Abstract:
A current filtering current buffer amplifier includes: a first port and a second input port configured to be coupled to and receive input current; a first output port and a second output port configured to be coupled to and provide current to a load; a buffer configured to transfer the received input current to the first and second output ports as an output current, the buffer having an input impedance and an output impedance where the output impedance is higher than the input impedance, the buffer including first and second amplifiers, the first amplifier being a common mode feedback amplifier; and a filter coupled to the first and second input ports and coupled to the first and second amplifiers, the filter having a complex impedance and being configured to notch filter the received input current.

Description:
BACKGROUND 
       [0001]    A buffer amplifier (a.k.a. buffer) is an electronic device that provides electrical impedance transformation from one circuit to another. Two main types of buffers exist: the voltage buffer and the current buffer. Typically a current buffer amplifier is used to transfer a current from a first circuit, having a low output impedance level, to a second circuit with a high input impedance level. The interposed buffer amplifier inhibits the second circuit from loading the first circuit unacceptably and interfering with its desired operation. 
         [0002]    In the ideal current buffer the input resistance is zero while the output resistance is infinite (impedance of an ideal current source is infinite). Other properties of the ideal buffer typically include perfect linearity regardless of signal amplitudes and instant output response regardless of the speed of the input signal. For a current buffer amplifier, if the current is transferred unchanged (the current gain is 1), the amplifier is called a unity gain buffer or a current follower because the output current “follows” or tracks the input current. The current gain of a current buffer amplifier is (approximately) unity. Existing current buffer amplifiers, while providing current buffering, do not provide current filtering. Also, existing current buffer amplifiers ordinarily do not provide near-perfect linearity at output. 
         [0003]      FIG. 1  illustrates a conventional current buffer amplifier  100  that comprises unipolar transistors  102 ,  104 ,  112 ,  114 ,  116 , and  118  (e.g., FET common gate connected transistors), differential amplifiers  106  and  110 , a phase shift amplifier  108 , and resistors  120  and  122 . The differential amplifiers  106  and  110  are common mode feedback (CMFB) amplifiers used to suppress common-mode signals. The drains of the transistors  112  and  114  are connected to the inputs of the amplifiers  106  and  108  as shown in  FIG. 1 . The two input ports of the buffer amplifier are connected to the drains of the transistors  112  and  114  respectively. The positive input port is connected to a negative input port of the amplifier  108 , the source of the transistor  102 , and a negative input port of the amplifier  106 . The negative input port is connected to a positive input port of the amplifier  108 , the source of the transistor  106 , and a positive input port of the amplifier  106 . The input current  i in is an output current from a first circuit (not shown) which is transferred to the second circuit (not shown) as output current  i out by the current buffer amplifier  100 . When both  i in +  and  i in− are applied, the CMFB amplifier  106  will amplify the  i in input signal and the FET transistors  102 ,  104 ,  112 ,  114  will invert the signal (180° phase) and substrate the common signal. For differential signal CMFB would not operate. In this case, the input current I in will pass through. Thus, the input impedance g m1  is kept low and the output impedance g m2  is kept high. 
         [0004]    Ideally, a current buffer amplifier is perfectly linear, with the output signal strength varying in direct proportion to the input signal strength. In a linear device, the output-to-input signal amplitude ratio is always the same, no matter what the strength of the input signal. A graph  200  in  FIG. 2  illustrates an ideal current transfer gain as a function of frequency. 
         [0005]    In reality, however, the type of ideal linearity illustrated in  FIG. 2  is difficult to accomplish. Even if an amplifier exhibits linearity under normal conditions, it will become nonlinear if the input signal is too strong due to overdrive. The amplification curve bends toward a horizontal slope as the input-signal amplitude increases beyond a critical point, producing distortion in the output. In analog applications such as amplitude-modulation (AM), wireless transmission and hi-fi audio, linearity is important. Nonlinearity in these applications results in signal distortion because the fluctuation in gain affects the shape of an analog output waveform with respect to the analog input waveform. Accordingly, a linearity issue may arise in the current buffer amplifier illustrated in  FIG. 1  when current is converted to voltage at the output. 
       SUMMARY 
       [0006]    An example of a current filtering current buffer amplifier comprises: a first port and a second input port configured to be coupled to and receive input current; a first output port and a second output port configured to be coupled to and provide current to a load; a buffer configured to transfer the received input current to the first and second output ports as an output current, the buffer having an input impedance and an output impedance where the output impedance is higher than the input impedance, the buffer comprising first and second amplifiers, the first amplifier being a common mode feedback amplifier; and a filter coupled to the first and second input ports and coupled to the first and second amplifiers, the filter having a complex impedance and being configured to notch filter the received input current. 
