Abstract:
A circuit including a pair of radio frequency signal devices to each of which are connected in parallel a respective input or output and a respective dc bias input device for biasing the respective radio frequency signal device; each dc bias input device including a radio frequency transistor and at least two different types of inductors.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a radio frequency signal biasing circuit, particularly but not exclusively to a pull-up circuit for a differential optical device driver.  
       BACKGROUND OF THE INVENTION  
       [0002]     Optical devices used in optical communications systems are generally driven with electrical signals in order to generate the optical signals. Driver circuits are available that can provide the required electrical signals to drive an optical device such as a laser diode or an external modulator. These optical device driver circuits are often available as integrated circuit (IC) packages, and can be easily incorporated into a driver system design.  
         [0003]     An optical device driver circuit typically provides a modulation current to the optical device that switches the optical device between two or more states in order to convey communication information. The modulation current is typically a radio frequency (RF) signal.  
         [0004]     An optical device driver system can typically provide the modulation current to an optical device either with a “single-ended” drive or a “differential” drive. With a single-ended drive, the modulation current is driven through the optical device in a single direction at different levels. With a differential driver, the modulation current is driven through the optical device in forward and reverse directions, or one output drives the optical device and the other output drives a dummy load. A general advantage of using differential drive is that the optical device can switch between states faster than with a single-ended driver. Differential drive also has the advantage of generating less electromagnetic emission, which means that there is less cross-talk from the transmitter to the receiver, and reduced atmospheric emission (to comply with FCC regulations and industry standards).  
         [0005]     A typical optical device driver circuit for differential drive typically comprises two complementary outputs (often denoted + and −) for driving the modulation current in forward and reverse directions, or for driving an optical device and a dummy load at the same time. The output stage of a typical optical device driver circuit for differential drive usually comprises a pair of transistors, one for each of the complementary outputs. These transistors are often connected in an open-collector configuration or may have a back terminating resistor between the collector and the power supply. In such a configuration, the collector of the output transistor is connected directly to the respective output of the optical device driver. In order for the transistor to operate, the collector connected to the output is biased using a dc voltage. This is achieved using a “pull-up” circuit.  
         [0006]     An optical device driver system  100  with a known pull-up circuit is shown in  FIG. 1 . A differential driver circuit  102  (such as a differential driver IC) has complementary inputs  104 ,  106  connected via AC-coupling capacitors  108 ,  110 . The output stage of the differential driver  102  also receives a DC supply voltage VCC at  112 . The differential driver has two complementary open-collector outputs  114 ,  116 . As stated above, these outputs need to be “pulled-up” by the supply voltage VCC, in order to bias the transistors in the output stage of the differential driver  102 . This has been achieved through the use of inductors  118 ,  120 , whereby one inductor is connected between one output of the differential driver and VCC, and the other inductor is connected between the other output of the differential driver and VCC.  
         [0007]     The inductors  118 ,  120  provide a low impedance path to the DC supply voltage VCC, thereby ensuring that the open-collector outputs of the differential driver are at a voltage close to the DC supply voltage, thereby biasing the output transistors. The inductors  118 ,  120  also provide a high impedance to the RF modulation signals from the outputs of the differential driver  102 , and this reduces the amount of RF modulation signal that is undesirably diverted away from the optical device  130  connected to outputs  126 ,  128 .  
         [0008]     The outputs  114 ,  116  of the differential driver  102  are also connected to AC-coupling capacitors  122 ,  124 , which provide a low impedance to the RF modulation signals, allowing the RF modulation signal to pass to the outputs  126 ,  128 , for further connection to said optical device, such as a laser diode or an external modulator. The capacitors  122 ,  124  also provide a high impedance to DC, thereby preventing the DC voltage provided to the outputs of the differential driver  114 ,  116  from the inductors  118 ,  120  from entering the outputs  126 ,  128  and affecting the rest of the system.  
       SUMMARY OF THE INVENTION  
       [0009]     It has been observed that there is a problem with this conventional approach to providing a pull-up for an optical device driver. If the modulation signal is a wideband RF signal, then it may comprise a range of frequencies from a relatively low RF component up to a relatively high RF component. In order to provide sufficient impedance to the low frequency RF component, a large value inductor was thought to be required, and large value inductors tend to be of a large physical size. Therefore, whilst using large value inductors can improve the impedance over a relatively wide RF frequency range, to do so is not conducive to reducing the size of the circuit and in particular is not conducive to fitting the circuit on a small printed circuit board (PCB) for, for example, a pluggable optical module.  
