Abstract:
Driver circuitry with a tuned output impedance is provided. Tuning is provided by an isolation circuit and matching network coupled to an output of the driver circuit. The isolation circuit isolates the capacitance associated portions of the driver circuit thereby reducing overall output capacitance. The matching network substantially compensates for reactive impedances associated with other portions of the driver circuit. These tuning circuits allow the driver circuit overcome intrinsic reactance and exhibit a substantially resistive output impedance characteristic.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to driver circuitry. More particularly, this invention relates to circuits and methods for providing driver circuitry with a tuned output impedance. 
     In the past, the semiconductor industry has utilized various configurations of “driver circuitry” for supplying power to loads that are external to an integrated circuit. Common examples of such external loads include transmission lines, communication systems, and electric motors. One characteristic of driver circuitry that is of concern to system designers is output impedance. As a general principle, it is desirable to match the output impedance of the driver circuitry as closely as possible to the input impedance of the load to maximize power transfer and minimize signal reflections. 
     In certain applications, such as those involving power transmission or lighting systems, a closely matched impedance is not critical. Other applications, however, such as high speed communications systems, often rely on near-perfect impedance matches to properly function. 
     Historically, impedance matching has been accomplished by coupling a precision resistor between the driver circuit and the load to provide proper line termination. One deficiency of this approach, however, is that it fails to account for the output capacitance associated with the driver circuit. Because the response time of the driver circuitry is directly dependent on output capacitance, it is generally desirable to minimize this value to approximate a substantially resistive output characteristic. This is particularly desirable in applications that involve high speed data transfer. 
     Thus, in view of the foregoing, it would therefore be desirable to provide circuits and methods that compensate for the effects of output capacitance on driver circuit performance. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide circuits and methods that compensate for the effects of output capacitance on driver circuit performance. 
     This and other objects are accomplished in accordance with the principles of the present invention by providing driver circuitry with a tuned output impedance. The tuning function of the present invention is provided by an isolation circuit and matching network coupled to an output of the driver circuit. The isolation circuit isolates the capacitance associated portions of the driver circuit thereby reducing overall output capacitance. The matching network substantially compensates for reactive impedances associated with other portions of the driver circuit. Employing these circuits simultaneously allows the driver circuit to overcome intrinsic reactance and exhibit a substantially resistive output impedance characteristic. 
     Furthermore, the components used in the isolation circuit and matching network may be selected so that the output impedance of the driver circuit substantially matches that of an external load such as a transmission line or light emitting element. This solution eliminates the need for external damping components and provides significantly improved high frequency performance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
     FIG. 1 is schematic diagram of a driver circuit constructed in accordance with the principles of the present invention; and 
     FIG. 2 is a schematic diagram another driver circuit constructed in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic diagram of a driver circuit  100  constructed in accordance with the principles of the present invention. Driver circuit  100  includes a sourcing circuit  110 , a sinking circuit  120 , inductors  130  and  140 , an isolation resistor  150 , and capacitors  160  and  170 . 
     Transmission line  180  and load  190  represent an external load driven by circuit  100 . Although other arrangements are possible, sourcing circuit  110  and sinking circuit  120  are preferably disposed on a semiconductor die  106  and inductors  130  and  140  may be formed from material present on a typical integrated circuit package  105  (e.g., bond wire). 
     In operation, sourcing circuit  110  may provide a substantially constant source current (I S ) to isolation resistor  150  and load  190  (through transmission line  180 ). Sourcing circuit  110  may include any circuit configuration suitable for providing current such as current mirror type bias circuitry. 
     The current I S  supplied to load  190  may be varied by periodically switching sinking circuit  120  ON and OFF, allowing a modulation current (I M ) to pass through it during an ON state, and acting as an open circuit during an OFF state. In this way, a voltage differential is applied across load  190  to provide signal modulation. 
     Although driver circuit  100  is suitable for driving a wide variety of loads, it is particularly suitable for use in optical communication systems wherein load  190  is a light emitting element such as a laser diode, a vertical cavity surface emitting laser (VCSEL), or light emitting diode (LED) etc. In this case, signal modulation is used to switch load  190  ON and OFF to produce binary optical signals. 
     In some embodiments of the present invention, sinking circuit  120  may be configured to turn ON and OFF partially to improve response time. Sinking circuit  120  may include any circuitry suitable for switching between ON and OFF states such as a transistor or armature type switch. 
     Capacitor  170  is coupled in parallel with isolation resistor  150  and preferably has a value large enough to support low frequency operation. 
     Isolation resistor  150  provides three important benefits to circuit  100 . First, it acts as a line termination to match the characteristic impedance of transmission line  180 . Second, it prevents high frequency signals generated by sinking circuit  120  from being introduced onto a power plane coupled to sourcing circuit  110  (not shown). Third, it reduces the overall output capacitance of driver circuit  100  by isolating the capacitance associated with sourcing circuit  110 . The reduction of output capacitance decreases the time constant of driver circuit  100 , improving high frequency response. 
     As shown in FIG. 1, the frequency response of driver circuit  100  may be further improved by the addition of a broadband matching network formed by inductors  130  and  140 . Because inductors  130  and  140  are coupled in parallel, the overall package inductance driven by sinking circuit  120  is lowered. Using this configuration, it is possible to obtain a substantially resistive output impedance characteristic by compensating for the capacitance associated sinking circuit  120 . This may be accomplished by selecting certain values for inductors  130  and  140  that satisfy the following equations. For example, the output impedance associated with sinking circuit  120  will be substantially resistive if: 
     
