Light-emitting element drive device

Abstract

To provide a light-emitting element drive device which solves the problem that power consumption and delay are large in signal transmission using optical communication in future.SOLUTION: A light-emitting element drive device 1 does not require signal processing using a DSP of large power consumption. Specifically, the light-emitting element drive device is a device which outputs a current signal for driving a light-emitting element 2 and comprises a load 10, a differential circuit 20, a balance / unbalance conversion circuit 30 and a dummy load 40. The balance / unbalance conversion circuit 30 includes a first input terminal 31, a second input terminal 32, a first output terminal 33 and a second output terminal 34. The first input terminal 31 is connected to a first node N1, and the second input terminal 32 is connected to a second node N2. The first output terminal 33 is connected to the light-emitting element 2. The second output terminal 34 is connected to the dummy load 40. The balance / unbalance conversion circuit 30 includes a balun 50. The balun 50 has a configuration in which a first inductor and a second inductor are mutually coupled in an electromagnetic induction manner.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a light-emitting element driving device.

Background Art

[0002] Patent Documents 1 to 3 disclose inventions of devices that output current signals for driving light-emitting elements (for example, laser diodes). The light-emitting element driving devices described in these documents include a load and a differential circuit, input a differential voltage signal into the differential circuit, output a current signal corresponding to this differential voltage signal, and drive the light-emitting element with this current signal. However, conventional light-emitting element driving devices, including those described in these documents, are difficult to use, for example, in computer network systems that are expected to become widespread in the future. Hereinafter, this will be described by taking the server system of a data center as an example.

[0003] The current server system of a data center includes a plurality of racks, and each rack includes a plurality of types of resources such as a CPU, a GPU, and a memory. Data transmission and reception are performed both within each rack and between different racks. Since the data transmission and reception within each rack are short-distance, they can be connected by PCIe (peripheral component interconnect express) that enables communication with low latency. However, since the data transmission and reception between different racks have a transmission distance of up to about 30 m, they are connected by Ethernet (registered trademark), but the latency is large. In this configuration, when the processing capacity of a certain rack reaches its limit, even if an attempt is made to distribute the processing to other racks with a processing capacity margin, it is difficult to improve the processing capacity because of the large latency in data transmission and reception between these two racks.

[0004] In a server system expected to become widespread in the future, each rack becomes a resource pool that aggregates one type of resource among resources such as CPUs, GPUs, and memories, and the racks are connected with low latency. Since all the resource pools can be connected to each other with low latency, there is no resource with a surplus of processing power, and it becomes possible to maximize the processing power. This new server system is called resource-disaggregated computing. For further improvement of processing power, higher speed and lower power consumption are also required.

[0005] PCIe 6.0 has been released as a new standard to meet various requirements of server systems. This standard aims to achieve higher speed and lower latency by adopting a lightweight FEC (3-way interleaved single symbol correction) as forward error correction. Also, in this standard, it is required to make the SER (Symbol Error Rate) before FEC smaller than that of the conventional one.

[0006] In addition, in the conventional PCIe transmission, data transmission has been performed by copper wiring, but it is difficult to achieve medium-distance low-latency transmission of about 30 m required to realize resource-disaggregated computing. Therefore, it has been proposed to realize medium-distance transmission by performing data transmission by optical communication instead of copper wiring. However, optical communication has problems such as high power consumption and latency due to reasons such as the use of DSP. Note that on the transmission side, the DSP is used as an equalizer, and on the receiving side, the DSP is used as a distortion correction circuit.

[0007] If we do not use a DSP to achieve low power consumption and low latency, it is important to suppress the deterioration of the eye of the transmission signal. On the other hand, in PCIe6.0, it is required to make the SER before FEC smaller than before. From these facts, it is required to suppress the noise component of the eye of the optical communication signal, and furthermore, in order to do so, it is required to make the optical transmission signal of the optical communication have a large amplitude due to the necessity of improving the signal-to-noise ratio on the optical signal receiving side of the optical communication.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above by taking the server system of the data center as an example, in view of the future development direction of signal transmission, it is important that various requirements are met in signal transmission by optical communication. However, the light emitting element driving devices described in Patent Documents 1 to 3 are difficult to meet these requirements.

[0010] In the light-emitting element driving device described in Patent Document 1, a PMOS transistor operating in the saturation region is used as a load, and since it is difficult to rapidly follow feedback, the rising waveform (Rise current waveform) of the output current signal becomes sluggish. When the light-emitting element is a vertical cavity surface emitting laser (VCSEL), when the output current signal rises, due to the influence of the differential resistance of the VCSEL, the output voltage rises. When the output voltage rises, the Vds of the PMOS transistor becomes small, and the current of the PMOS transistor instantaneously decreases. However, the feedback loop bandwidth does not increase up to several GHz or more, and the control to maintain the PMOS transistor current is not in time. Due to this transient influence, the current driving force of the PMOS transistor current source drops, and the Rise current waveform becomes sluggish. When the output current signal is a PAM4 signal, this sluggishness of the Rise current waveform leads to the closing of the aperture of the Top eye among the three eyes of Top, Middle, and Bottom. The effect of this sluggishness of the Rise current waveform becomes more prominent as the large signal amplitude increases. Therefore, it is difficult for the light-emitting element driving device described in Patent Document 1 to meet the requirements of high speed and large amplitude.

[0011] Also, Patent Document 1 describes a circuit configuration for dealing with the problem of the sluggish Rise current waveform. However, since this circuit includes a plurality of driver circuits, a plurality of delay circuits that give a delay to the signal to adjust the output timing, a plurality of distortion correction circuits that correct the distortion of the duty cycle, etc., the power consumption is large and the delay is also large. Further, since this circuit requires a Retimer in the front stage, the power consumption and delay due to the Retimer are additionally added.

[0012] In the light-emitting element driving device described in Patent Document 2, since the output impedance is small, it is difficult to meet the requirement of large-amplitude operation. In the light-emitting element driving device described in Patent Document 3, a PMOS transistor operating in the linear region is used as a load, and since it is difficult to perform high-speed follow-up of feedback, it is difficult to meet the requirement of high speed. Further, in the light-emitting element driving device described in Patent Document 3, since the output impedance is small, it is difficult to meet the requirement of large-amplitude operation.

[0013] As described above, the light-emitting element driving devices described in Patent Documents 1 to 3 cannot satisfy various requirements in signal transmission by future optical communication.

[0014] The present invention has been made to solve the above problems, and an object thereof is to provide a light-emitting element driving device that can satisfy various requirements in signal transmission by future optical communication.

Means for Solving the Problems

[0015] The light-emitting element driving device of the present invention is a device that outputs a current signal for driving a light-emitting element. A first aspect of the light-emitting element driving device of the present invention includes: (1) a load provided between a first reference potential supply terminal that supplies a first reference potential and a first node and a second node, and supplies current to each of the first node and the second node; (2) a differential circuit provided between a second reference potential supply terminal that supplies a second reference potential lower than the first reference potential and the first node and the second node, and outputs a signal corresponding to an input differential voltage signal from the first node and the second node; (3) a balun having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, in which a first inductor and a second inductor are magnetically inductively coupled to each other, the first inductor is provided between the first input terminal and the first output terminal, the second inductor is provided between the second input terminal and the second output terminal, the first input terminal is connected to the first node, the second input terminal is connected to the second node, and a balun-unbalun conversion circuit that outputs a current signal from the first output terminal; and (4) a dummy load provided between the second output terminal of the balun-unbalun conversion circuit and the reference potential supply terminal.

[0016] In the second aspect of the light-emitting element driving device of the present invention, in addition to the first aspect, the coupling coefficient between the first inductor and the second inductor of the balun, or the frequency characteristics of the signal path from the second input terminal of the balun through the second inductor and the second output terminal to the dummy load are variable.

[0017] In the third aspect of the light-emitting element driving device of the present invention, in addition to the second aspect, the balun circuit further includes a variable capacitance portion provided between an intermediate node of the second inductor and a reference potential supply terminal.

[0018] In the fourth aspect of the light-emitting element driving device of the present invention, in addition to the second or third aspect, the balun circuit further includes a third inductor provided between the first input terminal and the first inductor, a fourth inductor provided between the second input terminal and the second inductor, and a variable capacitance portion provided between a node between the second inductor and the fourth inductor and the reference potential supply terminal.

[0019] In the fifth aspect of the light-emitting element driving device of the present invention, in addition to any one of the second to fourth aspects, the balun circuit further includes a variable impedance element provided in parallel with the second inductor between the second input terminal and the second output terminal.

[0020] In the sixth aspect of the light-emitting element driving device of the present invention, in addition to any one of the second to fifth aspects, the balun circuit further includes a variable capacitance portion provided in parallel with the dummy load between the second output terminal and the reference potential supply terminal.

[0021] In the seventh aspect of the light-emitting element driving device of the present invention, in addition to any one of the second to sixth aspects, the balun circuit further includes a variable capacitance portion provided between the second input terminal and the reference potential supply terminal.

[0022] In the eighth aspect of the light-emitting element driving device of the present invention, in addition to any one of the first to seventh aspects, the differential circuit includes a first NPN transistor having a collector connected to the first node, a second NPN transistor having a collector connected to the second node, and a tail current source provided between the emitters of the first NPN transistor and the second NPN transistor and the second reference potential supply terminal, and a differential voltage signal is input to the bases of the first NPN transistor and the second NPN transistor respectively.

[0023] In the ninth aspect of the light-emitting element driving device of the present invention, in addition to any one of the first to seventh aspects, the differential circuit includes a first NPN transistor having a collector connected to the first node, a second NPN transistor having a collector connected to the second node, a first tail current source provided between the emitter of the first NPN transistor and the second reference potential supply terminal, a second tail current source provided between the emitter of the second NPN transistor and the second reference potential supply terminal, and a resistor provided between the emitter of the first NPN transistor and the emitter of the second NPN transistor, and a differential voltage signal is input to the bases of the first NPN transistor and the second NPN transistor respectively.

[0024] In the tenth aspect of the light-emitting element driving device of the present invention, in addition to any one of the first to ninth aspects, the load includes a first current source provided between the first reference potential supply terminal and the first node, and a second current source provided between the first reference potential supply terminal and the second node.

[0025] In the eleventh aspect of the light-emitting element driving device of the present invention, in addition to any one of the first to ninth aspects, the load includes a first resistor provided between the first reference potential supply terminal and the first node, and a second resistor provided between the first reference potential supply terminal and the second node.

[0026] In the twelfth aspect of the light-emitting element driving device of the present invention, in addition to any one of the first to eleventh aspects, the dummy load has impedance characteristics equivalent to those of the light-emitting element.

[0027] In the 13th aspect of the light-emitting element driving device of the present invention, in addition to the 12th aspect, the dummy load includes a resistor and a diode connected in series with each other.

Advantages of the Invention

[0028] According to the present invention, it is possible to provide a light-emitting element driving device that can meet various requirements in future signal transmission by optical communication.

Brief Description of the Drawings

[0029]

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Best Mode for Carrying Out the Invention

[0030] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant descriptions are omitted. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0031] FIG. 1 is a diagram showing a configuration example of a light-emitting element driving device 1. The light-emitting element driving device 1 is a device that outputs a current signal for driving a light-emitting element 2, and includes a load 10, a differential circuit 20, a balun circuit 30, and a dummy load 40. The light-emitting element 2 may be arbitrary, but is preferably a semiconductor light-emitting element, and among them, a laser diode. Further, the light-emitting element 2 is preferably a VCSEL among laser diodes.

[0032] The load 10 is provided between a first reference potential supply terminal that supplies a first reference potential (for example, a power supply potential) and a first node N1 and a second node N2. The load 10 supplies current to each of the first node N1 and the second node N2. The load 10 may be an active load or a passive load.

[0033] The differential circuit 20 is provided between a second reference potential supply terminal that supplies a second reference potential and the first node N1 and the second node N2. The second reference potential is at a lower potential than the first reference potential, for example, the ground potential. The differential circuit 20 outputs a signal corresponding to the input differential voltage signal (INP, INN) from the first node N1 and the second node N2. The configuration of the differential circuit 20 is arbitrary. The differential circuit 20 shown in this figure includes a first NPN transistor 21, a second NPN transistor 22, and a tail current source 23. The characteristics of the first NPN transistor 21 and the second NPN transistor 22 are the same as each other. The collector of the first NPN transistor 21 is connected to the first node N1. The collector of the second NPN transistor 22 is connected to the second node N2. The tail current source 23 is provided between the emitters of the first NPN transistor 21 and the second NPN transistor 22 and the second reference potential supply terminal. The differential voltage signal (INP, INN) is input to the bases of the first NPN transistor 21 and the second NPN transistor 22 respectively.

[0034] The balun circuit 30 has a first input terminal 31, a second input terminal 32, a first output terminal 33, and a second output terminal 34. The first input terminal 31 is connected to the first node N1. The second input terminal 32 is connected to the second node N2. The first output terminal 33 is connected to the light emitting element 2. The second output terminal 34 is connected to the dummy load 40.

[0035] The dummy load 40 is provided between the second output terminal 34 of the balun circuit 30 and the reference potential supply terminal. This reference potential supply terminal may be common with the second reference potential supply terminal or may be different. The dummy load 40 has impedance characteristics equivalent to those of the light emitting element 2.

[0036] FIG. 2 is a diagram showing a configuration example of the balun circuit 30. The balun circuit 30 includes a balun 50. The balun 50 has a configuration in which a first inductor 51 and a second inductor 52 are electromagnetically inductively coupled to each other. The first inductor 51 is provided between a first input terminal 31 and a first output terminal 33. The second inductor 52 is provided between a second input terminal 32 and a second output terminal 34. The balun circuit 30 including the balun 50 inputs a balanced signal to the first input terminal 31 and the second input terminal 32, and outputs an unbalanced signal corresponding to the balanced signal from the first output terminal 33. The balanced signals input to the first input terminal 31 and the second input terminal 32 are corresponding to differential voltage signals (INP, INN) input to the bases of the first NPN transistor 21 and the second NPN transistor 22 of the differential circuit 20, respectively. The unbalanced signal output from the first output terminal 33 is a current signal for driving the light emitting element 2.

[0037] FIG. 3 is a diagram showing a configuration example of the dummy load 40. The dummy load 40 has impedance characteristics equivalent to those of the light emitting element 2. When the light emitting element 2 is a laser diode, the dummy load 40 can have a configuration in which a resistor 41 and diodes 42 and 43 are connected in series. The diodes 42 and 43 can be configured by connecting the collector and the base of an NPN transistor to each other. By providing such a dummy load 40, the left-right symmetry of the circuit operation can be improved, the pattern dependence of the power supply current can be suppressed, and the power supply noise can be suppressed.

[0038] FIG. 4 is a diagram schematically showing signal waveforms at the first input terminal 31, the second input terminal 32, and the first output terminal 33 of the balun circuit 30. In this figure, the signal waveform of the first input terminal 31 is shown by a solid line, and the signal waveforms and inverted signal waveforms of the second input terminal 32 are shown by broken lines. In the light emitting element driving device 1, it is easy to increase the frequency characteristics of the differential circuit 20. Therefore, the waveforms of the signals at the first node N1 and the second node N2, that is, the signal waveforms at the first input terminal 31 and the second input terminal 32 of the balun circuit 30, have a slow rise while the fall is emphasized.

[0039] The signal output from the first output terminal 33 of the balun conversion circuit 30 is a signal in which a signal obtained by inverting the signal input to the second input terminal 32 is superimposed on the signal input to the first input terminal 31. The waveform of the signal obtained by inverting the signal input to the second input terminal 32 has a sharp rise and a slow fall. Therefore, the waveform of the signal output from the first output terminal 33 becomes a waveform in which only the rise is emphasized (pre-emphasis). Hereinafter, this rise emphasis is referred to as Rise edge emphasis.

[0040] By this Rise edge emphasis, the eye opening of the current signal output from the first output terminal 33 can be increased. When the current signal output from the first output terminal 33 is a PAM4 signal, it is possible to suppress the opening collapse of the Top eye among the three eyes of Top, Middle, and Bottom.

[0041] The effect of this Rise edge emphasis can be adjusted by providing other passive elements in addition to the balun 50 in the balun conversion circuit 30. Further, by making the parameters of the additional passive elements variable, the coupling coefficient between the first inductor 51 and the second inductor 52 in the balun 50 and the frequency characteristics of the dummy path (the signal path from the second input terminal 32 through the second inductor 52 and the second output terminal 34 to the dummy load 40) can be made variable, and the effect of the Rise edge emphasis can be made variable.

[0042] In this way, the light-emitting element driving device 1 can output a high-speed and large-amplitude current signal by enabling Rise edge emphasis with the balun conversion circuit 30 composed only of passive elements, and can suppress power consumption. Further, the light-emitting element driving device 1 can suppress eye degradation and eye noise components by high-speed and large-amplitude driving, so that other circuits (for example, DSP and Retimer that were conventionally required) can be made unnecessary, and low-latency operation is possible. The light-emitting element driving device 1 can meet various requirements in future signal transmission by optical communication.

[0043] Note that since the balun 50 utilizes electromagnetic induction coupling between the first inductor 51 and the second inductor 52, it is possible to maintain the frequency characteristics of the signal path without increasing the load of the signal path (the signal path from the first input terminal 31 through the first inductor 51 and the first output terminal 33 to the light-emitting element 2).

[0044] Next, another configuration example of the balun-unbalun conversion circuit 30 will be described. In the configuration examples shown in FIGS. 5 to 9, the balun-unbalun conversion circuit 30 includes other passive elements in addition to the balun 50, so that the coupling coefficient of the balun 50 or the frequency characteristics of the dummy path can be made variable, and the effect of Rise edge enhancement can be made variable.

[0045] The balun-unbalun conversion circuit 30A shown in FIG. 5 includes a variable capacitance section 61 in addition to the balun 50. The variable capacitance section 61 is provided between an intermediate node of the second inductor 52 in the dummy path and the reference potential supply terminal. The intermediate node of the second inductor 52 may be the center tap of the second inductor 52. This reference potential supply terminal may be common with the second reference potential supply terminal or may be different. In this configuration, when the capacitance value of the variable capacitance section 61 changes, the self-resonant frequency of the second inductor 52 changes, the effective inductance value at a specific frequency of the second inductor 52 changes, and the coupling coefficient between the first inductor 51 and the second inductor 52 in the balun 50 changes.

[0046] The balun-unbalun conversion circuit 30B shown in FIG. 6 includes, in addition to the balun 50, a third inductor 71, a fourth inductor 72, and a variable capacitance section 62. The third inductor 71 is provided between the first input terminal 31 and the first inductor 51. The fourth inductor 72 is provided between the second input terminal 32 and the second inductor 52. The third inductor 71 and the fourth inductor 72 are not electromagnetically inductively coupled to each other. The variable capacitance section 62 is provided between the node between the second inductor 52 and the fourth inductor 72 and the reference potential supply terminal. This reference potential supply terminal may be common with the second reference potential supply terminal or may be different. In this configuration, when the capacitance value of the variable capacitance section 62 changes, the self-resonant frequency of the second inductor 52 changes, the effective inductance value of the second inductor 52 at a specific frequency changes, and the coupling coefficient between the first inductor 51 and the second inductor 52 in the balun 50 changes.

[0047] The balun-unbalun conversion circuit 30C shown in FIG. 7 includes, in addition to the balun 50, a variable impedance element 81. The variable impedance element 81 is provided in parallel with the second inductor 52 between the second input terminal 32 and the second output terminal 34. The variable impedance element 81 may be a variable capacitance section or a variable resistor. In this configuration, when the impedance of the variable impedance element 81 changes, the self-resonant frequency of the second inductor 52 changes, the effective inductance value of the second inductor 52 at a specific frequency changes, and the coupling coefficient between the first inductor 51 and the second inductor 52 in the balun 50 changes.

[0048] The balun-unbalun conversion circuit 30D shown in FIG. 8 includes, in addition to the balun 50, a variable capacitance section 63. The variable capacitance section 63 is provided between the second output terminal 34 and the reference potential supply terminal. This reference potential supply terminal may be common with the second reference potential supply terminal or may be different. The variable capacitance section 63 is provided in parallel with the dummy load 40. In this configuration, when the capacitance value of the variable capacitance section 63 changes, the frequency characteristics of the dummy path change, and the Rise edge enhancement of the signal waveform at the first output terminal 33 changes.

[0049] The balun - unbalun conversion circuit 30E shown in FIG. 9 includes a variable capacitance section 64 in addition to the balun 50. The variable capacitance section 64 is provided between the second input terminal 32 and the reference potential supply terminal. This reference potential supply terminal may be common with or different from the second reference potential supply terminal. In this configuration, when the capacitance value of the variable capacitance section 64 changes, the self - resonance frequency of the second inductor 52 changes, and the frequency characteristics of the dummy path also change.

[0050] The balun - unbalun conversion circuit 30 may have a configuration that combines any two or more of the configuration examples shown in FIGS. 5 to 9. For example, the balun - unbalun conversion circuit 30 may include, in addition to the balun 50, the third inductor 71, the fourth inductor 72, and the variable capacitance section 62 in FIG. 6, and the variable capacitance section 63 in FIG. 8.

[0051] The load 10 and the differential circuit 20 of the light - emitting element driving device 1 are not limited to the configuration shown in FIG. 1, and may have the configurations shown in FIGS. 10 to 13.

[0052] The light - emitting element driving device 1A shown in FIG. 10 includes a load 10A as the load 10. This load 10A is an example of an active load and includes a first current source 11 and a second current source 12. The first current source 11 and the second current source 12 have similar configurations to each other. The first current source 11 is provided between the first potential supply terminal and the first node N1. The second current source 12 is provided between the first potential supply terminal and the second node N2. Each of the first current source 11 and the second current source 12 is constituted by a PMOS transistor, and by applying a bias voltage Vbias to the gate, a current flows from the first reference potential supply terminal to the nodes N1 and N2.

[0053] The light-emitting element driving device 1B shown in FIG. 11 is different from the configuration shown in FIG. 10 in that it includes a load 10B instead of the load 10A. The load 10B includes a first current source 11, a second current source 12, a first resistor 13, and a second resistor 14. The first current source 11 and the second current source 12 have similar configurations to each other. The first current source 11 and the first resistor 13 are connected in series with each other and provided between a first potential supply terminal and a first node N1. The second current source 12 and the second resistor 14 are connected in series with each other and provided between the first potential supply terminal and a second node N2.

[0054] The light-emitting element driving device 1C shown in FIG. 12 is different from the configuration shown in FIG. 10 in that it includes a differential circuit 20A instead of the differential circuit 20. The differential circuit 20A includes a first NPN transistor 21, a second NPN transistor 22, a resistor 24, a first tail current source 25, and a second tail current source 26. The first tail current source 25 is provided between the emitter of the first NPN transistor 21 and a second reference potential supply terminal. The second tail current source 26 is provided between the emitter of the second NPN transistor 22 and the second reference potential supply terminal. The resistor 24 is provided between the emitter of the first NPN transistor 21 and the emitter of the second NPN transistor 22. With such a configuration, emitter degeneration occurs and the transconductance of the differential pair is degenerated. The gain is suppressed, the input dynamic range is increased, and the linear operation of the light-emitting element driving device 1A becomes possible.

[0055] The light-emitting element driving device 1D shown in FIG. 13 is different from the configuration shown in FIG. 10 in that it includes a load 10D instead of the load 10A. The load 10D is a passive load and includes a first resistor 13 and a second resistor 14. The first resistor 13 is provided between the first reference potential supply terminal and the first node N1. The second resistor 14 is provided between the first reference potential supply terminal and the second node N2. Each of the first resistor 13 and the second resistor 14 may be a passive resistor or an active resistor. Even with such a configuration, the load 10D can supply current to each of the first node N1 and the second node N2.

Description of Reference Numerals

[0056] 1, 1A~1D... Light-emitting element driving device, 2... Light-emitting element, 10, 10A~10D... Load, 11... First current source, 12... Second current source, 13... First resistor, 14... Second resistor, 20, 20A... Differential circuit, 21... First NPN transistor, 22... Second NPN transistor, 23... Tail current source, 24... Resistor, 25... First tail current source, 26... Second tail current source, 30, 30A~30E... Balun, 31... First input terminal, 32... Second input terminal, 33... First output terminal, 34... Second output terminal, 40...Dummy load, 41... Resistor, 42, 43... Diode, 50... Balun, 51... First inductor, 52... Second inductor, 61~64... Variable capacitance section, 71... Third inductor, 72... Fourth inductor, 81... Variable impedance element, N1... First node, N2... Second node.

Claims

1. An apparatus for outputting a current signal for driving a light-emitting element, comprising: a load provided between a first reference potential supply terminal for supplying a first reference potential and a first node and a second node, and supplying current to each of the first node and the second node; a differential circuit provided between a second reference potential supply terminal for supplying a second reference potential lower than the first reference potential and the first node and the second node, and outputting a signal corresponding to an input differential voltage signal from the first node and the second node; a balun having a first input terminal, a second input terminal, a first output terminal, and a second output terminal, in which a first inductor and a second inductor are electromagnetically inductively coupled to each other, the first inductor being provided between the first input terminal and the first output terminal, the second inductor being provided between the second input terminal and the second output terminal, the first input terminal being connected to the first node, the second input terminal being connected to the second node, and a balun-unbalun conversion circuit for outputting the current signal from the first output terminal; a dummy load provided between the second output terminal of the balun-unbalun conversion circuit and a reference potential supply terminal; A light-emitting element driving device comprising the above.

2. The coupling coefficient between the first inductor and the second inductor of the balun, or the frequency characteristics of the signal path from the second input terminal of the balun through the second inductor and the second output terminal to the dummy load are variable, The light-emitting element driving device according to claim 1.

3. The balun-unbalun conversion circuit further includes a variable capacitance portion provided between an intermediate node of the second inductor and a reference potential supply terminal, The light-emitting element driving device according to claim 2.

4. The balun-unbalun conversion circuit includes: a third inductor provided between the first input terminal and the first inductor; a fourth inductor provided between the second input terminal and the second inductor; a variable capacitance portion provided between a node between the second inductor and the fourth inductor and a reference potential supply terminal; further including, The light-emitting element driving device according to claim 2.

5. The balun-unbalun conversion circuit further includes a variable impedance element provided in parallel with the second inductor between the second input terminal and the second output terminal, The light-emitting element driving device according to claim 2.

6. The balun circuit further includes a variable capacitance unit provided in parallel with the dummy load between the second output terminal and the reference potential supply terminal. The light-emitting element driving device according to claim 2.

7. The balun circuit further includes a variable capacitance unit provided between the second input terminal and the reference potential supply terminal. The light-emitting element driving device according to claim 2.

8. The differential circuit a first NPN transistor having a collector connected to the first node; a second NPN transistor having a collector connected to the second node; a tail current source provided between the emitters of the first NPN transistor and the second NPN transistor and the second reference potential supply terminal; and includes inputting the differential voltage signal to the bases of the first NPN transistor and the second NPN transistor respectively. The light-emitting element driving device according to claim 1.

9. The differential circuit a first NPN transistor having a collector connected to the first node; a second NPN transistor having a collector connected to the second node; a first tail current source provided between the emitter of the first NPN transistor and the second reference potential supply terminal; a second tail current source provided between the emitter of the second NPN transistor and the second reference potential supply terminal; a resistor provided between the emitter of the first NPN transistor and the emitter of the second NPN transistor; and includes inputting the differential voltage signal to the bases of the first NPN transistor and the second NPN transistor respectively. The light-emitting element driving device according to claim 1.

10. The load a first current source provided between the first reference potential supply terminal and the first node; a second current source provided between the first reference potential supply terminal and the second node; and includes The light-emitting element driving device according to claim 1.

11. The load a first resistor provided between the first reference potential supply terminal and the first node; a second resistor provided between the first reference potential supply terminal and the second node; and includes The light-emitting element driving device according to claim 1.

12. The dummy load has impedance characteristics equivalent to those of the light-emitting element. The light-emitting element driving device according to claim 1.

13. The dummy load includes a resistor and a diode connected in series with each other. The light-emitting element driving device according to claim 12.

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