Abstract:
A translinear variable-gain amplifier. The translinear variable gain amplifier receives a differential input voltage and produces a differential output current having a selected gain. The amplifier comprises a buffer amplifier that receives the differential input voltage and produces a differential input current. The amplifier further comprises a translinear gain cell coupled to receive the differential input current and produce the differential output current. The gain cell includes a first adjustable bias source that operates to set a linear input range of the gain cell, and a second adjustable bias source that operates to set a gain value of the gain cell.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This Application claims the benefit of priority of a co-pending U.S. Provisional Patent Application entitled “Translinear Variable Gain Amplifier” Serial No. 60/316,768 filed on Aug. 31, 2001, the disclosure of which is incorporated by reference herein in its entirety for all purposes. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to variable-gain amplifiers, and more specifically, to a translinear variable-gain amplifier.  
         BACKGROUND OF THE INVENTION  
         [0003]    The signals received and transmitted in wireless communications systems often vary in strength and thus require the use of variable gain amplifiers in the radio transceiver. These variable gain amplifiers operate to remove fluctuations in the received signal and to compensate for path losses in the transmitted signal.  
           [0004]    Ideally, the variable gain amplifier (VGA) provides amplification at low noise levels, adds little distortion, and consumes very little power. This is important because distortion produced by the receiver VGA limits its ability to reject interfering signals, while distortion produced by the transmitter VGA spills power into adjacent communication channels and thereby reduces system capacity. To minimize distortion, the bias current in the VGA and other circuits is typically high—an unwanted attribute for portable devices.  
           [0005]    It would therefore be advantageous to have a VGA with low distortion and low power consumption.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention includes a translinear variable-gain amplifier with adjustable gain and linearity. The amplifier operates over a wide control range, cascades easily, and allows broadband operation, while adding little distortion and consuming minimal power.  
           [0007]    In one embodiment included in the present invention, a translinear variable gain amplifier is provided that receives a differential input voltage and produces a differential output current having a selected gain. The amplifier comprises a buffer amplifier that receives the differential input voltage and produces a differential input current. The amplifier further comprises a translinear gain cell coupled to receive the differential input current and produce the differential output current. The gain cell includes a first adjustable bias source that operates to set a linear input range of the gain cell, and a second adjustable bias source that operates to set a gain value of the gain cell.  
           [0008]    In one embodiment included in the present invention, a variable gain amplifier with an adjustable linear input range and gain is provided. The amplifier comprises a pair of buffer amplifiers that receive differential input voltages and couple the input voltages to a pair of resistors that convert the input voltages to differential input currents. The amplifier also comprises a translinear gain cell coupled to receive the differential input currents and including a pair of diode-connected transistors and a first adjustable bias source. The first adjustable bias source used to set a linear input range. The translinear gain cell also includes a transistor differential pair and a second adjustable bias source that is operable to set a gain value of the translinear gain cell. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The foregoing aspects and the attendant advantages of this invention will become more readily apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:  
         [0010]    [0010]FIG. 1 shows a diagram of a current mirror circuit;  
         [0011]    [0011]FIG. 2 shows a diagram of a current gain cell circuit;  
         [0012]    [0012]FIG. 3 shows a detailed schematic of one embodiment of a translinear VGA in accordance with the present invention;  
         [0013]    [0013]FIG. 4 illustrates that the gain of the translinear VGA is easily adjusted by the bias current I G ;  
         [0014]    [0014]FIG. 5 illustrates that the linear input range of the translinear VGA is easily adjusted by the bias current I D ;  
         [0015]    [0015]FIGS. 6 a - b  illustrate the effective linearity of the translinear VGA; and  
         [0016]    [0016]FIG. 7 shows a detailed schematic of another embodiment of the translinear variable gain amplifier in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 shows a current mirror circuit that utilizes the translinear principle. In this circuit, the input current I in  flows through transistor Q 1  and develops a base-emitter voltage (V BE1 ) given by  
         V     B                 E1       =       V   T          ln        (       I     i                 n         I   S1       )                               
 
         [0018]    where V T  is the thermal voltage and I S1  is the saturation current of transistor Q 1 . The base-emitter voltage of transistor Q 1  is impressed upon or “mirrored” to the base-emitter junction of transistor Q 2 , producing an output current I out  equal to:  
         I     o                 u                 t       =       I   S2          exp        (       V     B                 E2         V   T       )                               
 
         [0019]    Rearranging these equations yields the following translinear relationship: 
         J Q1 =J Q2   
         [0020]    where the current densities (J) of the transistors are  
           J   Q1     =           I     i                 n         A     e        (   Q1   )                         a                 n                 d                   J   Q2       =       I     o                 u                 t         A     e        (   Q2   )               ,                         
 
         [0021]    with the emitter area (A e ) of each transistor proportional to the saturation current (I S ).  
         [0022]    [0022]FIG. 2 shows a current gain cell that also utilizes the translinear principle. In this circuit, the input currents I in+  and I in−  develop the following diode voltages:  
         V   D1     =       V   T          ln        (       I       i                 n     +         I   S       )                   V   D2     =       V   T          ln        (       I       i                 n     -         I   S       )                               
 
         [0023]    In one embodiment, diodes D 1  and D 2  are formed using “matched” transistors—devices having identical dimensions and thus equal saturation currents—with their bases and collectors connected. The associated difference voltage ΔV D  is expressed as:  
         Δ                   V   D       =         V   D1     -     V   D2       =       V   T          ln        (       I       i                 n     +         I       i                 n     -         )                                 
 