       
           Z   0   2   =L/C   N ;  (1) and 
       
     
     
       
           C   BR   =C   N /4  (2) 
       
     
     where Z 0  is the characteristic impedance of transmission line  180 , C N  is the output capacitance associated with sinking circuit  120 , C BR  is the capacitance between inductors  130  and  140 , and L is the total inductance of inductors  130  and  140 . 
     Thus, as can be seen from the above, combining an isolation resistor with a broadband matching network in circuit  100  essentially “tunes out” driver capacitance and package inductance and provides a substantially resistive output impedance (i.e., within about ±5% of a purely resistive impedance characteristic). 
     From a functional standpoint, this can be viewed as supplying a specified inductance to tune out the frequency effects of output capacitance or vice-versa. Because this technique is not frequency dependent, it eliminates the need for cumbersome external damping networks. Moreover, the resonant frequency of driver circuit  100  is substantially increased, permitting a broader range of high frequency operation (e.g., by a factor of about 1.41). This result is particularly desirable in optical communications systems that constantly strive to accommodate increasing data rates. 
     In some embodiments of the present invention, it may not be possible to obtain a substantially resistive impedance without isolating the capacitance associated with sourcing circuit  110 . For example, if the capacitance value needed to obtain a certain output characteristic is less than the intrinsic output capacitance of driver  100 , at least some isolation of sourcing circuit  110  is necessary. 
     Assume, for example, that inductors  130  and  140  each have a 2 nH value and transmission line  180  has a characteristic impedance of 50 Ohms. In this case, using equation 1, a substantially resistive impedance is achieved when the output capacitance of circuit  100  is 1.6 pF. Consequently, if the intrinsic output capacitance of driver circuit  100  is greater than 1.6 pF, at least some isolation of sourcing circuit  110  will be required. 
     Another driver circuit constructed in accordance with the principles of the present invention is shown in FIG.  2 . Driver circuit  200  is similar to driver circuit  100  in many ways and represents one specific embodiment of the present invention suitable for driving a light emitting element. As shown in FIG. 2, driver circuit  200  includes a modulation circuit  210 , a sinking circuit  220 , inductors  230 ,  240 , and  245 , an isolation resistor  250 , and capacitors  260  and  270 . 
     Transmission lines  280  and  285  and laser diode  290  represent an external load driven by circuit  200 . Although other arrangements are possible, modulation circuit  210  and sinking circuit  220  are preferably disposed on a semiconductor die  206  and inductors  230 ,  240 , and  245  may be formed from material on an integrated circuit package  205  (e.g., bond wire). 
     In operation, a current I L  flows from voltage source  295  (V cc ) through laser diode  290 , transmission lines  280  and  285 , isolation resistor  250 , and into sinking circuit  220 . Current I L  is typically set such that laser diode  290  is ON (i.e., in a conducting state) but with an optical output indicative of a “logic low ” signal. Sinking circuit  220  may be any circuit configuration suitable for biasing laser diode  290 . 
     The current drawn from laser diode  290  may be varied by periodically switching modulation circuit  210  ON and OFF, allowing a modulation current (I M ) to pass through it during an ON state, and acting as an open circuit during an OFF state. During an ON state, additional current is drawn from the cathode of laser diode  290 , turning it ON further and causing it to produce an optical output indicative of a “logic high” signal. Thus, optical data signals may be obtained from laser diode  290  by switching modulation circuit  290  between ON and OFF states. In some embodiments, modulation circuit  210  may be configured to turn ON and OFF partially to improve response time. Modulation circuit  210  may include any circuitry suitable for switching between ON and OFF states such as a transistor or armature type switch. 
     Capacitors  260  and  270  and isolation resistor  250  perform substantially the same functions as their counterpart components in FIG. 1 (i.e., capacitors  160  and  170  and resistor  150 ). Furthermore, circuit  200  may be arranged to provide a substantially resistive output characteristic by following the relationships set forth equations 1 and 2. 
     Providing the proper inductance values for the inductors shown in FIGS. 1 and 2 may be accomplished in a number of ways. One method involves the use of “static compensation ” technique. The first step in this method involves determining the output capacitance associated with the driver circuit. This may be accomplished by manufacture and measurement or by calculation. 
     Next, an inductor of the proper value is designed from material normally present on an integrated circuit package such as bond wire and/or a package lead. The value of these inductors may be altered by changing the length, width, spacing, or material from which they are constructed. Once designed, the inductors are incorporated into the integrated circuit manufacturing process so that the final product has the inductor present in the integrated circuit package, and thus the desired output impedance characteristic. This process is known as static compensation due to the difficulty involved with altering the inductance value on the chip after it has been produced. 
     Another way of providing a specific inductor value involves a “active compensation ” technique. With this method, an adjustable inductor is disposed on the die of the driver circuit rather than constructing it from packaging materials. After fabrication of the driver circuit is complete, either the manufacturer or a user may adjust the inductor (e.g., by trimming) to obtain a desired inductor value and thus a certain output impedance. 
     In some embodiments of the present invention it may be desirable to provide driver circuitry that is under compensated (i.e., with inductance values less than that needed to provide a substantially resistive output characteristic). This allows users to set the output impedance of the driver circuit to a specific desired value by adding additional external components. In this way, it is possible for users to match the impedance of the driver circuitry to a wide range of external loads such as transmission lines and light emitting elements. 
     Persons skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.