JP2008153544A - Current source circuit and light-emitting element drive circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a current source circuit and a light-emitting element drive circuit capable of reducing control delay when outputting current and surely turning OFF the current of a light-emitting element when outputting no current. <P>SOLUTION: A switch 19a switches base connection of a transistor 15a either to a differential amplifier 13 or the ground. A switch 19b switches over the connection of the connection point 14a and an inverting input terminal of the differential amplifier 13. The inverting input terminal of the amplifier 13 is connected with a connection point 14b. The switch 19b connects the base of the transistor 15a with the differential amplifier 13 when outputting the current Iout, and grounds the base of the transistor 15a when stopping the output of the current Iout. The second switch connects the connection point 14a with the inverting input terminal of the amplifier 13 when outputting the current, and disconnects the connection point 14a from the inverting input terminal of the amplifier 13 when stopping the current output. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a current source circuit and a light emitting element driving circuit.

  As a configuration of a conventional current source circuit for supplying a current to a laser diode (hereinafter, LD) for optical communication, for example, there is one described in Patent Document 1 or 2. The circuit described in Patent Document 1 is shown in FIG. The circuit 100a includes an NPN transistor 101, a differential amplifier 102 that supplies a base voltage to the transistor 101, and a resistor 103 connected between the emitter of the transistor 101 and a ground line. A connection point 104 between the emitter and the resistor 103 is connected to the inverting input terminal of the differential amplifier 102 to constitute a negative feedback circuit. The circuit 100 a supplies a current proportional to the voltage Vin input to the non-inverting input terminal of the differential amplifier 102 to the LD connected to the collector of the transistor 101.

FIG. 8B shows a circuit described in Patent Document 2. The circuit 100b includes an NPN transistor 110 and a resistor 111 connected between the emitter of the transistor 110 and a ground line. The circuit 100 b supplies a current proportional to the voltage Vin input to the base of the transistor 110 to the LD connected to the collector of the transistor 110.
JP 2003-163547 A JP-A-6-164038

  As shown in FIG. 8A, the current source having a negative feedback loop formed of a differential amplifier has an advantage that the voltage / current conversion gain (transconductance) is constant regardless of the output current. Often used as a current source (bias current source, modulation current source) of an LD driving circuit. However, when the output voltage Vout of the differential amplifier 102 is controlled to 0 [V] to make the LD non-light-emitting, no current flows from the output terminal of the differential amplifier 102, and the transconductance of the transistor 101 is 0. Thus, the negative feedback loop is not formed. When the LD is switched to the light emission state, the voltage Vin is raised by an auto power control (APC) circuit provided in the LD driving circuit, but reaches the base-emitter voltage necessary for turning on the transistor 101. Until the negative feedback loop is not formed, a control delay occurs. Therefore, when the output of the APC circuit changes at high speed (such as during burst control), a transient response (such as overshoot) may be exhibited due to this control delay.

  When the output of the differential amplifier 102 includes an offset component, the output voltage Vout does not become 0 [V] even when the voltage Vin is controlled to 0 [V]. And a negative feedback loop is formed. In this case, although the voltage Vin is 0 [V], a current flows through the LD, and in some cases, a current exceeding the threshold current of the LD flows. Even if there is no offset in the differential amplifier, the minimum input voltage of the non-inverting terminal of the differential amplifier is usually a potential where the minimum output voltage of the circuit driving the differential amplifier is slightly higher than the ground potential 0V (several tens of mV). Therefore, a negative feedback loop is similarly formed, and a current flows through the LD. In some cases, a current exceeding the threshold current of the LD flows.

  In addition, the transconductance of the circuit 100a illustrated in FIG. 8A is a constant value such as 1 / Ra [V / A] (Ra: the resistance value of the resistor 103), whereas the circuit illustrated in FIG. The transconductance of 100b is gm / (1 + gm · Rb) [V / A] (gm: the transconductance of the transistor 110, Rb: the resistance value of the resistor 111), and changes nonlinearly depending on the current flowing through the transistor 110. That is, the transconductance of the circuit 100b is close to 1 / Rb [V / A] when the amount of current is large and becomes a substantially constant value, but is 1 / gm [V / A] when the amount of current is small. Become. In the configuration of the circuit 100b, when the input voltage Vin (that is, the base voltage) becomes 0 [V], the LD current can be surely turned off. However, such nonlinear characteristics still cause a control delay.

  The present invention has been made in view of the above problems, and a current source circuit and a light emitting element that can suppress a control delay at the time of current output and can reliably turn off the current of the light emitting element when no current is output. An object is to provide a driving circuit.

  In order to solve the above problems, a first current source circuit according to the present invention is a current source circuit that supplies current to a light emitting element, and includes a differential amplifier and a first current that supplies current to the light emitting element. A source, a second current source connected to the output of the differential amplifier and suppressing saturation of the output of the differential amplifier, and a switch for switching a connection state between the output of the differential amplifier and the first current source. It is characterized by that.

