Abstract:
A modulator driver design is disclosed that employs a differential pair amplifier coupled to feedback amplifiers through tuning networks. Each tuning network comprises a set of inductors that enables a broadband response while reducing the loading effect of the feedback amplifier. An active load is placed at the output to serve multiple purposes, including: generating a high output swing, enabling a lower power supply voltage, and allowing the entire bias circuit to be monolithically integrated. A modulator driver comprises: a first amplifier stage (A 1 ) having inputs and outputs; a second amplifier stage (A 2 ), having inputs and outputs, the inputs of the second amplifier coupled to the outputs of the first amplifier; an active load having inputs and outputs, the inputs of the active load coupled to the outputs of the amplifier stage; and a feedback stage (A 3 ) having inputs and outputs, the inputs of the feedback stage coupled to the outputs of the second amplifier stage by means of a tuning network, and the outputs of the feedback stage coupled to the inputs of the second amplifier stage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to a co-pending U.S. patent application Ser. No. 10/034,023, entitled “FET Active Load and Current Source” by Carl Walter Pobanz, filed on Dec. 28, 2001, owned by the assignee of this application and incorporated herein by reference. 
    
    
     BACKGROUND INFORMATION 
     1. Field of the Invention 
     The present invention relates generally to electro-optical devices, and more specifically to modulator drivers for high-speed communications. 
     2. Description of Related Art 
     In a high-speed fiber optics communication system, the light is often transmitted into the fiber by a continuous-wave laser followed by an electro-optical modulator. The modulator, which turns the light ON and OFF to generate logical ones and zeros for transmission, is generally fabricated with a non-linear material such as Lithium Niobate (LiNbO 3 ) or Indium Phosphide (InP). These materials exhibit a change in optical refractive index or absorption as a function of an applied voltage. Due to the weakness of such electro-optic effects, a substantial voltage is required to maximize the ratio of optical power between on and off states (the extinction ratio) of the modulated light. Moreover, since the modulator operates at or near the full bit-rate of the communications system, the electrical input to the modulator has both the largest magnitude and highest bandwidth of any signal in the system. Producing this signal requires an amplifier, known as a modulator driver, which achieves high output amplitude and high speed without degrading the quality of the transmitted pulse. 
     One popular type of electro-optic modulator is the Mach-Zehnder (MZ) modulator, which uses the linear electro-optic (Pockels) effect—commonly in LiNbO 3 —and an interferometer to generate light pulses. A typical MZ modulator requires a 6-volt peak-to-peak drive applied to a single input (“single drive”) or applied differentially to, two inputs 180 degrees out of phase (“dual drive”). In a practical application, the modulator driver should be capable of producing 7 to 8 volts peak-to-peak to obtain adequate margin for variations due to temperature, losses in the materials, drift, etc. Electro-absorption modulators typically require single-ended drive voltages on the order of 3 to 4 volts peak-to-peak. 
     The challenge in modulator driver design is to create an amplifier with high voltage swing, wide bandwidth, and high-fidelity pulse response. For example, in a 40-Gigabit per second (40 Gbps) system the transmitted data signal may contain substantial energy at frequencies up to and beyond 40 GHz, depending on the rise and fall times of the data pulse edges. The modulator driver requires an amplitude response that is nearly constant versus frequency, along with constant group delay (i.e., linear phase response)—in other words, all frequencies are amplified equally and travel at the same speed through the amplifier. Deviations from this ideal affect the transmitted data signal, producing pattern-dependent jitter in the timing of the transitions and decreased contrast between logic states (“eye closure”). 
     One objective in driver-amplifier design is to produce a large amount of current, which typically requires large transistors. The parasitics of the transistors—capacitance, resistance, and in some cases inductance, which increase with the size of the transistor—slow down the response, as time is wasted in transferring energy to them rather than to the actual load, the modulator. One classic solution that achieves high speed with a large effective transistor size is the distributed amplifier  100  as shown in FIG. 1, either with a single output or a differential output. This approach defines a certain transistor size that is necessary for the desired output amplitude, and then divides up a large transistor into multiple smaller transistors, which are connected by inductors in such a way that the parasitic capacitance of each small transistor in conjunction with the inductors forms a transmission line. The inductors (or high-impedance transmission line sections) allow the smaller capacitors to charge in sequence, emulating the propagation of a wave in a transmission line, thus obtaining larger bandwidth at the acceptable expense of increased delay. Thus this circuit is also known as a “traveling wave amplifier.” 
