Abstract:
An amplifier apparatus for use with a sensor includes: (a) a first and a second amplifying circuit segment coupled with the sensor and cooperating to effect substantially balanced handling of signals received from the sensor; the first amplifying circuit segment includes a first transistor device; the second amplifying circuit segment includes a second transistor device; (b) a countercurrent unit coupled with the first and second amplifying circuit segments for receiving a first indicator signal from the first transistor device and a second indicator signal from the second transistor device; the first indicator signal represents a first parameter in the first transistor device; the second indicator signal represents a second parameter in the second transistor device; the countercurrent unit provides feedback signals to at least one of the first transistor and second transistor devices to reduce input impedance of the apparatus.

Description:
This application claims benefit of prior filed copending Provisional Patent Application Ser. No. 60/549,375, filed Mar. 2, 2004. 

   BACKGROUND OF THE INVENTION 
   The present invention is directed to signal amplifiers used with a sensor such as a read head in an information storage device, and especially to such signal amplifiers having reduced input impedance. 
   There are many important goals in designing and operating an amplifier for use with a sensor. One such goal is low band pass corner frequency. Sensors such as magneto-resistive sensing elements require a direct current (DC) bias applied across them to operate correctly. The presence of such a DC bias may cause problems if the DC signal is passed on to amplifying elements. A low band pass corner frequency permits sensing of lower frequency signals while still rejecting DC signals and therefore contributes to a truer sensing of signals indicated by the sensor. 
   Needs for greater magnetic storage density in industry have been answered by various innovations. One such innovation has been perpendicular magnetic recording in which the magnetic recording medium is magnetized perpendicularly to the film plane of the medium rather than in the plane of the medium. This new technology has occasioned new design challenges. One such challenge has been that when using perpendicular magnetic recording, low corner frequency f LF  is preferably about one-tenth the value of low corner frequency f LF  values required for longitudinal recording applications where the magnetic recording medium is magnetized substantially in the film plane of the medium. 
   Prior art signal amplifiers, especially signal amplifiers for use with a read head in an information storage device, have heretofore been difficult to produce economically with low band pass corner frequency f LF . 
   There is a need for a signal amplifier apparatus that accommodates economic design for low band pass corner frequency. 
   SUMMARY OF THE INVENTION 
   An amplifier apparatus for use with a sensor includes: (a) a first and a second amplifying circuit segment coupled with the sensor and cooperating to effect substantially balanced handling of signals received from the sensor; the first amplifying circuit segment includes a first transistor device; the second amplifying circuit segment includes a second transistor device; (b) a countercurrent unit coupled with the first and second amplifying circuit segments for receiving a first indicator signal from the first transistor device and a second indicator signal from the second transistor device; the first indicator signal represents a first parameter in the first transistor device; the second indicator signal represents a second parameter in the second transistor device; the countercurrent unit provides feedback signals to at least one of the first transistor and second transistor devices to reduce input impedance of the apparatus. 
   It is, therefore, an object of the present invention to provide a signal amplifier apparatus that accommodates economic design for low band pass corner frequency. 
   Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an electrical schematic illustration of a prior art differential amplifier for use with a read head. 
       FIG. 2  is an electrical schematic illustration of a first embodiment of the differential amplifier of the present invention. 
       FIG. 3  is an electrical schematic illustration of the preferred embodiment of the differential amplifier of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is an electrical schematic illustration of a first example of a prior art differential amplifier for use with a read head. In  FIG. 1 , a read amplifier circuit  10  (sometimes also referred to as a read front-end) is attached to a magneto-resistive element  12  via connection leads  14 ,  16  connected in parallel. Magneto-resistive element  12  is coupled with a supply voltage V CC  at a supply voltage locus  52  via a current source  70 . Magneto-resistive element  12  is also coupled with ground  28  (or another potential appropriate to establish the required DC bias for magneto-resistive element  12 ) via a current source  71 . Current sources  70 ,  71  apply DC (direct current) bias so that flux from a medium being read (not shown in  FIG. 1 ) by magneto-resistive element  12  may be converted to a voltage change or a current change for use by read amplifier circuit  10 . A capacitor  18  is coupled with connection lead  14 . A capacitor  20  is coupled with connection lead  16 . Capacitors  18 ,  20  block low frequency signals that appear on connection leads  14 ,  16 . 
   Bipolar Junction Transistor (BJT)  30  has an emitter  32 , a collector  34  and a base  36 . Bipolar Junction Transistor (BJT)  40  has an emitter  42 , a collector  44  and a base  46 . Emitters  32 ,  42  are coupled in common and with a ground locus  28  via a current source  50 . Base  36  is coupled with connection lead  14  via capacitor  18 . Base  36  is also coupled with ground  28  via a voltage source  74 , an electrical lead  75  and an inductor  72 . Voltage source  74  and inductor  72  cooperate to perform as a DC voltage source to provide an operating bias at base  36  for transistor  30 . Base  46  is connected with connection lead  16  via capacitor  20 . Base  46  is also coupled with ground  28  via voltage source  74 , an electrical lead  75  and an inductor  73 . Voltage source  74  and inductor  73  cooperate to perform as a DC voltage source to provide an operating bias at base  46  for transistor  40 . Inductors  72 ,  73  may be substituted by resistors or current sources (not shown in  FIG. 1 ). One of capacitor of  18 ,  20  may be omitted in some applications employing read amplifier circuit  10 . Collector  34  is coupled with a supply voltage V CC  at supply voltage locus  52  via a resistor  54 . Collector  44  is coupled with supply voltage V CC  at supply voltage locus  52  via a resistor  56 . Output signals are taken from collectors  34 ,  44  and presented at output loci  60 ,  62 . 
   Amplifier circuit  10  has a band pass frequency range between a lower frequency and an upper frequency within which amplifier circuit  10  has a significantly greater responsiveness than outside that band pass frequency range. If impedances of inductors  72 ,  73  are sufficiently higher than input impedance of transistors  30 ,  40 , amplifier circuit  10  advantageously permits setting low corner frequency f LF  (i.e., the lower frequency limit of the band pass frequency range of amplifier circuit  10 ) according to the relationship: 
   
