Abstract:
A driving circuit of an optical modulator using cascode output structure including transistors to output an output voltage uniformly shared between the cascode transistors. Thus, the driving circuit outputs high driving voltage exceeding the toleration voltage of single transistor.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to a driving circuit for an optical modulator including a cascode of electrical elements, wherein output voltage is uniformly shared between the transistors of the cascode electrical elements to increase the operating range of the output voltage. 
   2. Description of the Related Art 
   Optical transmission systems with high capacity are widely implemented. To meet requirements of high speed and long distance communication, optical transmitter employing continuous-wave (CW) laser diode operating with a high-speed optical modulator is crucial. The driving circuit of the optical modulator is typically manufactured by an integrated circuit process with high breakdown voltage to output high voltage between about 3˜10 volts, thus increasing manufacturing cost. Hence, a method for forming the high-voltage driver with a low-voltage integrated circuit process is necessary. 
     FIG. 1  is a circuit diagram of a conventional driving circuit. The conventional driving circuit comprises transistors  10  and  11  and a current source I m . Current source I m  is directed to flow through the transistors  10  or  11  toward the output terminals Vo 1  or Vo 2  by the differential signals respectively input to the input terminals S 1  and S 2 . The output voltages are output completely across the transistor  10  or  11 . Thus, the output voltage value is limited by the breakdown characteristics of the transistors  10  and  11 . 
     FIG. 2  is a circuit diagram of a conventional driving circuit comprising a cascode of transistors. The high voltage output terminal of the driving circuit comprises a plurality of transistors, wherein the second transistor  12 , the third transistor  13 , the fourth transistor  14  and the fifth transistor  15  comprise a cascode structure for sharing the high output voltage. The driving circuit however requires an extra source-follower to couple partial output voltage to the gate of the third transistor  13 . Thus, power consumption is increased and the increment of the gate delay deteriorates the high frequency output characteristics. 
   To solve the disadvantages of the conventional driving circuit, the present invention provides a driving circuit for outputting high voltage. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is thus to provide a driving circuit for an optical modulator including a cascode of output structures, wherein output voltage is uniformly shared between the transistors of the cascode structures to increase operating range of the output voltage. 
   In addition, output voltage of the driving circuit is uniformly shared between the cascode transistors by using feedback voltage provided by passive components, such as resistors and capacitors. Thus, a damage to the output transistors due to output voltage exceeding a voltage tolerance range is prevented. Additionally, output voltage is doubled by the cascode of output stages without influencing the circuit operation. 
   Moreover, a base control signal of the upper cascode transistors is generated by a gain unit and the value of the high frequency signal fed back from the output terminal is controlled by adjusting the capacitance ratio of two capacitors to obtain an optimized output waveform. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention. 
       FIG. 1  is a circuit diagram of a conventional driving circuit. 
       FIG. 2  is a circuit diagram of another conventional driving circuit. 
       FIG. 3  is a circuit diagram of a driving circuit according to first embodiment of the present invention. 
       FIG. 4  is a circuit diagram of a driving circuit according to second embodiment of the present invention. 
       FIG. 5  is a circuit diagram of a driving circuit according to third embodiment of the present invention. 
       FIG. 6  is a circuit diagram of a driving circuit according to fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a circuit diagram of a driving circuit according to first embodiment of the present invention. 
   The driving circuit according to the first embodiment of the present invention comprises input terminals S 1  and S 2 . First transistor T 1  is connected to input terminal S 1 , and second transistor T 2  is connected to input terminal S 2 . In addition, the emitters of first transistor T 1  and second transistor T 2  are connected and are further connected to ground through current source I m . 
   Additionally, first transistor T 1  is connected to third transistor T 3  in serial, and the connection point of first transistor T 1  and third transistor T 3  is connected to first current source I 1 . Second transistor T 2  is connected to fourth transistor T 4  in serial, and the connection point of second transistor T 2  and fourth transistor T 4  is connected to second current source I 2 . First current source I 1  and second current source I 2  respectively provide bias current of third transistor T 3  and fourth transistor T 4  to decrease switching time of transistors and avoid output waveform distortion. 
   In addition, third transistor T 3  connected to first transistor T 1  and first current source I 1  is further connected to first feedback circuit  20 . First feedback circuit  20  comprises a low frequency feedback circuit and a high frequency feedback circuit. The low frequency feedback circuit comprises a first resistor R 1  and a second resistor R 2 . A feedback signal with lower frequency is controlled by the ratio of first resistor R 1  to second resistor R 2 . The high frequency feedback circuit comprises an equivalent collector-base capacitor Ccb 1  of third transistor T 3  and first capacitor C 1 . A feedback signal with higher frequency is controlled by the ratio of collector-base capacitor Ccb 1  to first capacitor C 1 . 
   Fourth transistor T 4  connected to second transistor T 2  and second current source I 2  is further connected to second feedback circuit  21 . Second feedback circuit  21  comprises a low frequency feedback circuit and a high frequency feedback circuit as first feedback circuit  20 . The low frequency feedback circuit comprises a third resistor R 3  and a fourth resistor R 4 . A feedback signal with lower frequency is controlled by the ratio of third resistor R 3  to fourth resistor R 4 . The high frequency feedback circuit comprises an equivalent collector-base capacitor Ccb 2  of fourth transistor T 4  and second capacitor C 2 . A feedback signal with higher frequency is controlled by the ratio of collector-base capacitor Ccb 2  to second capacitor C 2 . Second resistor R 2  and fourth resistor R 4  are all connected to reference voltage VT 1 . Additionally, the cascode resistors of feedback circuits  20  and  21  can also act as back termination resistors to improve impedance matching. 
   The principal design spirit of the present invention is not limited by the first embodiment. Thus, other high voltage driving circuits with a cascode of structures are provided by other embodiments. 
     FIG. 4  is a circuit diagram of a driving circuit according to second embodiment of the present invention. The differences between first and second embodiments are described in the following. Third transistor T 3  is further connected to fifth transistor T 5  to form a cascode of structures with more stages. 
   Fifth transistor T 5  connected to third transistor T 3  is further connected to third feedback circuit  22 . Third feedback circuit  22  comprises a low frequency feedback circuit and a high frequency feedback circuit as feedback circuits  20  and  21 . A feedback signal with lower frequency is controlled by the ratio of fifth resistor R 5  to sixth resistor R 6 , and a feedback signal with higher frequency is controlled by the ratio of equivalent collector-base capacitor Ccb 3  of fifth transistor T 5  to third capacitor C 3 . 
   Fourth transistor T 4  is further connected to sixth transistor T 6  to form a cascode of structures with more stages. Sixth transistor T 6  connected to fourth transistor T 4  is further connected to fourth feedback circuit  23  comprising a low frequency feedback circuit and a high frequency feedback circuit. A feedback signal with lower frequency is controlled by the ratio of seventh resistor R 7  to eighth resistor R 8 , and a feedback signal with higher frequency is controlled by the ratio of equivalent collector-base capacitor Ccb 4  of sixth transistor T 6  to fourth capacitor C 4 . 
   Sixth resistor R 6  and eighth resistor R 8  are all connected to reference voltage VT 2 . The cascode resistors of feedback circuits  22  and  23  further act as back termination resistors to improve impedance matching. 
   The feature of the second embodiment of the present invention is the triple output voltage using a cascode of structures according to the design spirit of the present invention. 
     FIG. 5  is a circuit diagram of a driving circuit according to third embodiment of the present invention. The difference between second and third embodiments is first resistor R 1  of first feedback circuit  20  and third resistor R 3  of second feedback circuit  21  are respectively connected to output terminals Vo 1  and Vo 2 . Thus, low frequency feedback circuits of third feedback circuit  22  and first feedback circuit  20  are connected, and low frequency feedback circuits of fourth feedback circuit  23  and second feedback circuit  21  are connected. Other circuitry connections and operation are the same as in the second embodiment. 
   Moreover, the cascode resistors of feedback circuit  20  and  21  in first embodiment can be replaced by a voltage gain circuit. 
     FIG. 6  is a circuit diagram of a driving circuit according to fourth embodiment of the present invention. The emitters of first transistor T 1  and second transistor T 2  are connected to a first reference current source I 10 . Additionally, first transistor T 1  is connected to first input terminal S 1  and first resistor R 1 , second transistor T 2  is connected to second input terminal S 2  and second resistor R 2 , and resistors R 1  and R 2  are respectively connected to reference voltage VT 1 . 
   Input terminals S 1  and S 2  are further connected to third resistor T 3  and fourth resistor T 4 . The emitters of transistors T 3  and T 4  are connected to a second reference current source  111 . In addition, third transistor T 3  is connected to first current source I 1  and connected to fifth transistor T 5  in serial, and fourth transistor T 4  is connected to second current source I 2  and connected to sixth transistor T 6  in serial. Fifth transistor T 5  is connected to first current I 1 , and first current I 1  is connected to the connection point of third transistor T 3  and fifth transistor T 5 . The base of fifth transistor T 5  is connected to first capacitor C 1  and first resistor R 1 . A first high frequency feedback circuit comprises collector-base capacitor Ccb 1  of fifth transistor T 5  and first capacitor C 1 . In addition, sixth transistor T 6  is connected to second current source I 2 , and second current  12  is connected to the connection point of fourth transistor T 4  and sixth transistor T 6 . The base of sixth transistor T 6  is connected to second capacitor C 2  and second resistor R 2 . A second high frequency feedback circuit comprises collector-base capacitor Ccb 2  of sixth transistor T 6  and second capacitor C 2 . 
   Similar with first embodiment, the high frequency signal fed back from the output terminal is controlled by adjusting the capacitance ratio of first capacitor C 1  and collector-base capacitor Ccb 1 , and that of second capacitor C 2  and collector-base capacitor Ccb 2  to obtain an optimized output waveform. In addition, resistor R 1  and R 2  comprise a resistor set connected to reference voltage VT 1 . 
   In the fourth embodiment, resistors R 1 , R 2 , R 3 , and R 4  are replaced by a voltage gain circuit to provide a low frequency base control signal, hence output voltage is shared between the transistors of the cascode structures. 
   According to the embodiments of the present invention, output voltage is uniformly shared and high output voltage is easily obtained by the cascode output stage comprising a plurality of transistors. 
   The foregoing description of the invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.