Abstract:
Systems and methods for reducing the effects of imbalance between input signals in a gain controlled differential amplifier are shown. Distortion in the amplifier is reduced by modulating the gain control input based upon detected differences between the signal levels of the differential input signal source.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates in general to reducing second order distortion in electronic circuits more particularly to systems and methods for reducing the effects imbalances between differential input signals to gain-controlled amplifier circuits.  
         BACKGROUND  
         [0002]    One method of reducing second order distortions in electronic circuitry is to use a differential amplifier. In order to use a differential amplifier there must be differential (at least two) signals coming into the amplifier circuitry. Unfortunately, differential signals do not usually come to a circuit from external sources. The signals that typically arrive at a device from external sources are single-ended signals. A balun, which is a balance transformer, converts the single-ended signal to a differential signal. Baluns are relatively expensive, have bulk and do not always provide a balanced output. In some instances, the output of the balun is not fully differential. These “non perfect” balun outputs then cause problems, such as second order distortion, in the amplifier.  
         BRIEF SUMMARY  
         [0003]    It has been found that a primary cause of second order distortion in electronic circuits, and particularly in differential amplifiers, is due to signal imbalances. These imbalances can be either mismatches within the circuit itself or could be caused by imbalances in the applied input signal. In one embodiment, concepts of the present invention are applied to a variable gain amplifier which has its gain adjusted under control of an adjustment signal applied to one input of the amplifier. The adjustment signal may be provided, for example, through use of a variable resistor whose value is controlled by an externally applied gain control signal. It has been determined that such a variable resistor is a dominant source of second order distortion because the value of the resistor changes based on the input signals. By taking what is essentially the average voltage of the input signals, and injecting that average (or a fraction of that average) into the variable resistor adjustment signal, the modulation of the resistance can be significantly reduced.  
           [0004]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which:  
         [0006]    [0006]FIG. 1 is a simplified version of a prior art gain-controlled differential amplifier circuit;  
         [0007]    [0007]FIG. 2 is one embodiment of the invention providing a gain-controlled differential amplifier having second order distortions reduced. 
     
    
     DETAILED DESCRIPTION  
       [0008]    [0008]FIG. 1 is a simplified diagram of the input stage of prior art gain-controlled differential amplifier  10 . Input voltages at V IN  (such as differential input voltages V 1  and V 2 ) are converted into currents I 1  and I 2 . M 1  and M 2  are variable resistors where the resistance depends on the gate voltage for each device (V GS1  and V GS2 , respectively). This voltage is equal to V C  minus V CM. Any input signal (V 1 , V 2 ) imbalance coming into transistors Q 1  and Q 2  of circuit  10  translates to a voltage V CM  that varies depending on the input voltage. As a result of this variation in the voltage V CM , the voltage differential (V C -V CM ) also varies, causing the resistance of M 1  and M 2  to change. This resistance change causes problems in the signal processing circuit, such as second order distortion.  
         [0009]    As shown in FIG. 1, devices M 1  and M 2  are part of the degeneration network of the amplifier. When the input signal, V IN , is balanced (i.e., V 1 =V IN (common mode) + 
         V   1     =       V     IN                   (     common                 mode     )         +       V   d     2                             
 
         [0010]    and V 2 =V IN (common mode) − 
           V   2     =       V     IN                   (     common                 mode     )         -       V   d     2         ,                         
 
         [0011]    where V d =V 1 −V 2  and V IN (common mode) = 
             V     IN                   (     common                 mode     )         =       (       V   1     +     V   2       )     2       )     ,                         
 