         [0007]    Implementations of such an amplifier may comprise one or more of the following features. The filter includes an RC circuit having a resistance and a capacitance, the filter being coupled to positive and negative inputs of both of the first and second amplifiers. The resistance comprises first and second resistances, the first resistance coupled between the first input port and negative inputs of the first and second amplifiers, and the second resistance being coupled between the second input port and positive inputs of the first and second amplifiers. The capacitance is connected between the first and second resistances. The capacitance includes a first capacitance coupled between the positive inputs of the first and second amplifiers and ground, and a second capacitance coupled between the negative inputs of the first and second amplifiers and the ground. The amplifier further comprises a booster coupled to the buffer and configured to boost a common gate voltage of a transistor of the buffer to inhibit transfer gain in a pass band of the amplifier and in a stop band of the amplifier. The booster portion includes a first booster circuit coupled to the first input port via a third capacitance and a second booster circuit coupled to the second input port via a fourth capacitance, the third and fourth capacitances being configured to pass current of frequencies in the stop band of the amplifier to the first and second booster circuits, respectively. 
         [0008]    An example of a method of buffering current between first and second circuits includes: providing an input impedance to an output of the first circuit and an output impedance to an input of the second circuit, the output impedance being higher than the input impedance; and transferring current received from the first circuit to the second circuit by low-pass and notch filtering the current received from the first circuit such that: first current received from the first circuit having a frequency below a first frequency is transferred to the second circuit such that a first output amplitude is at least half of a first input amplitude of the first current; and second current received from the first circuit having a frequency above a second frequency is transferred to the second circuit such that a second output amplitude is less than one-tenth of a second input amplitude of the second current; where the second frequency is less than about two times the first frequency. 
         [0009]    Implementations of such a method may comprise one or more of the following features. The notch filtering causes a local minimum of transfer gain to occur at a local-minimum frequency that is between about 1.3 times the first frequency and about 1.7 times the first frequency. The method further comprises inhibiting transfer gain at least one of below the first frequency or above the local-minimum frequency. 
         [0010]    An example of a current buffer comprises: a first port and a second input port configured to be coupled to and receive input current; a first output port and a second output port configured to be coupled to and provide current to a load; a buffer portion configured to transfer the received input current to the first and second output ports as an output current, the buffer portion having an input impedance and an output impedance where the output impedance is higher than the input impedance; and filter means, coupled to the first and second input ports, the first and second output ports, and the buffer portion, for filtering the received input current such that the amplifier has transfer gains, for the received input current from the first and second input ports to the first and second output ports, above a first transfer gain value for frequencies up to a first frequency, has transfer gains below a second transfer gain value for frequencies above a second frequency that is higher than the first frequency, has a transfer gain of a third transfer gain value at a third frequency that is higher than the second frequency, and has a transfer gain of a fourth transfer gain value at a fourth frequency that is higher than the third frequency, the third transfer gain value being lower than the second transfer gain value and the fourth transfer gain value being higher than the third transfer gain value. 
         [0011]    Implementations of such a buffer may comprise one or more of the following features. The filter means are configured such that the first transfer gain value is about −3 dB, the second transfer gain value is about −10 dB, and the second frequency is about 1.2 times the first frequency. The third frequency is about 1.5 times the second frequency. The filter means comprise an RC circuit, including resistance and capacitance, coupled between the first and second input ports and the first and second output ports. Values of the resistance and capacitance determine the third frequency. 
