JP4889341B2 - Current output circuit - Google Patents

Description

  The present invention relates to a current output circuit for driving a laser diode. More specifically, the present invention relates to a current output circuit capable of supplying a stable high current at a low output for reading a laser diode and a low noise at a high output for writing.

  As shown in FIG. 4, a conventional current output circuit for driving a laser diode requires a current of 50 to 100 mA at the time of reading and a current of 200 to 500 mA or more at the time of writing. Required. Furthermore, at the time of reading, since it is necessary to identify a small signal with a small laser beam output, low noise is required to improve the S / N ratio.

  As such a current output circuit, for example, a configuration as shown in FIG. 5 is known. That is, in FIG. 5, an input resistor connected between one input terminal (positive terminal) of an amplifier AMP4 that generates a constant current using a virtual short circuit, together with the input terminal IN4, and ground to convert the current into a voltage. One end of R41 is connected. The output of the amplifier AMP4 is connected to the base of the NPN transistor Q41, the emitter of the NPN transistor Q41 is connected to the ground via the resistor R43, and the collector is connected to the power supply terminal Vcc.

The connection point between the emitter of NPN transistor Q41 and resistor R43 is connected to the base of NPN transistor Q42 for outputting a reference current and the base of NPN transistor Q43 for outputting an output current, respectively. The emitter is connected to the ground, the collector is connected to the collector of one PNP transistor Q44 constituting the current mirror circuit, the PNP transistor Q44 has its emitter connected to the power supply terminal V _CC , and its collector and base are connected. And connected to the base of the other PNP transistor Q45 constituting the current mirror circuit. The emitter of the other PNP transistor Q45 is connected to the power supply terminal V _CC , the collector is connected to the ground via the load resistor R42 of the amplifier AMP4, and the connection point between the collector and the load resistor R42 is the other input terminal (negative) of the amplifier AMP4. Terminal), the reference current of the transistor Q42 is turned back to the amplifier AMP4. The emitter of the NPN transistor Q43 is connected to the ground, and the collector is the output terminal OUT4.

In the conventional current output circuit shown in FIG. 5, the current output to the output terminal OUT4 adjusts the area ratio of the PNP current mirror circuit composed of transistors Q44 and Q45 and the output circuit composed of transistors Q42 and Q43. As a result, it is possible to set almost unlimited, so that a high output current for writing is easily obtained. However, the current output noise is the sum of the thermal noise 4KTR ₄₁ (K is the Boltzmann constant, T is the absolute temperature, R ₄₁ is the resistance value of the input resistor R41) of the input resistor R41 and the input converted noise voltage V _NO4 at the amplifier AMP4. in divided by the square of the resistance value R ₄₂ of the resistor R42, further multiplied by the current amplification factor of the current mirror circuit, adding the noise generated by the transistors Q44, Q45, further to the transistors Q42, Q43 Is multiplied by the current amplification factor and the noise generated in the transistors Q42 and Q43 is added.

That is, if the resistance value of the load resistor R42 can be increased and the current can be directly output from the load resistor R42, the noise of the output current can be reduced, but from the current mirror circuit and the transistors Q42 and Q43, The output noise is amplified through the output circuit, so that the noise of the previous stage is amplified by these transistors, and the noise of these transistors is also added. There is a problem that the noise at the can not be kept low.

  The present invention has been made to solve such a problem, and provides a current output circuit capable of reducing noise of an output current for low current and simultaneously obtaining an output current for high current. For the purpose.