     In recent modulator driver designs for next-generation optical networks that require transmission rates of 40G (OC-768), 50G, and beyond, the distributed amplifier approach has been widely used to achieve the required bandwidth and output amplitude. However, traveling-wave amplifiers occupy a significant amount of die space as the circuits themselves are physically large and make very inefficient use of substrate area. Furthermore, the individual frequency responses of the multiple small transistors produce numerous poles in the amplifier&#39;s transfer function that coincide near the upper bandwidth limit of the amplifier. Beyond this cutoff frequency, the gain decreases sharply and the phase becomes highly nonlinear, creating large variations in group delay as the band edge is approached. Discontinuities and mismatches between devices in the circuit cause unwanted reflections and reactive energy storage in the transmission lines, further degrading the amplitude and phase characteristics as frequency increases. Consequently, in order to mitigate these problems and increase the usable frequency range, designers end up creating a much larger and broader-band amplifier than is necessary, thereby producing an inefficient amplifier structure. 
     Another shortcoming in a conventional design is that it typically requires an external means of biasing, such as a bias tee, which separates DC and AC allowing the power supply and the output signal to coincide on the same line. A bias tee is generally impossible to integrate onto a chip due to the large inductor required to meet the low-frequency requirements of optical data signals (e.g. below 50 kHz). It must be added externally, increasing assembly cost and complicating the packaging of the chip as well as degrading the response of the amplifier. Furthermore, traveling-wave amplifiers produce relatively low gain, typically requiring a pre-driver in order to operate with standard inputs such as 400-millivolt peak-to-peak current-mode logic (CML) signals from multiplexer devices and the like. Increasing the gain is difficult without sacrificing bandwidth, and often means adding another traveling-wave amplifier in series, thus multiplying the shortcomings of the architecture two-fold. 
     Accordingly, there is a need to design an amplifier with a large output voltage and current swing capability, having optimum amplitude and phase characteristics over a broad frequency range for driving modulators in a high-speed fiber optic system. 
     SUMMARY OF THE INVENTION 
     The present invention provides a modulator driver design that employs a differential amplifier coupled to feedback amplifiers through tuning networks. Each tuning network comprises a set of inductors that enables a broadband response, while reducing the loading effect of the feedback amplifier. An active load is placed at the output to serve multiple purposes: generating a high output voltage swing, reducing the required power supply voltage, and allowing the bias circuits to be integrated on a chip. 
     A modulator driver comprises: a first amplifier stage (A 1 ) having inputs and outputs; a second amplifier stage (A 2 ), having inputs and outputs, the inputs of the second amplifier coupled to the outputs of the first amplifier; an active load having inputs and outputs, the inputs of the active load coupled to the outputs of the amplifier stage; and a feedback stage (A 3 ) having inputs and outputs, the inputs of the feedback stage coupled to the outputs of the second amplifier stage by means of a tuning network, and the outputs of the feedback stage coupled to the inputs of the second amplifier stage. 
     Advantageously, the present invention generates fully differential outputs, which enables operation with dual-drive optical modulators requiring only half the voltage swing, per input, of single-input modulators. The dual-drive capability also improves the performance of the optical system by allowing zero or non-zero adjustable chirp (phase shift) to be added to the optical signal for improved propagation in dispersive fiber. 
    
    
     Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a prior art circuit diagram illustrating a distributed amplifier. 
     FIG. 2 is a block diagram illustrating a modulator driver for high-speed communications that employs a differential amplifier in accordance with the present invention. 
     FIG. 3 is a circuit diagram illustrating a modulator driver for high-speed communication as shown in FIG. 2 in accordance with the present invention. 
     FIGS. 4A-4C are circuit diagrams illustrating examples of active loads in accordance with the present invention. 