     
       
         
           
             
               
                 
                   f 
                   LF 
                 
                 ∼ 
                 
                   1 
                   
                     2 
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     RC 
                   
                 
               
             
             
               
                 [ 
                 1 
                 ] 
               
             
           
         
       
     
       
       
         
           where˜indicates proportional to;
           R is the input impedance of amplifier circuit  10 ; and   C is the equivalent capacitance of capacitors  18 ,  20 .   
         
         
       
     
  
   Low corner frequency f LF  is commonly determined by varying the value of C in expression [1]. A larger value for C will reduce the value of low corner frequency f LF . 
   When flux from a medium being read (not shown in  FIG. 1 ) by magneto-resistive element  12  produces a read-out voltage across magneto-resistive element  12  having a polarity as indicated in  FIG. 1 , a small signal current i b1  flows in the direction indicated in  FIG. 1  into base  36  of transistor  30 , and a small signal current i b2  flows in the direction indicated in  FIG. 1  out of base  46  of transistor  40 . When read-out signal voltage across magneto-resistive element  12  has a polarity opposite to the polarity indicated in  FIG. 1 , small signal currents i b1 , i b2  flow in directions opposite to directions illustrated in  FIG. 1 . 
   Input impedance of apparatus  10  is inversely proportional to BJT (bipolar junction transistor) base currents i b1 , i b2 . Base currents i b1 , i b2  are determined by tail current i t  at current source  50  and β of BJT transistors  30 ,  40 . Tail current i t  is chosen to minimize noise in apparatus  10 , and such choices have typically resulted in C (expression [1]) having a value of about 100 picoFarads (pf) to achieve a 1 megaHertz low corner frequency. Such operational parameters have sufficed for use in longitudinal recording applications. 
   Needs for greater magnetic storage density in industry have been answered by various innovations. One such innovation has been perpendicular magnetic recording in which the magnetic recording medium is magnetized perpendicularly to the film plane of the medium rather than in the plane of the medium. This new technology has occasioned new design challenges. One such challenge has been that when using perpendicular magnetic recording, low corner frequency f LF  is preferably about one-tenth the value of low corner frequency f LF  values required for longitudinal recording applications. Inspection of expression [1] reveals that there are two straightforward ways to alter low corner frequency f LF . 
   One may increase the value of C by ten times to set low corner frequency f LF  at one-tenth its beginning value (that is, the value of C which is useful for longitudinal recording applications). There is a problem with this solution because increasing the value of C to such an amount would require significantly greater silicon area for its implementation. In these days of smaller packages for smaller products, such an increase in required area is not an attractive design solution. 
   Another approach to set low corner frequency f LF  at one-tenth its beginning value is to use high β transconductance transistors for transistors  30 ,  40  ( FIG. 1 ) to effect an increase in input impedance of apparatus  10  ( FIG. 1 ). However, such a design approach requires introducing special processing and contributes to noise characteristics of apparatus  10 . Higher input impedance could be achieved by using Metal Oxide Silicon (MOS) transistors for transistors  30 ,  40 . However, noise increase due to lower conductance of MOS transistors as compared with BJT transistors, and difficulties in matching MOS transistors would add complexity to manufacturing such a modified apparatus. 
   Increasing input impedance for an amplifier such as apparatus  10  ( FIG. 1 ) may be achieved by using the present invention, as described in connection with  FIGS. 2 and 3 . 
