Driver circuit

Abstract

This driver circuit is configured to convert an input signal (Vin) that is an input electric signal into an optical signal by driving a light-emitting element (LD) on the basis of the input signal, and comprises a first transistor (Tr1), a second transistor (Tr4), a third transistor (Tr3), and a first inductor (Lb). The first transistor supplies a current for driving the light-emitting element from a power supply to the light-emitting element. The second transistor includes a gate terminal to which the input signal (Vin) input to an input terminal (Tin) is supplied, and controls the amount of current supplied to the light-emitting element by drawing in at least a part of the current from the first transistor in accordance with the supplied input signal. The first inductor is disposed between the first transistor and the second transistor, and performs peaking of a frequency characteristic of a conversion gain from the input signal to the optical signal. The third transistor includes: a gate terminal to which a control signal for adjusting the amount of peaking is input; and a source terminal and a drain terminal connected to both ends of the first inductor.

Description

Driver Circuit

[0001] The present invention relates to a driver circuit for converting an input signal into an optical signal.

[0002] In recent years, the remarkable development of social networking services (SNS) has led to an increase in traffic volume worldwide year by year. Further increases in traffic volume are expected in the future due to the development of the Internet of Things (IoT) and cloud computing technologies. To support this massive traffic volume, there is a demand for increased communication capacity both inside and outside data centers. To accommodate this demand, the standardization of 100GbE for Ethernet, a key network standard, has been completed, and discussions are underway to standardize 400GbE, which aims for even greater capacity. Aiming to apply 400GbE to such networks, a directly modulated laser (DML) driver circuit (Non-Patent Document 1) has attracted attention from the perspective of low power consumption.

[0003] T. Kishi et al., “A 137-mW, 4 ch x 25-Gbps low-power compact transmitter flip-chip-bonded 1.3-mm LD-array-on-Si,” in Proc. Opti. Fiber Commun. Conf. Exhib., 2018, Paper M2D.2.

[0004] Conventional driver circuits such as those described in Non-Patent Document 1 do not have the function of compensating for the frequency characteristics of the light-emitting element that converts an electrical signal into an optical signal, and therefore the frequency characteristics of the conversion gain from an electrical signal to an optical signal in a transmission front end consisting of the driver circuit and the light-emitting element are rate-determined by the characteristics of the light-emitting element.

[0005] An object of the present invention is to make the frequency characteristic of the gain of conversion from an electrical signal to an optical signal variable.

[0006] In order to solve the above problem, the driver circuit of the present invention is a driver circuit that converts an input signal, which is an input electrical signal, into an optical signal by driving a light-emitting element, and includes: a first transistor for supplying a current from a power source to the light-emitting element to drive the light-emitting element; a second transistor having a gate terminal to which the input signal input to its input terminal is supplied, and controlling the amount of the current supplied to the light-emitting element by drawing at least a portion of the current from the first transistor in accordance with the supplied input signal; a first inductor arranged between the first transistor and the second transistor, which peaks the frequency characteristics of the conversion gain from the input signal to the optical signal; a gate terminal to which a control signal for adjusting the amount of peaking is input; and a third transistor having a source terminal and a drain terminal connected to both ends of the first inductor.

[0007] According to the present invention, the frequency characteristics of the conversion gain from an electrical signal to an optical signal can be varied.

[0008] FIG. 1 is a circuit diagram of a driver circuit according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating the operation of the driver circuit of FIG. 1. FIG. 3 is a graph illustrating simulation results of frequency characteristics of EO responses by the driver circuit of FIG. 1 and a driver circuit of a comparative example. FIG. 4 is a graph illustrating simulation results of frequency characteristics of EO responses by the driver circuit of FIG. 1. FIG. 5 is a diagram illustrating a 32 Gbps NRZ optical waveform and a diamond-shaped eye opening mask simulated by the driver circuit of the comparative example. FIG. 6 is a diagram illustrating a 32 Gbps NRZ optical waveform and a diamond-shaped eye opening mask simulated by the driver circuit of FIG. 1. FIG. 7 is a circuit diagram of a driver circuit according to a second embodiment of the present invention. FIG. 8 is a circuit diagram of a driver circuit according to a third embodiment of the present invention. FIG. 9 is a circuit diagram of a driver circuit according to a fourth embodiment of the present invention. FIG. 10 is a circuit diagram of a driver circuit according to a fifth embodiment of the present invention. FIG. 11 is a circuit diagram of a driver circuit according to a sixth embodiment of the present invention. FIG. 12 is a circuit diagram of a driver circuit according to a seventh embodiment of the present invention.

