Abstract:
A differential amplifier circuit for amplifying an input signal and for providing an output signal representative of the input signal includes first and second amplifier circuits, and first and second coupling circuits. The first and second amplifier circuits each include first and second transistors, a resistor, and a current generator. The first coupling circuit includes a transistor, a capacitor, and a current generator, and couples a first input signal node to the first transistor of the second amplifier circuit. The second coupling circuit includes a transistor, a capacitor, and a current generator, and couples a second input signal node to the first transistor of the first amplifier circuit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a read system for reading information from a magnetic storage medium using a magnetoresistive head and for providing an output signal representative of the information read. In particular, the present invention relates to a read system with improved bandwidth and high frequency noise performance.  
           [0002]    A popular method of magnetic data storage utilizes magnetoresistive (MR) heads to store and recover data on a magnetic data storage medium such as a magnetic disk. An MR head employs an MR element that changes in resistivity with changing magnetic flux from data patterns on an adjacent magnetic disk surface. A bias current having a constant value is passed through the MR element, and the change in resistivity is measured by sensing the change in voltage across the MR head.  
           [0003]    Amplifier circuits that sense signals from MR heads commonly include differential inputs and differential outputs. While there are a wide variety of differential amplifier circuit topologies, most include an input stage with two load resistors and symmetrical transistors for splitting current between the load resistors. Usually, the output voltage is taken as the difference in the voltage drops across the load resistors; in this manner, large variations in output voltages may be achieved with extremely small input voltage differentials.  
           [0004]    For all differential amplifier circuits there are associated therewith certain frequency response performance characteristics. These characteristics and others determine the usefulness of the amplifier circuit in any given application. The band of frequencies over which the gain of the amplifier circuit is almost constant is called the bandwidth. Signals whose frequencies are outside the bandwidth will experience lower gain, with the gain decreasing as the signals move farther away from the bandwidth. Normally, the amplifier circuit is designed so that its bandwidth coincides with the spectrum of signals it is required to amplify. If this were not the case, the amplifier circuit would distort the frequency spectrum of the input signal, with different components of the input signal being amplified by different amounts.  
           [0005]    One well-known modification to the differential amplifier circuit is the addition of two capacitors that are cross-coupled to the transistors of the input stage. The capacitive (or ac) cross-coupling causes the noise resistances of the input transistors to be connected in parallel instead of in series, thereby reducing the effective noise resistances of the input transistors. An example of such a circuit is shown in U.S. Pat. No. 5,559,646. However, the main disadvantage of this type of circuit is its limited bandwidth. Because the cross-coupled capacitors cause the input capacitances of the input transistors to be connected in parallel, the effective input capacitances of the input transistors are increased. Therefore, because upper cutoff frequency is inversely proportional to input capacitance for transistors, the high frequency bandwidth of the circuit is reduced and the high frequency noise is increased.  
           [0006]    Accordingly, there is a need for a differential amplifier circuit having a cross-coupled input stage with improved bandwidth and high frequency noise performance.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention is a differential amplifier circuit for amplifying an input signal and for providing an output signal representative of the input signal. First and second amplifier circuits each include first and second transistors, a resistor, and a current generator. A first coupling circuit includes a transistor, a capacitor, and a current generator, and couples a first input signal node to the first transistor of the second amplifier circuit. A second coupling circuit includes a transistor, a capacitor, and a current generator, and couples a second input signal node to the first transistor of the first amplifier circuit.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 shows a circuit schematic diagram of a read system embodying the present invention.  
         [0009]    [0009]FIG. 2 a  shows an equivalent input circuit schematic diagram of a read system embodying the present invention.  
         [0010]    [0010]FIG. 2 b  shows a simplified input circuit schematic diagram of a read system embodying the present invention.  
         [0011]    [0011]FIG. 3 shows a circuit schematic diagram of a prior art read system.  
         [0012]    [0012]FIG. 4 a  shows an equivalent input circuit schematic diagram of a prior art read system.  