The present invention is a current output circuit for driving a laser diode, comprising a current output circuit for low current and a current output circuit for high current at least having a common output terminal, wherein the current output circuit for low current comprises: (A) a first amplifier that generates a constant current using a virtual short circuit that is a characteristic of the amplifier; and (b) one end of the first amplifier is connected to one input end of the first amplifier together with an input terminal, and the other end is A first input resistor which is connected to ground and converts current into voltage and which can set an output current; and (c) one end connected to the other input end of the first amplifier and the other end A first load resistor of the first amplifier that is connected to ground, sets an output current value by a ratio with the first input resistor, and can reduce output current noise; and (d) Receives the differential output of the first amplifier and receives the signal from one transistor. A first pair of transistors whose bases are connected to each other, and (e) a first current source comprising a current mirror circuit of a transistor for supplying a current to the first pair of transistors; (F) An emitter or a collector is connected to the one end of the first load resistor, a base is connected to the output of the pair of transistors to form a feedback circuit, and a first output current is connected to the output terminal. A high-current current output circuit (g) a second amplifier that generates a constant current using a virtual short circuit that is a characteristic of the amplifier; A second input resistor for converting a current into a voltage with one end connected to one input terminal of the second amplifier together with the input terminal and the other end connected to the ground; and (i) the second amplifier. One end to the other input end A second load resistor having a second end connected to the ground and having a current value of a first reference current set by a ratio to the second input resistor; and (j) the second amplifier. A second pair of transistors having bases connected to each other for receiving a differential output and outputting a signal from one transistor; and (k) a transistor for supplying a current to the second pair of transistors. A second current source comprising a current mirror circuit; and (l) one end of the first resistor is connected to the emitter or collector, the other end is connected to ground, and the base is the output of the second pair of transistors. A first transistor connected to a first portion; (m) a second transistor that connects the one end of the first resistor to a base and outputs a second reference current from a collector; and the second reference Current A first current mirror circuit that is turned back by the amplification factor to be the first reference current; and (n) a base is connected to the one end of the first resistor, and a ratio of the area of the transistor to the second transistor is set to a predetermined value. And a second output transistor for supplying a second output current to the output terminal.

  According to the current output circuit of the present invention, the current output circuit for low current and the current output circuit for high current are configured separately, and both are output from the same output terminal, while the current output circuit for low current is output. Has a circuit configuration in which a feedback resistor (load resistor) that feeds back to the amplifier is provided immediately below the output extraction transistor, so that the output current can be made extremely low noise by increasing the resistance value of the load resistor of the amplifier. Can do. On the other hand, the current output circuit for high current has a different circuit configuration from the current output circuit for low current while sharing the output terminal with the current output circuit for low current, so there is no need to worry about the effects of noise. By adjusting the area ratio between the current mirror circuit and the circuit on the output transistor side, the current can be increased freely. As a result, a very low noise current can be supplied for low current operation during reading, and a current sufficient for writing can be supplied for high current operation during writing.

  In addition, since the capacitor is connected between the base of the output transistor and the ground, the output current noise in the high frequency band can be reduced, so that the bandwidth of the feedback circuit related to noise can be reduced, and further Noise can be reduced.

Next, the current output circuit of the present invention will be described with reference to the drawings. The current output circuit for driving a laser diode according to the present invention has at least an output terminal in common as shown in FIG. 1 as an equivalent circuit diagram of one embodiment, and a low current current output circuit A and a high current current. The output circuit B is connected in parallel, and the low-current current output circuit A is configured as follows. Although not shown, it is preferable that a switch element is inserted on the output terminal OUT1 side of each of the low-current current output circuit A and the high-current current output circuit B to divide the unused circuit. Further, in the example shown in FIG. 1, the input terminal IN1 of the low-current current output circuit A and the input terminal IN2 of the high-current current output circuit B are provided separately, but the input terminals are also shared, It can also be switched by a switch element.

  In the current output circuit A for low current according to the present invention, one end of an input resistor R11 is connected to one input end (positive terminal) of an amplifier AMP1 that generates a constant current using a virtual short circuit that is a characteristic of the amplifier together with the input terminal IN1. The other end of the input resistor R11 is connected to the ground. One end of the load resistor R12 of the amplifier AMP1 is connected to the other input terminal (negative terminal) of the amplifier AMP1, and the other end of the load resistor R12 is connected to the ground. The differential output of the amplifier AMP1 is input to the emitters of a pair of transistors Q12 and Q13 whose bases for outputting signals are connected to each other, and the output signal from one of the transistors Q13 is input to the base of the output transistor Q11. At the same time, it is connected to a current supply circuit comprising a current mirror circuit formed by three transistors Q14, Q15, Q16 and a current source I1 for supplying a current to the pair of transistors Q12, Q13. The emitter or collector of the output transistor Q11 is connected to one end of the load resistor R12 to constitute a feedback circuit. The collector or emitter is connected to the output terminal OUT1 and the output current Iref1 is taken out. .

