Abstract:
An optical receiver. The optical receiver includes a photodiode, a differential transimpedance amplifier, a transistor, and a current source. When the photodiode receives an optical signal, a current signal transmitted from a cathode of the photodiode to an anode thereof is generated. Two input terminals of the differential transimpedance amplifier couple the current signal, and the differential transimpedance amplifier converts the current signal to a voltage signal. In addition, voltage variation of the cathode is coupled to the anode through a voltage follower composed by the transistor and the current source. As a result, voltage of the cathode and that of the anode vary in phase, effectively decreasing a value of the photodiode parasitic capacitance and improving operating bandwidth.

Description:
This Non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 093102746 filed in Taiwan, Republic of China on Feb. 6, 2004, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND 
   The invention relates to an optical receiver, and in particular to an optical receiver comprising a voltage follower. 
     FIG. 1  is a circuit diagram of a conventional optical receiver with a differential current-sensing transimpedance amplifier. As shown in  FIG. 1 , a differential transimpedance amplifier T 1  comprises resistors R 10  and R 11  and a differential amplifier OP 1 . A cathode of a photodiode D 1  is coupled to a noninverting input terminal IN+, and an anode thereof is coupled to an inverting input terminal IN−. The resistor R 10  is coupled between the noninverting input terminal IN+ and an inverting output OUT−, and the resistor R 11  is coupled between the inverting input terminal IN− and a noninverting output terminal OUT+. While receiving an optical signal, the photodiode D 1  generates a current signal I 1  and the current signal I 1  is converted into a voltage signal Vout 1  by the following differential transimpedance amplifier T 1 . Compared with a conventional single-ended transimpedance amplifier, such as a common-cathode transimpedance amplifier or a common-anode transimpedance amplifier, the transimpedance gain and the signal-to-noise ratio (SNR) of the differential current-sensing transimpedance amplifier T 1  are increased by 6 dB and 3 dB respectively. Therefore a better receive sensitivity can be achieved theoretically. 
   In the optical receiver of  FIG. 1 , however, the current signal I 1  transmits from the noninverting input terminal IN+ to the cathode of the photodiode D 1  and then transmits from the anode thereof to the inverting input terminal IN−. Thus the voltage signals at the anode and the cathode of the photodiode D 1  are out of phase. The resulting large differential voltage across the photodiode D 1  leads to a large transient current component required for charging and discharging the photodiode parasitic capacitance Cd 1 . Therefore, both the operating bandwidth and the transimpedance gain of the optical receiver are strongly limited by the photodiode parasitic capacitance Cd 1 . It is assumed that the equivalent input resistance of each input terminal of the differential transimdepance amplifier T 1  is rin10 and an ideal differential transimpedance amplifier is used, the operating bandwidth of the optical receiver can be represented by the following equation: 
   
     
       
         
           B1 
           = 
           
             1 
             
               2 
               ⁢ 
               
                 π 
                 ⁡ 
                 
                   ( 
                   
                     2 
                     ⁢ 
                     
                       rin10 
                       · 
                       cd1 
                     
                   
                   ) 
                 
               
             
           
         
       
     
