Abstract:
An apparatus and a method for exponentially controlling a gain of a driver amplifier to drive a power amplifier are provided. The driver amplifier includes a plurality of cascode amplifier segments, wherein when a plurality of candidate gain values of the driver amplifier is arranged, the candidate gain values form a geometric sequence. A unit cost can be reduced by halving an area occupied by the driver amplifier.

Description:
PRIORITY 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed on Dec. 28, 2011 in the Korean Intellectual Property Office and assigned Serial No. 10-2011-0144189, the entire disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a variable amplifier for amplifying a Radio Frequency (RF) signal in a wireless transmitter. More particularly, the present invention relates to an apparatus and a method for exponentially controlling a gain of a driver amplifier to drive a power amplifier which provides a signal to an antenna stage. 
         [0004]    2. Description of the Related Art 
         [0005]    A general wireless transmitter uses a power amplifier for emitting an RF signal via an antenna. For example, the power amplifier is disposed at the end of a receiver and sends a signal with high power via the antenna. 
         [0006]    However, such an amplifier structure is unable to obtain both a significant gain and a significant increase in power, and exhibits an increased performance only when focusing on either the gain or the power. More particularly, it is difficult for the power amplifier to produce a high power and satisfy a high gain at the same time. Yet, both high power and high gain are necessary for characteristics of the transmitter. 
         [0007]    Thus, a separate high-gain amplifier is used in a front end to drive the power amplifier. An amplifier at the end of the transmitter combines a Driver Amplifier (DA) and a Power Amplifier (PA). 
         [0008]    Typically, a structure of the DA is a set of cascode amplifiers to attain a variable gain. A cascode structure has good input/output isolation, high output impedance, and high gain. For example, for a variable gain interval of 6 dB, a unit cascode amplifier includes eight sets which increase by multiples of two, for example, 1, 2, 4, 8, 16, 32, 64, and 128, and only the cascode amplifier corresponding to the intended gain operates among the cascode amplifier sets. 
         [0009]    However, eight digital switches are required to control six gain modes, which adopt a 3-to-8 decoder. Since only one cascode amplifier operates and other cascode amplifier sets do not operate in one mode, this process is inefficient. For example, the area 256 times the smallest unit cascode amplifier size is required for the 8-step gain control, which increases a unit cost. 
         [0010]    In addition, scalability is greatly degraded. In some cases, a structure of the wireless transmitter may need more various gains. The control based on 3 dB or 1 dB, rather than 6 dB, is quite complicated and feasible only based on an approximate value, and thus its accuracy also degrades. When the control mode uses 16 steps or 32 steps, not 8 steps, a control switch exponentially increases as such and the area is two times the branch of a maximum gain. As a result, the inefficiency still remains. 
         [0011]    Therefore, a need exists for an apparatus and a method for exponentially controlling a gain of a driver amplifier to drive a power amplifier which provides a signal to an antenna stage. 
         [0012]    The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention. 
       SUMMARY OF THE INVENTION 
       [0013]    Aspects of the present invention are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a driver amplifier using gain control which linearly responds to a dB unit which is given on a log scale. 
         [0014]    Another aspect of the present invention is to provide a control switch and a driver amplifier for setting a variable gain so that a user can intuitively perceive the variable gain using a control code. 
         [0015]    Yet another aspect of the present invention is to provide a variable transistor circuit for exponentially increasing a gain as a control code increases. 
         [0016]    According to one aspect of the present invention, a driver amplifier is provided. The driver amplifier includes a plurality of cascode amplifier segments. When a plurality of candidate gain values of the driver amplifier is arranged, the candidate gain values form a geometric sequence. 
         [0017]    According to another aspect of the present invention, a transmitter is provided. The transmitter includes a Digital to Analog Converter (DAC) for converting a digital signal to an analog signal, a Low Pass Filter (LPF) for removing a high frequency noise from an analog output signal of the DAC and filtering only a low frequency signal, a mixer for converting the filtered analog signal to a transmit carrier frequency, a driver amplifier for controlling a gain of the converted analog signal, and a power amplifier for power-amplifying the analog signal of which the gain is controlled by the driver amplifier. The driver amplifier comprises a plurality of cascode amplifier segments. When a plurality of candidate gain values of the driver amplifier is arranged, the candidate gain values form a geometric sequence. 
         [0018]    Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0020]      FIG. 1  is a diagram of a wireless transmitter according to an exemplary embodiment of the present invention; 
           [0021]      FIG. 2  is a diagram of a driver amplifier according to an exemplary embodiment of the present invention; and 
           [0022]      FIG. 3  is a flowchart of a method for exponentially controlling a driver amplifier gain in a wireless transmitter according to an exemplary embodiment of the present invention. 
       