  According to the first current source circuit, when the current output to the light emitting element is stopped, the output of the differential amplifier and the first current source can be disconnected by the switch. Therefore, the output current to the light emitting element can be reliably turned off without depending on the input voltage value to the non-inverting input terminal of the differential amplifier and the offset component of the differential amplifier. On the other hand, since the output of the differential amplifier is connected to the second current source, the differential amplifier continues to operate. In addition, when outputting current to the light emitting element, the first current source and the output of the differential amplifier can be connected by a switch to supply current to the light emitting element.

  Thus, according to the first current source circuit, the differential amplifier continues to operate with the second current source even while the current output to the light emitting element is stopped. When starting output, the connection relationship with the first current source is quickly formed, and the control delay can be suppressed. Further, when no current is output, the output of the differential amplifier and the first current source are disconnected from each other, so that the current of the light emitting element can be reliably turned off.

  The first current source circuit includes a first current source, a first transistor having one current terminal connected to the light emitting element, and the other current terminal of the first transistor between the ground potential line. A second resistor having one current terminal connected to the power supply potential line, and the other current terminal of the second transistor and the ground potential line. A inverting input terminal of the differential amplifier is connected to a second connection point between the other current terminal of the second transistor and the second resistor, and the switch is connected to the switch. Is connected between the control terminal of the first transistor and the output of the differential amplifier, and the input of the control terminal of the first transistor is connected to either the output of the differential amplifier or the ground potential line. The other current of the switch and the first transistor A second switch for switching a connection state between the first connection point of the child and the first resistor and the inverting input terminal of the differential amplifier, and the first switch outputs a current to the light emitting element. The control terminal of the first transistor is connected to the output of the differential amplifier, and when the current output to the light emitting element is stopped, the control terminal of the first transistor is connected to the ground potential line, and the second switch When the current is output to the light emitting element, the first connection point and the inverting input terminal of the differential amplifier are connected, and when the current output to the light emitting element is stopped, the first connection point and the differential input terminal are differentially connected. The inverting input terminal of the amplifier may be disconnected.

  In the first current source circuit, when the current output to the light emitting element is stopped, the control terminal of the first transistor is connected to the ground potential line by the first switch, so that the non-inverting input of the differential amplifier The output current to the light emitting element can be reliably turned off without depending on the input voltage value at the end or the offset component of the differential amplifier. At the same time, the connection point (first connection point) between the other current terminal of the first transistor and the first resistor and the inverting input terminal of the differential amplifier are disconnected by the second switch. Therefore, the negative feedback path from the first transistor to the differential amplifier is blocked. On the other hand, since the inverting input terminal of the differential amplifier is connected to the connection point (second connection point) between the other current terminal of the second transistor and the second resistor, the non-inversion of the differential amplifier A negative feedback circuit composed of a differential amplifier, a second transistor, and a second resistor is formed by a slightly higher potential (several tens of mV) from 0 V applied to the input. The differential amplifier performs a negative feedback loop operation. Will continue to maintain.

When outputting current to the light emitting element, the control terminal of the first transistor and the output of the differential amplifier are connected by the first switch, and the first connection point and the inverting input terminal of the differential amplifier are connected to the first switch. Since the two switches are connected to each other, a negative feedback circuit including a differential amplifier, a first transistor, and a first resistor is formed, and current is supplied to the light emitting element. At this time, the transconductance as a current source for supplying current to the light emitting element is 1 / R ₁ [V / A] (R ₁ : first resistance value), and is constant regardless of the output current.

  As described above, according to the current source circuit, the transconductance when the current is output to the light emitting element is constant, and the differential amplifier is connected to the second amplifier while the current output to the light emitting element is stopped. Since the negative feedback loop operation with the transistor is continuously maintained, a negative feedback loop is quickly formed with the first transistor when starting the current output to the light emitting element, and the control delay can be suppressed. Further, when no current is output, the control terminal of the first transistor is connected to the ground potential line, so that the current of the light emitting element can be reliably turned off.

  A second current source circuit according to the present invention is a current source circuit that supplies current to a light emitting element, and includes a differential amplifier, a first current source that supplies current to the light emitting element, and a differential amplifier. A second current source for suppressing output saturation and a switch for connecting the output of the differential amplifier to one of the first and second current sources are provided.

  According to the second current source circuit, when the current output to the light emitting element is stopped, the output of the differential amplifier is connected to the second current source by the switch, and the output of the differential amplifier and the first current source are connected. The current source can be disconnected. Therefore, the output current to the light emitting element can be reliably turned off without depending on the input voltage value to the non-inverting input terminal of the differential amplifier and the offset component of the differential amplifier. On the other hand, since the output of the differential amplifier is connected to the second current source, the differential amplifier continues to operate. In addition, when outputting current to the light emitting element, the first current source and the output of the differential amplifier can be connected by a switch to supply current to the light emitting element.

