Abstract:
The invention discloses an active back-end termination circuit, which comprises a first resistor, a first transistor, a second resistor, and a second transistor. The first resistor and the first transistor are connected in series for forming a first impendence unit. A first source of the first transistor is connected to a working voltage with V TT . The second resistor and the second transistor are connected in series for forming a second impendence unit. A second gate and a second drain of the second transistor are connected to the working voltage with V TT . Wherein, the first impendence unit and the second impendence unit are connected in parallel. The first transistor or the second transistor is switched on through a power source, and the first transistor and the second transistor change the impedance actively for matching a load according to the voltage source.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an active back-end termination circuit, in particular to a DC-coupled active back-end termination circuit for matching the impedance of the load. 
         [0003]    2. Description of the Related Art 
         [0004]    At present, optical communication technology is the mainstream of the high-speed communication network. A direct modulation with distributed feedback (DFB) laser is generally adopted to a middle-distance of the optical communication technology, and an external modulation with DFB laser is adopted to a long-distance of the optical communication technology. As to the transmission rate, present manufacturers adopt a mature network technology in system carrier, namely a coarse wavelength division multiplexing (CWDM) technology having a primary technical specification of OC48(2.488 Gb/s)˜ OC192(9.953 Gb/s), and the research works will be headed to OC768(39.812 Gb/s) continually. 
         [0005]    The traditional laser driver is designed with an open collector architecture, and uses an external matching network for driving laser diode. With the increasing operation speed of the optical communication network, the signal reflection between the laser driver and the laser diode is getting worse due to the impedance mismatch and it impacts the performance of the transmission. To solve this problem, the passive back-end termination circuit is used, but it costs 50% modulation current due to the internal resistor (Rint). In another prior art, the internal AC-coupled active back-end termination circuit disclosed in U.S. Pat. No. 6,667,661 is used, but it is very diffucult to design a high quality capacitor in chip process and it occupies large area. 
       SUMMARY OF THE INVENTION 
       [0006]    In view of the aforementioned problems of the prior art, the object of the present invention is to provide an active back-end termination circuit for matching the impedance of the load. 
         [0007]    According to another object of the present invention, an active back-end termination circuit is provided, comprising a first resistor, a first transistor, a second resistor and a second transistor. The first resistor comprises a first terminal and a second terminal The first transistor comprises a first source, a first gate and a first drain. The first gate and the first drain are connected to the second terminal, and a first matching unit is formed by connecting the first transistor and the first resistor in series. The first source is connected to a working voltage V TT . The second resistor comprises a third terminal and a fourth terminal. The second transistor comprises a second source, a second gate and a second drain. The second source is connected to the fourth terminal, and a second matching unit is formed by connecting the second resistor and the second transistor in series. The second gate and the second drain are connected to the working voltage V TT . The first matching unit and the second matching unit are connected in parallel by connecting the first terminal and the third terminal are connected to a circuit. The first transistor or the second transistor is applied with a bias voltage by connecting the first terminal and the third terminal to the voltage source. According to the bias voltage supplied by the voltage source is different, so that the first matching unit and the second matching unit actively change an impedance to match with a load according to the voltage source. In other word, the first transistor and the second transistor may be switchon or switch off according to the voltage source. 
         [0008]    If the voltage source is situated at a period of a positive half cycle, the voltage source drives the first matching unit, and if the voltage source is situated at a period of a negative half cycle, the voltage source drives the second matching unit. 
         [0009]    The voltage source outputs a voltage with output high level (VOH) or a voltage with output low level (VOL) by defining the working voltage V TT  as an amplitude origin. 
         [0010]    The voltage source outputs the working voltage V TT  without any loss of DC current. 
         [0011]    The load is a laser diode (LD) or an elector-absorption modulated laser (EML). 
         [0012]    When the voltage source outputs a voltage with output high level (VOH) or a voltage with output low level (VOL), the first transistor or second transistorprovides an impedance matching according to the bias voltage. 
         [0013]    Another objective of the present invention is to provide an active back-end termination circuit, comprising: a first resistor, a first transistor, a second resistor and a second transistor. The first resistor comprises a first terminal and a second terminal. The first transistor comprises a first source, a first gate and a first drain, and the first drain is connected to the second terminal, and a first matching unit is formed by connecting the first resistor and the first transistor in series, and the first source is connected to the working voltage V TT , and the first gate is connected to an external voltage source. The second resistor comprises a third terminal and a fourth terminal. The second transistor comprises a second source, a second gate and a second drain, and the second source is connected to the fourth terminal, and a second matching unit is formed by connecting the second resistor and the second transistor in series. The second drain is connected to the working voltage V TT , and the second gate is connected to an external voltage source. The first terminal and the third terminal are connected to a circuit, such that the first matching unit and the second matching unit are connected in parallel to the circuit. The first terminal and the third terminal are connected to the voltage source, first transistor or second transistor, and a bias voltage is provided according to the voltage source and the external voltage source. Now, the first transistor and the second transistor are variable resistors for actively matching an impedance of a load. 
         [0014]    If the voltage source is situated at a period of a positive half cycle, the voltage source drives the first matching unit, and if the voltage source is situated at a period of a negative half cycle, the voltage source drives the second matching unit. 
         [0015]    The voltage source outputs a voltage with output high level (VOH) or a voltage with output low level (VOL) by using the working voltage V TT  as an amplitude origin. 
         [0016]    The voltage source outputs the working voltage V TT  without any loss of DC current. 
         [0017]    The load is a laser diode (LD) or an elector-absorption modulated laser (EML). 
         [0018]    When the voltage source outputs a voltage with output high level (VOH) or a voltage with output low level (VOL), the first transistor or second transistor provides an impedance matching according to the bias voltage. 
         [0019]    In summation of the description above, the active back-end termination circuit of the present invention has one or more of the following advantages: 
         [0020]    (1) The active back-end termination circuit has a driving efficiency higher than the passive back-end termination circuit. 
         [0021]    (2) The active back-end termination circuit provides the impedance matching without designing a capacitor in a chip process, and thus no large chip area is occupied by the capacitor. 
         [0022]    (3) The active back-end termination circuit consumes no DC current when the voltage source outputs the working voltage with V TT . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic diagram of an active back-end termination circuit in accordance with the present invention; 
           [0024]      FIG. 2  is a schematic view of a sine-wave voltage source of the present invention; 
           [0025]      FIG. 3  is a schematic diagram of an active back-end termination circuit in accordance with a first preferred embodiment of the present invention; 
           [0026]      FIG. 4  is a Smith chart diagram of a first matching unit in accordance with a first preferred embodiment of the present invention; 
           [0027]      FIG. 5  is a Smith chart diagram of a second matching unit in accordance with a first preferred embodiment of the present invention; 
           [0028]      FIG. 6  is a schematic diagram of an active back-end termination circuit in accordance with a second preferred embodiment of the present invention; 
           [0029]      FIG. 7  is a Smith chart diagram of a first matching unit in accordance with a second preferred embodiment of the present invention; 
           [0030]      FIG. 8  is a Smith chart diagram of a second matching unit in accordance with a second preferred embodiment of the present invention; 
           [0031]      FIG. 9  is a schematic diagram of an active back-end termination circuit in accordance with a third preferred embodiment of the present invention; 
           [0032]      FIG. 10  is a Smith chart diagram of an active back-end termination circuit in accordance with a third preferred embodiment of the present invention; 
           [0033]      FIG. 11  is a schematic diagram of an active back-end termination circuit in accordance with a fourth preferred embodiment of the present invention; 
           [0034]      FIG. 12  is a Smith chart diagram of an active back-end termination circuit in accordance with a fourth preferred embodiment of the present invention; 
           [0035]      FIG. 13  is a schematic diagram of another active back-end termination circuit in accordance with the present invention; 
           [0036]      FIG. 14  is an eye diagram of a conventional passive back-end termination connected to an output stage circuit in 25 ohm driving system; 
           [0037]      FIG. 15  is an eye diagram of a DC-coupled active back-end termination connected to an output stage circuit in 25 ohm driving system in accordance with the present invention; 
           [0038]      FIG. 16  is an eye diagram of a conventional passive back-end termination circuit connected to an output stage in 50 ohm driving system; and 
           [0039]      FIG. 17  is an eye diagram of a DC-coupled active back-end termination circuit connected to an output stage in 50 ohm driving system in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    With reference to  FIG. 1  for a schematic diagram of an active back-end termination circuit in accordance with the present invention, the active back-end termination circuit  1  comprises a first matching unit  11  and a second matching unit  12 . The first matching unit  11  comprises a first resistor  111  and a first transistor  112 . The first resistor  111  comprises a first terminal  1111  and a second terminal  1112 . The first transistor  112  comprises a first source, a first gate and a first drain, and the first gate and the first drain are connected to the second terminal  1112 , such that the first resistor  111  and the first transistor  112  are connected in series, and the first source is connected to a working voltage V TT . 
         [0041]    The second matching unit  12  comprises a second resistor  121  and a second transistor  122 . The second resistor  121  comprises a third terminal  1211  and a fourth terminal  1212 . The second transistor  122  comprises a second source, a second gate and a second drain, and the second source is connected to the fourth terminal  1212 , such that the second resistor  121  and the second transistor  122  are connected in series, and the second gate and the second drain are connected to the working voltage V TT . 
         [0042]    The first terminal  1111  and the third terminal  1211  are connected to a circuit, such that the first matching unit  11  and the second matching unit  12  are connected in parallel with a circuit. If the first terminal  1111  and the third terminal  1211  are connected to a voltage source, the first transistor  112  or the second transistor  122  generates a bias voltage according to the voltage source. If the first transistor  112  and the second transistor  122  are switched on according to the bias voltage, the resistance will be different, so that the first transistor  112  and the second transistor  122  may be considered as variable resistors for actively matching an impedance of a load. The first transistor  112  or second transistor  122  may be an N-type metal oxide semiconductor (NMOS), a P-type metal oxide semiconductor (PMOS) or a bipolar junction transistor (BJT), but the present invention is not limited to such arrangements only. 
         [0043]    The voltage source may be a periodic wave voltage source, such as a sine wave voltage source, a square wave voltage source or a triangle wave voltage source. If the voltage source is a sine wave voltage source as shown in  FIG. 2 , the voltage source may define the working voltage V TT  as an amplitude origin to output a voltage with output high level (VOH) or a voltage with output low level (VOL). If the voltage source is situated at a period of a positive half cycle, the voltage source will drive the first matching unit  11 . If the voltage source is situated at a period of a negative half cycle, the voltage source will drive the second matching unit  12 . 
         [0044]    With reference to  FIG. 3  for a schematic diagram of an active back-end termination circuit in accordance with a first preferred embodiment of the present invention,  FIG. 4  for a Smith chart diagram of a first matching unit in accordance with a first preferred embodiment of the present invention, and  FIG. 5  for a Smith chart diagram of a second matching unit in accordance with a first preferred embodiment of the present invention respectively, if the voltage source outputs a voltage with output high level, such as a voltage value of 5V, and the operating frequency is approximately equal to 1 GHz, the first transistor  112  of the first matching unit  11  or the second transistor  122  of the second matching unit  12  will be affected by a bias voltage to obtain an impedance of 82 ohms of the first matching unit, and an impedance of 356 ohms of the second matching unit. 
         [0045]    With reference to  FIG. 6  for a schematic diagram of an active back-end termination circuit in accordance with a second preferred embodiment of the present invention,  FIG. 7  for a Smith chart diagram of a first matching unit in accordance with a second preferred embodiment of the present invention, and  FIG. 8  for a Smith chart diagram of a second matching unit in accordance with a second preferred embodiment of the present invention respectively, if the voltage source outputs a voltage with output low level such as a voltage value of 2V, and the operating frequency is approximately equal to l GHz, the first transistor  112  of the first matching unit  11  or the transistor  122  of the second matching unit  12  is affected by a bias voltage to obtain an impedance of 363 ohms for the first matching unit, and an impedance of 83 ohms for the second matching unit. 
         [0046]    In the aforementioned two preferred embodiments, if the voltage source is a voltage with output high level, an impedance of 82 ohms for the first matching unit and an impedance of 356 ohms for the second matching unit can be obtained. If the voltage source is a voltage with output low level, an impedance of 363 ohms for the first matching unit and an impedance of 83 ohms for the second matching unit can be achieved. The impendances of the whole set of matching units at a voltage with output high level or a voltage with output low level are very close and equal to 66.65 ohms and 67.56 ohms respectively. 
         [0047]    If the voltage source is at a working voltage V TT  such as a voltage of 3.5V, and the first source of the first transistor  112  of the first matching unit  11  is connected to the working voltage V TT , and both terminals have an equal electric potential, there will have no loss of DC current. Similarly, the second transistor  122  of the second matching unit  12  has no loss of DC current. 
         [0048]    With reference to  FIG. 9  for a schematic diagram of an active back-end termination circuit in accordance with a third preferred embodiment of the present invention, the active back-end termination circuit  1  is connected to an output stage circuit  2  to match a load. If the load is a laser diode (LD) in 25 ohm driving system, and four sets of active back-end termination circuits may be used to match the laser diode. 
         [0049]    With reference to  FIG. 10  for a Smith chart diagram of an active back-end termination circuit in accordance with a third preferred embodiment of the present invention, different operating frequencies such as 1 GHz, 5 GHz, 6.04 GHz, 7.09 GHz, 8.07 GHz, 9.05 GHz and 10.39 GHz give the impedance values of 30.6945 ohms, 29.7445 ohms, 29.2975 ohms, 28.8262 ohms, 28.3755 ohms, 27.9182 ohms and 27.2871 ohms respectively, and their impedance values are very close. Therefore, the active back-end termination circuit of the present invention may provide an impedance matching. 
         [0050]    With reference to  FIG. 11  for a schematic diagram of an active back-end termination circuit in accordance with a fourth preferred embodiment of the present invention, the active back-end termination circuit  1  is connected to the output stage circuit  2  to match a load. If the load is an elector-absorption modulated laser (EML) in 50 ohm driving system, and two sets of active back-end termination circuits may be used for matching the elector-absorption modulated laser. 
         [0051]    With reference to  FIG. 12  for a Smith chart diagram of an active back-end termination circuit in accordance with a fourth preferred embodiment of the present invention, different operating frequencies such as 1 GHz, 5 GHz, 6.04 GHz, 7.09 GHz, 8.07 GHz, 9.05 GHz and 10.39 GHz give impedance values of 61.2828 ohms, 56.7781 ohms, 54.8778 ohms, 52.9571 ohms, 51.1932 ohms, 49.4708 ohms and 47.1961 ohms respectively, and the values are very close at different frequencies, and thus the active back-end termination circuit of the present invention may provide an impedance matching. 
         [0052]    In the present invention, the number of active back-end termination circuits may be adjusted according to the load, and the load may be a 25 ohm, 50 ohm, 75 ohm or 100 ohm driving system, but the present invention is not limited to such arrangements only. 
         [0053]    With reference to  FIG. 13  for a schematic diagram of another active back-end termination circuit in accordance with the present invention, the active back-end termination circuit  3  comprises a first matching unit  31  and a second matching unit  32 . The first matching unit  31  comprises a first resistor  311  and a first transistor  312 . The first resistor  311  comprises a first terminal  3111  and a second terminal  3112 . The first transistor  312  comprises a first source, a first gate and a first drain, and the first drain is connected to the second terminal  3112 , such that the first resistor  311  and the first transistor  312  are connected in series, and the first source is connected to a working voltage V TT , and the first gate is connected to an external voltage source. 
         [0054]    The second matching unit  32  comprises a second resistor  321  and a second transistor  322 . The second resistor  321  comprises a third terminal  3211  and a fourth terminal  3212 . The second transistor  322  comprises a second source, a second gate and a second drain, and the second source is connected to the fourth terminal  3212 , such that the second resistor  321  and the second transistor  322  are connected in series, and the second drain is connected to a working voltage V TT , and the second gate is connected to an external voltage source. 
         [0055]    The first terminal  3111  and the third terminal  3211  are connected to a circuit, such that the first matching unit  31  and the second matching unit  32  are connected in parallel to the circuit. If the first terminal  3111  and the third terminal  3211  are connected to a voltage source, the first transistor  312  or the second transistor  322  is swithed on according to the voltage source and the external voltage source. If the first transistor  312  and second transistor  322  are switched on within different bias voltages, the resistance values will be different, and thus the first transistor  312  and the second transistor  322  may be considered as variable resistors for actively matching an impedance of a load, and the load may be a laser diode (LD) or an elector-absorption modulated laser (EML). 
         [0056]    The voltage source may be a periodic wave voltage source, such as a sine wave voltage source, a square wave voltage source or a triangle wave voltage source for outputting a voltage with output high level (VOH) or a voltage with output low level (VOL) by defining the working voltage V TT  as an amplitude origin. If the voltage source is situated at a period of a positive half cycle, the voltage source will drive the first matching unit  31 . If the voltage source is situated at a period of a negative half cycle, the voltage source will drive the second matching unit  32 . If the voltage source is situated at a working voltage V TT , one end of the circuit has the same electric potential of the working voltage V TT , and thus there will be no loss of DC current. 
         [0057]    The active back-end termination circuit  3  may also be connected to the output stage circuit  2  as shown in  FIGS. 9 and 11 . Now, the active back-end termination circuit  3  replaces the active back-end termination unit  1 . The external voltage source is used and connected to the first gate of the first transistor  312  and the second gate of the second transistor  322 , and the first transistor  312  and the second transistor  322  generate corresponding impedance values at different bias voltages for actively matching the impedance of the load. 
         [0058]    With reference to  FIG. 14  for an eye diagram of a conventional passive back-end termination circuit in 25-ohm driving system and  FIG. 15  for an eye diagram of a DC-coupled active back-end termination circuit in 25 ohm driving system in accordance with the present invention, the amplitude of the prior art ranges from −0.55 to 0.55, and the amplitude of the present invention ranges from −0.84 to 0.83. Obviously, the range of amplitudes of the present invention is greater than the range of amplitudes of the prior art, indicating that the present invention has a wider range of operating voltages to achieve a better driving efficiency and provide a better waveform quality. 
         [0059]    With reference to  FIG. 16  for an eye diagram of a conventional passive back-end termination circuit in 50 ohm driving system, and  FIG. 17  for an eye diagram of an active DC-coupled back-end termination circuit in 50 ohm driving system in accordance with the present invention respectively, the range of amplitudes of the prior art is from −1.159 to 1.053, and the range of amplitudes of the present invention is from −1.429 to 1.481. Obviously, the range of amplitudes of the present invention is greater than the range of amplitudes of the prior art, indicating that the invention has a wider range of operating voltages to achieve a better driving efficiency and provide a better waveform quality. 
         [0060]    While the invention has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope and spirit of the invention set forth in the claims.