         [0010]     It is an aim of the present invention to provide a new type of radio frequency signal device biasing circuit, and in particular it is an aim of the present invention to provide a new type of radio frequency signal device biasing circuit that can provide a good level of performance over a wide frequency range whilst at the same time being suitable for use in small devices.  
         [0011]     It is another aim of the present invention to provide a biasing circuit that is particularly suitable for differential radio frequency signal devices, such as differential optical device drivers.  
         [0012]     According to one aspect of the present invention, there is provided a circuit including a pair of radio frequency signal devices to each of which are connected in parallel a respective input or output and a respective dc bias input device for biasing the respective radio frequency signal device; each dc bias input device including a radio frequency transistor and at least two different types of inductors.  
         [0013]     In one embodiment, the pair of radio frequency signal devices together constitute part of a differential driver.  
         [0014]     In one embodiment, each dc bias input device includes a radio frequency transistor having a F T  value greater than or equal to 25 MHz.  
         [0015]     In one embodiment, the at least two types of inductors have different Q factors.  
         [0016]     In one embodiment, said two types of inductors include a coil inductor and a ferrite bead inductor.  
         [0017]     In one embodiment, the two dc bias input devices share a common power supply.  
         [0018]     According to another aspect of the present invention, there is provided a system for producing an optical signal, including a pair of radio frequency signal input devices to each of which are connected in parallel a respective output and a respective dc bias input device for biasing the respective radio frequency signal input device; each dc bias input device including a radio frequency transistor and at least two different types of inductors; and further including an optic device connected to at least one of said outputs.  
         [0019]     In one embodiment, said optic device is connected to both of said outputs.  
         [0020]     In one embodiment, said optic device is connected to only one of said outputs, and a dummy load is connected to another of said outputs.  
         [0021]     In one embodiment, the two radio frequency signal input devices constitute part of a differential driver. Providing two different types of inductors in combination with the radio frequency transistor facilitates the use of a radio frequency transistor having a lower transition frequency, and thereby facilitates the provision of a pull-up circuit displaying substantially identical performance for both differential outputs of the differential driver.  
         [0022]     In one embodiment, the optical device is a laser diode or an external modulator.  
         [0023]     In one embodiment, the two dc bias input devices share a common power supply.  
         [0024]     According to another aspect of the present invention, there is provided a circuit including a radio frequency signal device to which are connected in parallel an input or output and a dc bias input device for biasing the radio frequency signal device: 
        the dc bias input device including a radio frequency transistor and at least two different types of inductors.        
 
         [0026]     In one embodiment, the at least two types of inductors include a coil inductor and a ferrite bead inductor.  
         [0027]     In one embodiment, the at least two types of inductors have different Q factors.  
         [0028]     In one embodiment, the circuit further includes a capacitive element connected to said input or output in parallel with the dc bias input device and the radio frequency signal device.  
         [0029]     According to another aspect of the present invention, there is provided a system for producing an optical signal including: a radio frequency signal input device to which are connected in parallel an optical device for producing an optical signal and a dc bias input device for biasing the radio frequency signal input device, the dc bias input device including a radio frequency transistor and at least two different types of inductors.  
         [0030]     In one embodiment, the optical device is a laser diode or an external modulator.  
         [0031]     The radio frequency signal device may, for example, be a device for outputting a radio-frequency signal for driving an optical device, or an optical device for receiving a radio-frequency signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the following drawings in which:  
         [0033]      FIG. 1  shows an optical device driver system including a known pull-up circuit;  
         [0034]      FIG. 2  shows a differential optical device driver system including a hybrid pull-up circuit according to an embodiment of the present invention;  
         [0035]      FIG. 3  shows a single-ended optical device driver system including a hybrid pull-up circuit according to another embodiment of the present invention; and  
         [0036]      FIG. 4  shows a differential optical device driver system including a hybrid pull-up circuit according to another embodiment of the present invention, but with the optical device connected to only one of the driver outputs and the other driver output being connected to a dummy load.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0037]     Reference is first made to  FIG. 2 , in which is shown an optical device driver system  200  including a hybrid pull-up circuit according to an embodiment of the present invention. The system  200  comprises complementary inputs  104 ,  106  connected to a differential driver  102  (which may be a differential driver IC) via AC-coupling capacitors  108 ,  110 . The differential driver  102  is connected to a DC voltage supply VCC at  112 . The output stage of the differential driver  102  has two complementary outputs  114 ,  116  via which are output the RF modulation signals. The RF modulation signals pass through AC-coupling capacitors  122 ,  124  (which have low impedance at RF) to outputs  126 ,  128 , from which the RF modulation signals can be passed to an optical device  130 , such as a laser diode or an external modulator.  