     FIG. 5 is a circuit diagram illustrating a first embodiment of a predriver and the modulator driver in accordance with the present invention. 
     FIG. 6 is a circuit diagram illustrating a second embodiment of a predriver and the modulator driver in accordance with the present invention. 
     FIG. 7 is a detail circuit diagram illustrating the first embodiment of the predriver and the modulator driver as shown in FIG. 5 in accordance with the present invention. 
     FIG. 8 is a flow diagram illustrating the operational process of a predriver and a modulator driver in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 is a block diagram illustrating a modulator driver  200  for high-speed communications that employs a differential amplifier circuit. The modulator driver  200  comprises a first stage A 1   220  amplifier, a second stage amplifier or output driver A 2   240 , a feedback stage amplifier A 3   250   a , a feedback stage amplifier A 3   250   b , and an active load  260 . An input  210  is feed into the first stage Al  220  amplifier and an output  270  is generated to drive an external circuit or system, such as a modulator. The summing nodes  230   a  and  230   b  form a pair of differential feedback, where the summing node  230   a  is out of phase, but symmetric, with the summing node  230   b . A first set of inductors, L 1   241 , L 2   242 , L 3   243 , and L 4   252  are strategically placed for separating the output driver A 2   240  and the feedback stage A 3   250   a . A second set of inductors, L 5   245 , L 6   246 , L 7   247 , and L 8   256  are strategically placed for separating the output driver A 2   240  and the feedback stage A 3   250   b . These inductors may be realized as lumped circuits such as spirals, or as transmission line sections having relatively high characteristic impedance. 
     A circuit diagram  300  illustrating the block diagram of the modulator driver  200  is shown in FIG.  3 . The modulator driver  200  employs a differential pair A 2   240  with two large transistors Q 1   310  and Q 2   311 , which connect respectively to active loads  341  and  342  that convert an electrical current that is switched by the transistors into an output voltage  270 . To achieve wide bandwidth, strong feedback is provided around the differential pair  310  and  311  that forms the output stage. The feedback circuit is designed with followers Q 3   320  and Q 4   321 , either source followers with FETs or emitter followers with bipolar transistors. On a first arm of the differential pair A 2   240 , the follower  320  drives a resistor R 1   251  and the output current of the resistor R 1   251  is routed back to the input of the differential pair Q 1   310  in the modulator driver  300 , which forms a negative feedback loop, thereby reducing the gain and increasing the bandwidth. On a second arm of the differential pair A 2   240 , the follower  321  drives a resistor R 2   255  and the output current of the resistor R 1   255  is routed back to the input of the differential pair Q 2   311  in the modulator driver  300 , which forms a negative feedback loop, thereby reducing the gain and increasing the bandwidth. 
     To achieve wide bandwidth, the amplifier at this point has low input impedance as a result of the feedback current. The parasitic capacitance at the input terminals of the output stage has less of an effect because the preceding amplifier is able to charge the parasitic capacitance relatively fast. The feedback results in a broadband amplifier, but the resulting gain is reduced. In order to increase the gain, the differential pair A 1   220  operates as a first stage in the modulator driver  200 ; this first stage acting as a transconductance amplifier that receives a voltage input and generates a current output to directly drive the low-impedance input of the second differential pair A 2   240 . 
     The inductors L 1   241 , L 2   242 , L 3   243 , and L 4   252  are strategically placed in the modulator  200 , which separates the output driver A 2   240  and the feedback device  250   a . In a traditional design that strives for a large output swing, the follower device has to be large to effectively close the strong feedback loop. As a consequence, the follower device loads the output of the composite amplifier, resulting in slow response. In the present invention, the inductors L 1   241 , L 2   242 , and L 3   243  form a T-coil type of network, and split the cumulative effect of the parasitic capacitances of the output driver A 2   240 , the active load AL 1   260 , and the follower in the feedback stage A 3   250   a , which increases significantly the bandwidth of the modulator driver  200 . To compensate for the feedback and attain the desired transfer function of gain versus frequency, an inductor L 4   252  is added, which connects in series with the output of the follower A 3   250   a . The inductor L 4   252  affects the response of the feedback network to equalize the feedback gain in the presence of the network formed by the inductors L 1   241 , L 2   242 , and L 3   243 , which in turn increases the bandwidth and generates a desirable feedback response. By selecting the ratios between L 1   241 , L 2   242 , L 3   243 , L 4   252 , the resistance of the feedback resistor R 1   251  and the size of the follower device Q 3   230 , a designer can manipulate the complex poles and zeros thereby produced in the amplifier&#39;s transfer function and achieve a number of different frequency response characteristics. Therefore, a desirable transfer function can be obtained by selecting the value of the inductors L 1   241 , L 2   242 , L 3   243 , and L 4   252  to produce optimal pulse characteristics for a particular size of output driver A 2   240 . 
