Abstract:
The present invention, generally speaking, provides methods and apparatus for avoiding saturation of a transistor, with particular application to communications signal amplifiers. In accordance with one aspect of the invention, feedback circuitry is provided to control a transistor in accordance with a control signal. The combination of current monitor and a threshold comparator detects when the base current of the transistor exceeds a threshold, indicative of the onset of saturation. Feedback control is then modified to prevent saturation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to preventing saturation of transistors, with application to amplifier distortion reduction.  
           [0003]    2. State of the Art  
           [0004]    In the communications field, communications signals are required to be produced with low distortion. Particularly in wireless communications, low distortion is required in order to avoid adjacent channel interference (sometimes referred to as “splatter”). A principle cause of distortion in wireless communications is saturation of a final amplifier stage, resulting in signal clipping.  
           [0005]    U.S. Pat. No. 5,021,753 describes a linear amplifier including an arrangement for avoiding saturation of the amplifier. Conventional quadrature modulation is employed. In this arrangement an error signal is derived by comparing a desired signal component (I or Q) to a corresponding measured signal component obtained using a Cartesian feedback circuit. When the error exceeds a threshold, a pulse is generated. The frequency of occurrence of such pulses over time is monitored by a processor. The processor adjusts a scale factor within the circuit such that the frequency with which the error threshold is exceeded remains within a desired range.  
           [0006]    The use of Cartesian feedback to control amplifier saturation introduces significant circuit complexity, particularly if the effects of feedback delay are to be compensated for. Moreover, such a technique is not readily applicable to an amplifier following a polar architecture such as that described in High Efficiency Modulating RF Amplifier, U.S. patent application Ser. No. 09/637,269, filed Aug. 10, 2000, incorporated herein by reference.  
           [0007]    Accordingly, techniques are needed that avoid Cartesian feedback and that may be used with amplifier architectures other than the conventional quadrature modulator architecture. More generally, improved techniques are needed for avoiding transistor saturation.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention, generally speaking, provides methods and apparatus for avoiding saturation of a transistor, with particular application to communications signal amplifiers. In accordance with one aspect of the invention, feedback circuitry is provided to control a transistor in accordance with a control signal. The combination of current monitor and a threshold comparator detects when the base current of the transistor exceeds a threshold, indicative of the onset of saturation. Feedback control is then modified to prevent saturation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]    The present invention may be further understood from the following description in conjunction with the appended drawing. In the drawing:  
         [0010]    [0010]FIG. 1 is a diagram of a saturation prevention circuit in accordance with an exemplary embodiment of the invention.  
         [0011]    [0011]FIG. 2 is a detailed diagram of one implementation of the circuit of FIG. 1;  
         [0012]    [0012]FIG. 3 is a diagram showing one possible application of the circuit of FIG. 1; and  
         [0013]    [0013]FIG. 4 is a diagram showing a variation of the circuit of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Referring now to FIG. 1, a diagram is shown of a saturation prevention circuit in accordance with an exemplary embodiment of the invention. A transistor Q 1  is coupled to a power source, Vbat, and to a load L. In one possible application, the load L is a final amplifier stage of an RF amplifier as illustrated in FIG. 3 and described in greater detail in the foregoing co-pending application.  
         [0015]    In the configuration illustrated in FIG. 1, the transistor Q 1  is a bipolar transistor having an emitter terminal coupled to Vbat and a collector terminal coupled to the load L. The collector terminal is also coupled to a resistive network comprising series-connected resistors R 1  and R 2  coupled to ground. A voltage occurring at node A between the resistors R 1  and R 2  is proportional to the voltage applied to the load L. A resistor R 3  is coupled between the emitter terminal and the base terminal of the transistor Q 1 . The combination of the resistors R 1 -R 3  allows the gain of the transistor Q 1  to be set.  
         [0016]    A differencing operational amplifier (op amp)  101  is provided as part of a feedback circuit used to control the transistor Q 1  and thus set a voltage applied to the load L. A positive input terminal of the op amp is connected to node A of the circuit. In concept, the negative input terminal is coupled to a command input signal  103 , and an output signal of the op amp  101  is coupled to the base terminal of the transistor Q 1 . In the illustrated circuit, however, a current monitor  105  is inserted between the output signal of the op amp  101  and the base terminal of the transistor Q 1 .  
         [0017]    Furthermore, since in the illustrated circuit the command input signal is digital and the op amp  101  requires an analog input signal, a digital-to-analog converter (DAC)  107  is inserted in this path. The DAC  107  is a multiplying DAC, allowing a scale factor to be applied to the command input signal. The scale factor to be applied (at least in the absence of saturation) is stored in a multiplier register  108 . This value determines the power output to the load.  
         [0018]    Saturation prevention is carried out in response to the current monitor  105 , by a threshold comparator  109  and modification logic  111 . The threshold comparator is coupled to the current monitor  105  and to the modification logic  111 . The modification logic is coupled to the threshold comparator  109 , the multiplier register  108 , and the DAC  107 . Together, the modification logic  111 , multiplier register  108  and multiplying DAC  107  perform a scaling function represented by block  120 .  
         [0019]    Operation of the saturation prevention circuit is based on the following principle. In order to achieve a particular voltage at node A of the circuit, the required base current into the transistor Q 1  will varying linearly with the desired voltage throughout the linear range of the transistor Q 1 . However, as the transistor Q 1  approaches saturation, the base current will rapidly rise (by action of the feedback arrangement) in an unsuccessful attempt to raise the voltage at node A to the desired level. This rapid rise in base current is detected immediately by the current monitor  105  in combination with the threshold comparator  109 . The onset of saturation is thus signalled to the modification logic  111 . The modification logic then modifies downward the scale factor stored in the multiplier register such that an appropriately reduced scale factor is applied to the multiplying DAC  107 . As a result, the transistor Q is driven less heavily, and saturation is rapidly averted.  
         [0020]    The modification logic may vary from simple to complex, and may be implemented in hardware or as code executed by a processor (as in the foregoing U.S. Pat. No. 5,021,753, for example).  
         [0021]    Referring to FIG. 2, one particular implementation is shown, illustrating further details of the current monitor  107  and the threshold comparator  109 . The current monitor may take the form of an emitter-follower stage comprising a transistor Q 2  and resistors R 4  and R 5 . The threshold comparator may take the form of a common-emitter stage comprising a transistor Q 3  and resistors R 6  and R 7 . In operation, a current flows through the resistor R 7  that is proportional to the base current of the transistor Q 1 . Depending on that magnitude of that current, the output voltage developed at the comparator output will be either below or above a logic threshold of the scaling circuit  120 .  
         [0022]    The principle of the invention is applicable to various different types of active elements, including, for example, field-effect transistors (FETs). An example of such a circuit is shown in FIG. 4, in which the transistor Q 1  of FIG. 1 has been replaced by a FET M 1 . The current monitor  105  and the resistor R 3  are omitted from the circuit of FIG. 4. In addition, the threshold comparator of FIG. 1 is replaced by a voltage comparator  309 . As the transistor M 1  approaches saturation, the gate voltage will rapidly drop (by action of the feedback arrangement) in an unsuccessful attempt to raise the voltage at node A to the desired level. This rapid drop in gate voltage is detected immediately by the voltage comparator  309  to enable corrective action to be taken.  
         [0023]    Thus, there has been described saturation prevention techniques and circuits of general utility but having particularly advantageous application to the amplification of communications signals. The onset of saturation is quickly detected and corrective action taken. The circuit is simple in its realization but is highly effective in its operation.  
         [0024]    It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalents thereof are intended to be embraced therein.