Abstract:
A current amplifier has a variable resistor or capacitor to provide a high frequency boost. Additionally, additional transistors may be switched in and out of the circuit to provide different gains at lower frequency. The combination of variable resistors or capacitors and the switchable transistors provides control over the low frequency gain of the amplifier and the transition region from low gain to higher gain.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is filed simultaneously with commonly assigned United States patent application entitled “Differential Mode Current Amplifier with High Frequency Boost and Common Mode Rejection” by the present inventor, attorney docket number LSI59US01(02-6080), application Ser. No. ______, which is hereby specifically incorporated by reference for all it discloses and teaches. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    a. Field of the Invention  
           [0003]    The present invention pertains to electronic amplifier circuits and specifically to integrated circuit amplifier circuits.  
           [0004]    b. Description of the Background  
           [0005]    Amplifier circuits are used in may applications in integrated circuits. One common amplifier circuit is a cascode current mirror that is commonly used in integrated circuits. In general, the current mirrors are designed to generate a flat output. That is, the gain is constant throughout the frequency operating range.  
           [0006]    In some applications, there is need to increase or decrease the output of an amplifier in selective portions of the frequency range. For example, if an amplifier receives a signal from a device that has a gain that changes with frequency, the signal would need to be amplified so that the resultant gain was constant over the frequency range.  
           [0007]    It would therefore be advantageous to provide an amplifier circuit wherein a current signal may be amplified by different gains over different frequency ranges.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention overcomes the disadvantages and limitations of the prior art by providing a system and method for amplifying a current signal on an integrated circuit wherein a variable resistor is placed between the input line and the gate of a first transistor of a cascode amplifier and a variable capacitance may further be introduced to change the gain of the signal at different frequencies. Additionally, several transistors may be switched in or out of the circuitry to adjust the gain in the lower regions of input frequencies. The variable resistance, variable capacitance, and switchable transistors can be used to adjust the gain of the amplifier over different frequencies.  
           [0009]    The present invention may therefore comprise an equalizing current amplifier comprising: an input line; an output line; a first set of input transistors connected in parallel having the drains connected to the input line and the sources connected to ground, the first set of input transistors comprising at least one transistor; a variable resistor connected to the input line and the gates of the first set of input transistors; and a first set of output transistors connected in parallel having the gates connected to the input line, the sources connected to ground, and the drains connected to the output line, the first set of output transistors comprising at least one transistor.  
           [0010]    The present invention may further comprise an equalizing current amplifier comprising: an input line; an output line; a first set of input transistors connected in parallel having the drains connected to the input line, the gates connected to the input line, and the sources connected to ground, the first set of input transistors comprising at least one transistor; a variable capacitor connected to the gates of the first set of input transistors and ground; and a first set of output transistors connected in parallel having the gates connected to the input line, the sources connected to ground, and the drains connected to the output line, the first set of output transistors comprising at least one transistor.  
           [0011]    The present invention may further comprise an equalizing current amplifier comprising: an input line; an output line; a first set of input transistors connected in parallel having the drains connected to the input line and the sources connected to ground, the first set of input transistors comprising at least one transistor; a resistor connected to the input line and the gates of the first set of input transistors; a second set of input transistors connected in parallel having the drains connected to the input line, the gates connected to the gates of the first set of input transistors, and the sources connected to a switch, the switch being connected to ground, the second set of input transistors comprising at least one transistor; and a first set of output transistors connected in parallel having the gates connected to the input line, the sources connected to ground, and the drains connected to the output line, the first set of output transistors comprising at least one transistor.  
           [0012]    The advantages of the present invention are that current signals may be amplified with a tunable boost across a specific frequency range. The circuitry required is simple and therefore takes up a small portion of space on an integrated circuit. The signal boost may be tuned to a specific frequency range depending on the resistor value or capacitance value chosen. Further, the gain at lower frequencies may be adjusted by switching additional transistors in and out of the circuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    In the drawings,  
         [0014]    [0014]FIG. 1 is an illustration of a first embodiment of a current equalizer wherein a variable resistor or variable capacitor is used to affect the gain.  
         [0015]    [0015]FIG. 2 is an illustration of a plot of gain verses frequency for the embodiment illustrated in FIG. 1.  
         [0016]    [0016]FIG. 3 is an illustration of a second embodiment of the present invention of a current equalizer wherein additional transistors are switched in and out of the circuit.  
         [0017]    [0017]FIG. 4 is an illustration of a plot of the gain verses frequency of the embodiment illustrated in FIG. 3.  
         [0018]    [0018]FIG. 5 is an illustration of a third embodiment of the present invention wherein variable resistance, variable capacitance, and several transistors are switchable into the circuit.  
         [0019]    [0019]FIG. 6 illustrates a plot of the gain verses the frequency for the embodiment illustrated in FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    [0020]FIG. 1 illustrates a first embodiment of a current equalizer  100  wherein an input line  102  is connected to the drain of input transistor  104 , the gate of output transistor  106 , and variable resistor  108 . The variable resistor  108  is connected to the gate of transistor  104 , which is connected to a variable capacitor  110  that is connected to ground. The sources of transistors  104  and  106  are connected to ground.  
         [0021]    If the variable resistor  108  has no resistance, and the variable capacitor  110  is removed, the equalizer  100  functions as a common current mirror. When the resistance  108  or the capacitance  110  is added, the current draw of the input transistor  104  is reduced at high frequencies due to the coupling of the resistance, the capacitance, and the gate capacitance, causing more current to flow to the output transistor  106 , and thereby creating higher gain.  
