JPS6377172A - Circuit for driving light emitting element - Google Patents

Abstract

PURPOSE:To shorten a light irradiating delay time due to the signal input of a light emitting element by applying the output of a constant bias current supplying circuit together with the output of a light emitting element driving circuit to feed a constant current also to the element at the time of non signal. CONSTITUTION:A resistor R1 is added between the connecting position of an LED 1 and an npn Tr 4 and an earth to always feed a small predetermined current I0 to the LED 1. As a result, the LED 1 is not sufficient for charging in the electrostatic capacity of the LED 1 even if a signal '1' is input after a signal '0' is continued for a long time, and can have output characteristic having no light emitting time delay.

Description

[Detailed Description of the Invention] [Summary] In a drive circuit for a light emitting element (such as a light emitting diode (hereinafter referred to as LED)), the output of a constant bias current supply circuit is applied to the light emitting element to supply a constant current even when there is no signal. By flowing the light to the light emitting element, when a signal is input after a period of no signal, the delay time of light emission due to insufficient charging of the capacitance of the light emitting element is shortened.

[Industrial application field]

The present invention relates to improvements in LED drive circuits used in digital optical communications.

For example, in optical communication that transmits digital signals consisting of "1° and 0", "1" is the light emitting signal of the LED, and '
0” corresponded to no light emission.

At this time, if a large delay time occurs in the light emission corresponding to the signal "lo", the code error rate will increase especially in high-speed communication, so it is desirable that the delay time of the light emission be as short as possible.

[Conventional technology]

FIG. 6 is a diagram of a first conventional LED driving circuit.

FIG. 7 is a second conventional LED drive circuit diagram.

FIG. 8 is a diagram in which an input/output waveform diagram for a conventional example is superimposed on a characteristic diagram of an LED. In the figure, the horizontal axis shows drive currents IL and D, and the vertical axis shows optical output Po and voltage ED across the LED.

In FIG. 6, npn) transistor (hereinafter npnTr
An input signal (“1” or “O”) is added to 4 (referred to as “1” or “O”).

As shown in FIG. 8, when the human input signal is "1", the npn
A current flows through Tr4, driving LEDl to emit light.

On the other hand, when the input signal is “O”, 3inpnTr, 4inpnTr
No current flows through the LED 1, and the LED 1 does not emit light.

Next, a second conventional example will be explained.

In FIG. 7, npnTrs 6 and 7 are placed opposite each other, and an input signal is applied to the base of one npnTr via a NOR gate 5, and a signal obtained by inverting the input signal is applied to the base of the other npnTr.

As a result, when "1" is input to the base of npnTr7, a current flows to the npnTr7, a driving current also flows to the LED connected to the collector of the npnTr7, and the LED
DI emits light.

[Problem that the invention seeks to solve]

However, in the above-mentioned light-emitting element drive circuit, when the input signal is "0", no current is passed through the light-emitting element, so if the signal "1" is input after the signal "0" continues for a long time, , the electric charge charged in the capacitance of the light emitting element is discharged through the resistance of the light emitting element, and the voltage across the light emitting element decreases to near OV.

For this reason, time is required to charge the capacitance of the light emitting element, resulting in a problem that a light emission delay time as shown in FIG. 9 occurs.

Means for Solving Problem C] The above problem is caused by the problem that, as shown in FIG. A bias current supply circuit 3 is added, and the output of the constant bias current supply circuit 3 is added to the light emitting element 1 together with the output of the light emitting element drive circuit 2, so that a constant current is passed through the light emitting element 1 even when there is no signal. This problem is solved by the light emitting element drive circuit of the present invention, which shortens the delay time of light emission due to signal input of the light emitting element 1.

[Effect]

According to the present invention, by constantly causing a small current to flow through the light emitting element l by the constant bias current supply circuit 3, the signal "
Even when "1" is input after "0" continues for a long time, the capacitance of the light emitting element 1 will not be insufficiently charged, and the delay time of light emission can be shortened.

