Conversion circuit from single phase signal to differential phase signal

[0001] 1. Field of the Invention

[0002] The present invention relates to a differential circuit, in particular, the invention relates to a differential circuit with suppressed output offset.

[0003] 2. Related Prior Art

[0004] A Japanese Patent Application published as JP-2001-320249A has disclosed a circuit that includes a trans-impedance amplifier (hereafter denoted as TIA) and a differential amplifier. The TIA converts a photocurrent into a voltage signal. The differential amplifier amplifies this voltage signal and converts the single phase signal to two signals complementary to each other. The output of the TIA is filtered and led to one of inputs of the differential amplifier.

[0005] When the transistors implemented within the differential amplifier show lesser breakdown characteristic, namely, the collector current of the transistor increases at high collector biases even when the base bias current is kept low, the complementary signals output from the differential amplifier vary an offset voltage thereof depending on the input current.

[0006] An aspect of the present invention relates to a circuit that converts a single phase signal into two signals complementary to the others. The circuit comprises a differential circuit and a reference generator. The differential circuit includes two units and a current source commonly connected to the units. Each of units includes a differential transistor, a cascode transistor and a load resistor connected in series. One of units receives the single phase signal in the differential transistor thereof, while, the other of units receives an average voltage obtained by filtering the single phase signal. The reference generator generates a reference voltage which has a difference with respect to the average voltage. The cascode transistor in respective units is commonly biased by the reference voltage. A feature of the present invention, the difference between the reference voltage and the average voltage is substantially constant independent of an intensity of the signal phase input; accordingly, the collector bias of the differential transistor in respective units, which is lowered from the reference voltage by the base bias of the cascode transistor, may be substantially in constant independent of the single phase input and of the fluctuation of the power supply.

[0018] First, a conventional example of an optical receiver will be described. FIG. 1 is a circuit diagram of the conventional conversion circuit 101 that comprises a differential circuit 10, a filter 12, and a reference generator 14. The input terminal IN of the differential circuit 10 receives an output Vin of the TIA 40 which includes an amplifier 42 and a trans-impedance R_TIA. The TIA 40 receives a photocurrent Iin output from a photodiode 50 and converts the photocurrent Iin into a voltage Vin which is provided to the input IN. The filter 12 includes a series circuit of a resistor Rave and a capacitor Cave between the input IN and the ground. The voltage Vin appeared in the input IN may be filtered by the filter 12 and an average voltage Vave is output from a node between the resistor Rave and the capacitor Cave. The reference generator 14 includes two resistors, R₃ and R₄, connected in series between the power supply Vcc and the ground, and a current source I₂. The reference generator 14 generates a reference voltage Vn₁ between two resistors, R₃ and R₄.

[0019] The differential circuit 10 includes first to fourth transistors, Q₁ to Q₄, two emitter resistors, R₁ and R₂, two load resistors, RL₁ and RL₂, and a current source I₁. The emitter of the first transistor Q₁ is grounded through a series circuit of the first emitter resistor R₁ and the current source I₁; the base thereof receives the input voltage Vin; and the collector thereof is connected to the emitter of the third transistor Q₃. The base of the third transistor Q₃ receives the reference voltage Vn₁, while, the collector receives the power supply yea through the first load resistor RL₁. The first output OUT₁ of the differential circuit 10 is extracted from the collector of the third transistor Q₃.

[0020] Similarly, the second transistor Q₂ is grounded in the emitter thereof through the series circuit of the second emitter resistor R₂ and the current source I₁; the base thereof receives the average voltage Vave of the filter 12; and the collector thereof is connected to the emitter of the fourth transistor Q₄. The base of the fourth transistor Q₄ receives the reference voltage Vn₁, while, the collector thereof is coupled with the power supply Vcc through the second load resistor RL₂. The other output OUT₂ of the differential circuit is extracted from the collector of the fourth transistor Q₄. In the circuit shown in FIG. 1, two transistors, Q₂ and Q₄, operate as a cascade transistor for respective driving transistors, Q₁ and Q₂.

[0021] Two outputs, OUT₁ and OUT₂, output signals, Vout₁ and Vout₂, complementary to the others. Because the base of the third transistor Q₃ and that of the fourth transistor Q₄ are commonly connected to the node N₁, which is the output of the reference generator 14, and receive the reference voltage Vn₁; and the emitters thereof are connected to respective collectors of the paired transistors, Q₁ and Q₂; accordingly, the collectors of the transistors, Q₁ and Q₂, are biased in a level decreased from the reference voltage Vn₁ by the base-emitter voltage Vbe of the transistors, Q₃ and Q₄.