    
    
       [0023]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0024]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
         [0025]    The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention is provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
         [0026]    It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
         [0027]    By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
         [0028]    Exemplary embodiments of the present invention provide an apparatus and a method for exponentially controlling a gain of a driver amplifier in a wireless transmitter. 
         [0029]      FIGS. 1 through 3 , discussed below, and the various exemplary embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way that would limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged communications system. The terms used to describe various embodiments are exemplary. It should be understood that these are provided to merely aid the understanding of the description, and that their use and definitions in no way limit the scope of the invention. Terms first, second, and the like are used to differentiate between objects having the same terminology and are in no way intended to represent a chronological order, unless where explicitly stated otherwise. A set is defined as a non-empty set including at least one element. 
         [0030]      FIG. 1  is a diagram of a wireless transmitter according to an exemplary embodiment of the present invention. 
         [0031]    Referring to  FIG. 1 , a Digital to Analog Converter (DAC)  100  converts a digital signal generated by a Transmit Signal Processor (TSP) (not shown) to an analog signal and outputs the analog signal to a Low Pass Filter (LPF)  102 . 
         [0032]    The LPF  102  removes a high frequency noise, filters only a needed low-frequency signal, and outputs the filtered signal to a mixer  104 . The mixer  104  converts a signal output from the LPF  102  to a transmit carrier frequency and outputs the converted signal to a pre-power amplifier  106 . 
         [0033]    The pre-power amplifier  106  adjusts a gain of the signal output from the mixer  104  and sends the signal to an external power amplifier. 
         [0034]    The pre-power amplifier  106 , which is single-ended, needs a balun or a differential to single circuit and adjusts the gain using a Radio Frequency Variable Gain Amplifier (RFVGA) in an input stage. 
         [0035]    For example, the pre-power amplifier  106  can include the RFVGA, a converter for converting a differential signal to a single-ended signal, and a Driver Amplifier (DA). The DA addresses the lack of gain of the power amplifier and concurrently gives sufficient input power to the power amplifier. Although it is not depicted here, the power amplifier power-amplifies the signal of the compensated gain from the DA and outputs the signal via an antenna 
         [0036]    The DA is designed based on a maximum output power, and the gain control of the DA needed to obtain a dynamic range of a TX path can variously realize system requirements. For example, when the system requirements demand the maximum output power of 48 dB and the gain control at intervals of 6 dB, eight gain modes in total are adopted. The DA is further explained with reference to  FIG. 2 . 
         [0037]      FIG. 2  is a diagram of a DA according to an exemplary embodiment of the present invention. 
         [0038]    Referring to  FIG. 2 , the DA includes an input stage  200 , bias stages  202  and  204 , a power supply stage  206 , the output stage  208 , and a variable gain amplifier  210 . 
         [0039]    The input stage  200  provides an input signal to a first gate of the variable gain amplifier  210  through an input capacitor, and the power supply stage  206  supplies the power to a drain of the variable gain amplifier  210 . In so doing, for the stable power supply, a reactance, a resistor, and an input capacitor are connected in parallel to supply power. 
         [0040]    The output stage  208  outputs the signal which is input from the input stage  200  and amplified by the variable gain amplifier  210 , through an output resistor. 
