Patent Publication Number: US-2007103207-A1

Title: Source follower capable of increasing a voltage swing of an input terminal

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a source follower and, more particularly, to a source follower capable of increasing a voltage swing of an input terminal.  
      2. Description of Related Art  
      Typically, a source follower functions as a voltage buffer.  FIG. 1  shows a conventional circuit of a source follower, which consists of a current source I 1  and a PMOS transistor M 1 . As shown in  FIG. 1 , the PMOS transistor M 1  has a source connected to an output terminal V out , a gate connected to an input terminal V in , and a drain connected to a low voltage VSS. The current source has one terminal connected to a high voltage VDD and the other terminal connected to the source of the transistor M 1 .  
      When the source follower is in a saturation region, its input voltage V in  can reach to a minimum V in(min) =VSS+|V dsat1 |−|V gs1 |, which can be obtained by equation (1): 
 
 V   in   +|V   gs1   |−V   dsat1   |&gt;VSS,  
 
 V   in   &gt;VSS+|V   dsat1   |−|V   gs1 |,  (1) 
 
 where V dsat1  indicates a saturation voltage between the source and the drain of the transistor M 1 , and V gs1  indicates a voltage between the source and the gate of the transistor M 1 . In addition, the input voltage V in  can reach to a maximum V in(max)  of VDD−V I1c −|V gs1 |, which can be obtained by equation (2): 
 
 V   in   +V   I1c   +|V   gs1   |&lt;VDD,  
 
 V   in   &lt;VDD−V   I1c   −|V   gs1 |, (2) 
 
 where V I1c  indicates a voltage on the current source I 1 . 
 
      From equations (1) and (2), it is known that the voltage swing V swing  of the input voltage V in  is: 
 
 V   swing   =V   in(max)   −V   in(min) =( VDD−VSS )−( V   I1c   +|V   gs1 |). 
 
      Typically, V I1c  is about 0.2 V and |V gs1  is about 1 V. When the transistor M 1  is a native MOSFET, its voltage swing V swing  can be a value of (VDD−VSS)−(V I1c +|V dsat1 |), where |V dsat1 | is about 0.2 V. In this case, the voltage swing of the input voltage is increased, but the source follower has an insufficient driving capability for a next level.  
      To overcome this problem,  FIG. 2  shows an another circuit of source follower. The source follower in  FIG. 2  can eliminate the problem of the insufficient driving capability, but its input voltage swing equal to (VDD−VSS)−(V I1c +|V dsat1 |+V gs2 ) is reduced because of the subtracted voltage V gs2 , where V gs2  is a voltage between source and gate of a transistor M 2 . When VDD is 1.8 V, a low swing of an input voltage can limit the voltage magnitude of a circuit at the input terminal. In addition, when the input voltage has a slight offset, it can easily cause the circuit to present a saturation effect at the input terminal, resulting an error. Accordingly, the applications of the circuit are limited.  
      Therefore, it is desirable to provide an improved source follower to mitigate and/or obviate the aforementioned problems.  
     SUMMARY OF THE INVENTION  
      The object of the invention is to provide a source follower, which can increase the voltage swing of the input terminal and overcome the insufficient driving capability of the typical source follower.  
      In accordance with one aspect of the invention, a source follower is provided, which functions as a voltage buffer. The source follower includes a first current source, a first transistor, a second current source, a second transistor, a third current source and a third transistor. The first current source has one terminal connected to a high voltage and the other terminal connected to an output terminal. The first transistor has a source connected to the output terminal and a gate connected to an input terminal. The second current source has one terminal connected to a low voltage and the other terminal connected to a drain of the first transistor. The second transistor has a drain connected to the output terminal and a source connected to the low voltage. The third current source has one terminal connected to the high voltage and the other terminal connected to a gate of the second transistor. The third transistor has a source connected to the gate of the second transistor, a gate connected to the drain of the first transistor, and a drain connected to the low voltage, wherein a gate-source voltage of the second transistor counteracts a gate-source voltage of the third transistor to thereby increase a voltage swing of the input terminal to the first transistor.  
      In accordance with another aspect of the invention, a source follower is provided, which functions as a voltage buffer. The source follower includes a first current source, a first transistor, a second current source, a second transistor, a third current source and a third transistor. The first current source has one terminal connected to a high voltage. The first transistor has a drain connected to the other terminal of the first current source, a gate connected to an input terminal, and a source connected to an output terminal. The second current source has one terminal connected to a low voltage and the other terminal connected to the output terminal. The second transistor has a drain connected to the output terminal and a source connected to the high voltage. The third current source has one terminal connected to the low voltage and the other terminal connected to a gate of the second transistor. The third transistor has a source connected to the gate of the second transistor, a gate connected to the drain of the first transistor, and a drain connected to the high voltage, wherein a gate-source voltage of the second transistor counteracts a gate-source voltage of the third transistor to thereby increase a voltage swing of the input terminal to the first transistor.  
      Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a circuit diagram of a conventional source follower;  
       FIG. 2  is a circuit diagram of another conventional source follower;  
       FIG. 3  is a circuit diagram of a source follower in accordance with the invention; and  
       FIG. 4  is a circuit diagram of another source follower in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The inventive source follower can increase the voltage swing of its input terminal and overcome the problem of insufficient driving capability of the typical source follower. The source follower functions as a voltage buffer.  FIG. 3  is a circuit diagram of a source follower in accordance with the invention. The source follower includes a first current source  310 , a first transistor  320 , a second current source  330 , a second transistor  340 , a third current source  350  and a third transistor  360 . The first transistor  310  is PMOS transistor, the third transistor  360  is a PMOS transistor, and the second transistor  340  is an NMOS transistor. In other embodiments, the first transistor  320  can be a native PMOS transistor.  
      The first current source  310  has one terminal connected to a high voltage VDD and the other terminal connected to an output terminal V out . The first transistor  320  has a source connected to the output terminal V out  and a gate connected to an input terminal V in . The second current source  330  has one terminal connected to a low voltage VSS and the other terminal connected to a drain of the first transistor  320 .  
      The second transistor  340  has a drain connected to the output terminal V out  and a source connected to the low voltage VSS. The third current source  350  has one terminal connected to the high voltage VDD and the other terminal connected to a gate of the second transistor  340 . The third transistor  360  has a source connected to the gate of the second transistor  340 , a gate connected to the drain of the first transistor  320 , and a drain connected to the low voltage VSS, wherein a gate-source voltage V gs2  of the second transistor  340  counteracts a gate-source voltage V gs3  of the third transistor  360  to thereby increase a voltage swing of the input terminal to the first transistor  320 .  
      The minimum V in(min)  of the input voltage V in  of the source follower is a value of VSS+|V dsat1 |−|V gs1 |+V gs2 −|V gs3 |, which can be obtained by equation (3): 
 
