Parallel opto-electric structure for high sensitivity and wide bandwidth optical transceiver

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical transceiver, and more particularly to a receiver circuit for an optical transceiver having a plurality of photodiodes.

[0004] 2. Description of the Related Art

[0005] An optical receiver converts optical pulses to corresponding electrical signals. The optical receiver in an infrared (IR) optical transceiver typically utilizes an IR-sensitive PIN diode, which emits an electric current in proportion to the intensity of infrared light striking the active area of the diode.

[0006] For example, FIG. 1 illustrates the general structure of a conventional optical receiver circuit for an IR transceiver. A PIN diode D has its anode connected to the power Supply V_ddto create a bias voltage across the PIN diode D, and its cathode connected to the inverting input of an operational amplifier AMP. A feedback resistor R_fis connected between the output and the inverting input, so that the operational amplifier AMP operates as a transimpedance amplifier, in that its output voltage V_ois proportional to the input current I_pfrom the PIN diode:

[0007] V_o=−I_p*R_f  (1)

[0008] I_p=P_op*A*RE  (2)

[0009] where P_opis the incident optical power intensity, A is the active area of the PIN diode, and RE is the coefficient of responsiveness of the PIN diode.

[0010] In equation (1) a negative sign is added to I_p, because if I_pflows from V_dd(positive) to the inverting input of the operational amplifier AMP, the output voltage V_owill be negative. Substituting equation (2) into equation (1) yields:

[0011] V_o=−P_op*A*RE*R_f  (3)

[0012] The coefficient of responsiveness RE is determined by the active material used in the PIN diode. The wavelength to which the PIN diode is sensitive will dictate the material used in the diode. Therefore, once the wavelength is determined, RE is fixed and can be considered as a constant.

[0013] The design of any conventional IR receiver suffers from PIN diode parasitic capacitance when both high sensitivity and wide bandwidth are required. Generally, the bandwidth of the transimpedance amplifier itself can be very wide, but the total bandwidth of the front edge stage is limited by the single pole RC low pass filter formed by the input capacitance C_inof the operational amplifier AMP and the resistance of the feedback resistor R_f. A single pole RC low pass filter has the following gain-frequency relation: $\begin{array}{cc}G\left(f\right)=\frac{G\left(0\right)}{1+j*2*\pi *f*R*C_1}& \left(4\right)\end{array}$

[0014] where G(f) is the gain as a function of frequency, G(0) is the DC gain, f is the frequency, R is the resistance, C is the capacitance, and j is the imaginary unit.

[0015] The bandwidth BW of a single pole RC low pass filter is: $\begin{array}{cc}\mathrm{BW}=\frac{1}{2*\pi *R*C}& \left(5\right)\end{array}$

[0016] Applying equation (5), the bandwidth of the transimpedance amplifier described above can be expressed as: $\begin{array}{cc}{\mathrm{BW}}_t=\frac{1}{2*\pi *R_{\mathrm{in}}*C_{\mathrm{in}}}& \left(6\right)\end{array}$

[0017] where BW_tis the transimpedance amplifier bandwidth, C_inis the capacitance on the transimpedance amplifier inverting input, R_inis the input resistance of the transimpedance amplifier, R_in=R_f/G_open, and G_openis the open loop gain of the transimpedance amplifier.

[0018] The input capacitance C_inincludes PIN diode parasitic capacitance C_pand the operational amplifier input capacitance C_e. Based on current VLSI technology, C_ecan be much smaller than C_p(i.e., C_p>>C_e), so C_ecan be ignored and effectively

[0019] C_in=C_p  (7)

[0020] The parasitic capacitance C_pof a PIN diode is proportional to the PIN diode active area:

[0021] C_p=K*A  (8)

[0022] where K is a coefficient, and A is the PIN active area.

[0023] Substituting equations (7) and (8) into equation (6) yields:

[0024] BW_t=1/(2*π*R_in*K*A)  (9)

[0025] When a high sensitivity is required, a large active area A is needed to get more optical power. However, because the bandwidth BW_tis inversely proportional to the active area A, an increase of the area A results in a corresponding decrease in the bandwidth BW_tof the circuit. It can be seen from equations (3) and (9) that it is very difficult to achieve both high sensitivity and a wide bandwidth in a single large area PIN diode.

[0026] In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide an optical receiver circuit which replaces a single PIN diode having a large active area with a plurality of (e.g., preferably equal) smaller PIN diodes, each PIN diode being associated with a dedicated element transimpedance amplifier, the outputs of the element transimpedance amplifiers being connected to a summing amplifier which sums the voltages output from the element transimpedance amplifiers.