     FIG. 2  is an electrical schematic illustration of a first embodiment of the differential amplifier of the present invention. In  FIG. 2 , a read amplifier circuit  110  is attached to a magneto-resistive element  112  via connection leads  114 ,  116  connected in parallel. Magneto-resistive element  112  is coupled with a supply voltage V CC  at a supply voltage locus  152  via a current source  170 . Magneto-resistive element  112  is also coupled with ground  128  (or another potential appropriate to establish the required DC bias for magneto-resistive element  112 ) via a current source  171 . Current sources  170 ,  171  apply DC (direct current) bias so that flux from a medium being read (not shown in  FIG. 2 ) by magneto-resistive element  112  may be converted to a voltage change or a current change for use by read amplifier circuit  110 . A capacitor  118  is coupled with connection lead  114 . A capacitor  120  is coupled with connection lead  116 . Capacitors  118 ,  120  block low frequency signals that appear on connection leads  114 ,  116 . 
   Bipolar Junction Transistor (BJT) Q 1  has an emitter  132 , a collector  134  and a base  136 . Bipolar Junction Transistor (BJT) Q 2  has an emitter  142 , a collector  144  and a base  146 . Emitters  132 ,  142  are coupled in common and with a ground locus  128  via a current source  150 . Base  136  is coupled with connection lead  114  via capacitor  118  and base  146  is connected with connection lead  116  via capacitor  120 . 
   A countercurrent unit  200  includes BJT Q 3 , Q 4 . Bipolar Junction Transistor (BJT) Q 3  has an emitter  232 , a collector  234  and a base  236 . Bipolar Junction Transistor (BJT) Q 4  has an emitter  242 , a collector  244  and a base  246 . 
   Collector  234  is coupled with a supply voltage V CC  at supply voltage locus  152  via a resistor  154 . Collector  244  is coupled with supply voltage V CC  at supply voltage locus  152  via a resistor  156 . Output signals are taken from collectors  234 ,  244  and presented at output loci  160 ,  162 . 
   Emitter  232  is coupled with collector  144 . Emitter  242  is coupled with collector  134 . A diode  202  couples base  236  with base  136 , and a diode  204  couples base  246  with base  146 . Base  236  is also coupled with ground  128  via a voltage source  174 , an electrical lead  175  and an inductor  172 . Voltage source  174  and inductor  172  cooperate to perform as a DC voltage source to provide an operating bias at base  236  for transistor Q 3  and, via diode  202 , at base  136  for transistor Q 1 . Base  246  is also coupled with ground  128  via voltage source  174 , an electrical lead  175  and an inductor  173 . Voltage source  174  and inductor  173  cooperate to perform as a DC voltage source to provide an operating bias at base  246  for transistor Q 4  and, via diode  204 , at base  146  for transistor Q 2 . Inductors  172 ,  173  may be substituted by resistors or current sources (not shown in  FIG. 2 ). One of capacitor of  118 ,  120  may be omitted in some applications employing read amplifier circuit  110 . 
   Amplifier circuit  110  has a band pass frequency range between a lower frequency and an upper frequency within which amplifier circuit  110  has a significantly greater responsiveness than outside that frequency range. 
   If impedances of inductors  172 ,  173  are sufficiently higher than input impedance of transistors Q 1 , Q 2 , Q 3 , Q 4 , amplifier circuit  110  advantageously permits setting low corner frequency f LF  (i.e., the lower frequency limit of the band pass frequency range of amplifier circuit  110 ) according to the relationship set forth in expression [1] above. 
   When flux from a medium being read (not shown in  FIG. 1 ) by magneto-resistive element  112  produces a read-out voltage across magneto-resistive element  112  having a polarity as indicated in  FIG. 2 , a small signal current i b1  flows in the direction indicated in  FIG. 2  into base  136  of transistor Q 1 , and a small signal current i b2  flows in the direction indicated in  FIG. 2  out of base  146  of transistor Q 2 . When read-out signal voltage across magneto-resistive element  112  has a polarity opposite to the polarity indicated in  FIG. 2 , small signal currents i b1 , i b2  flow in directions opposite to directions illustrated in  FIG. 2 . 