  Thus, according to the second current source circuit, the differential amplifier continues to operate with the second current source while the current output to the light emitting element is stopped. When starting output, the connection relationship with the first current source is quickly formed, and the control delay can be suppressed. Further, when no current is output, the output of the differential amplifier is not connected to the first current source, so that the current of the light emitting element can be reliably turned off.

  The second current source circuit includes a first current source, a first transistor having one current terminal connected to the light emitting element, and the other current terminal of the first transistor between the ground potential line. A second resistor having one current terminal connected to the power supply potential line, and the other current terminal of the second transistor and the ground potential line. A third switch including a second resistor connected between the first and second transistors, wherein the switch connects the output of the differential amplifier to either the control terminal of the first transistor or the control terminal of the second transistor; And a fourth switch for connecting the control terminal of the first transistor to either the third switch or the ground potential line, and a differential amplifier. Invert input terminal A first connection point between the other current terminal of the first transistor and the first resistor, or a second connection point between the other current terminal of the second transistor and the second resistor. When the third and fourth switches output current to the light emitting element, the output of the differential amplifier and the control terminal of the first transistor are connected to stop the current to the light emitting element. The output of the differential amplifier and the control terminal of the second transistor are connected, the control terminal of the first transistor is connected to the ground potential line, and the fifth switch outputs a current to the light emitting element. Sometimes, connecting the inverting input terminal of the differential amplifier and the first connection point, and connecting the inverting input terminal of the differential amplifier and the second connection point when the current to the light emitting element is stopped. It is characterized by.

  In the second current source circuit, when the current output to the light emitting element is stopped, the control terminal of the first transistor is connected to the ground potential line by the fourth switch, so that the non-inverting input of the differential amplifier The output current to the light emitting element can be reliably turned off without depending on the input voltage value at the end or the offset component of the differential amplifier. At the same time, the first connection point and the inverting input terminal of the differential amplifier are disconnected by the fifth switch, so that the negative feedback path from the first transistor to the differential amplifier is blocked. On the other hand, the inverting input terminal of the differential amplifier is connected to the second connection point by the fifth switch, and the output terminal of the differential amplifier is connected to the control terminal of the second transistor by the third switch. A negative feedback circuit including a differential amplifier, a second transistor, and a second resistor is formed by a slightly higher potential (several tens of mV) from 0 V applied to the non-inverting input of the differential amplifier. A negative feedback loop operation is performed between the two transistors.

  When outputting current to the light emitting element, the control terminal of the first transistor and the output terminal of the differential amplifier are connected by the third and fourth switches, and the first connection point and the inverting input of the differential amplifier. Since the ends are connected by the fifth switch, a negative feedback circuit including a differential amplifier, a first transistor, and a first resistor is formed, and current is output to the light emitting element.

  According to the current source circuit, as in the first current source circuit described above, the transconductance when the current is output to the light emitting element is constant, and the current output to the light emitting element is stopped. Because the differential amplifier continues to maintain a negative feedback loop operation with the second transistor due to a slightly higher potential (several tens of mV) applied to the non-inverting input of the differential amplifier, a current is supplied to the light emitting element. Is outputted, a negative feedback loop is quickly formed between the first transistor and the control delay can be suppressed. Further, when no current is output, the control terminal of the first transistor is connected to the ground potential line, so that the current of the light emitting element can be reliably turned off.

  The light emitting element driving circuit according to the present invention is a light emitting element driving circuit for driving a light emitting element according to a transmission signal, and emits a modulation current according to the transmission signal and a bias unit for supplying a bias current to the light emitting element. And at least one of the bias unit and the modulation unit includes the current source circuit according to any one of claims 1 to 4. According to this light emitting element driving circuit, it is possible to suppress a control delay when supplying a current to the light emitting element, and to reliably turn off the current of the light emitting element when no current is supplied.

  According to the current source circuit and the light emitting element driving circuit of the present invention, it is possible to suppress a control delay at the time of outputting current and to reliably turn off the current of the light emitting element when no current is output.

  Embodiments of a current source circuit and a light emitting element driving circuit according to the present invention will be described below 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 description is omitted.

(First embodiment)
FIG. 1 is a diagram showing a configuration of a laser diode driving circuit (hereinafter referred to as an LD driving circuit) as a first embodiment of a light emitting element driving circuit according to the present invention. The LD drive circuit 1 is a circuit for driving the optical module 3. The optical module 3 includes a laser diode (hereinafter referred to as LD) 31 that generates signal light P, and a photodiode (hereinafter referred to as PD) 32 that detects (monitors) the signal light P. The LD 31 receives a drive current Id including a modulation current Imod corresponding to the transmission signal TX from the outside and a steady bias current Ibias from the LD drive circuit 1 and receives a signal light P which is a laser beam corresponding to the drive current Id. Output.