         [0024]    The circuit structure of the current gain cell mirrors this difference voltage to the inside differential pair consisting of matched transistors Q 3  and Q 4,  thereby generating output currents that are related as follows:  
           I   Q3       I   Q4       =     exp        (       Δ                   V     B                 E           V   T       )                             
 
         [0025]    where ΔV BE  is equal to ΔV D . Substituting the expression for ΔV D  into the above equation gives a current gain relationship expressed as:  
           I   Q3       I   Q4       =       I       i                 n     +         I       i                 n     -                               
 
         [0026]    which can be expressed in translinear form as J D1 J Q3 =J D2 J Q4 .  
         [0027]    Diode currents I D1  and I D2  sum into bias current I D . From this it follows that:  
         I   D1     =       I   D       1   +     exp        (     -       Δ                   V   D         V   T         )                     I   D2     =       I   D       1   +     exp        (       Δ                   V   D         V   T       )                                 
 
         [0028]    Similarly, transistor currents I Q3  and I Q4  sum into bias current I G , so that:  
         I   Q3     =       I   G       1   +     exp        (     -       Δ                   V     B                 E           V   T         )                     I   Q4     =       I   G       1   +     exp        (       Δ                   V     B                 E           V   T       )                                 
 
         [0029]    Noting that ΔV BE  is equal to ΔV D , the following important input-output relationship results:  
         I   Q3     =       I       i                 n     +            (       I   G       I   D       )                 I   Q4     =       I       i                 n     -            (       I   G       I   D       )                             
 
         [0030]    that clearly show that the bias current I G  controls the gain of the current amplifier.  
         [0031]    [0031]FIG. 3 shows a detailed schematic diagram of one embodiment of a translinear variable gain amplifier (VGA) in accordance with the present invention. The current gain cell requires linear input currents, I in+  and I in− , to operate properly. These currents are formed in the VGA of FIG. 3. Transistors Q 5  and Q 6  form emitter-follower amplifiers that buffer and transfer the input voltages, V in+  and V in− , to resistors R 1  and R 2 . The resistors convert the input voltages to linear currents that feed the current gain cell. Thus, the transistors Q 5 , Q 6  and resistors R 1  and R 2  form a buffer circuit that receives differential input voltages and produces differential input currents for input to the gain cell.  
         [0032]    The differential input voltage ΔV in  develops the following voltage drops: 
         ΔV in =V in+ −V in− =(V BE5 −V BE6 )+R(I D1 −I D2 )+(V D1 −V D2 ) 
         [0033]    where the value of resistors R 1  and R 2  is R. The terms (V BE5 −V BE6 ) and (V D1 −V D2 ) are designed to be comparatively small, thus the above the expression can be rewritten as:  
           I   D1     -     I   D2       ≈       Δ                   V     i                 n         R                           
 
         [0034]    Furthermore;  
         Δ                   I     o                 u                 t         =         I   Q3     -     I   Q4       ≈         I   G       I   D            (       Δ                   V     i                 n         R     )                               
 
         [0035]    where transistor currents I Q3  and I Q4  are I out+  and I out−  respectively. This is a fundamental expression for the translinear variable gain amplifier.  
         [0036]    [0036]FIG. 4 shows transfer function curves that illustrate how the gain of the translinear VGA is set by the adjustable bias current I G . The linear input range of the translinear variable gain amplifier (VGA) is set by the adjustable bias current I D  along with resistors R 1  and R 2 . The linear input range is effectively limited to a maximum value of V in(max) =I D R as illustrated in the transfer function curves of FIG. 5.  
         [0037]    [0037]FIGS. 6 a - b  illustrate the effective linearity of the translinear VGA. The gain of the translinear VGA is defined in terms of the transconductance parameter g m , defined as  
         g   m     =         ∂     I     o                 u                 t           ∂     V     i                 n           .                             
         [0038]    It&#39;s fairly constant near V in =0, but falls off as V in  approaches V in(max) . This illustrates the actual linearity of the translinear VGA, since gm is ideally constant and flat.  
         [0039]    [0039]FIG. 7 shows a detailed schematic of another embodiment of the translinear variable gain amplifier in accordance with the present invention. In many applications, a wide range of gain control is needed—wider than the range available from a single VGA stage. For these applications, multiple VGA stages like the one shown in FIG. 7 can be used. The circuit of FIG. 7 is an extension of the translinear VGA of FIG. 3 and easily interfaces to additional similar stages. The amplifier structure converts the output currents to a differential voltage (V out+ , V out− ) that can be directly connected to the next stage, enabling broadband operation from DC to GHz frequencies. Thus, the translinear VGA provides gain and linearity control, allowing optimum performance at minimum power consumption.  
         [0040]    The VGA is ideally suited for RF transmitters. Its power consumption tracks the RF output power level, which is set by a control signal. In one embodiment, a VGA constructed in accordance with the present invention is included in an RF transmitter circuit of a radio communication device. Because of its low power consumption, the VGA is ideally suited for use in portable radio communication devices, such as cell phones, PDAs, portable computers, and other handheld communication devices.  
         [0041]    The embodiments described herein are illustrative of the present invention and are not intended to limit the scope of the invention to the particular embodiments described. Accordingly, while one or more embodiments of the invention have been illustrated and described, it will be appreciated that various changes can be made to the embodiments without departing from their spirit or essential characteristics. Therefore, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.