     FIG. 3  is an electrical schematic illustration of the preferred embodiment of the differential amplifier of the present invention. In  FIG. 3 , a read amplifier circuit  310  (sometimes also referred to as a read front-end) is attached to a magneto-resistive element  312  via connection leads  314 ,  316  connected in parallel. Magneto-resistive element  312  is coupled with a supply voltage V CC  at a supply voltage locus  352  via a current source  370 . Magneto-resistive element  312  is also coupled with ground  328  (or another potential appropriate to establish the required DC bias for magneto-resistive element  312 ) via a current source  371 . Current sources  370 ,  371  apply DC (direct current) bias so that flux from a medium being read (not shown in  FIG. 3 ) by magneto-resistive element  312  may be converted to a voltage change or a current change for use by read amplifier circuit  310 . A capacitor  318  is coupled with connection lead  314 . A capacitor  320  is coupled with connection lead  316 . Capacitors  318 ,  320  block low frequency signals that appear on connection leads  314 ,  316 . 
   Bipolar Junction Transistor (BJT) Q 1  has an emitter  332 , a collector  334  and a base  336 . Bipolar Junction Transistor (BJT) Q 2  has an emitter  342 , a collector  344  and a base  346 . Emitters  332 ,  342  are coupled in common and with a ground locus  328  via a current source  350 . Base  336  is coupled with connection lead  314  via capacitor  318  and base  346  is connected with connection lead  316  via capacitor  320 . 
   A countercurrent unit  400  includes BJTs Q 3 , Q 4 . Bipolar Junction Transistor (BJT) Q 3  has an emitter  432 , a collector  434  and a base  436 . Bipolar Junction Transistor (BJT) Q 4  has an emitter  442 , a collector  444  and a base  446 . 
   Collector  434  is coupled with a supply voltage V CC  at supply voltage locus  352  via a resistor  354 . Collector  444  is coupled with supply voltage V CC  at supply voltage locus  352  via a resistor  356 . Output signals are taken from collectors  434 ,  444  and presented at output loci  360 ,  362 . Emitter  432  is coupled with collector  344 . Emitter  442  is coupled with collector  334 . 
   BJTs Q 5 , Q 6  are diode-coupled. BJT Q 5  has an emitter  412 , a collector  414  and a base  416 . Base  416  is coupled with collector  414  to establish diode coupling. BJT Q 6  has an emitter  422 , a collector  424  and a base  426 . Base  426  is coupled with collector  424  to establish diode coupling. BJT Q 5  couples base  436  with base  336 , and BJT Q 6  couples base  446  with base  346 . 
   Base  436  is also coupled with ground  328  via a voltage source  374 , an electrical lead  375  and an inductor  372 . Voltage source  374  and inductor  372  cooperate to perform as a DC voltage source to provide an operating bias at base  436  for transistor Q 3  and, via transistor Q 5 , at base  336  for transistor Q 1 . Base  446  is also coupled with ground  328  via voltage source  374 , an electrical lead  375  and an inductor  373 . Voltage source  374  and inductor  373  cooperate to perform as a DC voltage source to provide an operating bias at base  446  for transistor Q 4  and, via transistor Q 6 , at base  346  for transistor Q 2 . Inductors  372 ,  373  may be substituted by resistors or current sources (not shown in  FIG. 3 ). One of capacitor of  318 ,  320  may be omitted in some applications employing read amplifier circuit  310 . 
   Amplifier circuit  310  has a band pass frequency range between a lower frequency and an upper frequency within which amplifier circuit  310  has a significantly greater responsiveness than outside that frequency range. If impedances of inductors  372 ,  373  are sufficiently higher than input impedance of transistors Q 1 , Q 2 , Q 3 , Q 4 , amplifier circuit  310  advantageously permits setting low corner frequency f LF  (i.e., the lower frequency limit of the band pass frequency range of amplifier circuit  310 ) according to the relationship set forth in expression [1] above. 
   When flux from a medium being read (not shown in  FIG. 3 ) by magneto-resistive element  312  produces a read-out voltage across magneto-resistive element  312  having a polarity as indicated in  FIG. 3 , a small signal current i b1  flows in the direction indicated in  FIG. 3  into base  336  of transistor Q 1 , and a small signal current i b2  flows in the direction indicated in  FIG. 3  out of base  346  of transistor Q 2 . When read-out signal voltage across magneto-resistive element  312  has a polarity opposite to the polarity indicated in  FIG. 3 , small signal currents i b1 , i b2  flow in directions opposite to directions illustrated in  FIG. 3 . 