  The output characteristics of the LD 31 vary greatly depending on the temperature and the like. Therefore, in order to keep the signal light intensity and the extinction ratio constant, it is necessary to control the bias current Ibias and the modulation current Imod according to a temperature change or the like. Therefore, the LD drive circuit 1 supplies the drive current Id corresponding to the transmission signal TX to the LD 31, and the PD 32 detects the optical output at that time, and the drive current Id ( That is, the bias current Ibias and the modulation current Imod) are controlled.

  The LD drive circuit 1 includes a bias current source 4, a modulation current source 5, a modulation circuit unit 7, and an APC circuit 9. The bias current source 4 is connected to one end of the LD 31 and supplies a bias current Ibias having a magnitude corresponding to the signal Vb from the APC circuit 9 to the LD 31. Further, the modulation current source 5 and the modulation circuit unit 7 constitute a modulation unit for supplying the current Imod modulated in accordance with the transmission signal TX to the LD 31. The modulation current source 5 supplies a constant current Im having a magnitude corresponding to the signal Vm from the APC circuit 9 to the modulation circuit unit 7. A signal Sd for controlling light emission / non-light emission of the LD 31 is input from the outside of the LD drive circuit 1 to the bias current source 4 and the modulation current source 5. The bias current source 4 and the modulation current source 5 output a steady bias current Ibias and a constant current Im when the signal Sd indicates light emission, and stop the output when the signal Sd indicates non-light emission.

The modulation circuit unit 7 modulates the constant current Im according to the transmission signal TX, and generates a modulation current Imod. The modulation circuit unit 7 includes a pair of transistors 7a and 7b. The emitters of the transistors 7 a and 7 b are connected to each other and to the modulation current source 5. The collector of the transistor 7a is connected to the cathode of the LD 31, and the collector of the transistor 7b is connected to the power supply potential line Vcc via the resistor 8. Note that the anode of the LD 31 is connected to the power supply potential line Vcc. Bases of the transistors 7a, and the base of the transistor 7b are respectively transmit signals TX ₁ and the inverted signal TX ₂ from outside of the LD driving circuit 1 are input. Thus, the LD 31, a modulation current Imod is modulated in accordance with the transmission signal TX ₁ and the bias current Imod is supplied as the drive current Id.

  The APC circuit 9 generates signals Vb and Vm based on the signal Imon from the PD 32. That is, the APC circuit 9 controls the magnitudes of the bias current Ibias and the constant current Im so that the output signal Imon of the PD 32 becomes constant in order to keep the signal light intensity and extinction ratio of the LD 31 constant.

  FIG. 2 is a circuit diagram showing a common configuration (current source circuit) included in the bias current source 4 and the modulation current source 5 of the present embodiment. The current source circuit 10 shown in FIG. 2 includes a differential amplifier 13, a first transistor 15a, a second transistor 15b, a first resistor 17a, a second resistor 17b, a first switch 19a, and a second switch. 19b. Among these, the first transistor 15a and the first resistor 17a constitute a first current source for supplying current to the LD 31, and the second transistor 15b and the second resistor 17b are different from each other. A second current source for suppressing saturation of the output of the dynamic amplifier 13 is configured. The switches 19a and 19b constitute a switch for switching the connection state between the output of the differential amplifier 13 and the first current source.

  The current source circuit 10 outputs a current Iout having a magnitude corresponding to the signal Vb (or Vm) input to the input terminal 12a from the output terminal 12b. The output terminal 12b is connected to the LD 31 via the modulation circuit unit 7 shown in FIG. 1 or directly, and the current Iout is provided to the LD 31 as a bias current Ibias or a constant current Im. The current source circuit 10 outputs or stops the current Iout according to the value of the signal Sd input to the input terminal 12c.

  The transistors 15a and 15b are configured by, for example, NPN-type bipolar transistors. One current terminal (collector in the present embodiment) of the transistor 15a is connected to the output terminal 12b. A resistor 17a is connected between the other current terminal (emitter) of the transistor 15a and the ground potential line GND. The control terminal (base) of the transistor 15a is connected to the output terminal of the differential amplifier 13 or the ground potential line GND by the switch 19a. The differential amplifier 13 is a circuit for providing a control voltage Vc to each control terminal of the transistors 15a and 15b, and a signal Vb (or Vm) is input to the non-inverting input terminal via the input terminal 12a. .