         [0012]    Items and/or techniques described herein may provide one or more of the following capabilities. A current filtering current buffer amplifier may provide tunable notch filtering, reduced pass band peaking, and improved linearity compared to a conventional current buffer amplifier. A current buffer amplifier can be provided that is inexpensive and easy to tune, has a broad range and that will enhance diversity and a range of acceptable input and output circuits. Amplifiers are provided for use in electronic devices that employ circuits with low input impedance and high output impedance, for example, mobile electronic devices including portable computers, mobile telephones, personal digital assistants, and the like. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a circuit diagram of a conventional current buffer amplifier. 
           [0014]      FIG. 2  is a graph illustrating an ideal current transfer gain as a function of frequency. 
           [0015]      FIG. 3  is a circuit diagram of a current filtering current buffer amplifier. 
           [0016]      FIG. 4  is a graph of an input impedance as a function of frequency for the current filtering current buffer amplifier illustrated in  FIG. 3 . 
           [0017]      FIG. 5  is a current transfer gain as a function of frequency for the current filtering current buffer amplifier illustrated in  FIG. 3 . 
           [0018]      FIG. 6  is a graph of a total input impedance of a main common gate as a function of frequency with various resistance and capacitance values for the current filtering current buffer amplifier illustrated in  FIG. 3 . 
           [0019]      FIG. 7  is a graph of an input-to-output transfer function of the current filtering current buffer amplifier illustrated in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The described features generally relate to one or more improved methods and/or apparatus for current buffering. Further applicability of the described methods and apparatus will become apparent from the following detailed description, claims, and drawings. The detailed description and specific examples are given by way of illustration only, since various changes and modifications within the spirit and scope of the description will become apparent to those skilled in the art. Thus, the following description provides examples, and is not limiting of the scope, applicability, or configuration set forth in the claims. Changes may be made in the function and arrangement of elements discussed without departing from the spirit and scope of the disclosure. Various examples may omit, substitute, or add various procedures or components as appropriate. For instance, the methods described may be performed in an order different from that described, and various operations may be added, omitted, or combined. Also, features described with respect to certain examples may be combined with other examples. 
         [0021]      FIG. 3  illustrates an example circuit diagram of a current filtering current buffer amplifier  300  configured to provide tunable notch filtering and reduced band-pass peaking. The amplifier  300  will pass signals having frequencies within a certain range with little or no attenuation, and possibly with gain, and reject (i.e., significantly attenuate) signals having frequencies outside that range. The amplifier  300  includes a buffer portion  301  similar to, but different from, the current buffer  100  illustrated in  FIG. 1 . The buffer portion  301  includes transistors  302 ,  304 ,  312 ,  314 ,  316 ,  318 , resistances  320  and  322 , and amplifiers  306 ,  308 ,  310 . In addition, the buffer amplifier  300  includes common gate boost circuits  340 ,  350  that include unipolar transistors  342 ,  352  and are configured to boost common gate voltage for transistors  302 ,  304  respectively. The common gate boost circuits  340 ,  350  provide low impedance at high frequency by absorbing high-frequency signals, which helps prevent (inhibits) input impedance from becoming too high and helps reduce pass-band peaking (i.e., a maximum gain in the pass band). 
         [0022]    In the illustrative amplifier  300  shown in  FIG. 3 , the boost circuit  340  is common-gate connected at a node V 1  and the boost circuit  350  is common-gate connected at a node V 2 . The node V 1  of the boost circuit  340  is connected between the drain of the transistor  342  and a current source  343  that is connected to ground. Similarly, the node V 2  in the boost circuit  350  is connected between the drain of the transistor  352  and a current source  345  that is connected to ground. The boost circuits  340 ,  350  are respectively connected via capacitors  370 ,  372  to a positive input node  371  and a negative input node  373  of the buffer portion  301 . The input nodes  371 ,  373  are connected to drains of the transistors  312 ,  314 , gates of the transistors  302 ,  304 , and current sources  380 ,  382  that are connected to ground. A positive input current line  380  is connected to the input node  371  and carries an input current i in   + . A negative input current line  382  is connected to the input node  373  and carries an input current i in   − . 