         [0038]     The outputs  114 ,  116  are open-collector outputs, as described previously, and as discussed below are pulled-up by the supply voltage in order to bias the transistors in the output stage of the differential driver  102 .  
         [0039]     Identical pull-up circuitry is included for each of the two complementary outputs  114 ,  116 . The pull-up circuitry for the two complementary outputs comprises a high Q inductor, such as a coil inductor ( 202  for the “+” output  114 ,  204  for the “−” output  116 ), a low Q inductor, such as a ferrite bead inductor ( 206 ,  208  for the “+” and “−” outputs  114 ,  116 , respectively) and a transistor ( 210 A,  210 B for the “+” and “−” outputs  114 ,  116 , respectively). The transistors  210 A,  210 B are a matched pair of PNP bipolar transistors.  
         [0040]     The ferrite bead inductors  206 ,  208  are connected to the outputs  114 ,  116  of the differential driver. Ferrite beads have a relatively small physical size and are able to provide high impedance to relatively high frequency RF signals. The ferrite beads are connected to the coil inductors  202 ,  204 , which provide high impedance to mid-low RF frequencies. The combination of the passive ferrite bead inductors in series with the coil inductors provides the desired level of impedance over some of the required RF frequency range. The two different types of inductors give a combination of high Q and low Q inductors. Q is the quality factor for an inductor. Q is given by Q=X/R, where X is the inductive reactance and R is the equivalent series resistance.  
         [0041]     The transistors  210 A,  210 B provide a large impedance at the low frequency part of the wideband RF signal. The frequency range over which the transistor provides a high impedance is related to the transition frequency, f T , of the transistor, wherein the f T  value is the theoretical frequency at which the current gain (h fe ) of the transistor is unity (i.e. 0 dB).  
         [0042]     The collector of transistor  210 A is connected to coil inductor  202  and the collector of transistor  210 B is connected to coil inductor  204 . The emitter of transistors  210 A and  210 B are connected to the supply voltage VCC. A resistor  212  is connected between the base and the collector of transistor  210 A, and a resistor  214  is connected between the base and the collector of transistor  210 B. The resistors  212  and  214  bias the transistors  210 A,  210 B together with the differential driver back termination (which may typically be 75Ω). The values of resistors  212  and  214  are chosen to give the appropriate voltage in the bias circuit, and their values depend upon the back terminating resistor values in the driver  102  and the desired bias voltage. The base terminals of the transistors  210 A and  210 B are connected together.  
         [0043]     A capacitor  216  is connected between the supply voltage VCC and the base terminals of  210 A and  210 B. The capacitor creates a “virtual battery” which keeps the base emitter bias voltage constant for transistors  210 A and  210 B. Thus the current is constant in  210 A and  210 B. Capacitors  218 ,  220  bleed off any stray AC signals from the supply voltage line.  
         [0044]     The type of transistor used for  210 A and  210 B is deliberately chosen with a view to having two transistors of substantially identical characteristics, even if this means selecting a type of RF transistor that has a transition frequency lower than the highest that is available. In this embodiment, this is made possible by the co-use of the passive inductors  202 ,  204 ,  206 ,  208 , which together provide the desired level of impedance over the entire frequency range. A typical value for the transition frequency for the transistors shown in  FIG. 2  is ≧250 MHz. Typical values for the inductors are L 3 , L 4 =1 K@100 MHz (ferrite beads) and L 1 , L 2 =47 μH.  
         [0045]     Using the hybrid technique of combining active components (an RF transistor) and passive components (ferrite beads and coil inductors), a high level of impedance is achieved for wideband RF signals, thereby ensuring that the RF signal is not significantly diverted from the optical device, but without the components being too large in entirety to be used in a small module.  
         [0046]      FIG. 3  shows another embodiment of the present invention. In this embodiment, the driver for driving the optical device  304  is a single-ended ended optical device driver  302 . As a single-ended driver  302  only has a single output, only one transistor  210 A and two inductors  202 ,  206  are required. Otherwise, the operation of the circuit is identical to that described with reference to  FIG. 2 .  
         [0047]      FIG. 4  shows another embodiment of the present invention. It is identical to that shown in  FIG. 3 , except that the optical device  402  is connected to only one of the outputs, and the other output is connected to ground via a dummy load  404 .  
         [0048]     In each embodiment, the supply Vcc is for the driver&#39;s output stage and also the pull-up. The driver may get a second power supply for its preceding stages or other parts of the circuitry.  
         [0049]     The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any definitions set out above. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.