[0027] The parallel structure of the optical receiver circuit of the present invention provides the same output voltage as a single PIN diode having a large active area comparable to the sum of the active areas of the smaller PIN diodes in the optical receiver circuit of the present invention, and thus can achieve the same high sensitivity as the single PIN diode having a large active area. However, the summing amplifier can be designed to have a much wider bandwidth than that of the element transimpedance amplifiers. Since the total bandwidth of the optical receiver circuit depends on the bandwidth of the summed voltages at the input of the summing amplifier, the total bandwidth of the circuit is the sum of the bandwidths of the individual transimpedance amplifiers, which is much greater than that of a receiver circuit utilizing a single PIN diode with a comparable active area.

[0028] In a first aspect, the present invention provides an optical receiver circuit, including a plurality of element transimpedance amplifiers and a summing amplifier, the outputs of the element transimpedance amplifiers being connected through resistance elements to an input of the summing amplifier. When a photodiode connected to an input of each element transimpedance amplifier emits a current signal responsive to an optical signal striking the photodiode, a voltage is produced at the output of each element transimpedance amplifier proportional to an intensity of the optical signal, and the voltages output by the element transimpedance amplifiers are summed by the summing amplifier to produce an output electrical signal.

[0029] In another aspect, the present invention further provides a photoelectric transceiver having an optical receiver circuit including a plurality of element transimpedance amplifiers, a summing amplifier, the outputs of the element transimpedance amplifiers being connected through resistance elements to an input of the summing amplifier, and a photodiode connected to an input of each element transimpedance amplifier. When the photodiode connected to the input of each element transimpedance amplifier emits a current signal responsive to an optical signal striking the photodiode, a voltage is produced at the output of each element transimpedance amplifier proportional to an intensity of the optical signal, and the voltages output by the element transimpedance amplifiers are summed by the summing amplifier to produce an output electrical signal.

[0030] In a further aspect of the present invention, each transimpedance amplifier includes an operational amplifier having a resistance element connected between an output and an inverting input of the operational amplifier, each photodiode is connected to an inverting input of the respective element transimpedance amplifier and the outputs of the element transimpedance amplifiers are connected to an inverting input of the summing amplifier through an input resistor.

[0031] In a further aspect of the present invention, the optical signal includes an infrared signal and the photodiodes include PIN diodes.

[0032] The present disclosure relates to subject matter contained in Canadian Patent Application No. 2,311,433 filed Jun. 13, 2000, which is expressly incorporated herein by reference in its entirety.

[0033] The foregoing and other purposes, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

[0034] FIG. 1 is a schematic diagram of a conventional optical receiver circuit; and

[0035] FIG. 2 is a schematic diagram of an optical receiver circuit according to the present invention.

[0036] FIG. 2 illustrates a preferred embodiment of an optical receiver circuit according to the invention.

[0037] In FIG. 2, a plurality of photodiodes (e.g., in the preferred embodiment PIN diodes P₁, P₂. . . P_N) have their anodes connected to a voltage supply V_ddto supply a bias voltage across the diodes P₁, P₂. . . P_N. The cathode of a first PIN diode P₁is connected to the inverting input of a first element transimpedance amplifier (unreferenced), including an operational amplifier AMP₁having a resistance element such as feedback resistor R₁with a resistance R_f1connecting the output of the operational amplifier AMP₁to the inverting input of the operational amplifier AMP₁.

[0038] Similarly, the cathode of a second PIN diode P₂is connected to the inverting input of a second element transimpedance amplifier (unreferenced) including an operational amplifier AMP₂having a feedback resistor R₂with a resistance R_f2connecting the output of the operational amplifier AMP₂to the inverting input of the operational amplifier AMP₂. The construction is similar up to an Nth PIN diode P_Nwhich is similarly connected to an Nth element transimpedance amplifier including an operational amplifier AMP_Nhaving a feedback resistor R_Nwith a resistance R_fNconnecting the output of the operational amplifier AMP_Nto the inverting input of the operational amplifier AMP_N.

[0039] In the preferred, non-limiting embodiment shown, the non-inverting input of each operational amplifier AMP₁, AMP₂. . . AMP_Nis grounded, creating a constant voltage difference between V_ddand the respective inverting input of each element transimpedance amplifier when the PIN diodes are active.

[0040] The outputs V_o1, V_o2. . . V_oNof the element transimpedance amplifiers are each connected to the inverting input of a summing amplifier. The summing amplifier includes an operational amplifier AMP_Shaving a feedback resistor R_fSconnecting the output of the operational amplifier AMP_Sto the inverting input of the operational amplifier AMP_S. Resistance elements such as resistors 10, 12, 14 are the input resistors of the summing amplifier. Such resistors have the same value of R and the ratio of R_sf/r determines the gain of the summing amplifier. The output signal V_oSfrom the summing amplifier is fed to an IR receiver circuit for processing.

[0041] In operation, when optical radiation strikes the PIN diodes P₁, P₂. . . P_Neach PIN diode emits a current proportional to the optical power intensity of the incident optical light striking the active area of each diode. Each respective voltage V_o1, V_o2. . . V_oNoutput by the element operational amplifiers AMP₁, AMP₂. . . AMP_Nis proportional to the current I_p1, I_p2. . . I_pNfrom the respective PIN diode P₁, P₂. . . P_N. Since in the preferred embodiment all PIN diodes are identical and all element transimpedance amplifiers are identical, the element transimpedance amplifier output voltages V_o1, V_o2. . . V_oNare equal.