     The inductors L 5   245 , L 6   246 , L 7   247 , and L 8   256  are strategically placed in the modulator  200 , which separates the output driver A 2   240  and the feedback device  250   b . In a traditional design, in order to achieve a large output swing, the follower device has to be fairly large to effectively close the strong feedback loop, which slows down the output. In the present invention, the inductors L 5   245 , L 6   246 , L 7   247  form a T-coil network, and split the cumulative effect of the parasitic capacitance of the output driver A 2   240 , the active load  260 , and the follower in the feedback stage  250   b , which increases significantly the bandwidth of the modulator driver  200 . To compensate for the feedback and attain the desired transfer function of gain versus frequency, an inductor L 8   256  is added, which connects in series with the output of the follower A 3   250 b. The inductor L 8   256  affects the response of the feedback network to equalize the feedback gain in the presence of the network formed by the inductors L 5   245 , L 6   246  and L 7   247 , which in turn increases the bandwidth and generates a desirable feedback response. By selecting the ratios between L 5   245 , L 6   246 , L 7 , 247 , L 8   256 , the resistance of the feedback resistor R 2   255  and the size of the follower device Q 4   231 , a designer can manipulate the complex poles and zeros thereby produced in the amplifier&#39;s transfer function and achieve a number of different frequency response characteristics. Therefore, a desirable transfer function can be obtained by selecting the value of the inductors L 5   245 , L 6   246 , L 7   247  and L 8   256  to produce optimal pulse characteristics for a particular size of output driver A 2   240 . 
     To drive the output  270 , the driver stage comprises a composite of amplifiers. In this embodiment, the amplifier A 2   240  and the feedback amplifier A 3   250   a  and  250   b  drive each other, with a feedback signal going to the summing node  230   a  between the two amplifiers  240  and  250   a , and the summing node  230   b  between the two amplifiers  240  and  250   b . In order to increase the gain-bandwidth of the composite amplifier, the output driver  200  adds the predriver  220 , which is implemented with the third amplifier A 1   220  that precedes the driver stage A 2   240 . The third amplifier A 1   220  produces an amplified copy of the input signal  210  and delivers it with low output impedance. The low output impedance allows fast charging of parasitic capacitance associated with the driver A 2   240  and further enhances the bandwidth. 