[0009] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0010] 1, a driver circuit 10 according to the first embodiment is configured for a DML (Directly Modulated Laser) and is configured to drive a light-emitting element LD made of a laser diode. The driver circuit 10 drives the light-emitting element LD based on an input signal Vin, which is a modulated signal, thereby converting the input signal Vin, which is an electrical signal, into an optical signal.

[0011] The driver circuit 10 includes input terminals T1 to T4, a terminal Tcon, a terminal Tin, transistors Tr1 to Tr4, inductors La and Lb, resistors Rin and Rf, and a capacitor Cf. Transistors Tr1 and Tr2 are PMOS transistors here, but may be P-type bipolar transistors. Transistors Tr3 and Tr4 are NMOS transistors here, but may be N-type bipolar transistors.

[0012] The input terminal T1 is connected to the source terminal of the transistor Tr1. The input terminal T2 is connected to the gate terminal of the transistor Tr1. The source and drain terminals of the transistor Tr1 are connected to both ends of an element formed by a combination of a capacitor Cf and a resistor Rf connected in series.

[0013] The input terminal Tcon is connected to the gate terminal of the transistor Tr2. The source terminal of the transistor Tr2 is connected to the drain terminal of the transistor Tr1. The source terminal and drain terminal of the transistor Tr2 are connected to both ends of the inductor Lb, respectively.

[0014] The terminal T3 is connected to the gate terminal of the transistor Tr3, the drain terminal of which is connected to the drain terminal of the transistor Tr2.

[0015] The drain terminals of the transistors Tr2 and Tr3 are connected to the anode of the light emitting element LD, whose cathode is connected to ground.

[0016] The terminal T4 is connected to the gate terminal of the transistor Tr4 via a series connection of a resistor Rin and an inductor La. The terminal Tin is connected to a node connecting the resistor Rin and the inductor La, and is connected to the gate terminal of the transistor Tr4 via the inductor La. The source terminal of the transistor Tr4 is connected to ground.

[0017] Terminal T1 is connected to a power supply, and receives a drive current from the power supply to drive the light-emitting element LD. Terminals T2, T3, and T4 receive bias voltages to the gate terminals of transistors Tr1, Tr3, and Tr4, respectively. Terminal Tin receives an input signal Vin. Resistor Rin is an impedance matching resistor. Terminal Tcon receives a control signal Vcon, which is a voltage signal that controls transistor Tr2 (more specifically, the peaking amount described below).

[0018] The transistor Tr1 serves to supply a driving current from a power supply to the light emitting element LD, and functions as a constant current source.

[0019] The transistor Tr3 is connected in parallel with the light-emitting element LD as viewed from the transistor Tr1 and the inductor Lb. The transistor Tr3 controls the amount of drive current flowing through the light-emitting element LD by drawing at least a portion of the drive current from the transistor Tr1 in accordance with the voltage value of the input signal Vin supplied to its gate terminal. This control of the amount of drive current and the light emission of the light-emitting element LD by the drive current realizes the conversion of the input signal into an optical signal. Here, the higher the voltage value of the input signal Vin, the smaller the drive current to the light-emitting element LD. Therefore, the logic of the optical signal output from the light-emitting element LD is the inverse logic of the input signal Vin.

[0020] The transistors Tr3 and Tr4 are cascode-connected. As a result, even if the operating voltage of the light-emitting element LD exceeds the withstand voltage of the transistor Tr3 or Tr4 alone, the voltage applied to the transistors Tr3 and Tr4 is divided by the transistors Tr3 and Tr4. Therefore, breakdown of the transistors Tr3 and Tr4 is suppressed.

[0021] In this embodiment, the inductors La and Lb, the capacitor Cf, and the resistor Rf suppress excessive overshoot and undershoot in the optical waveform of the optical signal output from the light-emitting element LD, while peaking the frequency characteristics of the conversion gain from an electrical signal to an optical signal. More specifically, the flat frequency band where the conversion gain is near 0 dB can be expanded toward the high-frequency side, thereby improving the frequency characteristics of the conversion gain. Furthermore, the amount of peaking can be varied by the transistor Tr2. This allows the desired frequency characteristics to be obtained.

[0022] 2 is an explanatory diagram for explaining the operation of the driver circuit 10. When the transistor Tr1 functions as a constant current source, the transistor Tr1 can be regarded as a resistor Rtr1, and the transistor Tr2 can be regarded as a variable resistor Rcon whose resistance value changes according to the voltage value of the control signal Vcon. Here, the gate-source resistance and capacitance of the transistor Tr4 are respectively R １ , C １ and the potential of the gate terminal of the transistor Tr4 is V Ｇ The transconductance of the transistor Tr4 is g ｍ Furthermore, the resistance value of the resistor Rtr1, the resistance value of the resistor Rcon, the inductance of the inductor La, the inductance of the inductor Lb, the capacitance of the capacitor Cf, and the resistance value of the resistor Rf are respectively set as R ｔｒ１ , R ｃｏｎ , R ｆ , L ａ , L ｂ , C ｆ , R ｆ In addition, the potential of the input signal Vin is set to V ｉｎThe potential of the output signal of the driver circuit 10 input to the light emitting element LD is V ｏｕｔ In this case, |V Ｇ / V ｉｎ | and the gain Av are expressed by the following equations (1) and (2).

[0023] Regarding equation (1), 1-w ２ C １ L ａ When |V Ｇ / V ｉｎ If |>1 is satisfied, the peaking function is achieved. Therefore, in order to achieve the peaking function, the following expressions (3) and (4) must be satisfied.

[0024] Regarding equation (2), the voltage value V of the control signal Vcon ｃｏｎ When the value of is increased and the transistor Tr2 is turned off, Av increases as the frequency increases. Therefore, L is set so that Av increases from the frequency at which the value of equation (1) starts to decrease. ｂ , C ｆ , R ｆ By setting the value of , excessive overshoot and undershoot of the optical waveform are suppressed, while the frequency characteristic of the light emitting element LD is compensated for, and the frequency characteristic of the conversion gain from an electrical signal to an optical signal is peaked.

[0025] Also, L ｂ The higher the gain, the greater the gain. However, the light-emitting element LD has a characteristic that the group delay increases at its own relaxation oscillation frequency. Therefore, there is a trade-off between compensation for the frequency characteristics of the light-emitting element LD and the group delay characteristics. ｂ The value of must be determined taking into consideration the group delay characteristics.

[0026] Voltage value V of control signal Vcon ｃｏｎ When the value of is reduced and the transistor Tr2 is turned on, the impedance of the parallel connection of the transistor Tr2, i.e., the variable resistor Rcon and the inductor Lb, is reduced, and therefore Av is also reduced. ｃｏｎThe amount of peaking can be adjusted by changing the value of , and the band in which the frequency characteristic of the conversion gain is flat becomes variable.

[0027] 3 shows the frequency characteristics of the EO response (electrical signal to optical signal conversion gain) simulated by the driver circuit 10. Since group delay increases when the EO response exceeds 0 dB, maintaining 0 dB as much as possible while maintaining a flat frequency response can suppress excessive overshoot and undershoot. It can also be seen that the configuration of the driver circuit 10 improves the EO response compared to a comparative example that does not include inductors La and Lb, capacitor Cf, resistor Rf, and transistor Tr2.

[0028] The voltage value V ｃｏｎ The simulation results of the EO response in which the peaking amount is adjusted by adjusting the voltage value V ｃｏｎ When the value of V is increased, the transistor Tr2 is turned off, and the peaking effect of the inductor Lb increases the flat band of 0 dB in the frequency characteristics of the conversion gain. ｃｏｎ When the value of is decreased, the transistor Tr2 is turned on, so that the impedance of the parallel connection of the inductor Lb and the variable resistor Rcon becomes smaller, the amount of peaking decreases, and the flat band of 0 dB decreases.

[0029] 5 and 6 show simulated 32 Gbps NRZ optical waveforms and diamond-shaped eye opening masks. Fig. 5 shows the simulation results for the driver circuit of the comparative example, and Fig. 6 shows the simulation results for the driver circuit. Comparing Fig. 5 and Fig. 6 reveals that driver circuit 10 suppresses excessive overshoot and undershoot in the optical waveform while improving the eye opening in both the x-axis and y-axis directions compared to the comparative example by utilizing a high-frequency peaking function.

[0030] 7 has a circuit configuration in which the resistor Rf and the capacitor Cf are excluded from the driver circuit 10. For commercially available light-emitting elements LD that have poor bandwidth due to variations in characteristics, a circuit configuration in which the resistor Rf and the capacitor Cf are excluded and peaking is strongly applied can compensate for the frequency characteristics of the light-emitting element LD without deteriorating the group delay characteristics, and therefore the driver circuit 20 can be said to have a circuit configuration that is effective for light-emitting elements LD that have poor frequency characteristics.

[0031] 8 has a circuit configuration in which the transistor Tr3 is removed from the driver circuit 10. When the withstand voltage of a single transistor is high, the number of transistors and terminals can be reduced by not using a cascode configuration.

[0032] (Fourth Embodiment) A driver circuit 40 according to a fourth embodiment shown in FIG. 9 includes the same circuit configuration as the driver circuit 10, but also includes a combination of terminals T11 and T12 and transistors Tr11 and Tr12 upstream of transistor Tr1. The transistors constituting the power supply section, which serves as a current source, are configured in a multi-stage configuration of transistors Tr1, Tr11, and Tr12. The number of transistors and terminals, i.e., the number of stages, is optional. Furthermore, a driver circuit 30 includes the same circuit configuration as the driver circuit 10, but also includes a multi-stage configuration in which terminal T3 and transistor Tr3 are replaced with multiple terminals T31 and T32 and multiple transistors Tr31 and Tr32. The number of transistors and terminals, i.e., the number of stages, is optional. Each multi-stage configuration suppresses breakdown of the transistors. In particular, the breakdown voltage of individual transistors has recently been declining, making the multi-stage configuration of this embodiment particularly effective.

[0033] 10, the positions of the transistors Tr3 and Tr4 in the circuit configuration of the driver circuit 10 are interchanged, and accordingly, the positions of the terminals T3, T4, Tin, resistor Rin, and inductor La are also changed. In this way, the input position of the input signal may be the gate terminal of any of the multi-stage transistors.

[0034] 11 includes, in addition to the circuit configuration of the driver circuit 10, a resistor Rs having both ends connected to the source terminal of the transistor Tr4 and the ground, respectively, through which the current drawn by the transistor Tr4 flows. This improves the linearity of the driver circuit, allowing the driver circuit 60 to operate more linearly with respect to the input signal.

[0035] 12 includes a capacitor Cs connected in parallel with a resistor Rs between the source terminal of a transistor Tr4 and ground, in addition to the circuit configuration of the driver circuit 60. Compared to the circuit configuration of the driver circuit 60, the characteristics (attenuation) of the high-frequency band of the optical signal are improved.

[0036] (Summary of Each Embodiment) The following describes examples of configurations that can be understood from the above embodiments. Note that for each element below, the symbol of at least one corresponding element in each of the above embodiments is written in parentheses, as appropriate. (1) The driver circuit (10-70) may be configured to drive a light-emitting element (LD) based on an input signal (Vin), which is an input electrical signal, thereby converting the input signal into an optical signal, and may include a first transistor (Tr1), a second transistor (Tr4), a third transistor (Tr3), and a first inductor (Lb). The first transistor may be configured to supply a current from a power source to the light-emitting element to drive the light-emitting element. The second transistor may have a gate terminal to which the input signal (Vin) input to the input terminal (Tin) is supplied, and may be configured to control the amount of current supplied to the light-emitting element by drawing at least a portion of the current from the first transistor in accordance with the supplied input signal. The second transistor may be connected in parallel with the light-emitting element, for example, as viewed from the first transistor. The connection between the second transistor and the light-emitting element may be a direct connection as shown in FIGS. 8 and 10 , or an indirect connection via a transistor Tr3 as shown in FIG. 1 . The first inductor may be disposed between the first transistor and the second transistor and configured to peak the frequency characteristics of the conversion gain from an input signal to an optical signal. For example, one end of the first inductor is connected to a terminal (the drain terminal in the above example) that outputs a current from the first transistor, and the other end is connected to the light-emitting element and the second transistor, which are connected in parallel. The other end of the first inductor may be a node N1 directly connected to the light-emitting element, or may be connected to a node N1 (see FIG. 1 ) to which the second transistor is indirectly connected via a transistor Tr3 or the like. The third transistor may include a gate terminal to which a control signal for adjusting the amount of peaking is input, and a source terminal and a drain terminal connected to both ends of the first inductor.

[0037] According to this configuration, the amount of peaking caused by the first inductor is variable by the third transistor, so that the frequency characteristics of the conversion gain from the input signal to the optical signal can be varied, thereby obtaining the desired frequency characteristics.

[0038] (2) The driver circuit may further include a second inductor (La) having a first end connected to the gate terminal of the second transistor and a second end connected to the input terminal, and peaking the frequency characteristics.

[0039] According to this configuration, the second inductor peaks the frequency characteristics of the conversion gain from an electrical signal to an optical signal, improving the frequency characteristics.

[0040] (3) The driver circuit may further include an element consisting of a combination of a resistor (Rf) and a capacitor (Cf) connected in series, the two ends of which are connected to the source terminal and the drain terminal of the first transistor, respectively.

[0041] The element compensates for the frequency characteristics of the light emitting element LD while suppressing excessive overshoot and undershoot of the optical waveform, particularly in combination with the second inductance.

[0042] (4) The driver circuit may further include one fourth transistor (Tr3) or multiple fourth transistors (Tr31-Tr32) cascode-connected to the second transistor. The driver circuit may further include one or multiple fifth transistors (Tr11-Tr12), and the first transistor and the one or multiple fifth transistors may form a power supply unit that supplies a current to the light-emitting element to drive the light-emitting element.

[0043] With the above-described configuration, a multi-stage configuration using a plurality of transistors is adopted, thereby suppressing breakdown of the transistors.

[0044] (5) The driver circuit may further include a resistor (Rs) connected to the second transistor and through which the current drawn by the second transistor flows.

[0045] With the above configuration, the linearity of the driver circuit is improved.

[0046] (6) The driver circuit may further include a capacitor connected in parallel with the resistor.

[0047] With the above-described configuration, the characteristics (attenuation) of the high frequency band of the optical signal are improved compared to the circuit configuration of the driver circuit in (5) above, for example.

[0048] (Scope of the present invention) The present invention is not limited to the above-described embodiments. For example, the present invention includes various modifications to the above-described embodiments that can be understood by a person skilled in the art within the scope of the technical concept of the present invention. The configurations listed in the above-described embodiments can be combined as appropriate within a range that does not cause contradictions. Furthermore, it is also possible to delete any of the above-described configurations.

[0049] 10 to 70...driver circuit, LD...light emitting element, N1...node, Vcon...control signal, Vin...input signal.

Claims

1. A driver circuit that converts an input signal, which is an input electrical signal, into an optical signal by driving a light-emitting element, comprising: a first transistor for supplying a current from a power source to the light-emitting element to drive the light-emitting element; a second transistor having a gate terminal to which the input signal input to its input terminal is supplied, and controlling the amount of the current supplied to the light-emitting element by drawing at least a portion of the current from the first transistor in accordance with the supplied input signal; a first inductor disposed between the first transistor and the second transistor, for peaking the frequency characteristics of the conversion gain from the input signal to the optical signal; and a third transistor having a gate terminal to which a control signal for adjusting the amount of peaking is input, and a source terminal and a drain terminal connected to both ends of the first inductor.

2. The driver circuit of claim 1, further comprising a second inductor having a first end connected to the gate terminal of the second transistor and a second end connected to the input terminal, the second inductor peaking the frequency characteristic.

3. The driver circuit of claim 1, further comprising an element consisting of a combination of a resistor and a capacitor connected in series, both ends of which are connected to the source terminal and the drain terminal of said first transistor, respectively.

4. The driver circuit of claim 1, further comprising one or more fourth transistors cascode-connected with said second transistor.

5. The driver circuit according to claim 1, further comprising one or more fifth transistors, wherein the first transistor and the one or more fifth transistors constitute a power supply section that supplies a current for driving the light-emitting element to the light-emitting element.

6. The driver circuit of claim 1, further comprising a resistor connected to said second transistor and through which flows said current sunk by said second transistor.

7. The driver circuit of claim 6, further comprising a capacitor connected in parallel with said resistor.