Abstract:
An apparatus for use with a sensor includes first and second signal treating circuit segments coupled with the sensor for presenting a substantially balanced differential signaling representation of output signals from the sensor. Each respective signal treating circuit segment comprises a plurality of circuit elements having different electrical symmetries coupled in parallel and establishing a plurality of parallel signal paths having asymmetric signal handling characteristics. A feedback circuit is coupled with the first and second signal treating circuit segments and provides feedback signals to selected circuit elements in each of the first and second signal treating circuit segments. The feedback signals effect substantially balanced signal handling among the selected circuit elements having similar electrical symmetries.

Description:
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 for which some control is available regarding certain operating parameters associated with the amplifier. 
   There are many important goals in designing and operating an amplifier for use with a sensor, two such goals are: low band pass corner frequency and low noise. 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. Lower noise is desirable to reduce noise attributable to the sensor&#39;s read back signal. 
   Prior art signal amplifiers, especially signal amplifiers for use with a read head in an information storage device, have resulted in a compromise in noise performance when a very low band pass corner is needed. 
   There is a need for a signal amplifier apparatus that accommodates design for both low band pass corner frequency and lower noise. 
   SUMMARY OF THE INVENTION 
   An apparatus for use with a sensor includes first and second signal treating circuit segments coupled with the sensor for presenting a substantially balanced differential signaling representation of output signals from the sensor. Each respective signal treating circuit segment comprises a plurality of circuit elements having different electrical symmetries coupled in parallel and establishing a plurality of parallel signal paths having asymmetric signal handling characteristics. A feedback circuit is coupled with the first and second signal treating circuit segments and provides feedback signals to the circuit elements in each of the first and second signal treating circuit segments. The feedback signals effect substantially balanced signal handling among circuit elements having similar electrical symmetries. 
   It is, therefore, an object of the present invention to provide a signal amplifier apparatus that accommodates design for both low band pass corner frequency and lower noise. 
   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 a graphic representation of an amplifier output transfer function. 
       FIG. 2  is an electrical schematic illustration of a first example of a prior art differential amplifier for use with a read head. 
       FIG. 3  is an electrical schematic illustration of a second example of a prior art differential amplifier for use with a read head. 
       FIG. 4  is an electrical schematic illustration of a third example of a prior art differential amplifier for use with a read head. 
       FIG. 5  is an electrical schematic illustration of a fourth example of a prior art differential amplifier for use with a read head. 
       FIG. 6  is an electrical schematic illustration of the differential amplifier of the present invention. 
       FIG. 7  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 a graphic representation of an amplifier output transfer function. In  FIG. 1 , a graphic plot  100  includes a frequency response curve  101  is plotted against a vertical axis  102  representing signal strength in decibels (dB) and plotted against a horizontal axis  104  representing frequency in a parameter appropriate for the circuit or device involved, such as megaHertz (MHz; not indicated in  FIG. 1 ). Curve  101  varies up to a maximum signal strength of max dB. Curve  101  achieves a signal of MAX −3 dB at a frequency f LF . The frequency at a point  106  at which a frequency response curve (e.g., curve  101 ) is at a −3 dB signal level at the left end of the frequency response is commonly referred to as the low corner frequency of the frequency response curve. Frequency f LF  is the low corner frequency of frequency response curve  101 . In designing an amplifier circuit for a sensor, such as a read head, it is advantageous to establish low corner frequency f LF  as low as possible to permit the amplifier to respond to as low a frequency signal from the sensor as can be achieved without passing DC signals. 
     FIG. 2  is an electrical schematic illustration of a first example of a prior art differential amplifier for use with a read head. In  FIG. 2 , 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. 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 . 
   Metal-oxide silicon (MOS) transistor  30  has a source  32 , a drain  34  and a gate  36 . Metal-oxide silicon (MOS) transistor  40  has a source  42 , a drain  44  and a gate  46 . Sources  32 ,  42  are coupled in common and with a ground locus  28  via a current source  50 . Gate  36  is coupled with connection lead  14  via capacitor  18  and gate  46  is connected with connection lead  16  via capacitor  20 . A bias reference source  22  is connected via resistors  24 ,  26  to establish a predetermined bias potential at gates  36 ,  46 . Drain  34  is coupled with a supply voltage V CC  at a supply voltage locus  52  via a resistor  54 . Drain  44  is coupled with supply voltage V CC  at supply voltage locus  52  via a resistor  56 . Output signals are taken from drains  34 ,  44  and presented at output loci  60 ,  62 . 
   Amplifier circuit  10  advantageously permits setting low corner frequency f LF  ( FIG. 1 ) by resistors  24 ,  26  and capacitors  18 ,  20  according to the relationship: 
   