 V   in   +|V   gs1   |−|V   dsat1   |&gt;VSS+V   gs2   −|V   gs3 |, 
 
 V   in   &gt;VSS−|V   gs1   |+|V   dsat1   |+V   gs2   −|V   gs3 |,  (3) 
 
 where V dsat1  indicates a source-drain saturation voltage of the first transistor  320 , V gs1  indicates a source-gate voltage of the first transistor  320 , and V gs2  indicates a source-gate voltage of the second transistor  340 . The maximum V in(max)  of the input voltage V in  is a value of VDD−V I1c −|V gs1 |, which can be obtained by equation (4): 
 
 V   in   +V   I1c   +|V   gs1   |&lt;VDD′ 
 
 V   in   &lt;VDD−V   I1c   −|V   gs1 |′  (4) 
 
 where V I1c  indicates a voltage on a current source I 1 . 
 
      From equations (3) and (4), it is known that the voltage swing V swing  of the input voltage V in  is:  
                     V   swing     =       V     in   ⁡     (   max   )         -     V     in   ⁡     (   min   )                       =       (     VDD   -   VSS     )     -       (       V     I   ⁢           ⁢   1   ⁢   c       +          V     gs   ⁢           ⁢   1              )     .                     (   5   )             
 
      When the first transistor  320  is a native MOSFET, the voltage swing V swing  of the input voltage V in  further can be reduced to (VDD−VSS)−(V I1c +|V dsat1 |).  
       FIG. 4  is a circuit diagram of another source follower in accordance with the invention. The source follower includes a first current source  410 , a first transistor  420 , a second current source  430 , a second transistor  440 , a third current source  450  and a third transistor  460 . The first transistor  420  is an NMOS transistor, the third transistor  460  is an NMOS transistor, and the second transistor  440  is a PMOS transistor. In other embodiments, the first transistor  420  can be a native NMOS transistor.  
      The first current source  410  has one terminal connected to a high voltage VDD. The first transistor  420  has a drain connected to the other terminal of the first current source  410 , a gate connected to an input terminal V in , and a source connected to an output terminal V out . The second current source  430  has one terminal connected to a low voltage VSS and the other terminal connected to the output terminal V out . The second transistor  440  has a drain connected to the output terminal V out  and a source connected to the high voltage VDD.  
      The third current source  450  has one terminal connected to the low voltage VSS and the other terminal connected to a gate of the second transistor  440 . The third transistor  460  has a source connected to the gate of the second transistor  440 , a gate connected to the drain of the first transistor  420 , and a drain connected to the high voltage VDD, wherein a gate-source voltage V gs2  of the second transistor  440  counteracts a gate-source voltage V gs3  of the third transistor  460  to thereby increase a voltage swing of the input terminal to the first transistor  420 .  
      In view of the foregoing, it is known that the invention uses the counteraction of the voltage V gs2  and the voltage V gs3  to increase the voltage swing of the input terminal to the first transistor and also overcome the insufficient driving capability of the typical source follower. Accordingly, when VDD equal to 1.8 V is applied to the inventive source follower, the circuit is not likely to have the saturation effect on its input terminal due to the increased voltage swing of the input voltage, thereby avoiding the distortion.  
      Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.