         [0022]    By varying the resistance of the resistor  108  or the capacitance of the capacitor  110 , the output gain at high frequencies can be changed. The gain can thereby be tailored for the particular application.  
         [0023]    In some embodiments, the resistance and capacitance may be switched in and out in discrete intervals. For example, a series of resistors may be each connected to individual transistors capable of switching the resistance value into the circuit. Those skilled in the arts may use different mechanisms to accomplish a variable resistor and a variable capacitor while keeping within the spirit and intent of the present invention.  
         [0024]    Transistors as represented in a schematic of an integrated circuit may often be implemented as several small transistors operating in parallel. By varying the number of identical transistors in the first stage and second stage of the amplifier circuit, the output may be amplified or attenuated by the ratio of the number of transistors in each stage.  
         [0025]    [0025]FIG. 2 illustrates a plot  200  of gain  202  verses frequency  204  for the embodiment  100 . The curve  206  represents the gain with respect to frequency. The curve  208  illustrates the shape of the curve when a larger resistance or capacitance is present, while curve  210  illustrates the shape of the curve when a smaller resistance or capacitance is present. Either or both of the resistance or capacitance can be varied to achieve this effect. In some cases, it may be more economical to only vary either the resistance or capacitance to achieve the same result.  
         [0026]    [0026]FIG. 3 illustrates a second embodiment  300  of the present invention of a current equalizer. The input line  302  is connected to the drain of input transistors  304 ,  308 ,  310 , and  312 . A resistor  306  and capacitor  305  are connected the gates of transistors  304 ,  308 ,  310 , and  312 . The resistor  306  is connected to the input  302  and the capacitor  305  is connected to VSS. The source of transistor  304  is connected to ground while the respective sources of transistors  308 ,  310 , and  312  are connected to ground through the switches  309 ,  311 , and  313 . The input line  302  is connected to the gate of the output transistor  314 . The drain of output transistor  314  is connected to the output line  316  and the source of output transistor  314  is connected to ground.  
         [0027]    The switches  309 ,  311 , and  313  allow the transistors  308 ,  310 , and  312  to be individually switched into the circuit. In such a manner, the transfer function of the amplifier may be changed. In general, with the transistors removed from the circuit, the gain at lower frequencies will be higher than when the transistors are included in the circuit.  
         [0028]    In some embodiments, the addition or removal of the transistors  308 ,  310 , or  312  may cause the input current to be altered. In such embodiments, additional current drains may need to be switched into the circuit simultaneously with the transistors  308 ,  310 , and  312 . Several different means are available to those skilled in the arts for altering the input current when the transistors  308 ,  310 , and  312  are switched into the circuit. Such means may be used by those skilled in the art while keeping within the spirit and intent of the present invention.  
         [0029]    [0029]FIG. 4 illustrates a plot  400  of the gain  402  verses frequency  404  of the embodiment  300 . The curve  406  represents the gain over a frequency spectrum. The shape of the curve  408  is representative of the case when more transistors are switched into the circuit. The shape of the curve  410  is representative of the case when more transistors are switched out of the circuit.  
         [0030]    [0030]FIG. 5 illustrates a third embodiment  500  of the present invention wherein variable resistance, variable capacitance, and several transistors are switchable into the circuit. The input line  502  is connected to the drains of the transistors  504 ,  510 ,  512 , and  514 . A variable resistor  506  is connected from the input line  502  to the gates of the transistors  504 ,  510 , and  512 . A fixed resistor  519  is connected between the input line  502  and the gate of transistor  514 . A variable capacitor  508  is connected from the gates of transistor  504  to ground. The source of transistor  508  is connected to ground. The sources of transistors  510 ,  512 , and  514  are connected to ground through switches  511 ,  513 , and  515 . The input line  502  is connected to the gate of transistor  516 . The output line  518  is connected to the drain of transistor  516  while the source of transistor  516  is connected to ground.  
         [0031]    In some embodiments, the resistor  506  or capacitor  508  may be fixed components. For example, if the amplifier was being designed to amplify a specific signal and only one setting of the resistance is necessary, then a fixed resistor may be present. In this manner, the amplifier may be designed to operate without adjustment.  
         [0032]    In other embodiments, additional switches may be connected between the drains of transistors  504  and  518  for balancing purposes. Such switches may not be activated but may only serve to balance the loading when switches  511 ,  513 , or  515  are activated.  
         [0033]    The embodiment  500  combines the variable resistor and capacitor of embodiment  100  with the switchable transistors of embodiment  300 . The transfer function thereby has increased adjustability.  
         [0034]    [0034]FIG. 6 illustrates a plot  600  of the gain  602  verses the frequency  604 . Curve  606  represents a typical gain verses frequency curve for the embodiment  500 . The portion  608  of the curve represents a curve when many transistors are switched into the circuit. The portion  610  represents a curve when the switchable transistors are switched out of the circuit. The portion of the curve  612  represents a curve when high resistance and/or capacitance is present in the resistor  506  or capacitor  508 . The portion of the curve  614  represents a curve when low resistance or capacitance is present in the resistor  506  or capacitor  508 .  
         [0035]    The plot  600  illustrates the effects of changing the variable factors of the equalizer  500 . Both the low frequency gain and the transition point to the high frequency gain can be changed with the present embodiment. Those skilled in the arts will appreciate that various values of resistance, capacitance, and different transistors will result in various embodiments of the present invention while keeping within the spirit and intent of the present invention.  
         [0036]    The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.