         [0013]    [0013]FIG. 4 b  shows a simplified input circuit schematic diagram of a prior art read system.  
         [0014]    [0014]FIG. 5 shows a graph comparing the frequency response of a read system embodying the present invention to the frequency response of a prior art read system.  
         [0015]    [0015]FIG. 6 shows a graph comparing the noise of a read system embodying the present invention to the noise of a prior art read system. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1 shows a circuit schematic diagram of a read system  10  embodying the present invention. Read system  10  includes parallel amplifier circuits  12  and  14 , coupling circuits  16  and  18 , input signal nodes VMR 1  and VMR 2 , output signal nodes VO 1  and VO 2 , bias voltage VBIAS, and voltage potentials VCC and VEE.  
         [0017]    Parallel amplifier circuit  12  includes transistors Q 1  and Q 3 , resistor R 1 , and current generator I 1 . Transistors Q 1  and Q 3  are npn bipolar junction transistors each having a base, a collector, and an emitter. The emitter of transistor Q 1  is connected to input signal node VMR 1 , and the collector of transistor Q 1  is connected to the emitter of transistor Q 3 . The base of transistor Q 3  is connected to bias voltage VBIAS, and the collector of transistor Q 3  is connected to voltage potential VCC through resistor R 1 . Current generator I 1  is connected between the emitter of transistor Q 1  and voltage potential VEE. Output signal node VO 1  is connected to the collector of transistor Q 3 .  
         [0018]    Parallel amplifier circuit  14  includes transistors Q 2  and Q 4 , resistor R 2 , and current generator I 2 . Transistors Q 2  and Q 4  are npn bipolar junction transistors each having a base, a collector, and an emitter. The emitter of transistor Q 2  is connected to input signal node VMR 2 , and the collector of transistor Q 2  is connected to the emitter of transistor Q 4 . The base of transistor Q 4  is connected to bias voltage VBIAS, and the collector of transistor Q 4  is connected to voltage potential VCC through resistor R 2 . Current generator I 2  is connected between the emitter of transistor Q 2  and voltage potential VEE. Output signal node VO 2  is connected to the collector of transistor Q 4 .  
         [0019]    Coupling circuit  16  includes transistor Q 5 , capacitor C 1 , and current generator I 3 . Transistor Q 5  is a npn bipolar junction transistor having a base, a collector, and an emitter. The base of transistor Q 5  is connected to input signal node VMR 2 , the collector of transistor Q 5  is connected to voltage potential VCC, and the emitter of transistor Q 5  is coupled to the base of transistor Q 1  through capacitor C 1 . Current generator I 3  is connected between the emitter of transistor Q 5  and voltage potential VEE.  
         [0020]    Coupling circuit  18  includes transistor Q 6 , capacitor C 2 , and current generator I 4 . Transistor Q 6  is a npn bipolar junction transistor having a base, a collector, and an emitter. The base of transistor Q 6  is connected to input signal node VMR 1 , the collector of transistor Q 6  is connected to voltage potential VCC, and the emitter of transistor Q 6  is coupled to the base of transistor Q 2  through capacitor C 2 . Current generator I 4  is connected between the emitter of transistor Q 6  and voltage potential VEE.  
         [0021]    In operation, the voltage across an MR head is related to the signal that is retrieved from a data pattern on an adjacent magnetic disk surface. This voltage across the MR head is represented in FIG. 1 at input signal nodes VMR 1  and VMR 2 . The voltage difference between input signal nodes VMR 1  and VMR 2  is the input signal that is sensed by read system  10 . Variations in the voltage difference between input signal nodes VMR 1  and VMR 2  lead to variations in the currents through parallel amplifier circuits  12  and  14 , due to the constant values of resistors R 1  and R 2 . These variations in currents lead to voltage variations across resistors R 1  and R 2 , which in turn lead to variations in the voltage difference between output signal nodes VO 1  and VO 2 .  