  The amplifier AMP1 amplifies two input voltages, outputs a differential output, and fluctuates a current value flowing through a pair of transistors Q12 and Q13 that extract the output. The input resistor R11 converts a current input from the input terminal IN1 into a voltage and inputs the voltage to one input terminal (positive terminal) of the amplifier AMP1, and also has a function of setting an output current. One end of the load resistor R12 of the amplifier AMP1 is connected to the other input end (negative terminal) of the amplifier AMP1, and the other end is connected to the ground. The output current value is set by the ratio with the input resistor R11. As described later, by increasing the resistance value of the load resistor R12, the current noise is inversely proportional to the square of the resistance value, so that the current noise can be reduced and the current noise can be controlled. is doing. Then, one end of the load resistor R12 is connected to the emitter or collector of the output transistor Q11, for example, the emitter of the NPN transistor Q11, and the collector is connected to the output terminal OUT1 to constitute a circuit for supplying an output current. Yes. That is, the present invention is characterized in that the current noise can be reduced by connecting the load resistor R12 directly below the output transistor Q11.

  The pair of transistors Q12 and Q13 take out the differential output of the amplifier AMP1. For example, the bases of the NPN transistors Q12 and Q13 are connected to each other and to the collector of the NPN transistor Q12, and each collector is a current mirror. For example, it is connected to the collectors of a pair of PNP transistors Q15 and Q16 constituting the circuit. As described above, the collector of the transistor Q13 is connected to the base of the output transistor Q11 made of, for example, an NPN transistor.

The current mirror circuit constitutes a current supply circuit. For example, the collectors of the pair of NPN transistors Q12 and Q13 are connected to the collectors, for example, PNP transistors Q15 and Q16, and the bases and bases of the transistors Q15 and Q16 are mutually connected. For example, the PNP transistor Q14 and the current source I1 connected between the collector of the PNP transistor Q14 and the ground are connected to the pair of transistors Q12 and Q13. The emitters of the PNP transistors Q14, Q15, and Q16 are each connected to the power supply terminal Vcc, and the base and collector of the transistor Q14 are short-circuited and connected to the current source I1.

The high-current current output circuit B has the following configuration. That is, one end of the input resistor R21 is connected to one input end (positive terminal) of the amplifier AMP2 that generates a constant current using a virtual short circuit that is a characteristic of the amplifier, together with the input terminal IN2, and the other end of the input resistor R21 is Connected to ground. The input resistor R21 converts an input current into a voltage and sets a reference current I21. One end of the load resistor R22 of the amplifier AMP2 is connected to the other input terminal (negative terminal) of the amplifier AMP2, and the other end of the load resistor R22 is connected to the ground. This load resistor R22 sets the feedback output current I21 value by the ratio of the resistance value to the input resistor R21. The differential output of the amplifier AMP2 is input to the emitters of a pair of transistors Q21 and Q22 formed by, for example, NPN transistors Q21 and Q22, and supplies current to the pair of transistors Q21 and Q22 in order to make a signal output. Are connected to a current supply circuit comprising a current mirror circuit formed by three transistors Q26, Q27, Q28 and a current source I2.

  That is, the bases of the pair of transistors Q21 and Q22 are connected to each other and to the collector of the transistor Q21, and each collector is connected to the collectors of a pair of, for example, PNP transistors Q27 and Q28 constituting the current mirror circuit. As described above, the collector of the transistor Q22 is connected to the base of the transistor Q23. The emitters of the pair of transistors Q27 and Q28 constituting the current mirror circuit are connected to the power supply terminal Vcc, and the bases of the transistors Q27 and Q28 are connected to each other and supply current, for example, connected to the base of a PNP transistor Q26. The base and collector of Q26 are short-circuited and connected to the ground via the current source I2, and the collector is connected to the power supply terminal Vcc.

  An output signal from the collector of the transistor Q22 of the pair of transistors supplies an output current, for example, is input to the base of an NPN transistor Q23, the emitter of the transistor Q23 is connected to the ground via a resistor R23, and the collector is a power supply terminal. Connected to Vcc. The transistor Q23 supplies a base current to, for example, the NPN transistor Q24 for outputting the reference current I22, for example, and for outputting the output current Iref2, for example, an NPN transistor Q25, and a resistor R23 is connected to the transistors Q24, Q25. A base-emitter voltage Vbe is generated. The collector of the transistor Q24 is connected to the collector of a transistor Q210 that constitutes a current mirror circuit composed of, for example, PNP transistors Q29 and Q210 for turning the reference current I22 back to the reference current I21, and the emitter is connected to the ground. Further, the emitter of the output transistor Q25 that extracts the output current is connected to the ground, and the collector is connected to the output terminal OUT1, whereby the high output current Iref2 is extracted from the output terminal OUT1.

  Further, the transistors Q29 and Q210 of the current mirror circuit that returns the reference current I22 to the reference current I21 and feeds it back to the amplifier AMP2, for example, are PNP transistors, and their bases are connected to each other, and the collector and base of the transistor Q210 Are short-circuited and connected to the collector of the transistor Q24 described above, and the emitter is connected to the power supply terminal Vcc. The collector of the transistor Q29 is connected to the ground via the aforementioned load resistor R22, and the emitter is connected to the power supply terminal Vcc. As described above, the connection point between the load resistor R22 and the collector is connected to the other input terminal of the amplifier AMP2, so that the output current is fed back.

Next, the operation of this circuit will be described. First, in the current output circuit A for low current, the voltage value on the negative terminal side, that is, the voltage value of the load resistor R12 is set by the input voltage value or the input current × the voltage value of the input resistor R11 according to the virtual short circuit characteristic of the amplifier AMP1. Then, the output current Iref1 is set by the set voltage value / resistance R ₁₂ (resistance value of the resistor R12). That is, the output current Iref1 can be set by the ratio of the resistance of the input resistor R11 and the load resistor R12. At this time, the output current noise is the sum of the thermal noise generated by the input resistor R11 and the input conversion value of the noise generated by the amplifier AMP1 divided by the square of the resistance value of the resistor R12. It becomes current noise. Therefore, the output current noise can be reduced by increasing the resistance value of the load resistor R12. The maximum output current, since it is when the Vce of Vcc = Iref1 × R ₁₂ + Q11 of Vbe + Q16, to reduce the output current noise, a large resistance value R ₁₂ of the load resistor R12, the maximum output current Will decline. Therefore, when a high output current is used, the high output current circuit B is used.

In the high-output-current circuit B, the characteristics of the virtual short circuit of the amplifier AMP2, the voltage value of the resistance value R ₂₁ of the input voltage or input current × input resistor R21, the voltage value of the negative terminal side, that is, the voltage of the load resistor R22 set the value to set the reference current I21 by the resistance value R ₂₂ of the voltage values / load resistor R22 that the set, the reference current I21, using the current mirror circuit formed by transistors Q29, Q210, a reference current I22 Wrap as. In other words, the reference current I22 can be set to an arbitrary value of I22 = I21 × a, assuming that the current amplification factor a of the current mirror circuit of the transistors Q29 and Q210. The output current Iref2 can be set as Iref2 = I22 × b = I21 × a × b, where b is the transistor area ratio of the transistors Q24 and Q25. Maximum output current at this time, Vcc = I21 for is when a × R _22, adjusting the subsequent transistor Q29, the current amplification factor a and the transistor of the current mirror circuit of Q210 Q24, Q25 transistor area ratio b of Thus, it is possible to set the maximum output current indefinitely.

On the other hand, the output current noise, summing the input conversion value of the thermal noise and noise generated by the amplifier AMP2 of the input resistor R21, to it divided by the square of the resistance value R ₂₂ of the load resistor R22, a current mirror circuit Is multiplied by the noise generated in the transistors Q29 and Q210 of the current mirror circuit, and this value is multiplied by the area ratio b of the transistors Q24 and Q25, and further the noise generated in the transistors Q24 and Q25 is added. The combined result is output. Therefore, in the high output current circuit B, noise generated in the transistors Q29, Q210, Q24, and Q25 is included more than in the low output current circuit A, and thus it is difficult to obtain a low noise current output circuit. However, since the current output circuit of the present invention separately incorporates the low noise current output circuit A for low current, the low current output circuit A can be used when low noise is required. Since the current output circuit B can be used only when a large current value is required for writing or erasing the laser diode, there is no noise problem. That is, in the present invention, since a low-noise current output circuit for low current and a high-current current output circuit capable of obtaining a high output current are provided, both characteristics can be satisfied by using each circuit separately. Can do.