  One current terminal (collector in this embodiment) of the transistor 15b is connected to the power supply potential line Vcc. A resistor 17b is connected between the other current terminal (emitter) of the transistor 15b and the ground potential line GND, and a connection point 14b (second connection point) between the transistor 15b and the resistor 17b is differential. It is connected to the inverting input terminal of the amplifier 13. A control terminal (base) of the transistor 15 b is connected to the output terminal of the differential amplifier 13.

  The switch 19a switches the connection destination of the base of the transistor 15a to either the output terminal of the differential amplifier 13 or the ground potential line GND. The switch 19a is controlled by the signal Sd. When the signal Sd is, for example, logic 0 (when the current Iout is output), the output terminal of the differential amplifier 13 and the base of the transistor 15a are connected. When the output of Iout is stopped), the base of the transistor 15a is connected to the ground potential line GND. The switch 19b switches the connection state between the connection point 14a (first connection point) between the emitter of the transistor 15a and the resistor 17a and the inverting input terminal of the differential amplifier 13. The switch 19b is controlled by the signal Sd. When the signal Sd is, for example, logic 0, the connection point 14a and the inverting input terminal of the differential amplifier 13 are connected, and when the signal Sd is logic 1, they are not connected.

  The effects of the current source circuit 10 having such a configuration will be described. When the logic of the signal Sd becomes 1, since the base of the transistor 15a is connected to the ground potential line GND by the switch 19a, the input voltage value to the non-inverting input terminal of the differential amplifier 13 and the offset component of the differential amplifier 13 Without depending, the output current Iout is reliably turned off. At the same time, since the connection point 14a and the inverting input terminal of the differential amplifier 13 are disconnected by the switch 19b, the negative feedback path from the transistor 15a to the differential amplifier 13 is blocked. On the other hand, since the inverting input terminal of the differential amplifier 13 is connected to the connection point 14b, a negative feedback circuit is formed by the differential amplifier 13, the transistor 15b, and the resistor 17b. The differential amplifier 13 is the transistor 15b. The negative feedback loop operation is continuously maintained between and the output of the differential amplifier 13 is suppressed from being saturated.

When the logic of the signal Sd becomes 0, the base of the transistor 15a and the output terminal of the differential amplifier 13 are connected by the switch 19a, and the connection point 14a and the inverting input terminal of the differential amplifier 13 are connected by the switch 19b. Therefore, a negative feedback circuit is formed by the differential amplifier 13, the transistor 15a, and the resistor 17a, and the current Iout is output from the output terminal 12b. At this time, the transconductance as a current source for supplying the current Iout to the LD 31 is constant at 1 / R ₁ [V / A] (R ₁ : the value of the resistor 17a).

  As described above, according to the current source circuit 10 of the present embodiment, the transconductance when the current Iout is output is constant regardless of the current Iout, and the differential amplifier can be used while the output of the current Iout is stopped. 13 continues the operation of the negative feedback loop with the transistor 15b. Therefore, when starting to output the current Iout, the negative feedback loop is quickly formed with the transistor 15a, and the control delay can be suppressed. Further, when the current Iout is not output, the current of the LD 31 can be reliably turned off.

  Here, the problems of the conventional current source circuit will be mentioned. FIG. 3 shows how the optical output rises when the differential amplifier 102 has no offset (or when the offset is negative) in the conventional current source circuit 100a shown in FIG. It is a graph which shows the mode of a change of roughly. FIG. 4 is a graph schematically showing how the optical output rises and how each signal changes when the differential amplifier 102 has a positive offset in the conventional current source circuit 100a. 3 and 4, (a) is the input voltage Vin (graph G1) to the differential amplifier 102, the output voltage Vout (graph G2) from the differential amplifier 102, and the inverting input terminal of the differential amplifier 102. The feedback voltage (graph G3) to is shown. (B) shows the output current from the current source circuit 100a, and (c) shows the light output of the light emitting element.

In order to stop the current output to the LD, the input voltage Vin is set to 0 [V]. However, if the differential amplifier 102 has no offset or the offset is negative, the output voltage Vc is 0 [V] or less. Therefore, no current flows through the transistor 101, the transconductance of the transistor 101 becomes 0, and a negative feedback loop is not formed. When the LD is switched to the light emitting state, the voltage Vin is increased by the APC circuit, but the negative feedback loop is not formed until the voltage between the base and the emitter necessary for turning on the transistor 101 is reached, and FIG. As shown in a), a control delay (delay time t ₁ ) occurs. For this reason, when the output of the APC circuit changes at high speed (such as during burst control), a transient response (such as overshoot) as shown in FIGS. 3B and 3C may occur due to this control delay.

When the output of the differential amplifier 102 includes a positive offset, as shown in FIG. 4A, the output voltage Vout takes a positive value V ₁ even if the voltage Vin is controlled to 0 [V]. Since a voltage V ₂ corresponding to this voltage V ₁ is generated at both ends of the resistor 103 and a current flows through the transistor 101, a negative feedback loop is formed. Accordingly, as shown in FIG. 4 (b), it will be the current I _A flowing to the LD despite the input voltage Vin is 0 [V], exceeds the threshold current of the LD as the case, the LD light emission state (FIG. 4C).