〔Example〕

FIG. 2 is a diagram of a light emitting element driving circuit according to a first embodiment of the present invention.

FIG. 3 is a diagram of a light emitting element driving circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram in which an input/output waveform diagram for the embodiment of the present invention is superimposed on a characteristic diagram of the LED. In the figure, the horizontal axis represents the drive current ILID, and the vertical axis represents the optical output Po and the voltage VJD across the LED.

FIG. 5 is a diagram illustrating the present invention in detail.

The same reference numerals indicate the same objects throughout the figures.

In FIG. 2, by adding a resistor R1 between the connection point between the LEDI and the npnTr 4 and the ground, a constant small current 1o is caused to constantly flow through the LEDI, as shown in FIG. 4.

As a result, after the signal “0” continues for a long time, the LEDI “
Even when a signal of 1" is input, the capacitance of the LED 1 is not insufficiently charged, and as shown in FIG.
Output characteristics without time delay in light emission can be obtained.

Next, Fig. 3 shows a driving circuit diagram of the LEDI of the second embodiment using a switching circuit, but as in the case of Fig. 2, a constant small current 1o is constantly passed through the resistor R. do.

The LED drive circuit shown in Fig. 2 is suitable for relatively low frequencies where the input signal repetition frequency is approximately 30 Mb/s or less, and the LED drive circuit shown in Fig. 3 is suitable for high frequencies of approximately 30 Mb/s or more. Are suitable.

When the LED drive circuit shown in FIG. 3 is used with a crystal frequency signal, the pulse width of the input signal becomes narrower, and the effects of the present invention become more significant.

〔Effect of the invention〕

As explained above, according to the present invention, by adding a constant bias current supply circuit to the LED drive circuit and constantly supplying a small current to the LED, there is a delay in light emission when a signal is input after a long period of no signal. It can shorten the time.

[Brief explanation of the drawing]

Fig. 1 is a principle diagram of the present invention, Fig. 2 is an LED drive circuit diagram of the first embodiment of the invention, Fig. 3 is an LED drive circuit diagram of the second embodiment of the invention, and Fig. 4 is a diagram of the LED drive circuit of the second embodiment of the invention. A diagram in which the input/output waveform diagram for the embodiment of the present invention is superimposed on the LED characteristic diagram. Figure 5 is a diagram explaining the present invention in detail. Figure 6 is the LED drive circuit of the first conventional example. Figure 7 is the LED drive circuit diagram of the second conventional example, Figure 8 is a diagram in which input and output waveform diagrams for the conventional example are superimposed on the LED characteristic diagram, and Figure 9 is an example of light emitting output. It is a figure which shows the delay of a waveform. In the figure, 1 is a light emitting element, and 11 is an LED. 2 is a light emitting element drive circuit, 3 is a constant bias current supply circuit, 4.6.7 is an npnTr, 5 is a NOR gate Stop °j Memu ED, each diagram Z-layer light eq /) 2nd - refreshing example sword LED D, polar motion circuitry 3rd diagram LED characteristic diagram 1 piece 1 copy present兇佼] Ka4 Spring's admission or signal flow pattern beggar! Ruin of tatami; 2nd grade 4th episode -/I Aito example's LEDM * v 口路 6th g 2nd q fi * 1st row tf) L E D,,!
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Claims (1)

[Claims] In a light-emitting element drive circuit (2) that drives a light-emitting element (1) with an input signal, a constant bias current supply circuit (3) that supplies a constant bias current to the light-emitting element (1) is added. , the output of the constant bias current supply circuit (3) is added to the light emitting element (1) together with the output of the light emitting element driving circuit (2), and a constant current is caused to flow through the light emitting element (1) even when there is no signal. A light emitting element driving circuit characterized in that the delay time of light emission due to signal input of the light emitting element (1) is shortened.