  FIG. 2 is an equivalent circuit diagram similar to FIG. 1 showing another embodiment of the present invention. In this example, a capacitor C11 is connected between the base of the NPN transistor Q11, which is an output transistor of the current output circuit A for low current, for example, and the ground. The other parts of the low-current current output circuit A and the high-current current output circuit B are the same as the circuit shown in FIG. 1 described above, and the same parts are denoted by the same reference numerals and description thereof is omitted. This capacitor C11 short-circuits high-frequency noise and lowers the bandwidth of the feedback circuit for noise, thereby making the output current lower noise. That is, since the resistance component of the capacitor (capacitance value C) is 1 / jωC, a noise component having a high frequency is short-circuited. When the capacitor is connected to FIG. As shown in the graph of the noise characteristic with respect to the frequency, the noise in the high frequency region can be reduced.

As described above, according to the present invention, the current output circuit is configured separately from the low-current current output circuit for reading and the high-current current output circuit for writing, and the output terminal is shared. The current output circuit for low current is a current output circuit using the virtual short-circuit characteristic of the amplifier, and the load resistance of the amplifier is connected at the node closest to the output, so the resistance value of the load resistance of the amplifier is increased. And a low noise current can be supplied. On the other hand, at the time of writing, since a current output circuit for high current that can freely adjust the area ratio of the current mirror circuit and the output circuit is used, a large current can be obtained without any problem. As a result, reading data driven with a low current can be operated with very low noise, and the signal-to-noise ratio can be greatly improved, but a sufficient laser output can be obtained even in writing. Can be written.

It is an equivalent circuit diagram showing an embodiment of a current output circuit according to the present invention. It is an equivalent circuit diagram which shows other embodiment of the current output circuit by this invention. It is a figure which shows the frequency characteristic of the noise reduction by a capacitor. It is a figure which shows the relationship of the operating current with respect to reading, erasing, and writing of a laser diode. It is an equivalent circuit diagram of a current output circuit for driving a conventional laser diode.

Explanation of symbols

Q11 to Q13, Q21 to Q25 NPN transistors Q14 to Q16, Q26 to Q29, Q210 PNP transistors C11 Capacitors R11 to R12, R21 to R23 Resistors AMP1, AMP2 Amplifiers I1, I2 Current sources

Claims (1)

A current output circuit for driving a laser diode comprising a current output circuit for low current and a current output circuit for high current, at least having a common output terminal;
The current output circuit for low current includes (a) a first amplifier that generates a constant current using a virtual short circuit that is a characteristic of the amplifier, and (b) one input terminal of the first amplifier together with an input terminal. A first input resistor capable of converting a current into a voltage and setting an output current, and (c) another input terminal of the first amplifier. One end of the first amplifier is connected to the other end, and the other end is connected to the ground, the output current value is set according to the ratio with the first input resistance, and the output current noise can be reduced. (D) a first pair of transistors having bases connected to each other for receiving a differential output of the first amplifier and outputting a signal from one transistor; A transistor current for supplying current to the first pair of transistors. A first current source comprising a mirror circuit; and (f) an emitter or collector connected to the one end of the first load resistor, and a base connected to the output of the pair of transistors to constitute a feedback circuit. A first output transistor for supplying a first output current to the output terminal,
The current output circuit for high current includes (g) a second amplifier that generates a constant current using a virtual short circuit that is a characteristic of the amplifier, and (h) one input terminal of the second amplifier and an input terminal. The other end is connected to the ground, the second input resistor for converting the current into a voltage, and (i) one end connected to the other input end of the second amplifier, the other end A second load resistor having a portion connected to ground and setting a current value of the first reference current based on a ratio to the second input resistor; and (j) receiving a differential output of the second amplifier. And a second pair of transistors whose bases for outputting a signal from one transistor are connected to each other, and (k) a current mirror circuit of a transistor for supplying current to the second pair of transistors. A second current source and (l) one end of the first resistor A first transistor having a second end connected to the ground and a base connected to an output of the second pair of transistors; and (m) the one end of the first resistor. And a first transistor that outputs a second reference current from the collector, and a first current mirror circuit that turns the second reference current back with a predetermined current amplification factor and uses the first reference current as a first reference current (N) a base is connected to the one end of the first resistor, and the area ratio of the transistor to the second transistor is set to a predetermined area ratio, whereby a second output current is supplied to the output terminal. And a second output transistor for supplying the current output circuit.

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