   Apparatus  110  ( FIG. 2 ) and apparatus  310  ( FIG. 3 ) operate substantially the same because diodes  202 ,  204  and BJTs Q 5 , Q 6  participate in substantially the same way in their respective apparatuses  110 ,  310 . In order to avoid prolixity, only operation of apparatus  110  will be described herein. 
   Referring to  FIG. 2 , flux from a medium being read (not shown in  FIG. 2 ) by magneto-resistive element  112  produces a read-out voltage across magneto-resistive element  112  having a polarity as indicated in  FIG. 2 , small signal base current i b1  flows into BJT Q 1  (as shown in  FIG. 2 ). BJT Q 1  is thus conductive and current flows from emitter  132 , through collector  134 , through emitter  242  and collector  244  to output locus  162 . Those conditions cause small signal current i b4  to flow into base  246  in a direction as indicated in  FIG. 2 . The read-out signal voltage across magneto-resistive element  112  also causes small signal base current i b2  to flow out of BJT Q 2  (as shown in  FIG. 2 ). BJT Q 2  thus establishes a current flow from collector  234 , through base  236 , through diode  202  to base  136 . These conditions cause small signal current i b3  to flow out of base  236  in a direction as indicated in  FIG. 2 . Small signal currents i b2 , i b4  are both provided to capacitor  120  so that the sum of currents at capacitor  120  is substantially reduced from base current i b2 . Small signal currents i b1 , i b3  are both provided to capacitor  118  so that the sum of currents at capacitor  118  is substantially reduced from small signal current i b1 . 
   When flux from a medium being read (not shown in  FIG. 2 ) by magneto-resistive element  112  produces a read-out voltage across magneto-resistive element  112  having a polarity opposite to the polarity indicated in  FIG. 2 , base current i b2  flows into BJT Q 2 , and current flows from emitter  142 , through collector  144 , through emitter  232  and collector  234  to output locus  160 . Those conditions cause a small signal current i b3  to flow into base  236 . The read-out signal voltage across magneto-resistive element  112  also causes small signal base current i b1  to flow out of BJT Q 1 . BJT Q 1  thus establishes a current flow from collector  244 , through base  246 , through diode  204  to base  146 . These conditions cause a small signal current i b4  to flow out of base  246 . 
   Whatever the polarity of read-out signal voltage across magneto-resistive element  112 , small signal currents i b2 , i b4  are both provided to capacitor  120  so that the sum of small signal currents at capacitor  120  is substantially reduced from small signal current i b2 . Small signal currents i i1 , i b3  are both provided to capacitor  118  so that the sum of small signal currents at capacitor  118  is substantially reduced from small signal current i b1 . BJT small signal base currents i b1 , i b2 , i b3,  i b4  may vary due to mismatches during manufacturing. Such variances are usually less than 10% variation from design criteria applicable number, so at most about 1/10 of capacitor current will be realized at capacitors  118 ,  120  than would be present without offsetting currents i i3 , i b4 . 
   An important result is that the small signal currents at capacitors  118 ,  120  available to drive BJTs Q 1 , Q 2  are reduced. Input impedance of apparatus  110  is inversely proportional to the BJT base currents i b1 , i b2 . Base currents i b1 , i b2  are determined by tail current i t  at current source  150  and β of BJTs Q 1 , Q 2 . Base currents i b1 , i b2  are canceled out by base currents i b3 , i b4  which involve similar current amounts but opposite current direction with respect to base currents i b1 , i b2 . Input impedance of apparatus  110  is thereby increased. 
   Referring to expression [1], one may observe that increasing input impedance to apparatus  110  (i.e., increasing R in expression [1]), one may reduce low corner frequency f LF  without the disadvantages that attend increasing C or using MOS transistors or requiring special processing in manufacturing apparatus  110 , as discussed hereinabove in connection with  FIG. 1 . Such disadvantages include increased silicon area to accommodate a larger capacitor to increase C in expression [1], special processes to incorporate high β transconductance transistors to increase input impedance of apparatus  110  or using Metal Oxide Silicon (MOS) transistors which would add complexity to manufacturing. 
   It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: