JP2010129571A - Laser driving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To allow a feedback type laser driving circuit, having a buffer circuit, which includes an inductor as a load, to stably operate at a high speed with high quality by securing a breakdown voltage of a differential transistor of a buffer circuit. <P>SOLUTION: In the laser driving circuit including a laser bias circuit which supplies a bias current to a laser diode, and a modulation circuit constituted by connecting a first buffer circuit, an emitter follower circuit, and a second buffer circuit in order and outputting a modulation signal for modulating light emission intensity of the laser diode to the laser bias circuit, differential output terminals LDP and LDN of the modulation circuit and the laser diode circuit are connected by DC coupling, and a feedback element is connected between the differential output terminals LLDP and LDN and of the input of the emitter follower circuit to provide negative feedback; and an inductor as a load on the second buffer circuit is connected between a power source and the differential output terminals LDP and LDN, and a clamp circuit is connected between the second buffer circuit and the differential output terminals LDP and LDN. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a laser driving circuit using a semiconductor integrated circuit.

FIG. 8 shows a configuration example of a conventional laser driving circuit (Patent Document 1).
In FIG. 8, the conventional laser driving circuit includes a laser bias circuit 10 for supplying a bias current to the laser diode 11, a first buffer circuit 21, an emitter follower (EF) circuit 22, and a second buffer circuit 23. A modulation circuit 20 that outputs a modulation current that modulates the light emission intensity of the laser diode 11 and a control circuit 30 that adjusts a bias current and a modulation current are configured. Load resistors 24-1 and 24-2 are connected to the output terminals LDP and LDN of the second buffer circuit 23 of the modulation circuit 20, respectively.

FIG. 9 shows a configuration example of a conventional laser driving circuit (Non-Patent Document 1).
Here, the laser bias circuit 10 and the modulation circuit 20 are AC-coupled to each other through a capacitive element, and the inductor 25-1 is used instead of the load resistance at the output terminals LDP and LDN of the second buffer circuit 23 of the modulation circuit 20. 25-2 are connected.

  The modulation circuit 20 applies positive feedback by connecting feedback elements 27-1 and 27-2 between the differential output terminals LDP and LDN and the input of the EF circuit 22. At this time, there is a technique called “active back termination” in which a small impedance is driven, but synthesis is performed so that the output impedance seen from the load side becomes large. In the active back termination, a small output resistance can be boosted to an impedance that is double the amount of feedback due to positive feedback, so that a large signal loss that occurs in the case of passive termination can be avoided.

  In a laser driving circuit used for so-called continuous optical communication that continuously transmits data, AC coupling is generally performed as shown in FIG. 9 in order to avoid the need to consider the DC potential in the modulation output. On the other hand, if the transmission speed is low in burst mode transmission in which data is transmitted intermittently, there is no inconvenience even with AC coupling. However, when the transmission speed exceeds, for example, 10 Gbps, the data is actually input after being input to the drive circuit. Since it is necessary to shorten the laser on time (˜n seconds) until it is stably transmitted as an optical signal, DC coupling is more advantageous.

In addition, in order to realize a laser driving circuit at a low cost, it is necessary to realize a device used for the laser driving circuit by an inexpensive process. Therefore, a silicon process is generally more advantageous than using an expensive compound semiconductor. Become. However, silicon semiconductors are inferior in transistor cutoff frequency compared to compound semiconductors, and it is difficult to increase the speed. One technical element that becomes a problem in this speeding up is the back termination of the output line for the purpose of preventing multiple reflections from imperfect termination loads. In the prior art, the back termination is realized by a passive element such as a resistor. However, termination by a combined impedance using an active back termination using a feedback structure is known.
JP 2008-41902 A S. Morley, "A 3V 10.7Gb / s Differential Laser Diode with Active Back-Termination Output Stage", IEEE ISSCC Digest of Technical Papers, pp.220-221, 2005

  In a conventional laser drive circuit using an active back termination, the output terminal of the modulation circuit swings around the power supply voltage, so that it is difficult to ensure the withstand voltage of the differential transistor constituting the buffer circuit. In addition, it has been difficult to remove harmful effects by taking measures to ensure withstand voltage.

  The present invention provides a laser drive circuit capable of ensuring high withstand voltage of a differential transistor constituting a buffer circuit and capable of high-quality and stable high-speed operation in a feedback laser drive circuit including a buffer circuit having an inductor as a load. The purpose is to do.

  In the first invention, a laser bias circuit for supplying a bias current to the laser diode, a first buffer circuit, an emitter follower circuit, and a second buffer circuit are sequentially connected, and a modulation signal for modulating the emission intensity of the laser diode is obtained. In a laser driving circuit including a modulation circuit that outputs to a laser bias circuit, the differential output terminals LDP and LDN of the modulation circuit and the laser bias circuit are connected by DC connection (DC coupling), and the differential output terminals LDP and LDN A feedback element is connected negatively to the input of the emitter follower circuit, and an inductor is connected as a load of the second buffer circuit between the power supply and the differential output terminals LDP and LDN. A clamp circuit is connected between the dynamic output terminals LDP and LDN.

  In the second invention, a feedback element connected in a negative feedback connection between the differential output terminals LDP and LDN and the input of the emitter follower circuit in the first invention is connected in a positive feedback manner.

  In the third invention, a feedback element connected in positive feedback is added to a feedback element connected in negative feedback between the differential output terminals LDP and LDN and the input of the emitter follower circuit in the first invention. .

  The clamp circuit is composed of two clamp transistors that are cascode-connected to the differential transistor of the second buffer circuit. The clamp voltage of the clamp transistor is a value that fixes the emitter voltage of the clamp transistor so as not to exceed the collector-emitter breakdown voltage of the differential transistor and the collector-emitter breakdown voltage of the clamp transistor.

  Further, the termination circuit is connected between the emitter follower circuit and the second buffer circuit. Further, the differential output terminals LDP and LDN are connected to a termination circuit matched with the output impedance synthesized by the feedback structure of the modulation circuit. The termination circuit has a configuration using either a resistor, a T-type termination, a Thevenin termination, or a π-type termination.

The inductor has a capacitive coupling configuration that is connected to a power source via a capacitor.
The feedback element is a resistor, a parallel circuit of a resistor and a capacitive element, or a circuit in which a resistor is connected in series to a parallel circuit of a resistor and a capacitive element.

  The laser driving circuit according to the present invention can realize a feedback circuit configuration using an inductor load using a low-voltage transistor in a modulation circuit whose connection to the laser bias circuit is DC coupled, and at the same time, by reducing the mirror effect. The operation of the differential transistor can be speeded up. Therefore, it is possible to realize an increase in operation speed and a higher quality of output waveform than in the prior art. Here, since the level shift voltage of the laser diode is at least about 0.7 to 1V, and in some cases 1.5V or more is necessary, the power supply voltage of the laser drive circuit is typically 3.3V or more. On the other hand, since the high-speed bipolar device has a collector-emitter breakdown voltage as low as about 2 to 2.5 V, the present invention is extremely effective when operated with a 3.3 V power supply.

  In addition, the laser driving circuit of the present invention can reduce the increase in parasitic capacitance that is characteristic of the configuration of the present invention by connecting a termination circuit to the output terminal of the modulation circuit, and can further enhance the effects of the present invention. If the circuit constant of the termination circuit is designed according to the load impedance of the laser module to be connected or a laser module such as TOSA or BOSA, good impedance matching can be realized, and the waveform quality can be improved.

  Further, the laser driving circuit of the present invention can improve the band of the modulation circuit by negative feedback or positive feedback and increase the driving power of the laser diode. Further, by giving the feedback element frequency characteristics, it is possible to mitigate the influence caused by the frequency dependence of the parasitic capacitance (such as the base capacitance of the emitter follower) at high frequencies.

  The laser drive circuit of the present invention can provide a low-cost laser drive device used in a PON (Passive Optical Network) system that requires a burst operation at a high speed operation of 10 Gbps or more, particularly in the field of optical communication.

(First embodiment)
FIG. 1 shows a first embodiment of the laser drive circuit of the present invention.
In FIG. 1, a modulation circuit 20 that is DC-connected (DC coupled) to the laser bias circuit 10 of the laser drive circuit according to the present embodiment is connected between differential input terminals INP and INN and differential output terminals LDP and LDN. 1 buffer circuit 21, EF circuit 22, and second buffer circuit 23 are sequentially connected, and feedback elements 27-1 and 27-2 are negatively fed back between the differential output terminals LDP and LDN and the input of the EF circuit 22. The load inductors 25-1 and 25-2 are connected to the differential output terminals LDP and LDN instead of the load resistors, and clamped between the load inductors 25-1 and 25-2 and the second buffer circuit 23. The circuit 26 is connected. In the feedback circuit, the value obtained by dividing the output resistance of the circuit by the feedback amount becomes the output resistance, and similarly, the value obtained by dividing the open loop gain of the circuit by the feedback amount becomes the gain of the circuit. In the present embodiment and the embodiments described below, the control circuit 30 shown in the conventional configuration of FIGS. 8 and 9 is omitted.

  The feature of the laser drive circuit of the present invention is that the load resistance of the second buffer circuit 23 in the output stage of the conventional circuit is changed to an inductor even when the connection to the laser bias circuit 10 is DC coupled. If the inductor is simply replaced, the voltage at the differential output terminal will oscillate around the power supply voltage, making it difficult to ensure the withstand voltage of the differential transistor in the second buffer circuit 23. Therefore, the withstand voltage of the differential transistor is ensured by connecting a clamp circuit (clamp transistor) 26 between the load inductors 25-1 and 25-2 and the differential transistor of the second buffer circuit 23. Since the emitter voltage of the clamp transistor, that is, the collector voltage of the differential transistor is substantially fixed to a voltage obtained by subtracting the base-emitter voltage Vbe from the base voltage (clamp voltage) of the clamp transistor, both the clamp transistor and the differential transistor A breakdown voltage between the collector and the emitter can be secured. In addition, since the base-collector capacitance seen from the input of the differential transistor is reduced by mitigating the mirror effect by the cascode structure, it is advantageous for high-speed operation.

  However, since it is necessary for the clamp transistor to flow the same drive current as that of the differential transistor, the clamp transistor also needs to have the same size as the differential transistor. This causes an increase in parasitic wiring capacitance in the circuit layout, and causes deterioration of the waveform. In particular, since the rising and falling times (Tr / Tf) of the waveform become long, not only the margin of eye opening but also the increase of jitter becomes a problem. As the operating frequency increases, the parasitic capacitance gives the frequency characteristics to the operating characteristics of the drive circuit, such as the combined output impedance and buffer gain, causing further degradation of the output waveform by causing multiple reflections of signals due to increased jitter and impedance mismatching. Let In order to mitigate the influence accompanying the addition of the parasitic capacitance, it is conceivable to increase the buffer gain. However, inadvertent increase causes an increase in multiple reflection.

  Therefore, the laser drive circuit of the present invention adds a passive termination circuit between the EF circuit 22 and the second buffer circuit 23 or at least one of the differential output terminals of the second buffer circuit (details). Is shown in FIG. 2 and FIG. Incurs losses in terms of power, but includes a laser diode while forcibly terminating at the output end, taking advantage of the effect of the feedback circuit as an amplification circuit (extension of the band for negative feedback, increase of gain for positive feedback) Matching with the transmission system is easy to realize. Therefore, the circuit design is given flexibility and the output waveform is improved. This structure is effective when the laser bias circuit 10 and the modulation circuit 20 are DC-connected (DC coupling).

FIG. 2 shows a first circuit configuration example of the first embodiment of the present invention.
In this circuit configuration example, the first buffer circuit 21, the EF circuit 22, and the second buffer circuit 23 of the modulation circuit 20 shown in FIG. The load inductors 25-1 and 25-2 are denoted as LL1 and LL2, and the differential transistors of the second buffer circuit 23 are denoted as Q1 and Q2.

  The differential input terminals INP and INN of the modulation circuit 20 are connected to the positive input terminal and the negative input terminal of the first buffer circuit 21, respectively, a positive output terminal OPA that outputs a signal in phase with the INP, and an inversion phase with the INP. Negative output terminals ONA for outputting signals are respectively connected to the EF circuit 22. The positive output terminal OPC and the negative output terminal ONC of the EF circuit 22 are connected to the positive input terminal and the negative input terminal of the second buffer circuit 23, respectively, and the same phase signal as the OPC is the positive output terminal LDP of the second buffer circuit 23. The same phase signal as the ONC is output to the negative output terminal LDN of the second buffer circuit 23. The LDP and LDN are connected to a laser bias circuit 10 for supplying a bias current to the laser diode 11 by direct current connection (DC coupling), and control modulation of laser light emission.

  The power supply voltage VLD of the laser bias circuit 10 was set to 5V, and the power supply voltage VCC of the modulation circuit 20 was set to 3.3V. Each power supply voltage is not limited to this value, and both power supply voltages may be the same voltage. In the DC coupling, since the output of the modulation circuit 20 is DC-connected to the laser diode 11, there is a DC current transfer between the laser bias circuit 10 and the modulation circuit 20. Therefore, as shown in FIG. 2, when the power supply voltage of the modulation circuit 20 is lower than that of the laser bias circuit 10, current outflow from the modulation circuit 20 to the laser bias circuit 10 is reduced. It can save current and is effective in reducing power consumption. However, in DC coupling, if the power supply voltage of the modulation circuit 20 is made lower than the power supply voltage of the laser bias circuit 10 as it is, a large current flows from the power supply of the laser bias circuit 10 to the power supply of the modulation circuit 20. Therefore, the capacitive element C1 is connected between the load inductors LL1 and LL2 and the power supply voltage VCC. Further, the resistor R1 may be connected in parallel with the capacitive element C1. However, when the power supply voltages of the laser bias circuit 10 and the modulation circuit 20 are equal (VCC = VLD), the capacitive element C1 and the resistor R1 may be omitted.

  Further, in this circuit configuration example, the resistors RFB1 and RFB2 are negatively feedback connected as feedback elements. That is, the differential output terminals LDP and LDN are connected to the input terminals of the EF 22 to which signals having phases opposite to those of the terminals are input. The resistors RFB1 and RFB2 do not necessarily have the same resistance value. The feedback element may be a parallel circuit of a resistor and a capacitive element, or a circuit in which a resistor is connected in series to a parallel circuit of a resistor and a capacitive element, and the feedback element may have frequency characteristics.

  In this circuit configuration example, clamp transistors Q3 and Q4 are connected in cascode (vertically stacked) between the load inductors LL1 and LL2 and the differential transistors Q1 and Q2 of the second buffer circuit 23, and the bases of Q3 and Q4 are connected. A clamp voltage VCL is applied to the terminal. The potentials LDPP and LDNN of the emitter terminals of Q3 and Q4, that is, the collector terminals of Q1 and Q2, are substantially fixed to the potential obtained by subtracting the base-emitter voltage Vbe of Q3 and Q4 from VCL. For example, the collector-emitter breakdown voltage of the transistor is 2.5V. When the voltage amplitude of the differential output terminals LDP and LDN is 2V, the LDP and LDN swing with the power supply voltage 3.3V centered through the load inductors LL1 and LL2, so that the amplitude is in the range of 2.3V to 4.3V. . At this time, if VCL is fixed at 3.0V, the collector-emitter voltage Vce of the differential transistors Q1 and Q2 will not exceed 2.5V if the base-emitter voltage Vbe of Q3 and Q4 is about 0.7V. . Of course, the Vce of the clamp transistors Q3 and Q4 is also within the same 2V range as the output amplitude when LDPP and LDNN are fixed at 2.3V.

  As described above, even if a large output amplitude is ensured, the withstand voltage can be secured while securing a sufficient margin by fixing the collector potentials of the differential transistors Q1 and Q2. The value of the clamp voltage VCL may be optimally set according to the output amplitude condition and the input signal levels of the differential transistors Q1 and Q2.

  Further, an appropriate termination circuit 41 such as a Thevenin termination is connected between the outputs OPC and ONC of the emitter follower circuit 22 and the differential transistors Q1 and Q2. Then, if the impedances between the outputs OPC and ONC of the emitter follower circuit 22 and the differential transistors Q1 and Q2 are matched, and the input signal level to Q1 and Q2 is set appropriately, the above-described effects are exhibited. Can do.

FIG. 3 shows a second circuit configuration example of the first embodiment of the present invention.
In this circuit configuration example, in addition to the first circuit configuration example, a termination circuit 42 is connected to the differential output terminals LDP and LDN. When such a termination circuit 42 is added, a current flows between the differential output terminals LDP and LDN, so that it is obvious from the viewpoint of power efficiency, but it absorbs frequency-dependent impedance fluctuations. Therefore, significant waveform improvement is possible. Further, since it can be matched with the load impedance, reflection on the transmission line 45 with the laser bias circuit 10 is also reduced, and jitter can be significantly reduced. The types of termination circuits 41 and 42 are not particularly limited, and as shown in FIG. 4, (a) resistors, (b) T-type terminations, (c) Thevenin terminations, (d) π-type terminations, etc. Various termination circuits can be applied. These termination circuits can be monolithically mounted on the IC chip together with the modulation circuit.

  FIG. 5 shows a simulation comparison example of the output waveform (laser current waveform) when the conventional circuit and the circuit of the present invention are operated at a speed of 10 Gbps. In the circuit of the present invention, it can be seen that the band of the modulation circuit is increased and the waveform is improved.

(Second Embodiment)
FIG. 6 shows a second embodiment of the laser drive circuit of the present invention.
In the present embodiment, feedback elements 27-3 and 27-4 are connected in positive feedback between the differential output terminals LDP and LDN and the input of the EF circuit 22. That is, the feedback elements 27-3 and 27-4 are connected to the differential output terminals LDP and LDN of the modulation circuit 20 at the same phase input of the EF circuit 22. Since the gain of the second buffer circuit 23 can be increased according to the output impedance synthesized by the positive feedback, the rise / fall time of the output waveform can be improved. Also in this embodiment, the same effect can be expected by adding a termination circuit.

(Third embodiment)
FIG. 7 shows a third embodiment of the laser drive circuit of the present invention.
This embodiment is a combination of the negative feedback connection of the feedback elements 27-1 and 27-2 of the first embodiment and the positive feedback connection of the feedback elements 27-3 and 27-4 of the second embodiment. is there. By adjusting both positive and negative feedback amounts, the effect of each feedback can be controlled. Also in this embodiment, the same effect can be expected by adding a termination circuit.

  In the embodiment described above, all bipolar devices have been described, but a CMOS device can be similarly realized.

The figure which shows 1st Embodiment of the laser drive circuit of this invention. The figure which shows the 1st circuit structural example of the 1st Embodiment of this invention. The figure which shows the 2nd circuit structural example of the 1st Embodiment of this invention. The figure which shows the structural example of a termination circuit. The figure which shows the comparative example of the prior art and this invention in an output waveform (laser current waveform). The figure which shows 2nd Embodiment of the laser drive circuit of this invention. The figure which shows 3rd Embodiment of the laser drive circuit of this invention. The figure which shows the structural example of the conventional laser drive circuit. The figure which shows the structural example of the conventional laser drive circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Laser bias circuit 11 Laser diode 20 Modulation circuit 21 1st buffer circuit 22 Emitter follower (EF) circuit 23 2nd buffer circuit 24 Load resistance 25 Inductor 26 Clamp circuit 27 Feedback element 30 Control circuit 41, 42 Termination circuit 45 Transmission line

Claims (10)

A laser bias circuit for supplying a bias current to the laser diode;
A modulation circuit that sequentially connects a first buffer circuit, an emitter follower circuit, and a second buffer circuit, and outputs a modulation signal that modulates the light emission intensity of the laser diode to the laser bias circuit;
The differential output terminals LDP and LDN of the modulation circuit and the laser bias circuit are connected by direct current connection (DC coupling),
A feedback element is connected in a negative feedback manner between the differential output terminals LDP and LDN and the input of the emitter follower circuit.
An inductor is connected as a load of the second buffer circuit between a power supply and the differential output terminals LDP and LDN,
A laser drive circuit, wherein a clamp circuit is connected between the second buffer circuit and the differential output terminals LDP and LDN. A laser bias circuit for supplying a bias current to the laser diode;
A modulation circuit that sequentially connects a first buffer circuit, an emitter follower circuit, and a second buffer circuit, and outputs a modulation signal that modulates the light emission intensity of the laser diode to the laser bias circuit;
The differential output terminals LDP and LDN of the modulation circuit and the laser bias circuit are connected by direct current connection (DC coupling),
A feedback element is connected in positive feedback between the differential output terminals LDP and LDN and the input of the emitter follower circuit,
An inductor is connected as a load of the second buffer circuit between a power supply and the differential output terminals LDP and LDN,
A laser drive circuit, wherein a clamp circuit is connected between the second buffer circuit and the differential output terminals LDP and LDN. A laser bias circuit for supplying a bias current to the laser diode;
A modulation circuit that sequentially connects a first buffer circuit, an emitter follower circuit, and a second buffer circuit, and outputs a modulation signal that modulates the light emission intensity of the laser diode to the laser bias circuit;
The differential output terminals LDP and LDN of the modulation circuit and the laser bias circuit are connected by direct current connection (DC coupling),
A feedback element is connected between the differential output terminals LDP and LDN and the input of the emitter follower circuit by negative feedback connection and positive feedback connection,
An inductor is connected as a load of the second buffer circuit between a power supply and the differential output terminals LDP and LDN,
A laser drive circuit, wherein a clamp circuit is connected between the second buffer circuit and the differential output terminals LDP and LDN. In the laser drive circuit according to any one of claims 1 to 3,
The laser driving circuit, wherein the clamp circuit includes two clamp transistors that are cascode-connected to the differential transistor of the second buffer circuit. The laser drive circuit according to claim 4,
The clamp voltage of the clamp transistor is a value that fixes the emitter voltage of the clamp transistor so as not to exceed the collector-emitter breakdown voltage of the differential transistor and the collector-emitter breakdown voltage of the clamp transistor. Laser drive circuit. In the laser drive circuit according to any one of claims 1 to 3,
A laser drive circuit, wherein a termination circuit is connected between the emitter follower circuit and the second buffer circuit. In the laser drive circuit according to any one of claims 1 to 3,
A laser drive circuit, wherein a termination circuit matched with an output impedance synthesized by a feedback structure of the modulation circuit is connected to the differential output terminals LDP and LDN. In the laser drive circuit according to claim 6 or 7,
The termination circuit has a configuration using any of a resistor, a T-type termination, a Thevenin termination, or a π-type termination. In the laser drive circuit according to any one of claims 1 to 3,
The laser drive circuit, wherein the inductor has a capacitive coupling configuration connected to the power source via a capacitor. In the laser drive circuit according to any one of claims 1 to 3,
The laser driving circuit, wherein the feedback element is a resistor, a parallel circuit of a resistor and a capacitive element, or a circuit in which a resistor is connected in series to a parallel circuit of a resistor and a capacitive element.

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