         [0022]    Transistor Q 3  and resistor R 1  form a collector circuit, as do transistor Q 4  and resistor R 2 . Transistors Q 3  and Q 4  form a differential common-base stage, otherwise known as a cascode stage. The load resistance seen by transistor Q 1  is not resistor R 1  but is the much lower input resistance of transistor Q 3 . Similarly, the load resistance seen by transistor Q 2  is not resistor R 2  but is the much lower input resistance of transistor Q 4 . Because load resistance is inversely proportional to upper cutoff frequency for bipolar junction transistors, these reductions in the effective load resistances of transistors Q 1  and Q 2  lead to a considerable improvement in the amplifier circuit frequency response.  
         [0023]    Transistors Q 5  and Q 6  are the most important features of the present invention. By coupling input signal node VMR 2  to the base of transistor Q 1  using both transistor Q 5  and capacitor C 1 , the input capacitance of transistor Q 1  is reduced compared to using a capacitor alone. This is because transistor Q 5  is connected as an emitter follower and provides an emitter-base capacitance that, when connected in series with capacitor C 1 , reduces the net capacitance between input signal node VMR 2  and transistor Q 1 . Similarly, by coupling input signal node VMR 1  to the base of transistor Q 2  using both transistor Q 6  and capacitor C 2 , the input capacitance of transistor Q 2  is reduced compared to using a capacitor alone. This is because transistor Q 6  is connected as an emitter follower and provides an emitter-base capacitance that, when connected in series with capacitor C 2 , reduces the net capacitance between input signal node VMR 1  and transistor Q 2 . Because input capacitance is inversely proportional to upper cutoff frequency for bipolar junction transistors, these reductions in the effective input capacitances of transistors Q 1  and Q 2  lead to a higher upper cutoff frequency and thus a considerable increase in the bandwidth of the input stage, which in turn decreases high frequency noise.  
         [0024]    When analyzing transistor circuits, small-signal equivalent circuit models are often used to express the components of the transistors in terms of model parameters. In this way, it is possible to understand the signal operation of the transistors, and reduce the circuit to an equivalent circuit model consisting of more basic circuit elements. Model parameters which are useful in analyzing the effects of coupling circuits  16  and  18  of read system  10  include both the base-emitter input resistances and the base-emitter capacitances of transistors Q 1 , Q 2 , Q 5 , and Q 6 . The small-signal input resistance between the base and the emitter of a bipolar junction transistor, looking into the base, is denoted by Rpi. The emitter-base capacitance of a bipolar junction transistor is denoted by Cpi.  
         [0025]    [0025]FIG. 2 a  shows an equivalent input circuit schematic diagram of read system  10 . Equivalent input circuit  20  includes input signal nodes VMR 1  and VMR 2 , capacitors CC 1 , CC 2 , Cpi 1 , Cpi 2 , Cpi 5 , and Cpi 6 , and resistors Rpi 1 , Rpi 2 , Rpi 5 , and Rpi 6 . Capacitor Cpi 1  and resistor Rpi 1  (parameters representing transistor Q 1 ) are connected in parallel between input signal node VMR 1  and capacitor CC 1  (parameter representing capacitor C 1 ). Capacitor Cpi 6  and resistor Rpi 6  (parameters representing transistor Q 6 ) are connected in parallel between input signal node VMR 1  and capacitor CC 2  (parameter representing capacitor C 2 ). Capacitor Cpi 2  and resistor Rpi 2  (parameters representing transistor Q 2 ) are connected in parallel between input signal node VMR 2  and capacitor CC 2 . Capacitor Cpi 5  and resistor Rpi 5  (parameters representing transistor Q 5 ) are connected in parallel between input signal node VMR 2  and capacitor CC 1 . The value of capacitors CC 1  and CC 2  each greatly exceed the value of capacitors Cpi 1 , Cpi 2 , Cpi 5 , and Cpi 6 .  