[0022] Emitters of the transistors, Q₁ and Q₂, are commonly connected to the node N₂ through respective emitter resistors, R₁ and R₂. The emitter resistors, R₁ and R₂, may compensate an unbalanced state of the paired transistors, Q₁ and Q₂. That is, the base of the second transistor Q₂ receives the average voltage Vave of the filter 12, which is lowered from the input Vin by the bias current for the base of the second transistor Q₂ multiplied by the resistance of the resistor Rave. This input offset of the paired transistors, Q₁ and Q₂, may be compensated by differentiating the resistance of the emitter resistors, R₁ and R₂. Setting the resistance of the second resistor R₂ less than that of the first resistor R₁, the collector current Ic₂ of the second transistor Q₂ may be substantially equal to the collector current Ic₁ of the first transistor Q₁, which may set the output offset between two outputs, Vout₁ and Vout₂, complementary to the others. It is preferable to equalize the resistance of two load resistors, RL₁ and RL₂, not only for compensating the output offset but equalizing the voltage gain of the paired transistors, Q₁ and Q₂. Moreover, the desired output offset may be set by adjusting the balance of two emitter resistors, R₁ and R₂.

[0023] In the passive optical network system, which is often called as the PON system, the input optical level detected by the PD 50 widely diffuses depending on respective subscribers. Even in such cases, the differential circuit 10 may output signals, Vout₁ and Vout₂, complementary to each other by differentiating the input voltage signal Vin from the filtered average voltage Vave.

[0024] However, the circuit 101 fixes the voltage level Vn₁ at the node N₁ by the reference generator 14; accordingly, the paired transistors, Q₁ and ( ), are biased in the collector thereof by the voltage level Vn₁ lowered by the base-emitter voltage V_be of the transistors, Q₃ and Q₄.

[0025] FIG. 2 is a block diagram of an optical receiver circuit 110 that may be implemented with the conventional circuit 101 or a circuit 100 according to the present invention. The receiver circuit 110 includes the TIA 40, the circuit, 100 or 101, and buffers, 70 to 74. The TIA 40 receives the input Iin, while, the buffer 74 outputs signals, Dout₁ and Dout₂, which are complementary to each other. The circuit, 100 or 101, may provide a plurality of buffers, 70 to 74, in a downstream side thereof to ensure a necessary voltage gain. When the circuit 101 or 100, has an enough gain, the downstream buffers, 70 to 74, may decrease the number thereof, or may be fully omitted.

[0026] FIG. 3 shows behaviors of the collector bias, Vce₁ and Vce₂, of the paired transistors, Q₁ and Q₂, with respect to the input current Iin of the TIA 40. The collector biases, Vce₁ and Vce₂, widely vary as varying the input current Iin. For instance, as shown in FIG. 3, the collector biases, Vce₁ and Vce₂, vary by ΔVce for the input current Iin up to 3 mA. Because the inputs of the paired transistors, Q₁ and Q₂, are unbalanced as described above; specifically, the input of the transistor Q₂ is lowered by the resistor Rave and the base bias current flowing therein, the circuit, 100 or 101, may compensate this unbalance by adjusting the resistance of two emitter resistors, R₁ and R₂, so as to set the collector bias Vce₂ of the second transistor Q₂ slightly greater than that Vce₁ of the first transistor Q₁.

[0027] FIG. 5 shows the output offset voltage between two outputs, Vout₁ and Vout₂, with respect to the input current Iin of the TIA 40. In FIG. 5, filled circles correspond to simulation results, while, open circles correspond to measured results. In these analyses, four transistors, Q₁ to Q₄, whose static characteristics are shown in FIG. 4, are used. That is, transistors, Q₁ to Q₄, are a type of the hetero-bipolar transistor (HBT) made of InP based material. Such HBTs clearly show semi-breakdown behaviors when the collector bias Vce exceeds 2 volts as shown by a broken circle. The node voltage Vn₁, the power supply voltage Vcc, the resistance of resistors, R_ave, R₁, R₂, RL₁ and RL₂, are assumed to be 4.6V, −5.2V, 2 kΩ, 58.8Ω, 12.5Ω, 200Ω and 200Ω, respectively. The capacitance of the capacitor Cave is assumed to be 2.2 nF.

