Abstract:
An integrated circuit is disclosed. The integrated circuit includes an amplifier and a capacitor array coupled to the amplifier. The capacitor array is configured to be coupled in parallel to an inductor that is external to the integrated circuit, and the capacitor array and the external inductor comprise a tank circuit. The integrated circuit includes a resistor array coupled in parallel with the capacitor array. The resistor array is utilized to provide an overall frequency response of the capacitor array and resistor array that is opposite of a frequency response of the external inductor over a predetermined frequency range.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/776,041, filed on Mar. 11, 2013, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to integrated circuits and more particularly to improve the performance of an amplifier. 
       BACKGROUND 
       [0003]    A Frequency Modulated (FM) Transmitter typically utilizes an LC tank circuit coupled as a load. Due to the FM frequency range of 65 MHz˜108 MHz, it is not possible to implement both capacitor and inductor onto silicon die simultaneously. For practical silicon die area, only the capacitor is realized on-chip and made tunable for channels selection. An external inductor (L) is utilized (for example, L=120 nH) to form the required LC tank circuit. A flat gain response is desirable. 
         [0004]    A FM transmitter maintaining a flat gain for a frequency range of interest is essential for several reasons. Some of those reasons are enumerated below:
       For every dB increment in FM transmitter power, the N th  harmonic of the FM transmitter increases by N dB.   With a wide FM operating frequency range such as 65 MHz˜108 MHz, the harmonics level at the FM transmitter output become important. For example, on the higher end of FM frequency, the 9 th  harmonics fall inside a GSM band.   For the FM transmitter, the desired signal at 120 dBμV is to be delivered over the full frequency range of 65 MHz˜108 MHz. The corresponding harmonics that fall inside the cellular band must be at most −120 dBm (conducted) to avoid interfering with the cellular system.   If the gain of the FM transmitter is not flat, then to meet the desired signal level of 120 dBμV at 65 MHz will also result in higher output level at 108 MHz. And the corresponding cellular in-band harmonics will be compromised.       
 
         [0009]    However, if the FM transmitter gain is flat over the frequency range of interest the above identified issues can be minimized. 
         [0010]    However, there are limitations in providing a FM transmitter with a flat gain over a wide frequency range when utilizing an external inductor (L). Some of the limitations are described below:
       The Q of the external L varies across a wide operating frequency (for example, 65 MHz˜108 MHz)   The Q of the external L varies with manufacturers.   The frequency response of the external L itself varies greatly over frequency.       
 
         [0014]    With such external inductor characteristics, a flat gain response over 65 MHz&lt;f&lt;108 MHz is difficult to achieve without significantly affecting cost and potentially the size of the FM transmitter. 
         [0015]    Accordingly, what is needed is a system and method that addresses the above-identified issue. The system and method should be adaptable, cost effective and easily implemented in existing transmitters. The present invention addresses such a need. 
       SUMMARY 
       [0016]    An integrated circuit is disclosed. The integrated circuit comprises an amplifier and a capacitor array coupled to the amplifier. The capacitor array is configured to be coupled in parallel to an inductor that is external to the integrated circuit, wherein the capacitor array and the external inductor comprise a tank circuit. The integrated circuit includes a resistor array coupled in parallel with the capacitor array. The resistor array is utilized to provide an overall frequency response of the capacitor array and resistor array that is opposite of the frequency response of the external inductor over a predetermined frequency range. 
         [0017]    A system and method in accordance with and embodiment can be applied to any amplifier that requires flat gain response at its output. The embodiment does not require any digital control loop (sophisticated or simple), feedback loop, envelope detector etc to achieve a relatively flat gain response. 
         [0018]    In an embodiment, the gain response is auto adjusted with respect to the operating frequency without requiring any feedback circuit. Finally, a system and method in accordance with an embodiment does not require any sophisticated digital control signal to adjust the gain response. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIGS. 1A and 1B  are diagrams that illustrate the overall response of an LC tank circuit before predistortion of a capacitor array of the LC tank circuit and the overall response of the LC tank circuit after predistortion of the capacitor array. 
           [0020]      FIG. 2  is a first embodiment of an amplifier circuit in accordance with the present invention. 
           [0021]    FIGS.  3  and  4 A- 4 C illustrate the operation of the capacitor array in accordance with an embodiment. 
           [0022]      FIG. 5A  illustrates the series and parallel equivalent circuits of the capacitor array of the LC tank circuit in accordance with an embodiment. 
           [0023]      FIG. 5B  illustrates the series and parallel equivalent circuits of the external inductor of the LC tank circuit in accordance with an embodiment. 
           [0024]      FIG. 6  is a diagram of the LC equivalent tank circuit in accordance with an embodiment. 
           [0025]      FIG. 7  is a first embodiment of an implementation the resistor capacitor array. 
           [0026]      FIG. 8  a second embodiment of an implementation the resistor capacitor array. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The present invention relates generally to electrical circuits and more particularly to improve the performance of an amplifier. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0028]      FIGS. 1A and 1B  are diagrams that illustrate the overall response of an LC tank circuit before predistortion of a capacitor array of the LC tank circuit and the overall response of the LC tank circuit after predistortion of the capacitor array. To counter a gain imbalance of an amplifier over a range of frequencies of interest as a result of utilizing an external inductor, a technique to pre-distort the frequency response of a capacitor array utilized within the amplifier is disclosed. Instead of allowing a capacitor array associated with the LC circuit to have flat response as shown in  FIG. 1A , the technique provides for a change in the frequency response of the capacitor array in a direction opposite of a frequency response of the external inductor as shown in  FIG. 1B . 
         [0029]    By proportionally changing the on-chip capacitor array response in the opposite direction to the external inductor over the FM frequency by a resistor array, a flat response is provided. At low frequency, the resistor array introduces a larger parallel resistor (Rp) across the LC tank circuit. Therefore, the circuit Q is higher, resulting in smaller gain loss. At high frequency, the resistor array introduces a smaller parallel resistor (Rp) across the LC tank circuit and the circuit Q is lower, resulting in larger gain loss. Together with the external inductor L, the net result is a flat response of the amplitude of the amplifier. 
         [0030]    To describe the features of the present invention in more detail refer now to the following description in conjunction with the accompanying figures. 
         [0031]      FIG. 2  is a first embodiment of an amplifier circuit  100  in accordance with the present invention. The amplifier circuit comprises the amplifier  110  coupled to a tank circuit comprising capacitor array  102  and an external inductor (L)  112 . The inductor (L)  112  is an off-chip component. The capacitor array  102  is on-chip and in an embodiment is binary-weighted as it progresses from last significant bit (LSB) to most significant bit (MSB) of the capacitor array  102 . In an embodiment, the Q of the LC tank circuit comprising capacitor array  102  and an external inductor (L)  112  is maintained by adding a resistor array  104  in parallel to the capacitor array  102 . The frequency response of the capacitor array  102  may therefore be pre-distorted in an opposite direction of the external inductor (L)  112  utilizing the resistor array  104 , thus, cancelling the effect of the frequency response of the external inductor  112 . 
         [0032]    Accordingly, the resistor array  104  is utilized to beneficially affect the Q of LC tank ( 102 ,  112 ). If all of the resistors of the resistor array  104  are connected to ground, the Q of LC tank ( 102 ,  112 ) will be lower. If all of the resistors of the resistor array  104  are disconnected, then the Q will be higher. 
         [0033]    The resonance frequency of the tank circuit is given by the equation fs=1/(2π√L p C p ). Hence, in this implementation, for a higher frequency operation, a lesser number of branches of capacitors are required from capacitor array  102 . However, at this frequency, more resistor branches of the resistor array are connected in parallel to the LC tank ( 102 ,  112 ), which has the effect of a smaller parallel resistance (Rp); hence, there is a lower Q at a higher frequency operation. 
         [0034]    To operate at lower frequency, more branches of capacitors of the capacitor array  102  are used. At the same time, a lesser number of branches of resistors in the resistor array  104  are connected in parallel to the LC tank ( 102 ,  112 ). This results in a larger Rp; hence, there is a higher Q at lower frequency operation. 
         [0035]    At the resonance frequency, the peak amplitude of amplifier  100  is determined by the Q of the LC tank ( 102 , 112 ). The Q is given by the equation Q=R p /ωL p =ωR p  C p  Hence, if the Q of the tank ( 102 , 112 ) circuit is maintained throughout the FM frequency, the amplitude of the amplifier will also be maintained. 
         [0036]    FIGS.  3  and  4 A- 4 C illustrate the operation of the capacitor array  202  in accordance with an embodiment. Referring to  FIG. 3 , as is seen array  200   a  and array  200   b  are identical except array  200   a  represents the MOS switch for the selection of the individual capacitor branch by control bits CSEL&lt;8:0&gt; as switches  204  and in array  200   b  these MOS switches are represented as resistors  204  when they are ON. To resonate at a lowest frequency, a maximum number of capacitors in capacitor array  202  are used, i.e. ALL CSEL&lt;8:0&gt; are ON. The resonance frequency at f s =1/(2π√L p C max ) is the lowest. 
         [0037]    To resonate at highest frequency, a minimum number of capacitors of the in the capacitor array  202  are utilized; i.e. ONLY CSEL&lt;0&gt; is ON. The resonance frequency at f s =1/(2π√L p C min ) is the highest. 
         [0038]    Each branch of the capacitor array  202  has its corresponding Q given by Q=X c /R, where R is the ON resistance of a switch in each branch. A capacitor of the capacitor array  202  and a resistor of the resistor array  204  are doubled and halved respectively with respect to the previous branch of capacitor as LSB→MSB is traversed (shown in  FIG. 3 , and  FIGS. 4A-4C ,  302 ,  304  and  306  respectively). 
         [0039]    In so doing:
       1) Every branch of capacitor array  202  in  FIG. 3  has the same Q value       
 
         [0000]    
       
         
           
             
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                 Every branch of capacitor array  202  has the same Q s . 
               
             
             2) Any combination of the branch of capacitors in the capacitor array  202  in parallel will yield the same Q value. As an example ( FIGS. 4A-4C ,  302 ,  304 ,  306 ), the derivation for 2 branches in parallel is given: 
           
         
       
     
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         [0043]    With the Q value maintained, the capacitor array  202  will have a flat gain response over the operating frequency range of interest. Accordingly, due to its implementation approach, each branch of the capacitor array  202  or any combination of the capacitor branches has same Q. Consequently, the capacitor array  200  itself will result in a flat gain response. 
         [0044]      FIG. 5A  illustrates the series and parallel equivalent circuits  402  of the capacitor array of the LC tank circuit in accordance with an embodiment. At resonance, each capacitor in the capacitor array can be represented by its parallel equivalent circuits which are represented by the equations below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 In Series 
                 In Parallel 
               
               
                   
                   
               
             
             
               
                   
                 Q s  = X cs /R sc  = 1/ωR sc C s   
                 R pc  = (Q s   2  + 1)R sc   
               
               
                   
                   
                 C p  = [Q s   2 /(Q s   2  + 1)]C s   
               
               
                   
                   
                 Q p  = R pc /X cp   
               
               
                   
                   
               
             
          
         
       
     
         [0045]      FIG. 5B  illustrates the series and parallel equivalent circuits  404  of the external inductor of the LC tank circuit in accordance with an embodiment. At resonance, the external inductor can be represented by its parallel equivalent circuits which are represented by the equations below. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 In Series 
                 In Parallel 
               
               
                   
                   
               
             
             
               
                   
                 Q s  = X Ls /R sl   
                 R pl  = (Q s   2  + 1)R sl   
               
               
                   
                 R sl  = X Ls /Q s  = ωL s /Q s   
                 L p  = [(Q s   2  + 1)/Q 2 ]L s   
               
               
                   
                   
                 Q p  = R pl /X Lp   
               
               
                   
                   
               
             
          
         
       
     
         [0046]      FIG. 6  is a diagram of the LC equivalent tank circuit  500  in accordance with an embodiment. The equivalent LC tank  500  is given by a parallel network of RLC. The resonance frequency for circuit  500  is given by the equation, ω o =1/(√L p C p ). 
         [0000]    
       
         
           
             
               
                 
                   
                     Q 
                     p 
                   
                   = 
                     
                    
                   
                     
                       ( 
                       
                         
                           R 
                           dQeq 
                         
                          
                         
                            
                            
                         
                          
                         
                           R 
                           eq 
                         
                       
                       ) 
                     
                     / 
                     
                       X 
                       Lp 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     
                       
                         ( 
                         
                           
                             R 
                             dQeq 
                           
                            
                           
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                             eq 
                           
                         
                         ) 
                       
                       / 
                       
                         ω 
                         o 
                       
                     
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                       L 
                       p 
                     
                   
                 
               
             
             
               
                 
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                       ( 
                       
                         
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                          
                         
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                           eq 
                         
                       
                       ) 
                     
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                       √ 
                       
                         ( 
                         
                           
                             L 
                             p 
                           
                           / 
                           
                             C 
                             p 
                           
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
         [0047]    At resonance, the reactance cancels out, leaving only the (R dQeq |R eq ) as the load. The value of (R dQeq |R eq ) is determined by two resistive components:
       Parallel resistance of the resistor array R dQeq    502     Series resistance of external L  504 , which dominated the value of the equivalent parallel R eq  from capacitor array and external inductor       
 
         [0050]    Both of these resistances affects Q value of the LC tank circuit  500 . 
         [0051]    The amplifier gain is given by g m (R dQeq |R eq ), therefore to achieve a flat gain over the frequency of operation, it is essential for (R dQeq |R eq ) to remain fairly constant. 
         [0052]      FIG. 7  is a first embodiment of an implementation the resistor/capacitor array  600 . This implementation ensures that when a capacitor branch  602  is selected, its corresponding shunt resistor  604  is not selected. 
         [0053]    For the lowest frequency operation, all capacitors  602  are connected, all resistors  604  are disconnected, the effective parallel R is highest, and the de-Qing effect on the LC tank network by the resistor array is the lowest and there is lower attenuation at the lowest frequency. In contrast for the highest frequency operation, all capacitors  602  are disconnected, all resistors  604  are connected, the effective parallel resistance is the lowest, and the de-Qing effect is the largest on the LC tank network by the resistor array and there is higher attenuation at the highest frequency. 
         [0054]    The equivalent resistance (R) (contributed by each branch of the resistor and capacitor array) determines the amount of attenuation. Hence, if the equivalent R of each branch is further fine tuned, the desired gain flatness can be achieved. 
         [0055]      FIG. 8  a second embodiment of an implementation a capacitor array  702  in an amplifier circuit  700 . Here, the resistor array for degrading the quality value (de-Qing) the LC tank ( 702 ,  712 ) is embedded inside the capacitor array  702  as a bias resistor (RBIAS)  708 . 
         [0056]    When the EN_CAP signal=0, 
         [0057]    The switch  712 =OFF. Hence, the capacitor  706  is not shunted to GND by switch  712 . 
         [0058]    RBIAS  708  is pulled to VDD (AC GND). Hence, the capacitor  706  is connected to AC GND via RBIAS  708 . However, RBIAS is large (˜10 s kΩ). Hence, the capacitor sees a high impedance to AC ground which renders that particular capacitor ineffective in the LC tank ( 702 , 712 ). Hence, for the unselected capacitor  706  inside the capacitor array  702 , a series (R S1(OFF) |R BIAS )C network is formed. The equivalent parallel resistance contributed by this unselected capacitor  706  is R pc =(Q s   2 +1) (R S1(OFF) |R BIAS ) and its value is large since both R S1(OFF)  and RBIAS are large. 
         [0059]    When the EN_CAP signal=1, the capacitor  706  is used to form the LC tank resonance frequency. At the same time, RBIAS  708  is parallel to an ON resistance of switch  712 . These parallel resistances form the series equivalent R for each of the capacitor branches when EN_CAP=1. Since the ON resistance of switch  712  is small (few Ω˜100 s Ω), it dominates the value of the series equivalent R presenting a low impedance to GND for the capacitor  706  of the capacitor array  702 . Effectively, this makes the capacitor  706  of the capacitor array  702  part of the LC tank ( 702 , 712 ). 
         [0060]    The reason for pulling RBIAS  708  to VDD (&gt;1.4V) during EN_CAP=0 is to prevent the negative swing of node A from turning on the diode of the PN junction of switch  712 . By ensuring this PN junction is OFF, distortion in the signal is mitigated. Hence, the harmonic distortion of the power amplifier due to the PN junction is prevented. 
         [0061]    Hence, for higher frequency operation, a smaller number of capacitors  706  are required from capacitor array  702  and more RBIAS  708  branches are connected, resulting in a smaller equivalent parallel resistance R pc  to the LC tank ( 702 ,  712 ). Hence, lower Q at higher frequency operation. 
         [0062]    Similarly, for lower frequency operation, a larger number of branches of capacitors  706  are required from capacitor array  706  and less RBIAS  708  branches are connected, resulting in a larger equivalent parallel resistance R pc  to the LC tank. Hence, higher Q at lower frequency operation. 
         [0063]    The RBIAS  708  of each branch can be fine tuned further to achieve the desired de-Qing effect, hence, the flatness of the gain response. Besides de-Qing, RBIAS  708  also ensures the switch  712  stays OFF when the capacitor  706  of the capacitor array  702  is not selected (EN_CAP=0). The switch  712  is ensured OFF by biasing node A to a specific biasing voltage to prevent PN junction from turning ON as a result of the voltage swing. 
         [0064]    Advantages 
         [0065]    A circuit in accordance with an embodiment provides for area saving as a result of no requirement for feedback circuit. Furthermore the circuit has lower power consumption than conventional circuits a result of needing a smaller number of additional circuits (e.g. digital circuit, feedback loop, envelope detector etc). The circuit provides for a flat gain over a wide range of operating frequencies. 
         [0066]    Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the present invention.