         [0012]    the common node, V CM , is a virtual ground and there is an absence of an AC signal on V CM . When devices M 1  and M 2  are biased with the DC voltage, their gate-to-source voltages, V GS1  and V GS2 , are a constant DC voltage. However, when the input is unbalanced (e.g., V 1 =V IN (common mode) +V IN  and V 2 =V IN (common mode) ), V CM  is no longer a virtual ground and there exists an AC signal on V CM . Under this condition, a DC voltage at the gate of M 1  and M 2  results in an AC signal appearing between the gate and source of M 1  and M 2 . Because the resistance emulated by M 1  and M 2  depends on V GS1  and V GS2 , the degeneration resistance of the amplifier is modulated by the input signal. The modulation of the degeneration resistance is one of the contributing sources of second order distortion.  
         [0013]    Turning now to FIG. 2 where, in one embodiment, resistors R 2  and R 3  having common node  21  are added to circuit  20  providing a gain-controlled differential amplifier having second order distortions reduced according to an embodiment of the present invention. Voltage V D  at point  21  at any point in time is halfway between input signal V 1  and input signal V 2 . Voltage V D  is buffered in the illustrated embodiment by circuit  22 , which could be an amplifier, and by capacitor C 1 , and applied, along with gain adjustment signal V C  (via resistor R 1 ), to the input to variable resistors M 1  and M 2 . Voltage V D  is halfway between inputs V 1  and V 2  and thus counteracts the effect of the movement of voltage V CM  such as to maintain resistive values M 1  and M 2  relatively constant even though unbalanced inputs to Q 1  and Q 2  changes the value of V CM . By proper adjustment of the voltage at node  23  (by amplifier  22  or otherwise), the resistance values of M 1  and M 2  are held relatively unchanged and thus their distorting effect on further processing of the input signals is minimized. It is anticipated that this circuit will yield a 20 dB improvement in second order distortion performance.  
         [0014]    In the single ended case where only one side of the input (for example V 1 ) is moving, and the other side (V 2 ) is staying at a fixed voltage, then in the conventional circuit of FIG. 1, the resistance of M 1  would rise as V CM  rises under control of transistor Q 1 . Thus, variable resistor M 1  would then have a higher resistance than M 2 . Note that the value of M 1  rose because the voltage at the base of transistor Q 1  is different from the voltage across transistor Q 2  because the input signal values are not equal to each other. However, by injecting the differential signal of node  23  of FIG. 2 into the variable resistor control signal, the difference between those resistances is reduced significantly. As it is the delta between those resistors that have been found to be a significant cause the second order distortion, the illustrated embodiment of the present invention operates to significantly reduce second order distortion.  
         [0015]    It should be appreciated that the foregoing concept applies not just for a single ended case, but for other imbalances as well. The single-ended case is arguably the most extreme example when the problem is most severe because the inputs are completely unbalanced. If the inputs were perfectly differential, V CM  would be constant as the signal swung back and forth. With any unbalance, V CM  goes up and down at half the amplitude of the difference to the input signals, which in turn modulates the resistance of M 1  and M 2  which causes distortion.  
         [0016]    In the embodiment above M 1  and M 2  are preferably FETs and Q 1  and Q 2  are NPN transistors, but they could be FETs or any other device or combination of devices configured to provide differential amplification. The value of components R 2  and R 3  are not critical, but they should be equal so as to provide the average of the two signals, thereby establishing the magnitude of voltage applied to the V CM  node.  
         [0017]    The differential voltage buffer, which comprises in the illustrated embodiment component  22 , operates to minimize the amount of impact the use of this voltage has on the differential amplification circuit. If the resistance at node  21  were low this would impact the input impedance that is seen at V 1  and V 2 . Accordingly, it is desired to have the impedance of the differential voltage circuit of this embodiment of the present invention at circuit  22  balanced.  
         [0018]    V C  is preferably controlled by a control block (not shown) that sets the resistance of M 1  and M 2 , which then sets the gain for the differential input signal. This set value is what the average value of the gate voltage would need to be in order to control resistors M 1  and M 2 . Embodiments of the present invention operate to inject an AC signal on top of this DC signal so that the common mode signal (CV 1 , V 2 ) that appears at the input is added to the gate voltage that is applied to FETs M 1  and M 2 . Capacitor C 1  and Register R 1  effectively provide a pass characteristic where high frequencies are injected onto the gates of M 1  and M 2  while the DC value of that gate voltage is supplied by the external control circuitry, as discussed above. Another reason for the buffer is to isolate the input from the circuit by eliminating loading effects on the input.  
         [0019]    It is often important to avoid using feedback for cancellation purposes. When feedback is used there is always the risk of having the circuit become unstable. The desire is to create only one path and that path is through the buffer forward to the gate control voltage. It should be appreciated that the illustrated embodiment of the present invention provides a differential voltage control circuit to mitigate second order distortion without the use of feedback.  
         [0020]    In general, the illustrated embodiment injects the difference (delta) between the input voltages onto the gates of the FETs in order to reduce the modulation of the resistance. It should be appreciated that this delta can be obtained in different places according to embodiments of the present invention. For example, the difference can be obtained as voltages at the emitters of Q 1  and Q 2  which essentially track V 1  and V 2 . The delta could be obtained by sensing the V CM  voltage changes directly, which will also track the average between the two input voltages. The output signal could be sensed as well, although preferred embodiments do not implement feedback configurations.  
         [0021]    It should be noted that while corrections for second order distortion have been discussed above, the concepts of the present invention minimize the effects of imbalance between signals and thus, by minimizing the effects of such imbalance in any type of circuit, improvement in circuit function will result.  
         [0022]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.