  On the other hand, the state of rising of the optical output and the state of change of each signal in the current source circuit 10 of this embodiment are shown in FIGS. 5A shows the input signal Vb or Vm (graph G4) to the differential amplifier 13 shown in FIG. 2, the output voltage Vc from the differential amplifier 13, and the feedback voltage to the inverting input terminal of the differential amplifier 13. FIG. (Both are graphs G5). 5B shows the output current Iout, FIG. 5C shows the optical output of the LD 31, and FIG. 5D shows the logic transition of the signal Sd.

According to the current source circuit 10 of the present embodiment, the negative feedback loop operation of the differential amplifier 13 is maintained even when the current Iout is not output (period B in the figure). As shown, the output voltage Vc of the differential amplifier 13 is maintained at a certain value V ₃ (> 0), the control delay at the start of current output is suppressed, and the current is almost simultaneously with the rise of the input signal Vb or Vm (graph G4). The output (FIG. 5 (b)) can be raised. Further, since the current of the LD 31 can be reliably turned off in the period B as shown in FIG. 5B, the LD 31 can be reliably turned off as shown in FIG. 5C.

  Here, the reason why the transistor 15b and the resistor 17b are provided in this embodiment will be described in detail. If the transistor 15b and the resistor 17b are not provided, the output voltage Vc of the differential amplifier 13 becomes unstable because the negative feedback loop is interrupted by the switches 19a and 19b when the current output is stopped. When the input offset of the differential amplifier 13 is large, the output voltage Vc is swung to one of the power supply voltage Vcc and the ground potential GND.

  FIG. 6 is a graph showing how the optical output rises and how each signal changes when the output voltage Vc is swung to a high voltage close to the power supply voltage Vcc. 6A shows an input signal Vb or Vm (graph G6), an output voltage Vc (graph G7), and a feedback voltage (graph G8) to the inverting input terminal of the differential amplifier 13. FIG. FIG. 6B shows the output current Iout, FIG. 6C shows the optical output of the LD 31, and FIG. 6D shows the logic transition of the signal Sd.

  When the negative feedback circuit is operated by switching the switches 19a and 19b in a state where the output voltage Vc is swung to a high voltage close to the power supply voltage Vcc, an excessive output current is generated as shown in FIGS. Iout flows instantaneously. As a result, the optical output exceeding the standard may be output from the LD 31.

  When the output voltage Vc is swung to the ground potential GND, the base potential of the transistor 15a becomes 0 [V]. Therefore, the transistor 15a does not turn on immediately, and the base-emitter voltage required for turn-on It turns on after a delay of a fixed time determined by the drive capability of the differential amplifier 13, and a negative feedback loop is formed. In this case, the rising state of the optical output and the changing state of each signal are the same as those shown in FIGS.

  On the other hand, when the transistor 15b and the resistor 17b are provided as in the present embodiment, the differential amplifier 13 can always perform a negative feedback loop operation regardless of the current output state of the current source circuit 10, so that the signal Sd A stable transient response is obtained with respect to the change as shown in FIG.

  In the present embodiment, the value of the resistor 17b is preferably larger than the value of the resistor 17a. When the ratio of the resistor 17a and the resistor 17b is 1: M (M> 1), a reactive current having a magnitude of Iout / M flows through the transistor 15b. Therefore, the reactive current can be reduced by setting M large. Further, when the current source circuit 10 of this embodiment is realized by an integrated circuit, if the ratio of the emitter lengths of the transistors 15a and 15b is set to 1: M, the voltage generated on the emitter side of the transistors 15a and 15b is almost equal. Therefore, the voltage value of the inverting input terminal of the differential amplifier 13 can be adjusted after switching the switch 19b.

(Second Embodiment)
FIG. 7 is a circuit diagram showing a configuration of a second embodiment of the current source circuit according to the present invention. Referring to FIG. 7, the current source circuit 11 of the present embodiment is similar to the current source circuit 10 of the first embodiment in that the differential amplifier 13, the first transistor 15a, the second transistor 15b, and the first resistor 17a and a second resistor 17b. The current source circuit 11 includes a third switch 21a, a fourth switch 21b, and a fifth switch 21c instead of the first switch 19a and the second switch 19b of the first embodiment. As in the first embodiment, the first transistor 15a and the first resistor 17a constitute a first current source for supplying a current to the LD 31, and the second transistor 15b and the second resistor The resistor 17b constitutes a second current source for suppressing the saturation of the output of the differential amplifier 13. In the present embodiment, the switches 21a to 21c constitute a switch for connecting the output of the differential amplifier 13 to one of the first and second current sources.