     The circuit diagram of the modulator driver  300  in FIG. 3 is implemented with field-effect transistors (FETs). The FETs can be fabricated, for example, in InP or GaAs as high electron mobility transistors (HEMT) or pseudomorphic high electron mobility transistors (PHEMT), or in silicon-based processes as complementary metal-oxide semiconductor (CMOS) FETs. A variety of bipolar transistors may also be used. The transistors Q 1   310  and Q 2   311  operate as a differential pair A 2   240 , which forms the second stage of the modulator driver  300 . On the first arm of the differential pair A 2   240 , the inductors L 1   241 , L 2   242 , and L 3   243  form a T-coil type of network, which increases the output bandwidth, and decouples the loading effect of the feedback device Q 3   320 . On the second arm of the differential pair A 2   240 , the inductors L 5   245 , L 6   246 , and L 7   247 , form a T-coil type of network, which increases the output bandwidth, and decouples the loading effect of the feedback device Q 4   321 . The Q 3   320  transistor operates as a feedback follower device that closes the negative feedback loop from the output back to the input of Q 1   310 . The Q 4   321  transistor operates as a feedback follower device that closes the negative feedback loop from the output back to the input of Q 2   311 . The inductors L 4   252  and L 8   256  equalize the closed-loop gain of the second stage A 2   240  verses frequency. The first stage A 1   220  drives the low impedance and high output swing of the second stage A 2   240 . The first stage A 1   220  comprises the transistor Q 5   330  and the transistor Q 6   331 , which are transconductance amplifiers that receive voltage inputs and generate current outputs. Inductors L 9   350  and L 10   351  create a series peaking function between the two stages to speed up the transient response. The addition of resistors R 3   355  and R 4   356  provides a feedback function, which not only speeds up the input to the first stage A 1   220 , but also improves the settling time of the output driver by providing a bleed current to Q 3   320  and Q 4   321  feedback devices during operation so that the Q 3   320  and Q 4   321  transistors are not completely shut off, and always remain in a fast operating regime. As a result, the modulator driver design  300  produces high output swing and broadband response. Current sources CS 1   370  and CS 2   371  supply a constant current to the amplifier stages A 1   220  and A 2   240  respectively, and present high impedance at the source nodes to decrease the common mode gain and enable differential operation. A pair of bypass networks BP 1   360  and BP 2   361  ensure low power supply impedance for followers  250  in the modulator driver  300 . 
     FIGS. 4A-4C are circuit diagrams  410 ,  420  and  430 , illustrating examples of active loads, such as a “self-bootstrapped” load implemented with depletion-mode FETs. An active load is designed and selected so that the circuit has a significantly higher AC or dynamic resistance than its equivalent DC resistance. A more detailed description of the active loads has been disclosed in a co-pending patent application entitled “FET Active Load and Current Source,” assigned to the same assignee, and accorded an application number of Ser. No. 10/034,023, which is incorporated herein by reference in its entirety. 
     FIG. 5 is a circuit diagram illustrating a first embodiment of a broadband predriver circuit  510  and the modulator driver  300 . The function of the predriver  510  is to produce a desired broadband resistive input impedance, such as 50 ohms or 100 ohms differential, and thereby facilitate a standard interface between the modulator driver and other circuits. Concurrently, the output of the predriver  500  exhibits a low impedance, which drives the modulator driver stage  300  in order to obtain the optimal bandwidth. To achieve broadband low output impedance, the predriver amplifier  510  employs a differential pair, a Q 7  transistor  510  and a Q 8  transistor  511 , with a resistive feedback through R 7   520  and a resistive feedback R 8   521  that increase the bandwidth of the amplifier. Resistors R 5   530  and R 6   531  are chosen to achieve the desired low output impedance. Inductors L 11   540  and L 12   541  enhance the bandwidth by shunt peaking. The predriver  510  can be driven differentially at the input or with a single input, and provides a differential drive for the modulator driver. Therefore, the predriver  510  achieves low output impedance that produces the maximum bandwidth, and creates an unbalanced to balanced signal conversion, which allows the pre-driver  510  to operate with either a differential input or a single input. Resistors R 9   550  and R 10   551  provide the proper input DC bias level and also enhance the input impedance. 
     Supply voltages V 1   561  and V 4   564  are positive, relative to supply voltage V 3   563 . Supply voltages V 1   561 , V 2   562 , V 3   563 , and V 4   564  are selected to achieve the most efficient amplifier and keep the minimum power supply voltages as required by the output voltage swing. For example, the supply voltage V 1   561  is set to +5 volts, the supply voltage V 4   564  is set to 0 volts, and the supply voltage V 2   562  and the supply voltage V 3   563  are set to −5 volts. 
     FIG. 6 is a circuit diagram illustrating a second embodiment of a predriver  600  and the modulator driver  300  that serve the same functions as the predriver  500  in FIG. 5 in producing low output impedance to attain maximum bandwidth. The predriver  600  acts as a buffer. A pair of source followers Q 7   610  and Q 8   611  provides a low output impedance drive from the predriver  600  to the modulator driver  300 . Diodes D 1   621 , D 2   622 , D 3   623 , and D 4   624  are level shifting devices to ensure voltages at the output of the predriver  600  match with a common mode voltage or DC required by the modulator driver  300  for operation. Current sources CS 1   630  and CS 3   631  provide bias currents for the predriver  600 . 
