JPS6118231A - Driving circuit of light emitting element - Google Patents

Abstract

PURPOSE:To attain high-speed response of a light emitting element and to reduce current consumption by connecting an FET in parallel with the light emitting element to bring the FET to the pinch-off state at the lighting of the light emitting element and the FET to the operating state at the extinction of the light emitting element. CONSTITUTION:An anode of a light emitting diode PD2 and a drain of the FET5 are connected to a positive power supply terminal 1, a cathode of the PD2 is connected to the source of the FET5 and a resistor 6, a gate of the FET5 and a collector of a transistor (Tr)3 is connected respectively to the other end of the resistor 6. When the Tr3 is turned on, a current flows to the PD2 and the resistor 6, the PD2 is lighted, the FET5 is brought into the pinch-off state by a voltage drop across the resistor 6 and no current flows to the FET5. When the Tr3 is turned off and the PD2 is extinguished, the source and gate of the FET5 reach the equi-potential, the FET5 is operative, the electric charge stored in the capacity of the PD2 is discharged through the FET5 and high-speed response is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Application This invention relates to a light emitting element drive circuit for driving a light emitting element.

(B) Prior Art In the field of pulse-based optical communications such as pulse code modulation (PCM), semiconductor lasers or light emitting diodes are widely used as light emitting elements (light emitting sources). However, when these light emitting elements are driven at high speed, the higher the drive frequency becomes, the more distortion occurs in the light emission waveform, which tends to become less able to follow the drive current waveform.

FIG. 3 shows a circuit configuration diagram of a conventional light emitting element drive circuit,
In the figure, the anode of a light emitting diode 2 is connected to the positive terminal of a current source 1. A collector of an npn transistor 3 is connected to the cathode of the light emitting diode 2, and its emitter is grounded. The transistor 3 drives the light emitting diode 2, and a signal is input to its base.

FIG. 4 shows a waveform diagram of the light emitting element drive circuit of FIG. When the signal shown in Figure 4 is input to the base of the transistor 3, when the signal is 1, the transistor 3 is turned on and operates as a current source. A drive current flows as shown in FIG. 4 (ω). Due to this drive current, the light emitting diode 2 obtains a light emission output with an integral waveform shown in FIG. 4(C).

The reason for this is that since the light emitting diode is a capacitive load, its optical output waveform becomes an integral waveform of the drive current waveform shown in FIG. 4). Therefore, it is difficult to operate the light emitting diode at high speed.

In order to solve the above-mentioned problem, a light emitting element driving circuit configured as shown in FIG. 5 is known.

This circuit connects a resistor 4 in parallel to the light emitting diode 2 to improve the optical output waveform of the light emitting diode 2 and to enable high-speed operation.

(c) Problems to be Solved by the Invention In the light emitting device drive circuit shown in FIG. It is necessary to supply the sum from the transistor 3 acting as a current source, which has the disadvantage that the efficiency of using the current source is poor. moreover,
The lower the value of the resistor 4, the faster the light emitting diode 2 transitions from light emission to extinction, enabling high-speed operation. However, the current flowing through the resistor 4 during light emission becomes very large, and its value is naturally limited. Ru. For example, if the rise time of the light emitting diode 2 is 4.
If it is 6 nanoseconds, connecting a 60 ohm resistor 4 in parallel will take 2 nanoseconds, increasing the speed by a factor of 2.6.

However, regarding current consumption, the light emitting diode 2
If the resistor 4 is not connected to the resistor 4, the current is 60'mA, but if the 60 ohm resistor 4 is connected and the forward voltage of the light emitting diode 2 is 1.4V, the current flowing through the resistor 4 is 1.4M÷ 47 (mA) out of 30 (Q), and when combined with 60 mA of light emitting diode 2, it becomes 107 mA, which is an increase of about 1.8 times. For this reason, in addition to the need to strengthen the drive transistor by increasing the allowable power capacity due to the increase in current consumption, the light emitting output of the light emitting diode 20 will decrease due to the temperature rise due to the increase in heat generation.
Problems such as decreased reliability were occurring.

An object of the present invention is to provide a light-emitting element drive circuit that enables high-speed response of a light-emitting element and reduces current consumption.

2) Means for solving the problem In order to solve the above problem, a light emitting element, a resistor and a current source are connected in series, a field effect transistor is connected in parallel to the light emitting element, and its gate is connected to the resistor. and the current source. Figure 1 shows its configuration.

(E) Function As can be seen from Figure 1, when the light emitting diode emits light, the drive current increases and a voltage is generated across the resistor 6, which lowers the gate-source voltage and puts the field effect transistor in a pinch-off state. and reduce the current flowing through it. On the other hand, when the light emitting diode is turned off, the drive current is in a zero state, so the gate-source voltage becomes almost zero.
A constant current, generally defined as ID5S, flows through the field effect transistor, resulting in an immediate discharge of the charge stored in the capacitive component of the light emitting diode.

(F) Example Hereinafter, an example of the present invention will be described based on FIGS. 1 and 2. FIG. 1 shows a light emitting element driving circuit of the present invention, and the same parts as those in the circuit of FIG.

In FIG. 1, the drain of the field effect transistor 5 is connected to the anode of the light emitting diode 3 and the positive power supply terminal 1, and the source thereof is connected to the cathode of the light emitting diode 3. Further, one end of a resistor 6 is connected to the source of the field effect transistor 5 and the cathode of the light emitting diode 2, and the other end is connected to the gate of the field effect transistor 5 and the collector of the transistor 3. .

FIG. 2 is a diagram showing the relationship between the voltage between the gate and source of the field effect transistor 5, that is, the gate voltage VG, and the current flowing between the source and drain of the field effect transistor 5. As is clear from FIG. 2, when the gate voltage is increased in the opposite direction, the field effect transistor 5 becomes in a pinch-off state, and the current no longer flows. The voltage at this time is called a pinch-off voltage Vpo.

When the transistor 3 is ON, that is, when the light emitting diode 30 emits light, the drive current is applied from the positive power supply terminal 1 to the light emitting diode 2.
Flows through resistor 6 and transistor 3. As a result of the voltage drop caused by this drive current flowing through the resistor 6, the gate voltage of the field effect transistor 5 decreases, and the field effect transistor enters a pinch-off state or a high impedance state close to it. In this state, as shown in FIG. 2, the current I flowing through the field effect transistor 5 is zero or close to zero, eliminating unnecessary current consumption and operating the current source efficiently.

When the transistor 3 is off, that is, when the light emitting diode 3 is turned off, the drive current flowing through the resistor 6 is zero or close to zero, so the gate voltage of the field effect transistor 5 becomes almost equal to the source voltage, and as a result, the As shown in FIG. 2, a constant current corresponding to ID5S flows through the field effect transistor 5. ) in FIG. 1 shows the circuit at the time of extinction in FIG. and as a result,
The transition from emission to quenching occurs rapidly. In addition, the third
In the figure, for example, when a 60 ohm resistor 4 is connected in parallel, a voltage of 1.6 V is applied to the light emitting diode 2 at the moment of transition from light emission to extinction, so a voltage of 47 mA is applied to the light emitting diode 2. The current discharges the accumulated charge. In order to obtain a similar quenching effect, a field effect transistor 5') with a current ID5S of 47 mA should be selected.

In FIG. 1(5), when the value of the resistor 6 is R, the gate voltage vG when the field effect transistor 50 emits light is determined by R and the drive current II of the light emitting diode 2, and VG = RX Ip.

Therefore, the gate voltage VG in the above equation is converted to the pinch-off voltage VPO.
If designed so that the light emitting diode 2 emits light, no current flows through the field effect transistor 5, resulting in good current usage efficiency. In the case of a normal field effect transistor, VPO=-
If the current is 1.5'V% and the drive current ID is 60mA, then the above conditions are met with R-25Ω.

If a transistor is used in place of the field effect transistor 5, the operating speed of the transistor will be slowed down due to the influence of excessive accumulated base charge when the transistor performs a switching operation, thus defeating the original purpose of increasing the speed of the light emitting diode. be called. On the other hand, field effect transistors do not suffer from the above-mentioned phenomenon and can be switched at high speed, so that good characteristics can be obtained.

In the above embodiment, the light emitting diode 2 is used as the light emitting element, but the present invention is not limited to this, and the same improvement can be obtained even if a semiconductor laser is used.

(G) Effects This invention can improve the distortion of the optical output waveform by shortening the fall time of the light emitting element without increasing the current consumption.As a result, it is possible to respond quickly to input signals and to This type of optical communication system can be suitably used.

[Brief explanation of drawings]

Fig. 1 shows an embodiment of the present invention, Fig. 1 (5) is a circuit configuration diagram, ) in Fig. 1 is a discharge circuit diagram during extinction, and Fig. 2 is a gate voltage-current characteristic of a field effect transistor. Figure, 3rd
4 is a circuit diagram of a conventional light emitting element drive circuit, FIG. 4 is a waveform diagram of each part of the circuit of FIG. 3, and FIG. 5 is a circuit diagram of another conventional light emitting element drive circuit. 1... Positive power supply terminal, 2... Light emitting diode, 3...
・Transistor, 5... Field effect transistor, 6・
··resistance. Patent applicant: Sumitomo Electric Industries, Ltd. (5 others)
Figure 2 A School 3 Concave 47 Figure 5

Claims (1)

[Claims] A light emitting element, a resistor, and a current source are connected in series, a field effect transistor is connected in parallel to the light emitting element, and its gate is connected between the resistor and the current source. A light-emitting element driving circuit, characterized in that the field-effect transistor is controlled to be in a pinch-off state when the light-emitting element emits light, and the field-effect transistor is controlled to be in an active state when the light-emitting element is extinguished.