         [0041]    The main bias stage  202  supplies a bias current or voltage to the first gate of the variable gain amplifier  210 , and the cascode bias stage  204  supplies a bias current or voltage to a second gate of the variable gain amplifier  210 . 
         [0042]    The variable gain amplifier  210  includes therein a plurality of cascode amplifiers, and switches for controlling connection state of the segments. 
         [0043]    In this exemplary embodiment, transistor sizes are set to W/L, W/L, 3 W/L, 3 W/L, 15 W/L, 15 W/L, 45 W/L, and 45 W/L to attain the variable gain on 6 dB basis, and a control switch controls seven switches by combining a 3-bit digital signal b 2   b   1   b   0 . Accordingly, the largest transistor size is 45 times the smallest transistor size and is reduced to one third of the conventional 128. The total area is 128 times, which is reduced by half the conventional area. 
         [0044]    In various exemplary implementations, the size and the number of the transistors, and the digital bits for controlling the switches can vary. 
         [0045]    More specifically, exemplary embodiments of the present invention are in a transistor segment structure which can exponentially increase the gain with respect to an N-bit digital code (i.e., N is greater than 2). As an example, operations for controlling the DA gain according to a 3-bit digital control code b 0 , b 1  and b 2  are explained below. 
         [0046]    When the control code is zero (i.e., b 0 =0, b 1 =0, and b 2 =0), among eight cascode amplifier segments, the cascode amplifier segments excluding the first cascode amplifier segment cut their current and output only the unit current I. 
         [0047]    When the input is 1 (i.e., b 0 =1, b 1 =0, and b 2 =0), the switch b 0  is turned on, the current flows through the first cascode amplifier segment and the second cascode amplifier segment of the eight cascode amplifier segments, and thus 21 flows to the output stage. 
         [0048]    When the control code is 2 (i.e., b 0 =0, b 1 =1, and b 2 =0), the switch b 1  is turned on, the current flows through the first cascode amplifier segment and the third cascode amplifier segment of the eight cascode amplifier segments, and thus 4 I (I+3 I) flows to the output stage. 
         [0049]    When the control code is 3 (i.e., b 0 =1, b 1 =1, and b 2 =0), all of b 0 , b 1 , and b 2  are turned on and I+I+3 I+3 I=8 I flows to the output stage. For example, among the eight cascode amplifier segments, the current flows through the first through fourth cascode amplifier segments and 8 I flows to the output stage. 
         [0050]    When the control code is 4 (i.e., b 0 =0, b 1 =0, and b 2 =1), b 2  is turned on and I+15 I=16 I flows to the output stage. For example, among the eight cascode amplifier segments, the current flows through the first through fifth cascode amplifier segments and 16 I flows to the output stage. 
         [0051]    When the control code is 5 (i.e., b 0 =1, b 1 =0, and b 2 =1), b 0  and b 2  are turned on to output I+I+15 I+15 I=32 I. For example, among the eight cascode amplifier segments, the current flows through the first, fifth, and sixth cascode amplifier segments and 32 I flows to the output stage. 
         [0052]    When the control code is 6 (i.e., b 0 =0, b 1 =1, and b 2 =1), b 1  and b 2  are turned on to output I+3 I+15 I+45 I=64 I. For example, among the eight cascode amplifier segments, the current flows through the first, third, fifth, and seventh cascode amplifier segments and 64 I flows to the output stage. 
         [0053]    When the control code is 7 (i.e., b 0 =1, b 1 =1, and b 2 =1), all of the switches are turned on to output 128 I. For example, the current flows through all of the eight cascode amplifier segments and 128 I flows to the output stage. 
         [0054]    As a general expression, when k=4×b 2 +2×b 1 +b 0 , the output current is expanded to a Taylor series. All of b 2 , b 1 , and b 0  are ‘1’ or zero, b 2 N=b 2 , b 1 N=b 1 , and b 0 N=b 0 . As a result, Equation 1 can be obtained. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           OUT 
                         
