Abstract:
A low voltage regulated current source includes a feedback amplifier that forces a node voltage in both branches of the current mirror to equal to each other, by adjusting voltages in two branches of the current mirror to be equal to each other. The low voltage current mirror also has a higher output impedance compared to other current mirrors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/787,489 filed Mar. 31, 2006, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a current source, specifically a low-voltage regulated current source. 
       BACKGROUND OF THE INVENTION 
       [0003]    Modern low power integrated circuits typically operate with very low supply voltages. This creates a challenge for current sources in low power integrated circuits to supply a constant current because low voltage power supplies tend to have voltage variations due to the effects and power requirements of other circuit components. A widely used current source in integrated circuits is a current mirror. However, most current mirrors are susceptible to voltage and load variations, thus making them undesirable for use in low power integrated circuits. For low power integrated circuits, it is desirable to have a constant current source that is not susceptible to varying supply voltages and load requirements. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In one aspect, there is provided a current mirror circuit that comprises a reference current side, a load current side, and a feedback circuit. The reference current side of the current mirror is configured to generate a reference current. The reference current side further includes a plurality of transistors. The load current side is configured to generate a load current proportional to the reference current. The load current side includes a first transistor coupled to a second transistor of the plurality of transistors of the reference current side. The feedback circuit is configured to provide a voltage difference between a terminal of the first transistor of the load current side and a terminal of the second transistor to a terminal of a third transistor of the plurality of transistors. 
         [0005]    In another aspect of the present invention, the current mirror circuit uses an amplifier as a feedback circuit. The circuit further includes a first input, a second input, and an output of the amplifier being coupled to the terminal of the first transistor of the load current side, the terminal of the second transistor of the reference current side, and the terminal of the third transistor of the reference current side, respectively, wherein the terminal of the third transistor is a gate terminal. The circuit further includes a gate terminal of the first transistor of the load current side being coupled to a gate terminal of the second transistor of the reference current side. The current mirror circuit further includes a drain terminal of the third transistor being coupled to the gate terminal of the second transistor. 
         [0006]    An advantage of the present invention is that the current source has a high output impedance. 
         [0007]    A further advantage of the present invention is that the current source has a low compliance voltage, thus making it desirable for use in low voltage applications. 
         [0008]    Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure and particularly pointed out in the written description and claims hereof as well as the appended drawings. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0010]    The present invention is described with reference to the accompanying drawings. 
           [0011]      FIGS. 1-3  illustrate circuit diagrams of known current mirrors; 
           [0012]      FIG. 4  illustrates a circuit diagram of a current mirror according to an embodiment of the present invention. 
           [0013]      FIG. 5  illustrates a circuit diagram of a current mirror according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    This specification discloses one or more embodiments that incorporate the features of this invention. The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. An embodiment of the present invention is now described. While specific methods and configurations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the art will recognize that other configurations and procedures may be used without departing from the spirit and scope of the invention. 
         [0015]      FIG. 1  illustrates a commonly used current mirror  100  that includes transistors  102  and  104 . On a high level, when the source current of transistor  102  is held constant, the drain current of transistor  102  will remain constant. This holds true as long as the drain-to-source voltage (V DS ) is sufficiently large to keep transistor  102  in active mode (non-triode mode). In current mirror  100 , the gates of transistors  102  and  104  are tied together, which are also tied to the drain of transistor  102 . In this way, transistor  104  can replicate input current (I in )  106  and produce a proportional output current (I out )  108 . Current mirror  100  can be configured to produce I out    108  at various input-to-output ratios, 1:n. The input-to-output ratios are controlled by the relative transistor sizes. 
         [0016]    The design of current mirror  100  is elegant, but mirror  100  is susceptible to a varying output current due to varying supply voltage and load power requirements. For example in mirror  100 , when the power requirements of a load  110  changes, the V DS  of transistor  104  will also change. When this occurs, the V DS  of transistor  104  will be different with respect to the V DS  of transistor  102 . This causes I out    108  to be non-proportional to I in    106 . In the case where I out  is designed to be nominally equal to I in , a difference in the V DS  of transistor  102  and V DS  of transistor  104  will cause I in  and I out  to be unequal. 