         [0023]    The buffer portion  301  includes an RC circuit  360  connected between the positive and negative input nodes  371 ,  373 . The RC circuit  360  includes two resistors  362 ,  364  each of resistance R 1  and a capacitor  366  of capacitance C 1 . The RC circuit  360 , as shown, has the resistor  362  connected in series with the capacitor  366  connected in series with the resistor  364 , all connected in series between the nodes  371 ,  373 . A node  365  connected to both the resistor  362  and the capacitor  366  is also connected to negative input ports of the amplifiers  306 ,  308 . A node  367  connected to the resistor  364  and the capacitor  366  is also connected to positive input ports of the amplifiers  306 ,  308 . Thus, the input node  371  is connected via the resistor  362  to the capacitor  366  and the negative input port of each of the amplifiers  306 ,  308 , and the input node  373  is similarly connected via the resistor  364  to the capacitor  366  and the positive input port of each of the amplifiers  306 ,  308 . The capacitor  366  is connected between the positive and negative input ports of each of the amplifiers  306 ,  308 . While the RC circuit  360  is shown in  FIG. 3  as having the resistor  362  in series with the capacitor  366  in series with the resistor  364  between the nodes  371 ,  373 , alternative physical configurations are possible. For example, the node  371  can be connected via the resistor  362  to a capacitor that is connected to ground and the node  373  can be connected via the resistor  364  to another physically separate capacitor to ground. From an electrical standpoint, however, this is equivalent to the RC circuit  360  shown in  FIG. 3 . 
         [0024]    The RC circuit  360  serves to provide current notch filtering of the input signal as illustrated in  FIGS. 4-5  and described below. The notch filtering may be tuned by changing the capacitance value C 1  of the capacitor  366 . Capacitances C 2  of the capacitors  370 ,  372  that connect the boost circuits  340  and  350  to the input nodes  371  and  372  of the buffer portion  301  of the circuit  300  are used to block low frequencies by changing the frequency of the common mode signal input in the nodes  365  and  367 . Without the boost circuits,  3430 ,  350 , high-frequency signals would encounter a high impedance from the transistors  312 ,  314 , and be reflected back into the buffer  301 . The boost circuits  340 ,  350 , however, provide low impedances at high frequencies, thus helping the amplifier  300  to pass low-frequency current to the output and inhibiting high-frequency current from reaching the output. 
         [0025]    In operation, the current filtering current buffer amplifier  300  provides current buffering between two circuits with current filtering, where the filtering comprises passing low frequencies and notch filtering high frequencies as illustrated in  FIGS. 4-6 . 
         [0026]    Referring also to  FIGS. 4 and 5 , a graph  400  shows an input impedance as a function of frequency and a graph  500  shows a current transfer gain as a function of frequency for the current buffer amplifier  300 . A plot  402  illustrates the impedance as a function of frequency looking into nodes V 1 , V 2  indicated by arrows  390  and  392  in  FIG. 3 . A plot  404  illustrates the impedance (g m11 ) as a function of frequency looking into a node V 3  indicated by arrow  394  in  FIG. 3 . A plot  406  illustrates the combined input impedance. As shown in  FIG. 4 , the impedance  404  i.e., g m1 , peaks at a low frequency and subsides at a higher frequency. A plot  502  of the graph  500  illustrating the current transfer gain as a function of frequency shows that at low frequency f peak , there is a local/relative gain peak  504  of a peak gain G peak . In a pass band from 0 Hz to a pass-threshold frequency f PT1 , gain is above the dotted line (0 DB) and thus positive, indicating that signals are passed and some transfer gain is provided. Alternatively, the pass band may extend to a higher pass-threshold frequency f PT2  corresponding to a pass-threshold gain G PT  (e.g., −3 dB) with acceptably low attenuation of signals to be considered as passing these signals. At higher frequencies, signals are filtered and attenuated, reaching a stop-threshold gain G ST  of approximately −10 dB, although other levels may be acceptable and are determined by the circuit characteristics of the circuit values used at a stop-threshold frequency f ST , which is a tunable value, and reaching a local/relative minimum gain G notch  at a corresponding frequency f notch  at a “notch”  506  of the curve  502 . With the notch filtering provided by the RC circuit  360 , the gain of the amplifier  300  reaches the stop-threshold gain G ST  at a lower frequency than without the RC circuit  360  as shown by a gain curve  510  of gain provided by the amplifier  300  without the RC circuit  360 . Further, as indicated by a portion  507  of the plot  502  labeled “with parallel CG” and a plot  508  labeled “W/O parallel CG,” the gain with the parallel common gate boost circuits  340 ,  350  connected to the buffer portion  301  is lower at frequencies above the notch frequency than without the circuits  340 ,  350  connected. 
         [0027]    The frequency f notch  at the relative/local minimum gain G notch  will be approximately equal to the center frequency of the notch filter characteristics. The frequency f notch  corresponding to the local-minimum gain may be the center frequency of the notch filter characteristics or may be shifted to a slightly higher frequency due to the gain roll-off provided by the low-pass filter characteristics. The amount of difference between the center frequency of the notch filter characteristics and the local minimum-gain frequency f notch  will depend upon the gain characteristics (e.g., rate of gain roll-off) at and near the center frequency of the notch filter characteristics. The frequency f notch  corresponds to a local minimum gain as gain at frequencies above (at least above and near/adjacent) the notch frequency f notch  are higher than the local minimum gain G notch . 
         [0028]      FIG. 6  illustrates a graph  600  showing total input impedance Z in  of the main common gate as a function of frequency with various R 1  and C 1  values. Z in  may be calculated as follows: 
         [0000]    
       
