JP2738607B2 - Light emitting element drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device driving circuit, and more particularly to a circuit for stabilizing the light output of a light emitting device.

[0002]

2. Description of the Related Art FIG. 2 is a catalog of Sharp Corporation's catalog of semiconductor lasers for electronic components (created in February 1988).
FIG. 7 is a diagram showing a drive circuit of the conventional semiconductor light emitting device shown in FIG. 27-2. In the figure, 10
Reference numerals 1 and 102 denote a low-potential power supply and a high-potential power supply, respectively. Reference numeral 104 denotes a laser diode driving PNP transistor whose base is a PN diode 106 and a resistor 109.
It is connected to the output of the operational amplifier 103 via a. P
The constant voltage formed by the N diode 105 and the resistor 107 is applied to the non-inverting input of the operational amplifier. The output voltage of the photodiode formed by the photodiode 111, the resistor 108, and the variable resistor 112 is calculated by the operational amplifier 103.
Is applied to the inverting input. Reference numeral 113 denotes a capacitor connected between the high-potential power supply 102 and the base of the PNP transistor 104, and reference numerals 114 and 116 denote a capacitor and a resistor connected in parallel between the output and the inverting input of the operational amplifier 103.

Next, the operation will be described. The non-inverting input of the operational amplifier 103 is connected to a constant potential represented by a high potential voltage −V _F (PN diode 105).
The inverting input is connected to the cathode of a photodiode 111, and is connected to a high potential power supply via a resistor 108 and a variable resistor 112. Therefore, the laser diode 1
As the optical output of the optical amplifier 10 increases, the photovoltaic current generated in the photodiode 111 increases, and
03 decreases, the operational amplifier 103
Output voltage rises. As a result, the base potential and the emitter potential of the laser diode driving PNP transistor 104 increase, the laser diode driving current decreases,
The light output drops.

When the light output of the laser diode 110 decreases, the operation reverse to the above is performed, and the laser diode drive current increases, thereby increasing the light output. Although the light output of the semiconductor laser diode changes with a change in the ambient temperature, the circuit operation described above always provides a constant light output with respect to the change in the ambient temperature. The variable resistor 112 is used for adjusting the light output to a desired value in the initial setting.

[0005]

Since the conventional semiconductor light emitting element driving circuit is constructed as described above, it requires an operational amplifier and a large current output PNP transistor for driving a laser diode. There is a problem that the number of elements is large and that it is not suitable for integration into an integrated circuit.
The reason is that a large current output PNP transistor requires a large area in a wafer process used for a normal integrated circuit, and is inferior in characteristics to an NPN transistor.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to provide a light emitting element drive circuit which can be configured with a small number of elements and is suitable for integration into an integrated circuit. .

[0007]

Means for Solving the Problems A light emitting element driving circuit according to the present invention comprises a light emission element, the current source and the light-emitting element driving NP
And <br/> current drive circuit is a light - emitting element for current drive composed of a N-transistor, originating in proportion to the intensity of the light output of the light emitting element
A photodiode in which the amount of generated current changes,
To output the current corresponding to the current flowing in the photodiode
A first current mirror circuit composed of a PNP transistor and an output current corresponding to the output current of the first current mirror circuit.
A second current generating NPN transistor;
And a rent mirror circuit, in which the output of the second current mirror circuit constitutes a negative feedback circuit I by the be connected to the output of the current source.

[0008]

[0009]

The current mirror circuit according to the present invention is constructed as described above.
With this configuration , even if the output current increases or decreases due to the increase or decrease in the photoelectromotive current of the photodiode, a negative feedback operation is performed so that the current source output current decreases. even occurs, the operation to cancel this, row without a large current output PNP transistor and the operational amplifier.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. FIG. 1A shows a light emitting element driving circuit according to one embodiment of the present invention. In the figure, 1 and 2 are a low-potential power supply and a high-potential power supply, respectively. The constant current source 3 and the NPN transistor 4 serve as a current driving circuit 20 for the laser diode 10.
Is configured. The cathode of the photodiode 11 is PNP
The PNP current mirror circuit 3 is inputted to a PNP current mirror circuit 30 composed of transistors 7 and 8,
An output of 0 is an input of an NPN current mirror circuit 40 composed of NPN transistors 5 and 6. The output of the NPN current mirror circuit 40 is connected to the output of the current source 3.

Next, the operation will be described. The current source 3 and the NPN transistor 4 constitute a drive circuit 20 for the laser diode 10. When the light output of the laser diode 10 increases due to an increase in ambient temperature or the like, the photovoltaic current generated in the photodiode 11 also increases in proportion thereto.
The output current of the photodiode 11 is connected to the current source 3 via a PNP transistor current mirror circuit 30 including PNP transistors 7 and 8 and an NPN transistor current mirror circuit 40 including NPN transistors 5 and 6. Therefore, if the light output of the laser diode increases, the photovoltaic current generated in the photodiode increases, and the PNP transistor current mirror circuit 30 and the NPN transistor current mirror circuit 40
Since the current flowing increases, decreases the output current of the current source 3 along with this, NPN transistor 4 is turned on closer to the off state, to reduce the driving current of the laser diode, reduce the light output.

When the light output of the laser diode decreases, the photocurrent of the photodiode decreases, the output current of the current source 3 increases, the laser diode drive current increases, and the light output increases. The above operation makes the light output of the laser diode constant. Note that the relationship between the input current and the output current of the NPN transistor current mirror circuit can be made variable by the variable resistor 9, and the optical output of the laser diode can be set to a desired value in the initial setting.

As described above, in the above embodiment, the photovoltaic current generated in the photodiode is input to the PNP current mirror circuit, and the output of the PNP current mirror circuit is set to NPN.
A negative feedback circuit is configured by inputting the current to the current mirror circuit and connecting the output of the NPN current mirror circuit to the output of the current source. Therefore, even if the optical output of the laser diode fluctuates, An operation for canceling this can be realized with only a small number of elements suitable for integration.

In the above-described embodiment, a circuit configuration for stabilizing the light output is shown for the light-emitting element in which the cathode of the laser diode and the anode of the photodiode for detecting the light output are connected in common. b) shows another embodiment of the present invention in which the light output is made constant when the anode of the laser diode and the cathode of the photodiode are commonly connected.

In FIG. 1B, a current source 53 and an NPN
The transistor 54 forms a current driving circuit 60 for the laser diode 58. The photovoltaic current generated by the photodiode 59 for detecting the light output is equal to the NPN transistors 55 and 5.
6 is connected to the input of an NPN transistor current mirror circuit 70 composed of
Is connected to the output of the current source 53.

Next, the operation will be described. The photovoltaic current of the photodiode, which is proportional to the optical output of the laser diode 58, is input to the NPN current mirror circuit 70, and the output is connected to the output of the current source 53, so that the optical output of the laser diode and the drive current of the laser diode Constitutes a negative feedback circuit. Therefore, a constant light output is always obtained. The variable resistor 57 adjusts the relationship between the magnitude of the input current and the magnitude of the output current of the NPN transistor current mirror circuit 70, so that the optical output of the laser diode can be set to a desired value in the initial setting.

As described above, in the above embodiment, the negative feedback is obtained by connecting the output of the NPN current mirror circuit to the output of the current source by using the photovoltaic current generated in the photodiode as the input to the NPN current mirror circuit. Since the circuit is configured, even if the optical output of the laser diode fluctuates, an operation for canceling the fluctuation can be realized with only a smaller number of elements suitable for integration.

In each of the above embodiments, only the case where the light emitting element is a laser diode has been described. However, a normal light emitting diode may be used, and the same effects as those of the above embodiments can be obtained.

[0019]

As it is evident from the foregoing description, according to the light emitting element driving circuit according to the present invention, a light - emitting element, light emitting element driving current source
The light-emitting element is composed of an NPN transistor for
Operating current drive circuit and proportional to the light output intensity of the light emitting element
Photodiode that changes the amount of current generated
Out and soil, a current corresponding to the current flowing in the photo diode
A first current mirror circuit composed of a PNP transistor and an output current of the first current mirror circuit.
The second current of the NPN transistor
Of a current mirror circuit, since the output of the second current mirror circuit was Unishi by that make up the <br/> negative feedback circuit I by the be connected to the output of the current source, less element circuit Consist of numbers and circuit elements and circuits suitable for integrated circuits
Thus , there is an effect that a circuit can be manufactured at a low cost and an integrated circuit can be realized .

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a light emitting element driving circuit according to an embodiment of the present invention, (a) is a circuit diagram showing one embodiment thereof,
(b) is a circuit diagram showing a light emitting element drive circuit according to another embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional light emitting element drive circuit.

[Explanation of symbols]

 Reference Signs List 1,51 Low-potential power supply 2,52 High-potential power supply 3,53 Current source 4,54 NPN transistor 5,55 NPN transistor 6,56 NPN transistor 7 PNP transistor 8 PNP transistor 9,57 Variable resistor 10,58 Laser diode 11, 59 photodiode 12 variable resistor 13 capacitance 14 capacitance 20, 60 current drive circuit 30 PNP current mirror circuit 40, 70 NPN current mirror circuit

Claims (1)

(57) [Claims] A light emitting element; a current driving circuit comprising a current source and a light emitting element driving NPN transistor for driving the light emitting element with a current; and a large amount of current generated in proportion to the intensity of the light output of the light emitting element. A first current mirror circuit composed of a PNP transistor for outputting a current corresponding to a current flowing through the photodiode, a current corresponding to an output current of the first current mirror circuit, And a second current mirror circuit comprising an NPN transistor, wherein a negative feedback circuit is formed by connecting an output of the second current mirror circuit to an output of the current source .