         [0017]      FIG. 2  illustrates a current mirror  200  similar to current mirror  100 . In current mirror  200 , a resistor  202  is added to the input current side of current mirror  200  to better control the input current. In this way, the output current is also better controlled due to the added input current stability. However, current mirror  200  also suffers from the problem of varying output current due to varying load requirements that cause the V DS  of transistor  204  to change. 
         [0018]      FIG. 3  illustrates a current mirror  300  utilizing cascode stages to increase the output impedance and improve current matching. Current mirror  300  includes transistors  302 ,  304 ,  306 , and  308 . Transistor  304  is the current source. Transistor  306  helps keep the voltage at the drain of transistor  304  constant. In this way, the output current can be better controlled. However, current mirror  300  is not without disadvantages. Current mirror  300  requires a relatively large voltage source and is particularly sensitive to the input voltage swing. This makes current mirror  300  undesirable for use in low power integrated circuits. 
         [0019]      FIG. 4  illustrates an improved current mirror  400  according to an embodiment of the present invention. Current mirror  400  utilizes a cascode stage at the input side. Current mirror  400  includes a feedback circuit  402 , transistors  404 ,  406 , and  408 , and a current source  410 . In current mirror  400 , an input current (I in )  412  is replicated as an output current (I out )  414  using the transistor pair  406  and  408 , both of which are identical. Current mirror  400  can be configured to produce any desired I in  to I out  ratio. 
         [0020]    As shown in  FIG. 4 , the gates of transistors  404  and  408  are both coupled to node  420 , which is the drain terminal of transistor  402 . When node  420  is driven by I in    412 , transistor  406  will allow I in    412  to pass through its drain and source. At the same time, transistor  408  mirrors I in    412  and produces an equivalent current I out    414  because the voltage potential at its gate is the same as the voltage potential at the gate of transistor  406  and node  420 . 
         [0021]    In current mirror  400 , as shown in  FIG. 4 , output node  422  is coupled to a differential pair. It should be noted that any other load circuit can be coupled to output node  422 . The voltage at node  422  (V out ) is often time controlled by the load requirement of the differential pair or load circuit. A change in the differential pair load requirement will typically cause a change in the output voltage, V out . This change in the output voltage causes an imbalance in the current mirror system. For example, when V out  changes, the drain-to-source voltage of transistor  408  also changes, making it unequal to the drain-to-source voltage of transistor  406 . When this occurs, I out    414  will be different from I in    412 . To prevent the above problem from occurring, feedback circuit  402  is used to drive the gate of transistor  404 , as shown in  FIG. 4 . 
         [0022]    On a high level, feedback circuit  402  compares the voltage at nodes  422  and  424 . Based the voltage comparison, feedback  402  drives the gate of transistor  404  such that the voltage difference between nodes  422  and  424  will be substantially zero. This is achieved by biasing the gate voltage of transistor  404  such that the voltage drop across transistor  404  causes the voltage at node  424  (source voltage) to be the same as the voltage at node  422 . 
         [0023]      FIG. 5  illustrates a current mirror  500  that is one embodiment of the current mirror  400 , wherein an operational amplifier  502  is used as the feedback circuit. In current mirror  500 , the non-inverting and inverting inputs of amplifier  502  is coupled to the output node  504  and node  506 , respectively. This input-to-node coupling arrangement could also be reversed (i.e. inverting input coupled to node  504 ). Additionally, the output of amplifier  502  is coupled to the gate of transistor  510 . 
         [0024]    One advantage of using operational amplifier  502  as a feedback circuit is the fact that op-amp draws very little input current (in the range of nano and pico amps, as a practical matter). As a result, the current mirror system is not affected by the present of amplifier  502  (e.g. the output current at node  504  is not affected). When amplifier  502  senses a voltage difference between nodes  504  and  506 , amplifier  502  will drive the gate of transistor  510  such that voltages at nodes  504  and  506  will equalize. In this way, the headroom or operation voltage range of current mirror  500  is increased because of the stability added to the current mirror system by amplifier  502 . 
         [0025]    Although  FIGS. 4-5  show current mirrors  400  and  500  constructed with npn transistors, it should be understood by one skilled in the art that current mirrors  400  and  500  may also be constructed with pnp transistors such that it would not depart from the spirit and scope of the invention. 
       CONCLUSION 
       [0026]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.