Abstract:
High linearity is essential in audio circuitry. As sampling rates for audio applications are needed, high speed and high linearity are needed in analog and mixed signal portions of audio circuitry such as in current mirrors. A current mirror employs two current paths in an output. The first current path is driven by a fast acting transistor through a resistor. The second current path is driven by a differential amplifier coupled to another transistor through another resistor. The second current path is used to maintain linearity by causing the voltage across both transistors to be the same.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to generally to the semiconductor circuits and specifically with current mirrors. 
       BACKGROUND ART 
       [0002]    A current mirror is a basic building block of circuitry, particularly in current-mode circuits. A current mirror receives a current and generates a matching current. A current mirror has a wide variety of applications including digital to analog converter (DAC), automatic gain control, tunable time filter, etc. 
         [0003]      FIG. 1  shows a basic current mirror. It comprises field effect transistor (FET)  102  which has its drain and gate coupled together and FET  104  which has its gate coupled to the gate of FET  102 . When current flows through FET  102  the voltage registered at the gate of FET  102  controls the current through FET  104  to a matching current to that flowing through FET  102 . As a result, the input current I REF . is mirrored at output node  106  by output current I MIRROR . Current mirrors are can also be constructed from bipolar junction transistors (BJTs) in a similar fashion. 
       SUMMARY OF INVENTION 
       [0004]    A high speed highly linear current mirror is disclosed. The high speed current mirror comprises a transistor and a resistor in series in an input path. Two parallel output paths provide an output current through another resistor. A transistor coupled to the transistor in the input path controls one output path. Another transistor coupled to a differential amplifier controls the other output path. The differential amplifier measures the voltage difference between the two resistors and causes the voltage across the two resistors to be the same. 
         [0005]    In one embodiment, the transistors are FETs. In another embodiment, the transistors are bipolar junction transistors (BJTs) having a high β. In other embodiments, one or both of the resistors are variable resistors. In another embodiment both resistors have the same resistance. In yet another embodiment, the differential amplifier is an operational amplifier. 
         [0006]    One application of the high speed current mirror is a single ended DAC for use in audio applications. The DAC comprises a differential current steering DAC, an differential amplifier, such as an operational amplifier, a resistor and the high speed current mirror. The current mirror mirrors one of the outputs of the current steering DAC so that the difference between the outputs of the current steering DAC can be drawn through the resistor to produce a voltage signal. This voltage signal can then be used in an audio driver comprising a single-ended amplifier receiving the voltage signal and an output driver. 
         [0007]    Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0009]      FIG. 1  shows a basic current mirror; 
           [0010]      FIG. 2  shows an embodiment of the mixed signal and analog portions of an audio driver; 
           [0011]      FIG. 3  shows an embodiment of the mixed signal and analog portions of an audio driver; 
           [0012]      FIG. 4  shows an embodiment of a current mirror; 
           [0013]      FIG. 5  illustrates an analogous current mirror constructed using BJTs; 
           [0014]      FIG. 6  illustrates a current mirror with generic transistors; 
           [0015]      FIG. 7  shows an embodiment of a sourcing current mirror; 
           [0016]      FIG. 8  shows a variable gain embodiment of the current mirror; and 
           [0017]      FIG. 9  shows an alternate variable gain embodiment of the current mirror. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure as defined by the appended claims. 
         [0019]      FIG. 2  shows an embodiment of the mixed signal and analog portions of an audio driver. The audio driver comprises DAC  210 , amplifier  212  and output driver  214 . DAC  210  differentially drives amplifier  212  and output driver  214  drives speaker  216 . The connection between amplifier  212  and output driver  214  can be single ended or differential, as can the connection between output driver  214  and speaker  216 . As shown in this example, the driver has a two stage analog portion but in some embodiments, this can be one stage or three stage configuration. DAC  210  comprises current steering DAC  202  and resistors  204  and  206 . Current steering DACs are widely available and have become a common building block in mixed signal circuits due to their performance and availability. Resistor  204  receives current I OUTN  and provides output voltage V OUTN  and resistor  206  receives current I OUTP  and provides output voltage V OUTP . Thus the resistors provide a differential voltage output for DAC  210 . While the conversion of the differential current output of current steering DAC  202  to a differential voltage is straight forward. It is more complex to use a current steering DAC to provide a single ended output. 
         [0020]      FIG. 3  shows another embodiment of the mixed signal and analog portions of an audio driver. The audio driver comprises DAC  310 , amplifier  308 , and output driver  214 . DAC  310  provides a single output to single-ended amplifier  308 . The connection between amplifier  308  and output driver  214  can be single ended or differential, as can the connection between output driver  214  and speaker  216 . As shown in this example, the driver has a two stage analog portion, but in some embodiments this can be one stage or three stage among other configurations. 
         [0021]    DAC  310  is comprised of current steering DAC  202 , current mirror  302 , resistor  304 , and a differential amplifier shown here as operational amplifier  306 . Current mirror  302  draws I OUTN  from I OUTP , so that the net current flow through resistor  304  is I OUTP −I OUTN . Thus, the voltage across resistor  304  is V OUTP −V OUTN  and operational amplifier stably forces one terminal of the resistor at ground while permitting the other terminal which is coupled to DAC  310 &#39;s output to take the value of V OUTP −V OUTN . 
         [0022]    As audio drivers operate at faster sampling rates, greater demands are placed on components within DAC  310 . For example, it becomes desirable for current mirror  302  to react very quickly to changes in the current. The basic current mirror shown in  FIG. 1  can adapt quickly to changes in input current, but at the expense of the linearity of the current mirror. In other words, the voltage seen at the terminal of current mirror is not linearly proportional to the current drawn. Non-linearity in audio circuits often equate to distortion experienced by the listener. Therefore, a fast moving linear current mirror is highly desirable in any audio circuit using a current mirror, but in particular, the DAC within an audio driver. 