   
   wherein, B 1  is the operating bandwidth of the optical receiver, cd 1  is a value of the photodiode parasitic capacitance Cd 1 . 
   According to the above equation, the operating bandwidth of the differential-receiving optical receiver is reduced to one half that using a single-ended transimpedance amplifier. The bandwidth shrinkage is due to the out-of-phase relationship between the voltage signals at the two terminals of the photodiode D 1 . If the differential voltage signal across the photodiode D 1  can be reduced, then the undesirable transient effect due to the photodiode parasitic capacitance Cd 1  will be significantly suppressed. Moreover, in the circuitry of the optical receiver in  FIG. 1 , since an appropriate reverse bias cannot be provided to the photodiode D 1 , the optical receiver is not suitable for applications demanding high transmission rate and wide dynamic range. 
   SUMMARY 
   Accordingly, embodiments of the invention provide an optical receiver that ameliorates disadvantages of the related art. 
   Accordingly, the invention provides an optical receiver comprising a photodiode, a differential transimdepance amplifier, a transistor, and a first current source. The photodiode has a first terminal and a second terminal and generates a current signal transmitted from the first terminal to the second terminal while receiving an optical signal. The differential transimdepance amplifier has a first input terminal coupled to the first terminal and a second input terminal and converts the current signal to a first voltage signal. The transistor has a control terminal coupled to the first terminal, a third terminal coupled to the second terminal, and a fourth terminal coupled to the second input terminal. The first current source is coupled between the third terminal and a ground. When the photodiode receives the optical signal, the current signal is coupled to the first input terminal directly and to the second input terminal through the transistor. The transistor couples a voltage variation of the first terminal to the second terminal, such that voltage signals of the first and the second terminals vary in phase. 
   Embodiments of the invention further provide an optical receiver comprising a photodiode, a differential transimdepance amplifier, a transistor, and a first current source. The photodiode has a first terminal and a second terminal and generates a current signal transmitted from the first terminal to the second terminal when receiving an optical signal. The differential transimdepance amplifier has a first input terminal and a second input terminal coupled to the second terminal and converts the current signal to a first voltage signal. The transistor has a control terminal coupled to the second terminal, a third terminal coupled to the first terminal, and a fourth terminal coupled to the first input terminal. The first current source is coupled between the third terminal and a voltage source. When the photodiode receives the optical signal, the current signal is coupled to the second input terminal directly and to the first input terminal through the transistor, and the transistor couples a voltage variation of the second terminal to the first terminal, such that voltage signals of the first and the second terminals vary in phase. 
   A detailed description is given in the following embodiments with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various aspects of embodiments of the invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
       FIG. 1  is a circuit diagram of a conventional optical receiver with a differential-sensing transimpedance amplifier. 
       FIG. 2  is a circuit diagram of an optical receiver according to a first embodiment of the invention. 
       FIG. 3  is a circuit diagram of an optical receiver according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   First Embodiment 
     FIG. 2  is a circuit diagram of an optical receiver according to a first embodiment of the invention. The optical receiver  2  comprises a photodiode D 2 , a differential transimpedance amplifier T 2 , an NMOS transistor Tx 2 , and current sources SC 20  and SC 21 . The differential transimpedance amplifier T 2  comprises resistors R 20  and R 21  and a differential amplifier OP 2 . The current sources SC 20  and SC 21  provide bias current for the transistor Tx 2 . The amplifier OP 2  has two input terminals coupled to a noninverting input terminal IN+ and an inverting input terminal IN− of the differential transimpedance amplifier T 2  respectively and two output terminals coupled to a noninverting output terminal OUT+ and an inverting output terminal OUT− thereof respectively. The photodiode D 2  has a parasitic capacitor Cd 2 . The resistor R 20  is coupled between the noninverting input terminal IN+ and the inverting output terminal OUT− of the differential transimpedance amplifier T 2 , and the resistor R 21  is coupled between the inverting input terminal IN− and the noninverting output terminal OUT+ thereof. The resistors R 20  and R 21  are substantially equal in value. The transistor Tx 2  has a gate coupled to a cathode Dc 2  of the photodiode D 2 , a source coupled to an anode Da 2  thereof, and a drain coupled to the inverting input terminal IN−. The cathode Dc 2  is directly coupled to the noninverting input terminal IN+, and the anode Da 2  is coupled to the inverting input terminal IN− through the transistor Tx 2 . The current source SC 20  is coupled between a ground GND and the source of the transistor Tx 2 , and the current source S 21  is coupled between a voltage source VR 2  and the drain of the transistor Tx 2 . 
   When receiving an optical signal, the photodiode D 2  generates a current signal I 2  transmitted from the cathode Dc 2  to the anode Da 2 . Thus a voltage signal SV 2  is generated at the cathode Dc 2 . A voltage follower comprising the transistor Tx 2  and the current source SC 20  couples the voltage signal SV 2  to the anode Da 2 . As a result, voltage signals of the cathode Dc 2  and the anode Da 2  vary in phase and the differential voltage signal across the photodiode parasitic capacitance Cd 2  is greatly reduced. Therefore, the negative effect of the photodiode parasitic capacitance Cd 2  on the operating bandwidth is suppressed. 
   The transistor Tx 2  serves as not only a voltage follower but also a unit-gain current buffer. The current I 2  is directly coupled to the noninverting input terminal IN+ and it is also coupled to the inverting input terminal IN− through the transistor Tx 2 . The differential transimpedance amplifier T 2  converts the current signal I 2  to a voltage signal Vout 2 . The voltage signal Vout 2  is equal to the voltage difference between the output terminals OUT− and OUT+, generated by the differential transimpedance amplifier T 2  according the current I 2 , and is provided to back-end devices for data decision. 
   In the first embodiment, since the cathode Dc 2  is coupled to the gate of the transistor Tx 2 , a voltage Vgs between the gate and the source thereof can serve as a reverse bias applied to the photodiode D 2 . Thus, the optical receiver  2  of this embodiment does not require an extra reverse-bias control circuit. 
   The effect of the grounded parasitic capacitance of the cathode Dc 2  and anode Da 2  are further described with reference to  FIG. 2 . The grounded parasitic capacitance of the cathode Dc 2  and the anode Da 2  are respectively represented as capacitors Cc 2  and Ca 2 . Charging/discharging electric charges of the cathode Dc 2  and the anode Da 2  to the ground GND are represented by the following equations:
 
 Qc=cc 2 ×ΔVc 
 
 Qa=ca 2 ×ΔVa 
 
   wherein, Qc and Qa are the charging/discharging electric charges of the cathode Dc 2  and the anode Da 2  to the ground GND, cc 2  and ca 2  are values of the capacitors Cc 2  and Ca 2 , ΔVc and ΔVa are variations of voltage signals of the cathode Dc 2  and the anode Da 2 , respectively. 
   According to the phase relationship between the voltage signals of the cathode Dc 2  and the anode Da 2 , the charging/discharging phenomena of the capacitor Cc 2  causes a bandwidth degradation at the cathode Dc 2 , while the charging/discharging phenomena of the capacitor Ca 2  causes a bandwidth enhancement at the cathode Da 2 . It is assumed that the values cc 2  and ca 2  of the capacitor Cc 2  and Ca 2  are equal to a value c 2 . Considering a differential current signal provided to the differential transimpedance amplifier T 2  through the cathode Dc 2  and the anode Da 2 , the differential charging/discharging electric charges generated by the capacitors Cc 2  and Ca 2  are represented by the following equation:
 
 Qdiff=Qc−Qa=c 2×(Δ Vc−ΔVa )=( c 2 ×i 2)/ Gm 
 
   wherein, Qdiff is the differential charging/discharging electric charges due to the capacitors Cc 2  and Ca 2 , i 2  is a value of the current signal I 2 , Gm is the transconductance of the transistor Tx 2 . 
   According to the above analysis, when Gm is increased, the differential charging/discharging electric charge Qdiff is decreased. Thus, the effect on the operating bandwidth of the optical receiver by the capacitors Cc 2  and Ca 2  is automatically cancelled. It leads to an enlarged tolerance against the grounded parasitic capacitance. 
   Second Embodiment 
     FIG. 3  is a circuit diagram of a second embodiment of an optical receiver according to the invention. The optical receiver  3  comprises a photodiode D 3 , a differential transimpedance amplifier T 3 , a PMOS transistor Tx 3 , and current sources SC 30  and SC 31 . The differential transimpedance amplifier T 3  comprises resistors R 30  and R 31  and a differential amplifier OP 3 . The current sources SC 30  and SC 31  provide bias current for the transistor Tx 3 . The photodiode D 3  has a parasitic capacitance Cd 3 . The amplifier OP 3  has two input terminals coupled to a noninverting input terminal IN+ and an inverting input terminal IN− of the differential transimpedance amplifier T 3  respectively and two output terminals coupled to a noninverting output terminal OUT+ and an inverting output terminal OUT− thereof respectively. The resistor R 30  is coupled between The noninverting input terminal IN+ and The inverting output terminal OUT− of the differential transimpedance amplifier T 3 , and the resistor R 31  is coupled between The inverting input terminal IN− and The noninverting output terminal OUT+ thereof. The resistors R 30  and R 31  are substantially equal in value. The transistor Tx 3  has a gate coupled to an anode Da 3  of the photodiode D 3 , a source coupled to a cathode Dc 3  thereof, and a drain coupled to the noninverting input terminal IN+. The anode Da 3  is directly coupled to the noninverting input terminal IN−, and the cathode Dc 3  is coupled to the inverting input terminal IN+ through the transistor Tx 3 . The current source SC 30  is coupled between a voltage source VR 3  and the source of the transistor Tx 3 , and the current source S 31  is coupled between a ground GND and the drain of the transistor Tx 3 . 
   When receiving an optical signal, the photodiode D 3  generates a current signal I 3  transmitted from the cathode Dc 3  to the anode Da 3 . Thus a voltage signal SV 3  is generated at the anode Da 3 . A voltage follower comprising the transistor Tx 3  and the current source SC 30  couples the voltage signal SV 3  to the cathode Dc 3 . As a result, voltage signals of the anode Da 3  and the cathode Dc 3  vary in phase and the differential voltage signal across the photodiode parasitic capacitance Cd 3  is greatly reduced. Therefore, the negative effect of the photodiode parasitic capacitance Cd 3  on the operating bandwidth is suppressed. 
   The transistor Tx 3  serves as not only a voltage follower but also a unit-gain current buffer. The current I 3  is directly coupled to the inverting input terminal IN− and it is also coupled to the noninverting input terminal IN+ through the transistor Tx 3 . The differential transimpedance amplifier T 3  converts the current signal I 3  to a voltage signal Vout 3 . The voltage signal Vout 3  is equal to the voltage difference between the output terminals OUT− and OUT+, generated by the differential transimpedance amplifier T 3  according the current I 3 , and is provided to back-end devices for data decision. 
   In the second embodiment, since the cathode Dc 3  is coupled to the source of the transistor Tx 3 , a voltage Vsg between the source and the gate thereof can serve as reverse bias applied to the photodiode D 2 . Thus, the optical receiver  3  of this embodiment does not require an extra reverse-bias control circuit. 
   Moreover, according to the previously described analysis of the first embodiment, the effect caused by the grounded parasitic capacitance of the cathode Dc 3  and anode Da 3  is automatically cancelled due to the in-phase relationship between the voltage signal of the cathode Dc 3  and anode Da 3 . 
   As previously described, in an optical receiver of the invention, the voltage signal of a cathode and an anode of a photodiode vary in phase, and the differential voltage across the photodiode is greatly reduced. Thus, the negative effect of the photodiode parasitic capacitance on the operating bandwidth is significantly suppressed. Moreover, due to in-phase relationship between the voltage signal of the cathode and anode of the photodiode, the effect on the operating bandwidth of the differential transimdepance amplifier by the grounded parasitic capacitance is automatically cancelled. 
   In the described embodiments, the transistor Tx 2  can be an NMOS transistor, an NPN bipolar transistor, or any other element with equivalent circuitry according to requirements, and the transistor Tx 3  can be a PMOS transistor, a PNP bipolar transistor, or any other element with equivalent circuitry according to requirements. 
   While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.