         [0026]    [0026]FIG. 2 b  shows a simplified input circuit schematic diagram of read system  10 . Simplified input circuit  22  is a reduced form of equivalent input circuit  20 . Capacitors CC 1 , CC 2 , Cpi 1 , Cpi 2 , Cpi 5 , and Cpi 6  of equivalent input circuit  20  can all be reduced to a single effective capacitance Ceff. This is because capacitors connected in series can be replaced by a single equivalent capacitor, which is related to the individual capacitors by the formula  
         1     C   eq       =       1     C   1       +     1     C   2       +   …   +     1     C   n                               
 
         [0027]    And capacitors connected in parallel can be replaced by a single equivalent capacitor which is equal to the sum of the individual capacitors. Similarly, resistors Rpi 1 , Rpi 2 , Rpi 5 , and Rpi 6  of equivalent input circuit  20  can all be reduced to a single effective resistance Reff. This is because resistors connected in series can be replaced by a single equivalent resistor which is equal to the sum of the individual resistors. And resistors connected in parallel can be replaced by a single equivalent resistor, which is related to the individual resistors by the formula  
         1     R   eq       =       1     R   1       +     1     R   2       +   …   +     1     R   n                               
 
         [0028]    Simplified input circuit  22  includes input signal nodes VMR 1  and VMR 2 , capacitor Ceff, and resistor Reff. Capacitor Ceff and resistor Reff are connected in parallel between input signal nodes VMR 1  and VMR 2 . The value of capacitor Ceff is equal to 3(Cpi 1 )/2, where capacitor Cpi 1  is from equivalent input circuit  20 . The value of resistor Reff is equal to the value of Re in parallel with the value of Rpi 1 , which is approximately Re (the value of Rpi 1  greatly exceeds the value of Re), where resistor Re is the small-signal base-emitter input resistance of transistor Q 1  looking into the emitter, and resistor Rpi 1  is from equivalent input circuit  20 .  
         [0029]    In order to appreciate the improvements of the present invention, the input capacitance, bandwidth, and noise of read system  10  are compared to the same characteristics of a prior art read system shown in FIG. 3.  
         [0030]    [0030]FIG. 3 shows a circuit schematic diagram of a prior art read system  30 . Prior art read system  30  is similar to read system  10  with the exception that transistors Q 5  and Q 6 , and current generators I 3  and I 4  are not present. Instead, capacitor C 1  is directly connected between input signal node VMR 2  and the base of transistor Q 1 , and capacitor C 2  is directly connected between input signal node VMR 1  and the base of transistor Q 2 . Due to the lack of additional emitter-base capacitances between the input signal nodes and the input transistors, prior art read system  30  lacks the reduced input capacitance caused by the emitter-base capacitances of transistors Q 5  and Q 6  in read system  10 . Because input capacitance is inversely proportional to upper cutoff frequency for bipolar junction transistors, prior art read system  30  possesses a more limited high frequency bandwidth than read system  10 , and therefore exhibits greater high frequency noise than read system  10 .  
         [0031]    [0031]FIG. 4 a  shows an equivalent input circuit schematic diagram of prior art read system  30 . Equivalent input circuit  40  includes input signal nodes VMR 1  and VMR 2 , capacitors CC 1 , CC 2 , Cpi 1 , and Cpi 2 , and resistors Rpi 1  and Rpi 2 . Capacitor Cpi 1  and resistor Rpi 1  (parameters representing transistor Q 1 ) are connected in parallel between input signal node VMR 1  and capacitor CC 1  (parameter representing capacitor C 1 ), and capacitor CC 1  is connected to input signal node VMR 2 . Capacitor Cpi 2  and resistor Rpi 2  (parameters representing transistor Q 2 ) are connected in parallel between input signal node VMR 2  and capacitor CC 2  (parameter representing capacitor C 2 ), and capacitor CC 2  is connected to input signal node VMR 1 . The value of capacitors CC 1  and CC 2  each greatly exceed the value of capacitors Cpi 1  and Cpi 2 .  
