Patent Publication Number: US-7595626-B1

Title: System for matched and isolated references

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. provisional application Ser. No. 60/677,912, entitled SYSTEM FOR MATCHED AND ISOLATED REFERENCES, filed May 5, 2005, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to bias reference circuits and, more particularly, to a system for matched and isolated bias references. 
     BACKGROUND OF THE INVENTION 
     Radio receivers and transmitters integrate together low noise amplifiers, mixers, RF oscillators, filters, variable gain amplifiers, and high-power driver amplifiers. Each system operates over a wide dynamic range and requires extensive isolation. 
     In practice, inadequate isolation due to circuit or layout coupling limits the achievable dynamic range. Circuit coupling can occur through circuits shared by multiple components, such as reference circuits, as these circuits offer only limited isolation. For example, strong signals processed by low noise amplifiers, RF Oscillators, and PA drivers can affect common bias sources. It would therefore be advantageous to have reference circuits that are isolated from other system components. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention relates to a system and method for providing matched and isolated references. In one exemplary embodiment, a network is provided wherein multiple bias sources are substantially matched and isolated. 
     In one aspect the present invention is directed to a reference current generator which includes a primary reference generator operative to produce a first reference current. The reference current generator further includes a duplicate reference generator operative to produce a second reference current. An adjustment circuit coupled to the primary reference generator and the duplicate reference generator is configured such that the first reference current is substantially matched to and isolated from the second reference current. 
     In another aspect the present invention relates to a method for generating matched current references. The method includes generating a primary reference current in response to a reference voltage. A comparison voltage is produced based upon a comparison of the reference voltage and a mirrored voltage related to the primary reference current. The method further includes adjusting a value of a digital control word in accordance with the comparison voltage. A compensation voltage is provided based upon the digital control word. A duplicate reference current is then adjusted in accordance with the compensation voltage so as to match the duplicate reference current to the primary reference current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects of the embodiments described herein will become more readily apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  shows a diagram of a radio transceiver; 
         FIG. 2  shows a practical reference circuit; 
         FIG. 3  shows one embodiment of a novel reference network for generating matched and isolated references; 
         FIG. 4  shows a diagram of one embodiment of a bi-directional D/A converter. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows a block diagram of a radio transceiver  100  comprising a receiver portion  110  and a transmitter portion  120 . The radio receiver  110  operates to receive potentially weak signals and to reject strong interfering signals, covering a wide dynamic range. The radio transmitter  120  forms the transmit signal and generates sufficient power to overcome various wireless impairments. Most communication networks also include power control to minimize interference, while some networks, like CDMA networks, require control over a very wide range. 
     The receiver  110  comprises a low noise amplifier  130 , down-converting mixers  132 , frequency synthesizer (PLL and RF oscillator)  134 , variable gain amplifiers (VGAs)  136 , filters  140 , and A/D converters  142 . The transmitter  120  includes D/A converters  150 , filters  152 , a direct I/Q modulator  154 , frequency synthesizer  158 , RF variable gain amplifiers  160 , and PA driver amplifier  162 . In general, these circuits receive bias signals from reference circuits (not shown) designed to optimize performance. Accordingly, the reference circuits may emphasize precision, matching, and/or specify a certain temperature behavior. Ideally, the reference circuits resemble current sources with infinite output impedance or voltage sources with zero source impedance. 
       FIG. 2  shows an exemplary reference circuit  200 . As known to those skilled in the art, it is currently impossible to realize ideal reference circuits such as current or voltage sources. The reference circuit of  FIG. 2  presents real impedances. It generates a reference current and reference voltage described by; 
     
       
         
           
             
               I 
               REF 
             
             = 
             
               
                 V 
                 
                   REF 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
               
                 R 
                 1 
               
             
           
         
       
       
      
       V 
       REF 
       =V 
       REF1 
       +V 
       gs  
      
     
     where V REF1  is a precision voltage source (e.g., such as a bandgap generator), and V gs  is the gate-source voltage of the MOS transistor N 1 . The real impedances presented by each reference are given by;
 
 r   out1 =(1+ g   m   R   1 ) r   o   +R   1 
 
               r     out   ⁢           ⁢   2       =       r   op       1   +     A   op               
where r out1  is the impedance of the current source, g m  is the transconductance and r o  is the output resistance of transistor N 1 , r out2  is the impedance of the voltage reference, and r op  is the output resistance and A op  the gain of the operational amplifier. Note that the impedance of the current source r out1  decreases at high frequencies as g m  falls. Similarly, the gain of the operational amplifier also decreases at high frequencies, increasing r out2 .
 
     The real impedances of the reference circuits adversely affect the circuit elements driven by them by causing a bias change to occur as these circuit elements draw signal current. Specifically, the bias changes according to:
 
 V   REF   →V   REF   −i   radio   r   out2  
 
where i radio  represents the signal current drawn from the reference circuit by the radio circuits. This effect consequently couples together radio circuits that share the same reference circuit and thereby limits isolation and dynamic range.
 
     A bandgap circuit generates a precise and temperature stable voltage, making it suitable for generating the V REF  voltage. It also means that the reference current I REF  shares the same characteristics as resistor R 1 . This is important since integrated resistors typically show excellent matching but poor accuracy. Fortunately, a variety of circuits can be designed to take advantage of the excellent matching property while they minimize the impact of poor accuracy. However, many radio circuits operating at RF frequencies use inductive elements and therefore require precise bias settings. This is only possible with a precise resistor, which may only be available as an external element. Furthermore, at these frequencies, both g m  and A op  fall, making the reference impedances far from ideal. 
     Isolated references are needed for RF circuits to operate properly. One approach to achieving such isolation involves designing multiple references with separate external resistors. However, this is generally not practical since the result would consume more power and use additional device pins. 
       FIG. 3  shows one embodiment of a novel reference network  300  of the present invention that generates matched and isolated bias current sources using at most a single external resistor. The reference network  300  comprises a primary reference circuit  310  and a duplicate reference circuit  320  that are coupled together by an adjustment circuit  330 . In one embodiment, the adjustment circuit  330  comprises a pair of D/A converters  340  controlled by the same digital code. The D/A converters  340  adjust the reference network  300  so that the duplicate reference circuit  320  effectively matches the primary reference circuit  310 . 
     The reference network  300  of  FIG. 3  operates as follows. Operational amplifier OP 1 , transistor N 1 , and resistor R 1  establish the primary reference current; 
     
       
         
           
             
               I 
               1 
             
             = 
             
               
                 V 
                 REF 
               
               
                 R 
                 1 
               
             
           
         
       
     
     Transistors P 1  and P 2  mirror current I 1  to resistor R 2 , which adds to current ΔI 1  generated by the D/A converter  340   a  to establish the voltage V 2  given by;
 
 V   2 =( I   1   +ΔI   1 ) R   2  
 
     The comparator  350  senses this voltage, compares it to the reference voltage V REF , and adjusts the digital register (REG)  360  that drives the D/A converter  340   a  until voltage V 2  equals V REF . The current ΔI 1  required to be produced by the D/A converter  340   a  depends on the relationship between resistors R 1  and R 2 . If,
 
 R   2   =R   1 (1+α)
 
then ΔI 1  equals;
 
     
       
         
           
             
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               
                 I 
                 1 
               
             
             = 
             
               
                 
                   
                     V 
                     REF 
                   
                   
                     R 
                     2 
                   
                 
                 - 
                 
                   I 
                   1 
                 
               
               = 
               
                 
                   
                     V 
                     REF 
                   
                   
                     
                       R 
                       1 
                     
                     ⁡ 
                     
                       ( 
                       
                         1 
                         + 
                         α 
                       
                       ) 
                     
                   
                 
                 - 
                 
                   I 
                   1 
                 
               
             
           
         
       
     
     Note that the REG  360  also drives a second D/A converter  340   b . The D/A converter  340   b  generates an output current ΔI 2  that matches ΔI 1  and feeds the duplicate reference circuit  320 . The duplicate reference circuit  320  nominally generates a current I 2  described by 
               I   2     =       V   REF       R   3             
where R 3  matches resistor R 2 . Current ΔI 2  alters the current pulled through transistor P 3  such that;
   I   3   =I   2   −ΔI   2    
which gets mirrored to the output. It follows then that;
 
               I   out     =           V   REF       R   3       -     (         V   REF       R   2       -     I   1       )       =     I   1             
which equals the original reference current. In this way a pair of effectively matched and isolated reference current sources I out  and I 1  are made available for use by external circuits (not shown). Additional matched and isolated current references are possible by replicating operational amplifier OP 2 , transistors N 2 , P 3 -P 4 , resistor R 3 , and the D/A converter.
 
       FIG. 4  shows a diagram of an implementation of a bi-directional D/A converter capable of being utilized as the D/A converters  340 . As shown, the bi-directional D/A converter of  FIG. 4  comprises a current generator and a series of selectable current mirrors. The current generator, consisting of operational amplifier OP 3 , transistor N 3 , and resistor R 4 , produces the current; 
               I   bias     =       V   REF       R   4             
which scales to the output based on transistors N 4  plus P 5 -P 9 , resistor R 5 , and switches S 1 -S 4 . Accordingly,
 
               I   dac     =       m   ⁢       V   REF       R   4         -       V   REF       R   5               
where m represents the combined gate width of selected transistors P 6 -P 9  divided by the gate width of transistor P 5 . Adding transistor N 4  and resistor R 5  allows for a bi-directional output current I doc . In the exemplary embodiment the value of this current with transistors P 6 -P 9  selected is set to be one-half of the maximum scaled PMOS current (equal to mI bias ) by appropriately sizing transistor N 4  and resistor R 5 . Note that resistors R 4 -R 5  must match sensing resistors R 2  and R 3  (see  FIG. 3 ) to track any changes.
 
     Referring again to  FIG. 3 , the only physical link between the primary reference circuit  310  and the duplicate reference circuit  320  is the digital register REG  360 . The resulting digital signals possess extensive isolation, which means they are capable of tolerating very large coupling factors—even from very strong signals such as a power amplifier (PA) driver signal. The network  300  is designed to operate properly provided that favorable element matching, which is inherent to integrated circuit technology, is achieved. 
     Resistor R 1  can be realized as an external or integrated element. This allows the reference circuit to generate precise and well-matched bias sources with specific temperature behavior. Note that any temperature sensitivity can be readily designed into the voltage reference (V REF ). 
     The novel reference network produces multiple bias references that are both well matched and effectively completely isolated. Thus, embodiments of the reference network are suitable for in any type of circuit such as a receiver, transmitter, amplifier, or any other circuit that may utilize multiple bias references. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well-known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following Claims and their equivalents define the scope of the invention.