     FIG. 7 is a detailed circuit diagram illustrating the first embodiment of the predriver  500  and the modulator driver  300  as shown in FIG.  5 . The current sources are explicitly made out of FETs and resistors. Significantly, transistors Q 15   710  and Q 16   711  serve as a variable current source for the predriver  500 ; this allows the gain of the predriver  500  to be changed, which in turn changes the output amplitude of the driver given a fixed input signal. Because this adjustment is made at the predriver stage, the output stage driver remains operating at the same bias point as before, and therefore its performance is not degraded during the adjustment of the output amplitude. Hence, variable output amplitudes can be achieved while retaining high pulse fidelity. 
     FIG. 8 is a flow diagram illustrating the operational process  800  of the predriver  500  and the modulator  300 . At predriver  500 , an amplifier Q 7   510  receives an input voltage  801  from either a single input or from differential inputs, and generates an output voltage with low impedance in step  805 . The predriver amplifier Q 7   510  and Q 8   511  is also able to convert an unbalanced signal to a balanced signal. At the first driver stage A 1  in step  810 , the output voltage from the predriver  500  is converted to a current that is applied to the inputs of the second stage A 2  by way of the summing node in step  860 . At the second stage A 2  in step  820 , the large transistor Ql  310  amplifies this input signal to produce an output current in the first arm of the differential amplifier A 2   240 . The tuning network, which comprises inductors L 1   241 , L 2   242 , and L 3   243  separates  830  the feedback stage A 3   250   a  from the second stage amplifier A 2   240 , thereby reducing the loading effect of the feedback stage amplifier A 3   250   a . At step  840 , the feedback stage amplifier A 3   250   a , which comprises the source follower Q 3   320 , in conjunction with resistor R 1   251 , reduces both the input impedance and gain and increases the bandwidth of the second stage amplifier A 2  through negative feedback. The inductor L 4   252  equalizes  850  the gain, which in turn increases the bandwidth and generates a desirable feedback response. At step  860 , the summing node  230   a  sums the current output from the amplifier A 1   220  with the feedback current from the feedback stage amplifier A 3   250   a  through resistor R 1   251 . At step  870 , the active load AL 1   341  receives the output current from amplifier A 2   240  and converts it to a final output voltage  880 . 
     Similarly, in the second arm of a differential amplifier, at the predriver  500  an amplifier Q 7   510  and Q 8   511  receives an input voltage  801  from either a single input or from differential inputs, and generates an output voltage with low impedance in step  806 . The predriver amplifier Q 7   510  and Q 8   511  is also able to convert an unbalanced signal to a balanced signal. The resulting signal voltages and currents in the second arm of the differential amplifiers are precisely 180 degrees out of phase with those in the first arm. At the first driver stage A 1  in step  811 , the output voltage from the predriver  500  is converted to a current that is applied to the inputs of the second stage A 2  by way of the summing node in step  861 . At the second stage A 2  in step  821 , the large transistor Q 2   311  amplifies this input signal to produce an output current in the second arm of the differential amplifier A 2   240 . The tuning network, which comprises inductors L 5   245 , L 6   246 , and L 7   247  separates  831  the feedback stage A 3   250   b  from the second stage amplifier A 2   240 , thereby reducing the loading effect of the feedback stage amplifier A 3   250   a . At step  841 , the feedback stage amplifier A 3   250   b , which comprises the source follower Q 4   321 , in conjunction with resistor R 2   255 , reduces both the input impedance and gain and increases the bandwidth of the second stage amplifier A 2  through negative feedback. The inductor L 8   256  equalizes  851  the gain, which in turn increases the bandwidth and generates a desirable feedback response. At step  861 , the summing node  230   b  sums the current output from the amplifier A 1   220  with the feedback current from the feedback stage amplifier A 3   250   b  through resistor R 2   255 . At step  871 , the active load AL 2   342  receives the output current from amplifier A 2   240  and converts the current to a final driver output voltage  881 . 
     The above embodiments are only illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the appended claims.