         [0023]      FIG. 4  shows an embodiment of a current mirror. The current mirror comprises FET  402 , FET  404 , FET  406 , a differential amplifier shown here as operational amplifier  408 , resistor  410 , and resistor  412 . Resistors  410  and  412  can have the same resistance. FET  402  and FET  404  are configured as traditional current mirrors. Operational amplifier  408  compares the voltages across resistors  410  and resistors  412 . In this configuration, the current mirrored is the combined current flowing through FET  404  and FET  406 . 
         [0024]    However, the current drawn through FET  404  is susceptible to error. The fast path of the current mirror is fabricated so that FET  404  is smaller than FET  402 . The result is that FET  404  has a higher impedance than FET  402 , so rather than precisely mirroring the current flowing through FET  402 , the current flowing through FET  404  is a current proportional and smaller than the current flowing through FET  402 . For example, if the impedance of FET  402  is 90% that of FET  404 , the current flowing through FET  404  would be 90% that of FET  402 . Other ratios can be employed but for most applications a ratio between 80-90% is effective. As a design criteria, the ratio should be sufficient to prevent current flowing through FET  404  to exceed that of FET  402  with error taken into account. For example, if the ratio is 90% then an error of 10% is tolerated. 
         [0025]    Operational amplifier  408  measures the difference in voltages across resistor  412  and resistor  410 . It generates a voltage proportional to the difference causing FET  406  to pass current until the voltage across resistor  412  matches that across resistor  410 . Because FET  404  and FET  406  are in a parallel arrangement, the total current passing through FET  404  and FET  406  passes through resistor  412 . This current is the total current drawn by the current mirror. If resistors  410  and  412  have the same resistance, the current I MIRROR  drawn through resistor  412  would be substantially the same as the current I REF  flowing through resistor  410  in order to have the same voltage across the two resistors. The bulk of the current is drawn by fast acting FET  404  but operational amplifier  408 , resistors  410  and  412  use FET  406  to maintain linearity. In the absence of FET  404 , the circuit would still perform as a current mirror. However, operational amplifiers are often slow acting and such a current mirror would not be suitable for high speed applications. 
         [0026]      FIG. 5  illustrates an analogous current mirror constructed using BJTs. It comprises BJT  502 , BJT  504 , BJT  506 , a differential amplifier shown here as operational amplifier  508 , resistor  410  and resistor  412 . Current mirror  500  is similar in basic functionality to current mirror  400 . However, the physics are quite different. Chief among the differences is that BJTs use current into and out of the base to control current flowing from the collector to the emitter. If significant current flows between the bases of BJT  402  and BJT  404 , linearity is not maintained. However, if the BJTs are selected with a high β value, the current flowing through this path is negligible. 
         [0027]    Because FET and BJT fail to exhibit common terminology, for the purposes of describing a generic current mirror. The term control terminal should refer to the base of a BJT or the gate of an FET. The term input terminal should refer to the collector of a BJT or the drain of an FET. The term output terminal should refer to the emitter of a BJT or the source of an FET. With this terminology in place,  FIG. 6  illustrates a current mirror with generic transistors. Current mirror  600  comprises transistor  602 , transistor  604 , transistor  606 , a differential amplifier shown here as operational amplifier  608 , resistor  410  and resistor  412 . In one embodiment, transistors  602 ,  604  and  606  are FETs and hence current mirror  600  becomes current mirror  400 . In another embodiment, transistors  602 ,  604 , and  606  are BJTs and hence current mirror  600  becomes current mirror  500 . 
         [0028]    It should be noted that in the previous examples, the resistors are coupled to a reference voltage which is shown to ground. The current mirror also operates when the reference voltage is tied to another voltage level. As shown in  FIG. 7 , when the reference voltage is the positive supply rail, the direction of current flow is reversed. Instead of a sinking current mirror, current mirror  700  is a sourcing current mirror. Structurally, current mirror  600  and  700  are topologically the same, though current mirror  700  is now drawn upside down to adhere the convention of having the positive supply on top. However, current mirror  700  uses the positive supply rail rather than ground. 
         [0029]    Current mirrors  400 ,  500 ,  600 , and  700  maintain linearity even when resistors  410  and  412  do not have the same resistances. Rather than functioning as a unity gain current mirror, the effect is the current mirror functions with a gain proportional of the ratio of resistor  410  to resistor  412 . For example, if the resistance of resistor  410  is twice that of resistor  412 , the current mirror would have a gain of 2. 
         [0030]    The gain of the current mirror could be made adjustable, by replacing either resistor  410  and/or resistor  412  with a variable resistor. By adjusting the resistance of the variable resistor, the gain of the current mirror could be adjusted. 
         [0031]      FIG. 8  shows a variable gain embodiment of the current mirror  800 . It is similar to current mirror  400  but comprises variable resistor  802  instead of resistor  412 . By adjusting the resistance of variable resistor  802 , the gain can be adjusted. The gain is inversely proportional to the resistance of variable resistor  802 . 
         [0032]      FIG. 9  shows an alternate variable gain embodiment of the current mirror. Current mirror  900  is similar to current mirror  400  but comprises variable resistor  902  instead of resistor  410 . By adjusting the resistance of variable resistor  902 , the gain can be adjusted. The gain is proportional to the resistance of resistor  902 . The choice of using current mirror  800  or  900  depends on the type of adjustment to the gain that is desired. 
         [0033]    It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.