[0028] As shown in FIG. 5, the collector biases, Vce₁ and Vce₂, increase as the input current Iin becomes large, which causes the semi-breakdown in the transistors, Q₁ and Q₂, as shown in FIG. 3 and the output offset voltage increases. Thus, when the transistors, Q₁ and Q₂, have lesser performance in the breakdown voltage for the collector bias Vce, the output offset voltage between complementary outputs, Vout₁ and Vout₂, depends on the input. That is, the output offset voltage is hard to be kept in a preset level for the conventional circuit 101.

[0029] Next, preferred embodiments according to the present invention will be described in detail, where the embodiments may suppress the output offset voltage between the complementary signals.

[0030] FIG. 6 is a circuit diagram according to the first embodiment of the present invention. The differential circuit 100 substitutes the reference generator 14 shown in FIG. 1 with another reference generator 20 to generate a base bias for the cascode transistors. The reference generator 20 includes a differential amplifier 22 that provides a non-inverting input for receiving the filtered output Vave and two diodes, D₁ and D₂, forwardly connected between the output and the inverting input of the differential amplifier 22. According to the circuit shown in FIG. 6, the output voltage Vn₁ of the reference generator 20 becomes higher than the input Vave thereof by a value twice of a turn-on voltage Vt of a junction diode; that is:
Vn₁=Vave+2×Vt.
Thus, the reference generator 20 may set the voltage difference between the input Vave and the node voltage Vn₁. Moreover, this voltage difference may be variable by selecting a number of diodes connected in series between the output and the inverting input thereof.

[0031] Because the base biases Vbe of the transistors, Q₁ to Q₄, are automatically set to be the turn-on voltage Vt of a diode, which is typically about, 0.7V, the collector bias Vce₂ of the second transistor Q₂ may become substantially equal to twice of the turn-on voltage 2×Vt. Although the collector bias Vce₁ of the first transistor Q₁ becomes higher than that Vce₂, ˜2×Vt, of the second transistor Q₂ by a voltage drop by the third resistor Rave due to the base bias current therefore, the emitter resistors, R₁ and R₂, may compensate this unbalanced bias.

[0032] According to the first embodiment of the present invention, the reference generator 20 may keep the collector bias Vce of the transistors, Q₁ and Q₂, measured from the base input Vave of the second transistor Q₂ substantially in constant independent of the input current Iin. The output offset voltage of the complimentary signals, Vout₁ and Vout₂, may be independent of the input current Iin.

[0033] FIG. 7 is a circuit diagram which specifically reflects the circuit shown in FIG. 6. The TIA 40 and the PD 50 are not shown in FIG. 7. The reference generator 20 to generate the base bias voltage includes 2 differential amplifiers, 24 and 26, connected in series and an emitter follower 28. The first differential amplifier 24 includes 4 transistors, Q₅ to Q₈, two load resistors, R₁₁ and R₁₂, and 2 current sources, I₂ and I₃. The resistor two transistors, Q₇ and Q₅, and the current source I₂ are connected in series between the power supply Vcc and the ground. Specifically, the emitter of the transistor Q₅ is grounded through the current source I₂, while the collector thereof is connected to the emitter of the other transistor Q₇. The collector of the transistor Q₇ is biased by the power supply Vcc through the resistor R₁₁. Similarly, the resistor R₁₂, two transistors, Q₈ and Q₆, and the current source I₂ are connected in series in this order between the power supply Vcc and the ground. The emitter of the transistor Q₅ is grounded through the current source I₂, while, the collector thereof is coupled with the emitter of the other transistor Q₈. The collector of the other transistor Q₈ is biased by the power supply Vcc through the resistor R₁₂. Bases of two transistors, Q₇ and Q₈, are commonly connected to the node N₁, which is the output of the reference generator 20, that is, the anode of the diode D₂ connected in series. The cathode of the diode D₁ is coupled with the base of the transistor Q₆ but grounded through the other current source I₃. The output Vave of the filter 12 is led to the base of the transistor Q₅.

