor, an input portion, and an output portion. A first terminal and a third terminal of a first transistor  301  are connected to a high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of a second transistor  302 , a third terminal of a fourth transistor  304 , and a third terminal of a fifth transistor  305 . A second terminal of the second transistor  302  is connected to a low-potential-side power supply line V ss , and a third terminal thereof is connected to an input portion V in  of the buffer circuit  300 . A first terminal of a third transistor  303  is connected to the high-potential-side power supply line V dd , a second terminal thereof is connected to a first terminal of the fourth transistor  304  and a third terminal of a sixth transistor  306 , and a third terminal thereof is connected to the input portion V in  of the buffer circuit  300 . A second terminal of the fourth transistor  304  is connected to the low-potential-side power supply line V ss . A first terminal of the fifth transistor  305  is connected to the high-potential-side power supply line V dd , and a second terminal thereof is connected to a first terminal of the sixth transistor  306  and an output portion V out  of the buffer circuit  300 . A second terminal of the sixth transistor  306  is connected to the low-potential-side power supply line V ss . The second terminal of the first transistor  301  is connected to the output portion V out  of the buffer circuit  300  through a capacitor  307 . 
     Here, when the gain of the first transistor  301  is expressed by g 1 , the gain of the second transistor  302  is expressed by g 2 , the gain of the third transistor  303  is expressed by g 3 , the gain of the fourth transistor  304  is expressed by g 4 , the gain of the fifth transistor  305  is expressed by g 5 , the gain of the sixth transistor  306  is expressed by g 6 , the angular frequency of an input signal is expressed by s (=2πf (f represents frequency)), and the electrostatic capacitance of the capacitor  307  is expressed by C, the gain of the buffer circuit  300  illustrated in  FIG. 3  can be expressed by the following mathematical formula (11) when the frequency is high. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     H 
                   
                   = 
                   
                     - 
                     
                       
                         
                           g 
                           3 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               g 
                               2 
                             
                             + 
                             
                               g 
                               6 
                             
                           
                           ) 
                         
                       
                       
                         
                           
                             g 
                             1 
                           
                           ⁢ 
                           
                             g 
                             3 
                           
                         
                         - 
                         
                           
                             g 
                             4 
                           
                           ⁢ 
                           
                             g 
                             6 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     On the other hand, the gain of the low-frequency component is expressed by the following mathematical formula (12). 
     
       
         
           
             
               
                 
                   [ 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     L 
                   
                   = 
                   
                     - 
                     
                       ( 
                       
                         
                           
                             
                               g 
                               2 
                             
                             ⁢ 
                             
                               g 
                               4 
                             
                             ⁢ 
                             
                               g 
                               6 
                             
                           
                           
                             
                               g 
                               1 
                             
                             ⁢ 
                             
                               g 
                               3 
                             
                             ⁢ 
                             
                               g 
                               5 
                             
                           
                         
                         + 
                         
                           
                             g 
                             2 
                           
                           
                             g 
                             1 
                           
                         
                         + 
                         
                           
                             g 
                             6 
                           
                           
                             g 
                             5 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     The gain of the fifth transistor  305  is not included in the above mathematical formula (11); thus, in the buffer circuit  300  illustrated in  FIG. 3 , the gain of the fifth transistor  305  does not contribute to an increase in gain of the high-frequency component. That is, the transistor of the last stage, which is connected to the high-potential-side power supply line, does not contribute to an increase in gain. On the other hand, as described above, in the buffer circuit  100  illustrated in  FIG. 1 , the transistor of the last stage, which is connected to the high-potential-side power supply line, contributes to an increase in gain; thus, the gain of the transistor of the last stage is increased by, for example, making the channel length of the transistor large. In such a manner, the gain of the buffer circuit can be increased. 
     As described in this embodiment, the buffer circuit illustrated in  FIG. 1 , which is one embodiment of the present invention, is novel and has advantageous effects in comparison to conventional buffer circuits. The buffer circuit can be applied to a driver circuit of a display device, and the buffer circuit and a pixel portion can be formed over the same substrate. 
     [Embodiment 2] 
     In this embodiment, a buffer circuit provided with p-channel transistors, which is one embodiment of the present invention, will be described. 
       FIG. 4  illustrates a buffer circuit  400  which is one embodiment of the present invention. 
     The buffer circuit  400  illustrated in  FIG. 4  includes first to sixth transistors, a capacitor, an input portion, and an output portion. A first terminal of a first transistor  401  is connected to a high-potential-side power supply line V dd , a second terminal thereof is connected to a first terminal of a second transistor  402  and a third terminal of a fourth transistor  404 , and a third terminal thereof is connected to an input portion V in  of the buffer circuit  400 . A second terminal and a third terminal of the second transistor  402  are connected to a low-potential-side power supply line V ss . A first terminal of a third transistor  403  is connected to the high-potential-side power supply line V dd , a second terminal thereof is connected to a first terminal of the fourth transistor  404  and a third terminal of a sixth transistor  406 , and a third terminal thereof is connected to the input portion V in  of the buffer circuit  400 . A second terminal of the fourth transistor  404  is connected to the low-potential-side power supply line V ss . A first terminal of a fifth transistor  405  is connected to the high-potential-side power supply line V dd , a second terminal thereof is connected to a first terminal of the sixth transistor  406  and an output portion V out  of the buffer circuit, and a third terminal thereof is connected to the input portion V in  of the buffer circuit  400 . A second terminal of the sixth transistor  406  is connected to the low-potential-side power supply line V ss . The second terminal of the first transistor  401  is connected to the output portion V out  through a capacitor  407 . 
     The buffer circuit  400  illustrated in  FIG. 4  has high driving capability and high gain of a high-frequency component. Further, such a buffer circuit has high slew rate. Since such a buffer circuit can be formed using transistors having the same conductivity type, it can be manufactured through a simple process. In other words, the slew rate of a buffer circuit provided with transistors that have the same conductivity type can be improved, which has been conventionally difficult. In addition, the buffer circuit can be formed with six transistors and one capacitor, whereby the area occupied by the buffer circuit can be made small. 
     As described in this embodiment, the buffer circuit illustrated in  FIG. 4 , which is one embodiment of the present invention, is novel and has advantageous effects in comparison to conventional buffer circuits. The buffer circuit can be applied to a driver circuit of a display device, and the buffer circuit and a pixel portion can be formed over the same substrate. 
     This application is based on Japanese Patent Application serial no. 2010-110995 filed with Japan Patent Office on May 13, 2010, the entire contents of which are hereby incorporated by reference.