         
           
             
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       Where 
       [0029]    s=jw,
 
C out  is the output capacitance of the feedback amplifier  308 ,
 
R out  is the output resistance of the feedback amplifier  308 .
 
g m2  is the transconductance of the feedback amplifier  308 ,
 
f p  is the peak frequency of the curves shown in the graph  600 .
 
The different plots shown in the graph  600  correspond to different experimental values of R 1  and C 1 .
 
         [0030]    A graph  700  in  FIG. 7  illustrates an input-to-output transfer function  700  of the current buffer amplifier  300 . The graph  700  shows the magnitude of current transfer gain of the buffer amplifier  300  in dB values as a function of frequency. As shown in  FIG. 7 , the peaking gets reduced, i.e. peaks illustrated in  FIG. 7  go down, with the boost circuits  340 ,  350  being used. The transfer value H CFCB (f) shown in  FIG. 7  may be calculated as follows: 
         [0000]    
       
         
           
             
               
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         [0000]    where C gs1  is the parasitic capacitance between gate and source of the transistors  312 ,  314 . The different curves shown in  FIG. 7  correspond to different experimental values of 
         [0031]    Referring to  FIGS. 3 ,  5 , and  7 , the amplifier  300 , and in particular the buffer  301 , is configured to provide desired characteristics of the peak  504  and the notch  506  and desired relationships of the peak  504  to the notch  506 . The peak gain G peak , peak-gain frequency f peak , notch gain G notch , and notch-gain frequency f notch  may have various values depending on the design of the amplifier. For example, referring to  FIG. 7 , the peak gain G peak  is about 10 dB (here from about 9 dB to about 12 dB), the peak-gain frequency f peak  may be between about 12 MHz and about 30 MHz (here from about 10 MHz to about 32 MHz), the notch gain G notch  may be about −20 dB (her from about −18 dB to about −21 dB), and the notch-gain frequency f notch  may be between about 25 MHz and about 60 MHz (here from about 23 MHz to about 63 MHz). A ratio of the notch-gain frequency f notch  to the peak-gain frequency f peak  is about 2 to 1 (here from about 1.9 to 1 to about 2.1 to 1). A ratio of the notch-gain frequency f notch  to the 3 dB pass-threshold frequency f PT2  is preferably between about 1.3 to 1 and about 1.7 to 1, here about 1.5 to 1 (from about 1.4 to 1 to about 1.6 to 1). A ratio of the notch-gain frequency f notch  to the 0 dB pass-threshold frequency f PT1  is about 1.65 to 1 (here from about 1.55 to one to about 1.75 to 1). A ratio of the stop-threshold frequency f ST  (with the stop-threshold gain G ST  being −10 dB) to the −3 dB pass-threshold frequency f PT2  is preferably less than about 2 to 1, here about 1.2 to 1 (here from about 1.15 to 1 to about 1.25 to 1). 
         [0032]    The previous description is provided to enable a person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. The disclosure is not limited to the examples and designs described herein but is accorded the widest scope consistent with the principles and features disclosed herein.