Abstract:
A preamplifier amplifies a differential signal from a magneto-resistive read head. The preamplifier is designed to maximize gain and minimize introduction of noise, while maintaining wide bandwidths and common mode rejection performance. The emitters of the differential amplifier are coupled together by a third transistor and a capacitor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to amplifiers in general; and, in particular, to preamplifiers for read heads in magnetic disk drives. 
     2. Discussion of the Related Art 
     The preamplifier in this application processes a differential signal from a magneto-resistive read head for a disk drive. In addition to signal amplification, the preamplifier contributes noise. If the preamplifier has a high gain, noise introduced by subsequent stages can be rendered, as compared to noise introduced by the preamplifier, relatively insignificant. Thus, noise in a preamplifier should be minimized, while the preamplifier must also maximize gain and bandwidth. 
     FIG. 1 shows a prior art preamplifier  100  receiving a differential signal across terminals  110  and  111 , which are amplified by differential stages  100   a  and  100   b . Differential stages  100   a  and  100   b  include emitter-coupled transistor pairs  101  and  102 , and  105  and  106 . Differential stages  100   a  and  100   b  are used because of the high gain required. For example, a bias of several hundred millivolts in the differential signal across terminals  110  and  111  (i.e., the signal provided by the read head) can exceed the rail-to-rail voltage. However, preamplifier  100  of FIG. 1 is undesirable because of the noise in four transistors. 
     As shown in FIG. 1, each of differential stages  100   a  and  100   b  is used as a single-ended amplifier. The signal at terminal  110  is coupled to the base terminal of transistor  101  and the signal at terminal  111  is coupled to base terminal of transistor  105 . Current sources  113  and  114  provide bias currents to differential stages  100   a  and  100   b , respectively. Transistors  115 - 118 , which are each biased by a common reference voltage source  119 , form cascodes to provide an AC ground. Differential amplifiers  120  and  121  drive differential stages  100   a  and  100   b  respectively to provide an output differential voltage at terminals  122  and  123 . Capacitors  124  and  125  filters out high frequency AC noise signals in input terminals  110  and  111 , respectively. Resistors  126 - 129  are typically given the same resistance. 
     Preamplifier  100 &#39;s gain can be set by selecting a suitable resistance value for resistors  126 - 129 . Roughly, the gain is determined by the ratio of this selected resistance to the emitter resistance in each of transistors  101 ,  102 ,  105  and  106 . Gain is increased by a higher resistance or a larger current in each of transistors  115 - 118 . 
     Since differential stages  100   a  and  100   b  are essentially two single-ended amplifiers, preamplifier  100  does not have a good common mode rejection characteristics. 
     FIG. 2 shows a preamplifier  150  in the prior art, which is a variation of preamplifier  100 , for use with multiple read heads. (To simplify discussion, like elements in the figures of the application are provided like reference numerals.) In preamplifier  150 , multiple differential signals from multiple read heads share differential stages  150   a  and  150   b . For example, as shown in FIG. 2, the differential output signal from a first read head is provided at terminals  110  and  111  and coupled into differential stages  150   a  and  150   b  via the base terminals of transistors  101  and  105 , respectively. In like manner, the differential output signal of a second read head is provided at terminals  151  and  152  and coupled into differential stages  150   a  and  150   b  via the base terminals of transistors  153  and  154 , respectively. A mechanism (not shown) is provided to select which one of the output differential signals from the multiple read head is coupled into preamplifier  150 . 
     To minimize shot noise, transistors  101 ,  102 ,  115  and  116  are made as large as practical. At the same time, however, without reducing the resistances in resistors  126 - 129 , increasing the currents in transistors  101 ,  102 ,  115  and  116  limits the dynamic range of the output signals. Reducing the resistances of resistors  126 - 129  reduces the gain of the preamplifier. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a preamplifier includes (i) an indirectly coupled transistor pair receiving an input differential signal and (ii) an AC coupling circuit indirectly coupling the transistors of the pair to each other. 