     
       
         
           
             
               
                 
                   f 
                   LF 
                 
                 ∼ 
                 
                   1 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     R 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     C 
                   
                 
               
             
             
               
                 [ 
                 1 
                 ] 
               
             
           
         
       
     
       
       
         
           where˜indicates proportional to;
           R is the resistance value of resistors  24 ,  26 ; and   C is the capacitance of capacitors  18 ,  20 .   
         
         
       
     
  
   A further advantage of amplifier circuit  10  is that setting of low corner frequency f LF  is independent of the currents or physical dimensions of MOS transistors  30 ,  40 . 
   A disadvantage of amplifier circuit  10  is that it requires that MOS transistors  30 ,  40  be large in order to limit noise at the input of amplifier circuit  10 —e.g., at gates  36 ,  46 , and a large MOS transistor  30 ,  40  will provide large input capacitance so that gates  36 ,  46  establish a capacitive divider that will effect significant attenuation on all frequency signals. Another disadvantage of amplifier circuit  10  is that a low noise design of amplifier circuit  10  requires that a large bias current be provided by current source  50  that contrasts the desire for low power. 
     FIG. 3  is an electrical schematic illustration of a second example of a prior art differential amplifier for use with a read head. In  FIG. 3 , a read amplifier circuit  110  (sometimes also referred to as a read front-end) is attached to a magneto-resistive element  112  via connection leads  114 ,  116  connected in parallel. 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 transistor  130  has an emitter  132 , a collector  134  and a base  136 . Bipolar transistor  140  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  and base  146  is connected with connection lead  116 . A bias reference source  122  is connected via resistors  124 ,  126  to establish a predetermined bias potential at bases  136 ,  146 . Collector  134  is coupled with a supply voltage V CC  at a supply voltage locus  152  via a resistor  154 . Collector  144  is coupled with supply voltage V CC  at supply voltage locus  152  via a resistor  156 . Output signals are taken from collectors  134 ,  144  and presented at output loci  160 ,  162 . 
   Amplifier circuit  110  advantageously requires less bias current than amplifier  10  ( FIG. 2 ) because bipolar transistors  130 ,  140  usually require less bias current than MOS transistors  30 ,  40  ( FIG. 2 ) for the same noise performance. Further, bipolar transistors  130 , 140  are physically smaller (i.e., require less die space) than MOS transistors  30 ,  40  ( FIG. 2 ) for a given noise level. 
   A disadvantage of amplifier  110  vis-à-vis amplifier  10  is that amplifier  110  has a low corner frequency f LF  and noise level that are both functions of the bipolar device collector current I C  and are in opposing relationship. In order to achieve low noise, collector current IC varies according to the relationship: 
   
     
       
         
           
             
               
                 
                   V 
                   NOISE 
                 
                 = 
                 
                   4 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   k 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     T 
                     ⁡ 
                     
                       ( 
                       
                         
                           r 
                           b 
                         
                         + 
                         
                           
                             V 
                             T 
                           
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               I 
                               C 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 [ 
                 2 
                 ] 
               
             
           
         
       
     
       
       
         
           Where V NOISE  is voltage level of noise present;
           k is Boltzmann&#39;s constant;   T is temperature;   r b  is related to transistor emitter geometry;   V T  is transconductance voltage of a bipolar transistor; and   I C  is collector current of a bipolar transistor.   
         
         
       
     
  
   Requiring high I C  conflicts with the need for high r π  to yield a low corner frequency f LF  according to the relationships: 
   
     
       
         
           
             
               
                 
                   r 
                   π 
                 
                 = 
                 
                   β 
                   · 
                   
                     
                       V 
                       T 
                     
                     
                       I 
                       C 
                     
                   
                 
               
             
             
               
                 [ 
                 3 
                 ] 
               
             
           
         
       
     
       
       
         
           Where β is current gain of a bipolar transistor;
           V T  is transconductance voltage of a bipolar transistor; and   I C  is collector current of a bipolar transistor.   
         
         
       
     
  
   
     
       
         
           
             
               
                 
                   f 
                   LF 
                 
                 ∼ 
                 
                   1 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       r 
                       π 
                     
                   
                 
               
             
             
               
                 [ 
                 4 
                 ] 
               
             
           
         
       
     
       
       
         
           
             
               where˜indicates proportional to;
               r π  is calculated according to Expression [3]; and   C is the capacitance of capacitors  118 ,  120 .   
             
             
           
         
       
     
  
     FIG. 4  is an electrical schematic illustration of a third example of a prior art differential amplifier for use with a read head. In  FIG. 4 , a read amplifier circuit  210  (sometimes also referred to as a read front-end) is attached to a magneto-resistive element  212  via connection leads  214 ,  216  connected in parallel. A capacitor  218  is coupled with connection lead  214 . A capacitor  219  is coupled with connection lead  216 . Capacitors  218 ,  219  block low frequency signals that appear on connection leads  214 ,  216 . 
   A bipolar transistor  220  has an emitter  222 , a collector  224  and a base  226 . A bipolar transistor  230  has an emitter  232 , a collector  234  and a base  236 . A bipolar transistor  240  has an emitter  242 , a collector  244  and a base  246 . A bipolar transistor  250  has an emitter  252 , a collector  254  and a base  256 . Emitters  222 ,  232  are coupled in common and with a ground locus  213  via a current source  211 . Emitters  242 ,  252  are coupled in common and with a ground locus  217  via a current source  215 . Base  226  is coupled with connection lead  214 . Base  246  is connected with connection lead  214  via capacitor  218 . Base  256  is coupled with connection lead  216 . Base  236  is connected with connection lead  216  via capacitor  219 . Collectors  224 ,  244  are coupled in common and are coupled with a supply voltage V CC  at a supply voltage locus  270  via a resistor  274 . Collectors  234 ,  254  are coupled in common and are coupled with supply voltage V CC  at supply voltage locus  270  via a resistor  272 . Base  236  is connected with a reference voltage V REF1  at a reference voltage locus  263  via a resistor  260 . Base  246  is connected with reference voltage V REF2  at reference voltage locus  264  via a resistor  262 . Output signals are taken from collectors  224 ,  244  connected in common and taken from collectors  234 ,  254  connected in common and presented at output loci  280 ,  282 . 
   Amplifier circuit  210  is improved over amplifier circuit  110  ( FIG. 3 ) in that cross-coupling of capacitors  218 ,  219  results in low corner frequency f LF  (for a given capacitor size) being determined by the relationship: 
   
     
       
         
           
             
               
                 
                   f 
                   LF 
                 
                 ∼ 
                 
                   1 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     π 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     C 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       
                         4 
                         ⁢ 
                         
                           r 
                           π 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 [ 
                 5 
                 ] 
               
             
           
         
       
     
   
   Capacitance (C) of capacitors  218 ,  219  in amplifier circuit  210  {expression [5]) may be significantly smaller—on the order of one-fourth—than capacitors  118 ,  120  ( FIG. 3 ; expression [3]) to yield the same low corner frequency f LF . Lower valued capacitors means smaller die size, which is advantageous. Moreover, smaller capacitors  218 ,  219  means that fewer parasitics are present so that better high frequency performance is experienced having better bandwidth and lower high frequency noise in amplifier circuit  210  than are experienced in amplifier circuit  110  ( FIG. 3 ). 
   However, amplifier circuit  210  still has the problem of bipolar transistor noise operating counter to improving low corner frequency f LF , as discussed in connection with amplifier circuit  110  ( FIG. 3 ) and expressions [2], [3] and [4] above. 
     FIG. 5  is an electrical schematic illustration of a fourth example of a prior art differential amplifier for use with a read head. In  FIG. 5 , 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. A capacitor  318  is coupled with connection lead  314 . A capacitor  319  is coupled with connection lead  316 . Capacitors  318 ,  319  block low frequency signals that appear on connection leads  314 ,  316 . 
   A metal-oxide silicon (MOS) transistor  320  has a source  322 , a drain  324  and a gate  326 . A MOS transistor  330  has a source  332 , a drain  334  and a gate  336 . A MOS transistor  340  has a source  342 , a drain  344  and a gate  346 . A MOS transistor  350  has a source  352 , a drain  354  and a gate  356 . Sources  322 ,  332  are coupled in common and with a ground locus  313  via a current source  311 . Sources  342 ,  352  are coupled in common and with a ground locus  317  via a current source  315 . Gate  326  is coupled with connection lead  314 . Gate  346  is connected with connection lead  314  via capacitor  318 . Gate  356  is coupled with connection lead  316 . Gate  336  is connected with connection lead  316  via capacitor  319 . Drains  324 ,  344  are coupled in common and are coupled with a supply voltage V CC  at a supply voltage locus  370  via a resistor  374 . Drains  334 ,  354  are coupled in common and are coupled with supply voltage V CC  at supply voltage locus  370  via a resistor  372 . Gate  336  is connected with a reference voltage V REF1  at a reference voltage locus  363  via a resistor  360 . Gate  346  is connected with reference voltage V REF2  at reference voltage locus  364  via a resistor  362 . Output signals are taken from drains  324 ,  344  connected in common and taken from drains  334 ,  354  connected in common and presented at output loci  380 ,  382 . 
   Amplifier circuit  310  enjoys advantages similar to advantages experienced by amplifier circuit  210  ( FIG. 4 ) because of the cross-coupling of capacitors  318 ,  319 . That is, capacitors  318 ,  319  in amplifier circuit  310  may be significantly smaller—on the order of one-fourth—than capacitors  118 ,  120  ( FIG. 3 ) to yield the same low corner frequency f LF . Lower valued capacitors means smaller die size. Moreover, smaller capacitors  318 ,  319  means that fewer parasitics present so that better high frequency performance having better bandwidth and lower high frequency noise is experienced in amplifier circuit  310  as compared with amplifier circuit  110  ( FIG. 3 ). Amplifier circuit  310  enjoys further advantages similar to amplifier circuit  10  ( FIG. 2 ) in that setting of low corner frequency f LF  is independent of the currents or physical dimensions of MOS transistors  320 ,  330 ,  340 ,  350 . However, noise characteristics of amplifier circuit  310  are not as good as noise characteristics of amplifier circuit  110  ( FIG. 3 ) or amplifier circuit  210  ( FIG. 4 ). 
   The present invention combines advantages of bipolar and MOS transistor implementations of amplifier circuits. This design proved difficult to achieve because balanced performance by bipolar and MOS transistors must be achieved. The preferred embodiment of the amplifier circuit of the present invention employs asymmetric amplifier structures in each of two parallel circuit segments that operate symmetrically and cooperate to effect balanced signal amplification overall. 
     FIG. 6  is an electrical schematic illustration of the differential amplifier of the present invention. In  FIG. 6 , a read amplifier circuit  410  (sometimes also referred to as a read front-end) is attached to a magneto-resistive element  412  via connection leads  414 ,  416  connected in parallel. A capacitor  418  is coupled with connection lead  414 . A capacitor  419  is coupled with connection lead  416 . Capacitors  418 ,  419  block low frequency signals that appear on connection leads  414 ,  416 . 