[0042] The element transimpedance amplifier output signals V_o1, V_o2. . . V_oNare transmitted to the inverting input of the summing amplifier AMP_Sthrough resistors 10, 12, 14. Thus, the output voltage V_oSfrom the summing amplifier is proportional to the current I_totalat the inverting input of the summing amplifier AMP_oS, represented by V_o1*R_fS/R+V_o2*R_fs/R+ . . . +V_on*R_fs/R.

[0043] Thus, according to the invention, a large area PIN diode is replaced with N (preferably but not necessarily equal) smaller PIN diodes cumulatively providing substantially the same active area as the large PIN diode, and each of the PIN diodes having its own dedicated element transimpedance amplifier. For each element transimpedance amplifier:

[0044] V_o1=P_op*RE*R_f1*A/N

[0045] V_o2=P_op*RE*R_f2*A/N

[0046] .

[0047] .

[0048] V_oN=P_op*RE*R_fN*A/N

[0049] AMP_oSis a general operational amplifier which sums all of the output signals V_o1, V_o2. . . V_oNfrom the element transimpedance amplifiers. Where all element transimpedance amplifier feedback resistors R_f1, R_f2. . . R_fnare equal and R_fs=R, the output V_oSis: $\begin{array}{cc}\begin{array}{c}{V}_{\mathrm{oS}}=V_{\mathrm{o1}}+V_{\mathrm{o2}}+\dots +V_{\mathrm{oN}}\\ =N*\left(P_{\mathrm{op}}*\mathrm{RE}*R_f*A/N\right)\\ =P_{\mathrm{op}}*\mathrm{RE}*R_f*A\end{array}& \left(10\right)\end{array}$

[0050] From a comparison of equations (3) and (10), it will be apparent that the parallel structure according to the invention has the same output voltage value (i.e., the negative sign can be ignored) as that of a single large area PIN diode optical receiver circuit in which the diode has an active area equal to the sum of the active areas of the smaller diodes P₁, P₂. . . P_N. Thus, the parallel structure according to the invention achieves the same high sensitivity as the single large area PIN diode optical receiver.

[0051] Applying equations (4) and (9) to each element transimpedance amplifier yields: $\begin{array}{cc}{G}_i\left(f\right)=\frac{G_i\left(0\right)}{1+j*2*\pi *R_{\mathrm{in}}*K*A/N}& \left(11\right)\\ {\mathrm{BW}}_i=\frac{1}{2*\pi *R_{\mathrm{in}}K*A/N}=\frac{N}{2*\pi *R_{\mathrm{in}}*K*A}& \left(12\right)\end{array}$

[0052] where i=1, 2 . . . N.

[0053] Thus, the bandwidth of each element transimpedance amplifier is N times that of the bandwidth of the single large area PIN diode optical receiver.

[0054] According to the general definition of gain, each element transimpedance amplifier has the following relation among gain, input current and output voltage:

[0055] G_i(f)=V_oi/I_pior V_oi=G_i(f)*I_pi

[0056] where i=1, 2, 3 . . . N.

[0057] For the output signal V_oS $\begin{array}{c}{V}_{\mathrm{oS}}=V_{01}+V_{02}+V_{03}+\dots +V_{0N}\\ =G_i\left(f\right)*\left(I_{\mathrm{p1}}+I_{\mathrm{p2}}+I_{\mathrm{p3}}+\dots +I_{\mathrm{pN}}\right)\\ =G_i\left(f\right)*I_{\mathrm{total}}\end{array}$

[0058] The output voltage has a gain G(f)

[0059] G(f)=V_oS/I_total=G_i(f)  (13)

[0060] Equation (13) shows that the parallel element transimpedance amplifier structure in the optical receiver circuit of the invention has the same gain-frequency relationship as that of each element transimpedance amplifier. Thus, the bandwidth of the parallel structure of the circuit of the present invention is the same as the bandwidth of each element transimpedance amplifier, which is N times the bandwidth of the single large area PIN diode optical receiver in which the PIN diode has a large active area substantially equal to the sum of the active areas of the PIN diodes P₁, P₂. . . P_N.

[0061] Thus, the optical receiver circuit according to the present invention has the same sensitivity as the single large area PIN diode optical receiver, but N times the bandwidth of the single large area PIN diode optical receiver in which the PIN diode has an active area comparable to the sum of the active areas of the PIN diodes P₁, P₂. . . P_N. Hence, the optical receiver circuit of the present invention achieves both high sensitivity and a wide bandwidth.

[0062] While a preferred embodiment of the invention has been thus described by way of example, it will be apparent to those skilled in the art that certain modifications and adaptations may be made without departing from the scope of the invention. The invention includes all such modifications and adaptations as fall within the scope of the claims.