[0034] The emitter follower 28 includes two transistors, Q₉ and Q₁₀, two diodes, D₃ and D₄, and two current sources, I₄ and I₅. One of units including the transistor Q₉, the diode D₃ and the current source I₄ is connected between the power supply Vcc and the ground; while, the other unit including the transistor Q₁₀, the diode D₄ and the current source I₅ is also connected between the power supply Vcc and the ground. Two diodes, D₃ and D₄, may adjust the output level led from the cathode thereof with respect to the input provided to the base of the transistors, Q₉ and Q₁₀. Although the embodiment shown in FIG. 7 provides only one diode in respective units, two or more diodes may be implemented.

[0035] Signals whose level is dropped by the emitter follower 28 are led to the second differential amplifier 26. The differential amplifier 26 includes two transistors, Q₁₁ and Q₁₂, two resistors, R₂₁ and R₂₂, and the current source I₆. The transistor Q₁₁ and the resistor 21 constitute the left unit; while, the transistor Q₁₂ and the resistor R₂₂ constitute the right unit. Two units are commonly connected to the current source I₆. The output of the second differential amplifier 26, which is provided from the collector of the transistor Q₁₁ is led to the node N₁ with the voltage of Vn₁.

[0036] FIG. 8 shows behaviors of the collector bias Vce₁ of the transistor Q₁ against the input current Iin to the TIA 40. In this analysis, transistors, Q₁ to Q₁₂, are assumed to be a type of InP-HBT and have a characteristic substantially same with those shown in FIG. 4. An operating temperature of the transistors, the turn-on voltage Vt, the resistance of the load resistors, R₁₁ and R₁₂ and the resistance of the load resistors, R₂₁ and R₂₂, are 45° C., 0.7V, 3.2 kΩ, and 3.0 kΩ, respectively. One of load resistors R₂₂ in the second differential amplifier 26 may be replaced to a diode. In FIG. 8, the solid line is obtained in a condition where the power supply is 5.2V, the broken line corresponds to a condition where the power supply is increased by +10% from the standard (5.2V), while, the dotted line corresponds to a condition where the power supply is decreased by −10% from the standard. Even when the input current Iin varies to 3 mA; moreover even the power supply fluctuates, the collector bias Vce₁ of the transistor Q₁ may be kept substantially in constant.

[0037] FIG. 9 shows the output offset voltage Vout₁-Vout₂ of the circuit 100 against the input current Iin. Solid, broken, and dotted lines correspond to the conditions of the power supply of 5.2V, 5.2V+10%, and 5.2V−10%, respectively. As shown in FIG. 9, the output offset voltage may be kept small, less than 30 mV, even the input current Iin and the power supply Vcc vary.

[0038] A circuit according to the third embodiment of the invention simplifies the differential amplifier in the reference generator 20 of the second embodiment. FIG. 10 is a circuit diagram of the reference generator 20a according to the third embodiment of the invention. The reference generator 20a of the present embodiment includes three transistors, Q₅ to Q₇, a resistor R₅ and two current sources, I₂ and I₃. The transistors, Q₇ and Q₅, constitute the left unit, while the resistor R₅ and the transistor Q₆ constitute the right unit. Two units are commonly connected to the current source I₂ and grounded through the current source I₂. The collector of the transistor Q₇ is directly connected to the power supply Vcc, while, the transistor Q₆ is biased by the power supply through the resistor R₅. The reference generator 20a of the present embodiment further includes two diodes, D₁ and D₂, and another current source I₃. Two diodes, D₁ and D₂, are forwardly connected between the collector and the base of the transistor Q₆, while, the other current source I₃ extracts the current from the diodes, D₃, and D₂.

[0039] The reference generator 20a includes a differential amplifier that outputs the node voltage Vn₁ by receiving two inputs, Vave and Vave′. The latter inputs Vave′ are fed back from the output Vn₁ through two diodes, D₁ and D₂. Thus, the inverting input Vave′ may be lowered by a value which is twice of the turn-on voltage of the diode, 2×Vt; in other words, the output Vn₁ of the reference generator 20a may be kept higher than the input Vave′ of the differential circuit by the value twice of the turn-on voltage 2×V_t.

[0040] While there has been illustrated and described what are presently considered to be example embodiments of the present invention, it will be understood by those skilled in the art that various modifications may be made, and equivalents may be substituted, without departing from the true scope of the invention. Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. For instance, embodiments above described adjust the emitter resistors, R₁ and R₂, so as to suppress the output offset voltage; however, the emitter resistors, R₁ and R₂, may be set in the resistance thereof so as to set a preset offset voltage in the output of the receiver circuit 110. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims.