Patent Publication Number: US-2007109040-A1

Title: Stacked CMOS current mirror using MOSFETs having different threshold voltages

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This is a Continuation-in-Part of U.S. application Ser. No. 11/354,944 filed Feb. 16, 2006, the entire content of which is incorporated herein by reference. This application claims priority from Korean Patent Application 10-2006-0047565 filed on May 26, 2006 and Korean Patent Application No. 10-2005-0013260 filed on Feb. 17, 2005, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to a stacked complementary metal oxide semiconductor (CMOS) current mirror. More particularly, the present invention relates to a stacked CMOS current mirror capable of sufficiently securing an output voltage swing range by decreasing the minimum saturation output operating voltage of the current mirror using metal oxide semiconductor field effect transistors (MOSFETs).  
      2. Description of the Related Art  
      Recently, as the integration of integrated circuit is increased, it has been required to design an integrated circuit which operates at low voltage and has an improved characteristic. However, it is difficult to implement an integrated circuit which operates at low voltage. In particular, it is difficult to implement a current mirror for an MOS analog circuit that operates at low voltage and has an improved characteristic. In an MOS analog circuit, a current mirror is used in the case where a stabilized and predictable DC reference current is produced at a certain point of the circuit and it is required that DC currents in proportion to the DC reference current are created.  
      The current mirror is generally composed of MOSFETs. The respective MOSFETs should operate in a saturation region in order for the current mirror to operate properly. Further, it is necessary to decrease the minimum voltage at which the MOSFETs begin to operate in the saturation region (hereinafter referred to as the “minimum saturation operating voltage”) in order to operate the current mirror at low voltage. Furthermore, it is necessary to increase the output resistance of the current mirror in order to sufficiently secure the output voltage swing range.  
      In the conventional current mirror, however, both the minimization of the minimum saturation operating voltage and the securing of the output voltage swing range are not satisfied.  
       FIG. 1  is a circuit diagram of a conventional CMOS current mirror. The conventional CMOS current mirror as illustrated in  FIG. 1  is a CMOS current mirror illustrated and described in the publication entitled “CMOS Analog Circuit Design” by P. E. Allen and D. R. Holberg.  
      As shown in  FIG. 1 , a MOSFET M 1  has a drain and a gate commonly connected to a reference current source Iref, and a source connected to ground. A MOSFET M 2  has a drain connected to a current source Iout, a gate connected to the gate of the MOSFET M 1 , and a source connected to ground. In the above construction, the minimization of the minimum saturation operating voltage is satisfied because only the minimum voltage headroom Δ 1  is required to operate the MOSFETs M 1  and M 2  in the saturation region.  
       FIG. 2  is a view explaining a problem of the conventional CMOS current mirror. Referring to  FIG. 2 , the conventional CMOS current mirror does not satisfy the securing requirement of the output voltage swing range because an error in that the reference current Iref does not coincide with the current-mirrored current occurs due to the small output resistance of the current mirror.  
     SUMMARY OF THE INVENTION  
      Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.  
      The present invention provides a stacked CMOS current mirror which can minimize the minimum saturation operating voltages of the MOSFETs and sufficiently secure an output voltage swing range by using MOSFETs having different threshold voltages for improving the output resistance of the current mirror.  
      According to an aspect of the present invention, there is provided a stacked CMOS current mirror, according to the present invention, which comprises a first MOSFET having a drain and a gate which are connected to an input current terminal, a second MOSFET having a drain connected to a source of the first MOSFET, a gate connected to the gate and the drain of the first MOSFET, and a source connected to ground, a third MOSFET having a drain connected to an output current terminal and a gate connected to the drain and the gate of the first MOSFET and the gate of the second MOSFET, and a fourth MOSFET having a drain connected to a source of the third MOSFET, a gate connected to the drain and the gate of the first MOSFET, the gate of the second MOSFET and the gate of the third MOSFET, and a source connected to the ground.  
      The first and third MOSFETs may be n-channel field effect transistors (nFETs), and the second and fourth MOSFETs may be low power nFETs (LpnFETs).  
      A threshold voltage of the second and fourth MOSFETs may be higher than a threshold voltage of the first and third MOSFETs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and/or other aspects of the present invention will be more apparent by describing certain exemplary embodiments of the present invention with reference to the accompanying drawings, in which:  
       FIG. 1  is a circuit diagram of a conventional CMOS current mirror;  
       FIG. 2  is a view explaining the problem of the conventional CMOS current mirror;  
       FIG. 3  is a circuit diagram of a stacked CMOS current mirror according to an exemplary embodiment of the present invention;  
       FIG. 4  is an equivalent circuit diagram of half of a stacked CMOS current mirror according to an exemplary embodiment of the present invention;  
       FIG. 5  is a view explaining an effect of a stacked CMOS current mirror according to an exemplary embodiment of the present invention; and  
       FIG. 6  is a view explaining another effect of a stacked CMOS current mirror according to an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS  
      Certain exemplary embodiments of the present invention will be described in greater detail with reference to the accompanying drawings.  
      In the following description, same drawing reference numerals are used for the same elements even in different drawings. The matters defined in the description such as a detailed construction and elements are nothing but the ones provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention can be carried out without those defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.  
       FIG. 3  is a circuit diagram of a CMOS current mirror according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 3 , the stacked CMOS current mirror according to the present invention includes four MOSFETs M 1 , M 2 , M 3  and M 4 .  
      The MOSFET M 1  has-a drain and a gate which are connected to an input current (I 1 ) terminal. The MOSFET M 2  has a drain connected to a source of the MOSFET M 1 , a gate connected to the drain and the gate of the MOSFET M 1 , and a source connected to ground.  
      The MOSFET M 3  has a drain connected to an output current (I 2 ) terminal and a gate connected to the drain and the gate of the MOSFET M 1  and the gate of the MOSFET M 2 . The MOSFET M 4  has a drain connected to a source of the MOSFET M 3 , a gate connected to the drain and the gate of the MOSFET M 1 , the gate of the MOSFET M 2  and the gate of the MOSFET M 3 , and a source connected to ground.  
      The MOSFETs M 1  and M 2  and the MOSFETs M 3  and M 4  form pairs, respectively, and two pairs of the opposite transistors form the current mirror.  
      The MOSFETs M 2  and M 4  have a high threshold voltage (highV T ), and the MOSFETs M 1  and M 3  have a regular threshold voltage (regV T ).  
      Table 1 below shows MOSFETs that can be put in the stacked CMOS current mirror and their characteristics.  
                           TABLE 1                       FET   condition   Wdesign/Ldesign   V T                    nFET   VD = 1.5 V, VB = 0 V   10/0.12   0.350 ± 0.055       pFET   VD = 1.5 V, VB = 0 V   10/0.12   0.300 ± 0.055       lpnFET   VD = 1.2 V, VB = 0 V   10/0.12   0.500 ± 0.055       lppFET   VD = 1.2 V, VB = 0 V   10/0.12   0.450 ± 0.055                  
 
      As shown in Table 1, lpnFETs having a high threshold voltage of 0.500±0.055 may be used as the MOSFETs M 2  and M 4 , and nFETs having a regular threshold voltage of 0.350±0.055 may be used as the MOSFETs M 1  and M 3 . Equation (1) represents a condition under which the MOSFET M 4  operates in a saturation region.
 
 V   GS4   −V   T4   ≦V   DS4   (1)
 
      In Equation (1), V GS4  indicates the gate-source voltage of the MOSFET M 4 , V T4  indicates the threshold voltage of the MOSFET M 4 , and V DS4  indicates the drain-source voltage of the MOSFET M 4 . In order for the MOSFET M 4  to operate in the saturation region, the voltage difference between the drain-source voltage V DS4  and the gate-source voltage V GS4  should be lower than the threshold voltage V T4 . This condition may be represented as Equation (1). Since the gate-source voltage V GS4  is equal to the voltage Δ 3 +V T3 +V A  at node B, the V GS4 −V T4  is given by V GS4 −V T4 =Δ 3 +V T3 +V A −V T4 . Here, Δ 3  indicates a micro-voltage of the MOSFET M 3  that is higher than 0, and V T3  indicates the threshold voltage of the MOSFET M 3 . Since the drain-source voltage V DS4  is equal to the voltage V A  at node A, Equation (1) may be represented as Δ 3 +V T3 +V A −V T4 ≦V A , which may be arranged as Δ 3 ≦V T4 −V T3 . Here, since Δ 3  is the micro-voltage higher than 0, Δ 3 ≦V T4 −V T3  may be represented as V T4 −V T3≧o. It can be known by the derived V   T4 −V T3  that the threshold voltage V T4  of MOSFET M 4  is higher than the threshold voltage V T3  of MOSFET M 3 .  
      Provided all of the MOSFETs M 1 , M 2 , M 3  and M 4  operate in the saturation region, V A  is calculated by Equation (2).  
                     V   A     =       ⁢       Δ   4     +     V     T   ⁢           ⁢   4       -     (       Δ   3     +     V     T   ⁢           ⁢   3         )                     V   A     =       ⁢           2   ⁢           ⁢     β   1       I       +     V     T   ⁢           ⁢   4       -     (           2   ⁢           ⁢     β   2       I       +     V     T   ⁢           ⁢   3         )                       V   A     =       ⁢           2   ⁢           ⁢     β   1       I       -         2   ⁢           ⁢     β   2       I       +     (       V     T   ⁢           ⁢   4       -     V     T   ⁢           ⁢   3         )         ,       β   1     =     β   2                       V   A     ~       ⁢     V     T   ⁢           ⁢   4         -     V     T   ⁢           ⁢   3                     (   2   )             
 
      As is derived from Equation (2), if  
           Δ   4     =         2   ⁢           ⁢     β   1       I         ,       Δ   3     =             2   ⁢           ⁢     β   2       I       ⁢           ⁢   and   ⁢           ⁢     β   1       =     β   2         ,         V   A     ~     V     T   ⁢           ⁢   4         -     V     T   ⁢           ⁢   3             
 
 is calculated. 
 
      Equation (3) is an equation that calculates the minimum saturation operating voltage according to the present invention using Equation (2).
 
 V   min =Δ 3+ ( V   T4   −V   T3 )  (3)
 
      Accordingly, if V A  derived from Equation (2) is applied to the voltage (V C =V min =Δ 3 +V A ) at node C, the minimum saturation operating voltage V min  results in V min =Δ 3+ (V T4 −V T3 ).  
       FIG. 4  is an equivalent circuit diagram of half of a stacked CMOS current mirror according to an exemplary embodiment of the present invention.  
      As shown in  FIG. 4 , a voltage source gm 2 *v gs3  and a resistor r o3  are connected in parallel between voltage v gs3  and voltage v gs4 , and a voltage gm 1 *v gs4  and a resistor r o4  are connected in parallel between voltage v gs4  and ground. Here, gm 2  denotes the transconductance of the MOSFET M 3 , gm 1  denotes the transconductance of the MOSFET M 4 . Since v gs4 =0 at a point of AC signal, the MOSFET M 1  has an output resistance r 04  only, and V gs3 +v a =0.  
      Equations (4-1) to (4-4) are equations that calculate the output current at the output node.  
               i   out     =         gm   2     ⁢     v     gs   ⁢           ⁢   3         +       (       v   out     -     v   a       )       r     o   ⁢           ⁢   3                   (     4   ⁢     -     ⁢   1     )                 i   out     =         gm   2     ⁡     (     -     v   a       )       +       (       v   out     -     v   a       )       r     o   ⁢           ⁢   3                   (     4   ⁢     -     ⁢   2     )                 v   a     =       i   out     ⁢     r     o   ⁢           ⁢   4                 (     4   ⁢     -     ⁢   3     )                 i   out     =         gm   2     ⁡     (       -     i   out       ⁢     r     o   ⁢           ⁢   4         )       +       (       v   out     -       i   out     ⁢     r     o   ⁢           ⁢   4           )       r     o   ⁢           ⁢   3                   (     4   ⁢     -     ⁢   4     )             
 
      Since v gs3 =−v a  in v gs3 +v a =0, Equation (4-2) is obtained by substituting this in Equation (4-1). Also, Equation (4-4), which calculates the output current at the output node, is obtained by substituting Equation (4-3) in Equation (4-2).  
      Equation 5 is an equation that calculates the output resistance.  
               r   out     =         v   out       i   out       =       r   04     +     r   03     +       gm   2     ⁢     r   04     ⁢       r   03     ~     gm   2       ⁢     r   04     ⁢     r   03                   (   5   )             
 
      By substituting Equations (4-4) in Equation (5), the output resistance  
         r   out     =         v   out       i   out       =       r     o   ⁢           ⁢   4       +     r     o   ⁢           ⁢   3       +       gm   2     ⁢     r     o   ⁢           ⁢   4       ⁢     r     o   ⁢           ⁢   3                 
 
 is obtained. Here, upon ignoring the resistances r 04  and r 03  having small values, the output resistance may be calculated from r out ˜gm 2 r 04 r 03  as its approximate value. 
 
       FIG. 5  is a view explaining an effect of a stacked CMOS current mirror according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 5 , an output current according to the change of the output voltage of the proposed current mirror and the output current according to the change of the output voltage of the conventional single current mirror are illustrated. It can be seen that the stacked CMOS current mirror according to the present invention has the minimum saturation operating voltage of 350 mV that is significantly lower than that of the conventional single current mirror. The minimum saturation operating voltage of the stacked CMOS current mirror according to the present invention is calculated using Equation (3). Accordingly, the minimum saturation operating voltage at which the MOSFETs that constitute the stacked CMOS current mirror begin to operate in the saturation region is decreased, and thus the CMOS current mirror can operate at low voltage.  
       FIG. 6  is a view explaining another effect of a stacked CMOS current mirror according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 6 , the output resistance according to the change of the output voltage of the proposed current mirror and the output resistance according to the change of the output voltage of the conventional single current mirror are illustrated. It can be seen that the stacked CMOS current mirror according to the present invention has the output resistance that is significantly higher than that of the conventional single current mirror. The output resistance of the stacked CMOS current mirror according to the present invention is calculated using Equation (5). As shown in  FIG. 5 , as the output resistance increases, a sufficient output voltage swing range is secured. Accordingly, the linearity of the current mirror increases and this causes the characteristic of the current source to be improved.  
      As described above, according to the present invention, both the minimization of the minimum saturation operating voltage and the securing of the output voltage swing range are satisfied, and thus current mirror can operate at low voltage with its linearity increased. This invention is used in a CMOS differential amplifier, a low noise amplifier and a mixer as well as in the current mirror so as to increase the operation at low voltage and the linearity.  
      The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.