  The switch 21a switches the connection destination of the output terminal of the differential amplifier 13 to either the base of the transistor 15a or the base of the transistor 15b. The switch 21b is connected between the switch 21a and the transistor 15a, and switches the connection destination of the base of the transistor 15a to either the switch 21a or the ground potential line GND. The switches 21a and 21b are respectively controlled by a signal Sd, and connect the output terminal of the differential amplifier 13 and the base of the transistor 15a when the signal Sd is, for example, logic 0 (when the current Iout is output). In the case of logic 1 (when the output of the current Iout is stopped), the switch 21a connects the output terminal of the differential amplifier 13 and the base of the transistor 15b, and the switch 21b connects the base of the transistor 15a to the ground potential line. Connect to GND.

  The switch 21c switches the connection destination of the inverting input terminal of the differential amplifier 13 to one of the connection points 14a and 14b. The switch 21c is controlled by the signal Sd. When the signal Sd is, for example, logic 0, the inverting input terminal of the differential amplifier 13 and the connection point 14a are connected. When the signal Sd is logic 1, the inverting input terminal of the differential amplifier 13 is connected. The connection point 14b is connected.

  The effects of the current source circuit 11 having such a configuration will be described. When the logic of the signal Sd becomes 1, the base of the transistor 15a is connected to the ground potential line GND by the switch 21b, so that the input voltage value to the non-inverting input terminal of the differential amplifier 13 and the offset component of the differential amplifier 13 Without depending, the output current Iout is reliably turned off. At the same time, the connection point 14a and the inverting input terminal of the differential amplifier 13 are disconnected by the switch 21c, so that the negative feedback path from the transistor 15a to the differential amplifier 13 is blocked. On the other hand, the inverting input terminal of the differential amplifier 13 is connected to the connection point 14b by the switch 21c, and the output terminal of the differential amplifier 13 is connected to the base of the transistor 15b by the switch 21b. , And a resistor 17b is formed, and the differential amplifier 13 performs a negative feedback loop operation with the transistor 15b.

When the logic of the signal Sd becomes 0, the base of the transistor 15a and the output terminal of the differential amplifier 13 are connected by the switches 21a and 21b, and the connection point 14a and the inverting input terminal of the differential amplifier 13 are connected by the switch 21c. Since they are connected, a negative feedback circuit is formed by the differential amplifier 13, the transistor 15a, and the resistor 17a, and the current Iout is output. At this time, the transconductance as a current source for supplying the current Iout to the LD 31 is 1 / R ₁ [V / A] (R ₁ : the value of the resistor 17a) as in the current source circuit 10 of the first embodiment. It becomes constant.

  Thus, according to the current source circuit 11 of the present embodiment, as in the current source circuit 10 of the first embodiment, the transconductance when the current Iout is output is constant regardless of the current Iout, and the current Since the differential amplifier 13 continues the operation of the negative feedback loop with the transistor 15b while the output of Iout is stopped, the negative feedback loop quickly with the transistor 15a when the output of the current Iout is started. Formed, and control delay can be suppressed. Further, when the current Iout is not output, the current of the LD 31 can be reliably turned off.

  In the current source circuit 11 of the present embodiment, unlike the current source circuit 10 of the first embodiment, no current (invalid current) flows through the transistor 15b and the resistor 17b while the current Iout is being output. It is possible to reduce power consumption when outputting Iout.

  The current source circuit and the light emitting element driving circuit according to the present invention are not limited to the above-described embodiments, and various other modifications are possible. For example, in each of the above embodiments, bipolar transistors are exemplified as the first and second transistors. However, a field effect transistor such as a MOSFET may be used in the present invention. In this case, the control terminal in each embodiment is read as the gate, and the current terminal is read as the source or drain.

  In each of the above embodiments, the current source circuit according to the present invention is applied to both the bias current source and the modulation current source. However, the current source circuit of the present invention may be applied to only one of them. .