         [0032]    [0032]FIG. 4 b  shows a simplified input circuit schematic diagram of prior art read system  30 . Simplified input circuit  42  is a reduced form of equivalent input circuit  40 . For similar reasons discussed above, capacitors CC 1 , CC 2 , Cpi 1 , and Cpi 2  of equivalent input circuit  40  can all be reduced to a single effective capacitance Ceff, and resistors Rpi 1  and Rpi 2  of equivalent input circuit  40  can be reduced to a single effective resistance Reff. Simplified input circuit  42  includes input signal nodes VMR 1  and VMR 2 , capacitor Ceff, and resistor Reff. Capacitor Ceff and resistor Reff are connected in parallel between input signal nodes VMR 1  and VMR 2 . The value of capacitor Ceff is equal to 2(Cpi 1 ), where capacitor Cpi 1  is from equivalent input circuit  40 . The value of resistor Reff is equal to the value of Re in parallel with the value of Rpi 1 , which is approximately Re (the value of Rpi 1  greatly exceeds the value of Re), where resistor Re is the small-signal base-emitter input resistance of transistor Q 1  looking into the emitter, and resistor Rpi 1  is from equivalent input circuit  40 .  
         [0033]    Comparing simplified input circuit  22  of the present invention to simplified input circuit  42  according to the prior art, it can be seen that the value of capacitor Ceff of simplified input circuit  42  is approximately 33% greater than the value of Ceff of simplified input circuit  22 . Therefore, the input capacitance of prior art read system  30  is approximately 33% greater than the input capacitance of read system  10 .  
         [0034]    [0034]FIG. 5 shows a graph comparing the frequency response  50  of read system  10  to the frequency response  52  of prior art read system  30 . The graph shows the gain (dB) as a function of frequency (Hz). The band of frequencies over which the gain is almost constant, to within a certain number of decibels, is called the bandwidth. The bandwidth of read system  10  extends approximately from 2*10 6 Hz to 2*10 9 Hz. The bandwidth of prior art read system  30  extends approximately from 2*10 6 Hz to 1*10 9 Hz. Therefore, the bandwidth of read system  10  extends approximately 1*10 9 Hz further than the bandwidth of prior art read system  30 .  
         [0035]    [0035]FIG. 6 shows a graph comparing the input referred noise  60  of read system  10  to the input referred noise  62  of prior art read system  30 . The graph shows the noise (10 −9 V) as a function of frequency (Hz). The range of frequencies over which read system  10  exhibits noise less than 1.0*10 −9 V extends approximately from 1.6*10 7 Hz to 1.2*10 9 Hz. The range of frequencies over which prior art read system  30  exhibits noise less than 1.0*10 −9 V extends approximately from 1.6*10 7 Hz to 6*10 8 Hz. Therefore, the range of frequencies over which read system  10  exhibits less than 1.0*10 −9 V of noise extends approximately 4.2*10 8 Hz further than the range of frequencies over which prior art read system  30  exhibits less than 1.0*10 −9 V of noise.  
         [0036]    Therefore, the present invention provides a read system having a cross-coupled input stage with improved bandwidth and high frequency noise performance. By implementing coupling circuits with both emitter followers and capacitors, the read system allows a reduced net capacitance between the input signal nodes and the input transistors. The reduced effective input capacitances extend the upper cutoff frequencies of the input transistors, and increase the bandwidth of the read system. This results in reduced high frequency noise, and greater accuracy and capability in detecting data recorded on a magnetic disk with an MR head.  
         [0037]    Although the preferred embodiment of the present invention is shown using npn bipolar technology, the present invention may also be practiced using pnp bipolar and FET technologies, the topology for either being readily derived from the small-signal models associated with the npn embodiment. Furthermore, the present invention may be practiced using either discrete or integrated circuit designs. Workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.