     In one embodiment, a current bypass circuit is also included for decreasing the emitter resistance and raising the gain while observing voltage overhead restrictions. 
     Thus, the present invention provides indirect AC coupling of a transistor pair in a preamplifier, allowing the preamplifier to function with fewer primary noise-contributing transistors and to perform common mode rejection. 
     The present invention is better understood upon consideration of the detailed description below and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art preamplifier using two emitter-coupled pairs. 
     FIG. 2 is a prior art preamplifier for a disk drive with multiple read heads. 
     FIG.  3   a  shows a preamplifier circuit  300  in accordance with one embodiment of the invention. 
     FIG.  3   b  shows a preamplifier circuit  350 , which is a variation of preamplifier circuit  300  of FIG.  3   a , accommodating multiple read heads. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the present invention is shown in preamplifier circuit  300  of FIG.  3   a . In preamplifier  300 , an input differential signal from a read head (not shown) appears across terminals  301  and  302 , which are respectively coupled into the base terminals of transistors  303  and  304 , respectively. A current source  306  provides a bias current split between transistors  303  and  304 . The present invention indirectly couples the input differential signal of terminals  301  and  302  through an AC coupling circuit  325  to provide an output differential signal at terminals  311  and  312 . AC coupling circuit  325  includes transistor  305  and capacitor  316 . 
     In preamplifier  300 , feedback amplifier  313  provides the base current of transistor  305 , based on the differential output signal across terminals  311  and  312 . The low-pass filter formed by capacitor  315  and resistor  314  filters out high frequency AC signals. In addition, resistor  317  isolates capacitors  315  and  316 , such as to allow a dynamic range of voltage at the base terminal of transistor  305  in response to feedback to the output differential signal across terminals  311  and  312   
     In one embodiment, capacitor  316  is provided integrally with the other components of preamplifier  300  on an integrated circuit, thereby eliminating inductance and a potential bandwidth limitation resulting from an external capacitor. For an AC signal (i.e., a signal of a frequency between DC and less than the frequency attenuated by the low-pass filter of resistor  314  and capacitor  315 ), AC coupling circuit  325  behaves like a diode between the emitter of transistor  303  and the emitter terminal of transistor  304 , having a coupling impedance of r e , which is the same emitter resistance as either of the emitter resistances of transistors  303  and  304 . Consequently, the emitter impedance of preamplifier  300  is 50% higher, and hence the gain 50% lower, than a similarly size differential stage, such as differential stage  100   a  of the prior art. 
     However, since only three transistors (i.e., transistors  303 ,  304  and  305 ) are involved in amplifying the differential input signal across terminals  301  and  302 , rather than four transistors, as in the prior art preamplifier  100  of FIG. 1, this reduction in the number of noise-contributing transistors represents a significant improvement in performance. 
     Transistors  307  and  308  are coupled to the collector terminals of transistors  303  and  304  respectively. Although not necessary for some applications, transistors  303  and  304  provide additional bandwidth for wide bandwidth applications. Voltage source  316  biases transistors  307  and  308  and provides an AC ground. 
     The gain of preamplifier  300  is largely determined by the resistances of resistors  309  and  310  in relation to emitter resistances (i.e., r e ) of transistors  303 ,  304  and  305 . The current in current source  306  can be selected to provide a predetermined dynamic range for the output differential signal across terminals  311  and  312 . Alternatively, resistors  309  and  310  can be eliminated, if the emitter currents of transistors  304  and  305  is used as a differential current signal. 
     Preamplifier  300  exhibits a higher common mode rejection performance than prior art preamplifier  100 . 
     FIG.  3   b  shows a preamplifier  350 , which is a variation of preamplifier  300  of FIG.  3   a , accommodating multiple read heads. To simplify discussion, elements of preamplifier circuits  300  and  350  in FIGS.  3   a  and  3   b , respectively, which perform substantially similar functions are provided like reference numerals. 
     The above-detailed descriptions are provided to illustrate the specific embodiments of the present invention and is and not intended to be limiting. Various modifications and variations within the scope of the present invention are possible. The present invention is set forth in the following claims.