FIG. 1 is a diagram showing a configuration of an LD driving circuit as a first embodiment of a light emitting element driving circuit according to the present invention. FIG. 2 is a circuit diagram showing a common configuration (current source circuit) included in the bias current source and the modulation current source of this embodiment. FIG. 3 shows how the optical output rises when the differential amplifier has no offset (or when the offset is negative) in the conventional current source circuit shown in FIG. It is a graph which shows a mode roughly. FIG. 4 is a graph schematically showing how the optical output rises and how each signal changes when the differential amplifier has a positive offset in the conventional current source circuit shown in FIG. is there. FIG. 5 is a graph showing how the optical output rises and how each signal changes in the current source circuit of the first embodiment. (A) shows the input signal to the differential amplifier (graph G4), the output voltage from the differential amplifier, and the feedback voltage to the inverting input terminal of the differential amplifier (both graph G5). (B) shows the output current. (C) shows the optical output of the LD. (D) shows the logic transition of the signal for controlling each switch. FIG. 6 is a graph showing how the optical output rises and how each signal changes when the output voltage of the differential amplifier is swung to a high voltage close to the power supply voltage. (A) shows the input signal to the differential amplifier (graph G6), the output voltage from the differential amplifier (graph G7), and the feedback voltage to the inverting input terminal of the differential amplifier (graph G8). (B) shows the output current. (C) shows the optical output of the LD. (D) shows the logic transition of the signal for controlling each switch. FIG. 7 is a circuit diagram showing a configuration of a second embodiment of the current source circuit according to the present invention. FIGS. 8A and 8B are circuits described in Patent Documents 1 and 2, respectively.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... LD drive circuit, 3 ... Optical module, 4 ... Bias current source, 5 ... Modulation current source, 7 ... Modulation circuit part, 9 ... APC circuit, 10, 11 ... Current source circuit, 13 ... Differential amplifier, 14a ... 1st connection point, 14b ... 2nd connection point, 15a ... 1st transistor, 15b ... 2nd transistor, 17a ... 1st resistance, 17b ... 2nd resistance, 19a ... 1st switch, 19b ... 2nd switch, 21a ... 3rd switch, 21b ... 4th switch, 21c ... 5th switch.

Claims (5)

A current source circuit for supplying current to the light emitting element,
A differential amplifier;
A first current source for supplying a current to the light emitting element;
A second current source connected to the output of the differential amplifier to suppress saturation of the output of the differential amplifier;
A current source circuit comprising: a switch for switching a connection state between the output of the differential amplifier and the first current source. The first current source includes a first transistor having one current terminal connected to the light emitting element, and a first transistor connected between the other current terminal of the first transistor and a ground potential line. Including resistance,
The second current source includes a second transistor having one current terminal connected to a power supply potential line, and a second transistor connected between the other current terminal of the second transistor and a ground potential line. Including resistance,
The inverting input terminal of the differential amplifier is connected to a second connection point between the other current terminal of the second transistor and the second resistor,
The switch is connected between a control terminal of the first transistor and an output of the differential amplifier, and an input of the control terminal of the first transistor is either an output of the differential amplifier or a ground potential line. A first switch connected to the first switch, and a first connection point between the other current terminal of the first transistor and the first resistor, and a first switch that switches a connection state between the inverting input terminal of the differential amplifier Including two switches,
The first switch connects the control terminal of the first transistor and the output of the differential amplifier when outputting current to the light emitting element, and stops outputting current to the light emitting element. Connecting the control terminal of the first transistor to a ground potential line;
The second switch connects the first connection point and the inverting input terminal of the differential amplifier when outputting a current to the light emitting element, and stops outputting the current to the light emitting element. The current source circuit according to claim 1, wherein the first connection point and the inverting input terminal of the differential amplifier are disconnected. A current source circuit for supplying current to the light emitting element,
A differential amplifier;
A first current source for supplying a current to the light emitting element;
A second current source for suppressing saturation of the output of the differential amplifier;
And a switch for connecting an output of the differential amplifier to one of the first and second current sources. The first current source includes a first transistor having one current terminal connected to the light emitting element, and a first transistor connected between the other current terminal of the first transistor and a ground potential line. Including resistance,
The second current source includes a second transistor having one current terminal connected to a power supply potential line, and a second transistor connected between the other current terminal of the second transistor and a ground potential line. Including resistance,
The switch includes a third switch that connects an output of the differential amplifier to either a control terminal of the first transistor or a control terminal of the second transistor, the third switch, and the first transistor. A fourth switch for connecting the control terminal of the first transistor to either the third switch or a ground potential line, and an inverting input terminal of the differential amplifier. , A first connection point between the other current terminal of the first transistor and the first resistor, or a second connection point between the other current terminal of the second transistor and the second resistor. Including a fifth switch connected to either
The third and fourth switches, when outputting current to the light emitting element, connect the output of the differential amplifier and the control terminal of the first transistor, and stop the current to the light emitting element. Sometimes, connecting the output of the differential amplifier and the control terminal of the second transistor, connecting the control terminal of the first transistor to a ground potential line,
The fifth switch connects an inverting input terminal of the differential amplifier and the first connection point when outputting a current to the light emitting element, and stops a current to the light emitting element. The current source circuit according to claim 3, wherein an inverting input terminal of the differential amplifier is connected to the second connection point. A light emitting element driving circuit for driving a light emitting element in response to a transmission signal,
A bias unit for supplying a bias current to the light emitting element;
A modulation unit for supplying a modulation current corresponding to the transmission signal to the light emitting element,
4. The light emitting element driving circuit according to claim 1, wherein at least one of the bias unit and the modulation unit includes the current source circuit according to claim 1.

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