                         = 
                           
                          
                         
                           
                             I 
                             × 
                             
                               2 
                               k 
                             
                           
                           = 
                           
                             I 
                             × 
                             
                               2 
                               
                                 
                                   4 
                                   × 
                                   b 
                                    
                                   
                                       
                                   
                                    
                                   2 
                                 
                                 + 
                                 
                                   2 
                                   × 
                                   b 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                                 + 
                                 
                                   b 
                                    
                                   
                                       
                                   
                                    
                                   0 
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           I 
                           × 
                           
                             ( 
                             
                               1 
                               + 
                               
                                 
                                   k 
                                   · 
                                   log 
                                 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               + 
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         k 
                                         · 
                                         log 
                                       
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     ) 
                                   
                                   2 
                                 
                                 / 
                                 
                                   2 
                                   ! 
                                 
                               
                               + 
                               
                                 
                                   
                                     ( 
                                     
                                       
                                         k 
                                         · 
                                         log 
                                       
                                        
                                       
                                           
                                       
                                        
                                       2 
                                     
                                     ) 
                                   
                                   3 
                                 
                                 / 
                                 
                                   3 
                                   ! 
                                 
                               
                               + 
                               … 
                             
                              
                             
                                 
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             ( 
                             
                               I 
                               + 
                               
                                 I 
                                 × 
                                 b 
                                  
                                 
                                     
                                 
                                  
                                 0 
                               
                               + 
                               
                                 3 
                                  
                                 
                                     
                                 
                                  
                                 I 
                                 × 
                                 b 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                               + 
                               
                                 3 
                                  
                                 
                                     
                                 
                                  
                                 I 
                                 × 
                                 b 
                                  
                                 
                                     
                                 
                                  
                                 0 
                                  
                                 b 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                             ) 
                           
                           × 
                           
                             ( 
                             
                               1 
                               + 
                               
                                 15 
                                  
                                 
                                     
                                 
                                  
                                 b 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0055]    For control at more precise intervals, the Taylor expansion is applied by setting the output as follows. For example, for the control at 3 dB intervals, the segments are constructed by expanding IOUT=I×2 k/2. For the control at 1.5 dB intervals, the segments are constructed by expanding IOUT=I×2 k/4. For example, when the current 6 dB intervals are segmented into M steps, the output current is generalized to Equation 2. 
         [0000]        I   OUT   =I× 2 k/M    (2)
 
         [0056]    When M is 6, the control is conducted at 1 dB intervals. When M is 6/5, the control at 5 dB intervals is feasible. 
         [0057]      FIG. 3  is a flowchart of a method for exponentially controlling a DA gain in a wireless transmitter according to an exemplary embodiment of the present invention. 
         [0058]    Referring to  FIG. 3 , the DA receives the digital bits or the control code in step  300  and directly drives a first switching group according to the digital bits in step  302 . For example, in  FIG. 2 , the switching operations of the switches of the second, third, and fifth cascode amplifier segments are determined directly by the digital bits. 
         [0059]    In step  304 , the DA drives a second switching group based on an AND operation result of some digital bits. For example, in  FIG. 2 , the switch of the fourth cascode amplifier segment operates based on the AND operation of the first bit b 0  and the second bit b 1  of the digital code. The switch of the sixth cascode amplifier segment operates based on the AND operation of the first bit b 0  and the third bit b 2  of the digital bits. The switch of the seventh cascode amplifier segment operates based on the AND operation of the second bit b 1  and the third bit b 2  of the digital bits. The switch of the eighth cascode amplifier segment operates based on the AND operation of the first bit b 0 , the second bit b 1 , and the third bit b 2  of the digital bits. 
         [0060]    In step  306 , the DA drives a first cascode amplifier segment group according to the first switching group and drives a second cascode amplifier segment group according to the second switching group. 
         [0061]    For example, when the digital code is zero in  FIG. 2 , among the eight cascode amplifier segments, the cascode amplifier segments excluding the first cascode amplifier segment cut their current and output only the unit current I. When the control code is 1 (i.e., b 0 =1, b 1 =0, and b 2 =0), the switch b 0  is turned on, the current flows through the first cascode amplifier segment and the second cascode amplifier segment of the eight cascode amplifier segments, and thus 2 I flows to the output stage. When the control code is 2 (i.e., b 0 =0, b 1 =1, and b 2 =0), the switch b 1  is turned on, the current flows through the first cascode amplifier segment and the third cascode amplifier segment of the eight cascode amplifier segments, and thus 4 I (i.e., I+3 I) flows to the output stage. When the control code is 3 (i.e., b 0 =1, b 1 =1, and b 2 =0), all of b 0 , b 1 , and b 2  are turned on, the current flows through the first through fourth cascode amplifier segments among the eight cascode amplifier segments, and 8 I flows to the output stage. When the control code is 4 (i.e., b 0 =0, b 1 =0, and b 2 =1), b 2  is turned on, the current flows through the first through fifth cascode amplifier segments among the eight cascode amplifier segments, and 16 I flows to the output stage. When the control code is 5 (i.e., b 0 =1, b 1 =0, and b 2 =1), b 0  and b 2  are turned on, the current flows through the first, fifth, and sixth cascode amplifier segments among the eight cascode amplifier segments, and 32 I flows to the output stage. When the control code is 6 (i.e., b 0 =0, b 1 =1, and b 2 =1), b 1  and b 2  are turned on, the current flows through the first, third, fifth, and seventh cascode amplifier segments among the eight cascode amplifier segments, and 64 I flows to the output stage. When the control code is 7 (i.e., b 0 =1, b 1 =1, and b 2 =1), all of the switches are turned on, the current flows through all of the eight cascode amplifier segments, and 128 I flows to the output stage. Thereafter, the DA completes this process. 
         [0062]    As set forth above, the control interval of the DA, which was controlled only by multiples of 2, adopts the digital control with the arbitrary linear exponential unit based on dB. Thus, the efficient configuration can reduce the area occupied by the DA by half and reduce the unit cost. 
         [0063]    While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.