Abstract:
The present invention improves on the topology of a conventional current source by interposing an RC circuit and additional MOS between the output of a buffer and the output of the current source. The topology of the present invention advantageously provides a clean current output by shunting noise to ground.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims priority of U.S. Provisional Patent Application No. 60/350,616 filed on Jan. 22, 2002 entitled “High PSRR Current Source,” and the teachings are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to current sources, and in particular, current sources capable of providing a high power supply rejection ratio (“PSRR”). 
     BACKGROUND OF THE INVENTION 
     PSRR refers to the change in input offset voltage of an operational amplifier (“op-amp”) to the change in the power supply voltage that causes it. The offset voltage refers to the difference in voltage at the two inputs of an op-amp required to bring the output voltage to zero. 
     SUMMARY OF THE INVENTION 
     The present invention improves on the topology of a conventional current source by interposing an RC circuit and additional MOS between the output of the buffer and the output of the current source. The topology of the present invention advantageously provides a clean current output by shunting noise to ground. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic circuit diagram of a conventional current source using a CMOS device; and 
     FIG. 2 is a schematic circuit diagram of a current source using a CMOS device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The numerous innovative teachings of the present application will be described with particular reference to the disclosed embodiment. However, it should be understood that this embodiment provides only one example of the many advantageous uses and innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others. Detailed descriptions of known functions and constructions unnecessarily obscuring the subject matter of the present invention have been omitted for clarity. Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications. 
     A conventional current source  100  is illustrated in FIG.  1 . It comprises a reference voltage input  10  coupled to the non-inverting input of buffer  11 . The output of buffer  11  is coupled to the gate of MOS transistor  12 . The drain of MOS transistor being coupled to the inverting input of the buffer  11  and to pin  13 . An external resistor  14  couples the current generator  100  to ground  15 . The source of MOS transistor  12  is coupled to output pin  16 . Disadvantageously, current generator  100  is unable to shunt noise to ground  15  and thus, noise is outputted at pin  16  with the current signal. 
     An embodiment of the present invention is disclosed as current source  200  in FIG.  2 . As seen therein, current source  200  comprises a voltage reference input terminal  20  coupled to the non-inverting input of buffer  21 . In the disclosed embodiment, buffer  21  comprises an operational amplifier buffer. The output of buffer  21  is coupled to the gate of first MOS transistor  22 . The source of first MOS transistor  22  is coupled to first pin  23 . The drain of first MOS transistor  22  is coupled to the inverting input of buffer  21 . The first terminal of first resistor  24  is coupled to the gate of first MOS transistor  22 . The second terminal of first resistor  24  is coupled to the gate of second MOS transistor  26 . The first terminal of capacitor  25  is coupled to the gate of the second MOS transistor  26  and the second terminal of capacitor  25  is coupled to the inverting input of buffer  21 . The drain of second MOS transistor  26  is coupled to second pin  27  and the source of the second MOS transistor  26  is coupled to output terminal  28 . In the disclosed embodiment of the present invention, the first MOS transistor  22  comprises a PMOS transistor and second MOS transistor  26  comprises an NMOS transistor. As shown in the disclosed embodiment, the second pin  27  is coupled to a first terminal of second resistor  29  and the second terminal of second resistor  29  is coupled to ground  30 . As shown, the second resistor  29  is external to current source  200 . 
     Advantageously, current source  200  is operable to shunt noise to ground  30  and a cleaned current signal to output terminal  28 . 
     Although a disclosed embodiment of the present invention has been illustrated in FIG.  2  and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.