   A bipolar transistor  420  has an emitter  422 , a collector  424  and a base  426 . A MOS transistor  430  has a source  432 , a drain  434  and a gate  436 . A bipolar transistor  450  has an emitter  452 , a collector  454  and a base  456 . A MOS transistor  440  has a source  442 , a drain  444  and a gate  446 . Emitter  422  and source  432  are coupled in common and with a ground locus  413  via a current source  411 . Emitter  452  and source  442  are coupled in common and with a ground locus  417  via a current source  415 . Base  426  is coupled with connection lead  414 . Gate  446  is connected with connection lead  414  via capacitor  418 . Base  456  is coupled with connection lead  416 . Gate  436  is connected with connection lead  416  via capacitor  419 . Collectors  424 ,  454 , drains  434 ,  444  and gates  436 ,  446  are coupled with a transconductance feedback and signal combining unit  460  (hereinafter referred to as feedback/combining unit  460 ). Feedback/combining unit  460  is coupled with a supply voltage V CC  at a supply voltage locus  462  and coupled with output loci  470 ,  472  at which output signals are presented. 
   Feedback/combining unit provides feedback to gates  436 ,  446  to ensure balanced performance for each MOS/bipolar transistor pair  430 / 420  and  440 / 450 . Amplifier circuit  410  has advantages from using MOS transistors  430 ,  440  in that low corner frequency f LF  is set substantially independent of the currents or physical dimensions of MOS transistors  430 ,  440 . Amplifier circuit  410  has advantages from using bipolar transistors  420 ,  450  in that noise is partly determined by expressions [2] and [3], but requiring high I C  does not conflict achieving a low corner frequency f LF . This is so because low corner frequency f LF  is substantially set by transconductance feedback from feedback/combining unit  460  so the need for high r π  (which is counter to the need for high I C  to reduce noise) for a bipolar transistor to yield a low corner frequency f LF  is not a consideration. 
     FIG. 7  is an electrical schematic illustration of the preferred embodiment of the differential amplifier of the present invention. In  FIG. 7 , a read amplifier circuit  510  (sometimes also referred to as a read front-end) is attached to a magneto-resistive element  512  via connection leads  514 ,  516  connected in parallel. A capacitor  518  is coupled with connection lead  514 . A capacitor  519  is coupled with connection lead  516 . Capacitors  518 ,  519  block low frequency signals that appear on connection leads  514 ,  516 . 
   A bipolar transistor  520  has an emitter  522 , a collector  524  and a base  526 . A MOS transistor  530  has a source  532 , a drain  534  and a gate  536 . A bipolar transistor  550  has an emitter  552 , a collector  554  and a base  556 . A MOS transistor  540  has a source  542 , a drain  544  and a gate  546 . Emitter  522  and source  432  are coupled in common and with a ground locus  517  via a current source  511 . Emitter  552  and source  542  are coupled in common and with a ground locus  517  via a current source  515 . Base  526  is coupled with connection lead  514 . Gate  546  is connected with connection lead  514  via capacitor  518 . Base  556  is coupled with connection lead  516 . Gate  536  is connected with connection lead  516  via capacitor  519 . Collectors  524 ,  554 , drains  534 ,  544  and gates  536 ,  546  are coupled with a transconductance feedback and signal combining unit  560  (hereinafter referred to as feedback/combining unit  560 ). Feedback/combining unit  560  includes a transconductance unit  562  connected with collectors  524 ,  554 , connected with gates  536 ,  546  and connected with a reference voltage V REF  at a reference voltage locus  564 . 
   Transconductance unit  562  provides an error current to gates  536 ,  546  that is related with voltages received at collectors  524 ,  554 . Transconductance unit  562  provides the required feedback to adjust collector currents at collectors  524 ,  554  and drain currents at drains  534 ,  544  to cause bipolar transistors  520 ,  550  to operate substantially symmetrically and to cause MOS transistors  530 ,  540  to operate substantially symmetrically. 
   Collector  524  is coupled with a supply voltage V CC  at a supply voltage locus  566  via a resistor  570 . Drain  534  is coupled with supply voltage V CC  at supply voltage locus  566  via a resistor  572 . Drain  544  is coupled with supply voltage V CC  at supply voltage locus  566  via a resistor  574 . Collector  554  is coupled with supply voltage V CC  at supply voltage locus  566  via a resistor  576 . 
   An amplifier  580  is connected to receive output signals from between resistor  572  and drain  534 , and is also connected to receive output signals from between resistor  574  and drain  544 . An amplifier  582  is connected to receive output signals from between resistor  570  and collector  524 , and is also connected to receive output signals from between resistor  576  and collector  554 . Thus, each of amplifiers  580 ,  582  receives inputs from only one type of transistor-amplifier  580  receives inputs from MOS transistors  530 ,  540  and amplifier  582  receives inputs from bipolar transistors  520 ,  550 . Amplified signals are provided by amplifiers  580 ,  582  to a summer  584 . Summer  584  is coupled with output loci  590 ,  592  at which output signals are presented. 
   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.