JPH02205367A - Light emitting element driving circuit - Google Patents

Abstract

PURPOSE:To shorten the rising and breaking time of a current by providing a switching circuit for completely varying a current flowing to a light emitting element in response to a binary signal, and a prebiasing circuit for applying a voltage of the degree not exceeding a light emitting threshold voltage to the light emitting element. CONSTITUTION:A switching circuit 6 conducts and cuts off the current flowing to a LED 5 in response to an input binary signal. Therefore the current flowing to the LED 5 is intermittently interrupted, and the LED 5 repeats emitting of a light and distinction of the light in response to the input binary signal. Since a voltage of the degree not exceeding a light emitting threshold voltage is applied to the LED 5 by a prebiasing circuit 7, the voltage applied to the LED 5 is rapidly arrived at a predetermined voltage. Thus, the rising and breaking time of the current flowing to the LED 5 are shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light emitting element drive circuit incorporated in a transmitting circuit for optical digital communication.

[Conventional technology]

When driving a large-capacity element such as an LED (light emitting diode), its switching, especially the switching at the falling edge, becomes slow. FIG. 4 is a waveform diagram showing this situation. As shown in FIG. 4 (B) for the binary input signal shown in FIG. Switching is delayed. Various countermeasures have been taken in the past to deal with this problem.

The first method is to connect circuits 2 and 3 consisting of passive elements such as resistors, capacitors, or coils in series or in parallel with the LEDI, as shown in Figures 5 (A) and (B). , there is a method to equivalently eliminate the capacitance of LED1. As a second method, as shown in Fig. 6, at the rise and fall of the input signal,
There is a method of applying peaking to the current flowing through the LED. As a third method, as shown in FIG. 7, there is a method in which a pre-bias current IP is always superimposed on the LED current.

[Problem to be solved by the invention]

However, since the first method uses only passive components, it is cumbersome because it is necessary to change the circuit constants each time the capacitance of the LED or the dielectric resistance changes. Furthermore, it is relatively difficult to integrate passive components into ICs, which hinders miniaturization. The second method would overload the LED. Furthermore, the circuit scale becomes larger and the power supply voltage also becomes higher. In the third method, a drive current flows through the LED even when the input signal is at a low level, so the LED emits light, albeit slightly. Therefore, it is necessary to monitor whether a signal is coming on the receiving side.

An object of the present invention is to solve these problems.

[Means to solve the problem]

For this reason, the present invention includes a switching circuit that changes the current flowing through the light emitting element by 100% according to a binary signal, and a prebias circuit that applies a voltage to the light emitting element to an extent that does not exceed the emission threshold voltage. It is prepared.

[Effect]

By applying a predetermined voltage to the light-emitting element in advance using a pre-bias circuit, the voltage applied to the light-emitting element quickly reaches the predetermined voltage, and the rise and fall times of the current flowing through the light-emitting element are shortened. be converted into

〔Example〕

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of the present invention.

The switching circuit 6 switches the LE according to the input binary signal.
The current applied to D5 is changed by 100%. Therefore, the current supplied to the LED 5 is interrupted, and the LED 5 repeats light emission and extinction depending on the input binary signal.

Further, a pre-bias circuit 7 applies a voltage to the LED 5 that does not exceed the light emission threshold voltage. Therefore, the voltage applied to the LED 5 quickly reaches the predetermined voltage, and the rise time and fall time of the current applied to the LED 5 are shortened.

Next, this embodiment will be described in detail below with reference to a more detailed circuit diagram shown in FIG.

The switching circuit 6 and pre-bias circuit 7 described above
The internal details of each are configured as follows.

In other words, the switching circuit 6 is composed of two NPN transistors 14 and 15 and a constant current source 16 commonly connected to the emitters of these transistors, the collector of the transistor 4 is connected to the cathode of the LED 5, and the transistor 15 collectors are grounded. Further, digital signals v and n, which are binary signals, are input to the base of the transistor 14, and digital signals 2 and 2 are input to the base of the transistor 5. A signal V 1 having an average level voltage of is input, and each transistor 14, 15 has a differential configuration.

The pre-bias circuit 7 is composed of a series circuit of diodes 17 and 18 and a constant current [19], and a series circuit of a resistor 20 and a PNP transistor 21.
The anode of the transistor 7 and one end of the resistor 20 are grounded, and the base of the transistor 21 is connected to the cathode of the diode 18.

The emitter of the transistor 21 is connected to the cathode of the LED 5, and a pre-bias voltage is applied to the cathode of the LED 5. That is, the cathode of LED 5 has about 1
．． The difference voltage between the constant voltage of 4V and the approximately Q,7V generated between the base and emitter of the transistor 21 is approximately 0.7V.
A pre-bias voltage of is applied. Note that the value of this pre-bias voltage can be changed by varying the value of the constant current generated by the constant current source 19, or by changing the value of the constant current generated by the constant current source 19.
By using a Zener diode or the like for 18, it is possible to set the value to an appropriate value.

In such a configuration, the operation of this embodiment will be described below with reference to the waveform diagrams of FIGS. 3(A) to 3(C).

The horizontal axis of FIG. 2A represents time, and the vertical axis represents signal voltage, and the signal waveform shown in the diagram is the waveform of the digital signal V1o applied to the base of the transistor 14. In other words, the signal v1n is at a low level (L) from time 0 to TI for 1 time T.
It is at a high level (H) until l-T2, and is at a low level after 2 hours T2. When this digital signal vIn is applied to the base of the transistor 14, the transistor 14
is in an off state for one hour from time 0 to T1, and on for one hour Tl-T2, and is in an off state for one hour after T2. Therefore, a current is applied to the LED 5 during the time period T1 to T2, and the LED 5 emits light. Note that by appropriately setting the amplitude of the digital signal v1n, the current switched for each time period can be changed by 100%.

In addition, as mentioned above, the cathode of LED5 has approximately 0.7
Since the pre-bias voltage of V is constantly applied, when the transistor 14 is turned on during the time Tl-72, the LE
The voltage applied between the terminals of D5 quickly reaches a predetermined voltage value. Therefore, this voltage waveform becomes the waveform shown by the solid line in Figure (B) (the horizontal axis represents time according to the same scale as Figure (A), and the vertical axis represents the applied voltage), and the rise time of the applied voltage waveform and the fall time is short as shown.

The extent of this shortening becomes clear by comparing it with the conventional applied voltage waveform shown by a dotted line in which no pre-bias voltage is applied, shown by vb in FIG. In other words, the rising and falling waveforms of the voltage waveform of this embodiment shown by the solid line and the conventional voltage waveform shown by the dotted line are usually similar, and by applying the pre-bias voltage V, The reference voltage line moves upward as shown, and the rising and falling waveforms of the voltage waveform according to this embodiment become steeper.

Further, the pre-bias voltage Vb is set to such an extent that it does not exceed the light emission threshold voltage of the LED 5, which is indicated by Vth in FIG. 5(B). In other words, the light emission threshold voltage V of LED5
Since th is usually about 1.2 to 1.4V, this approximately 0
．． Even if a pre-bias voltage Vb of 7v is applied to the cathode of LED5, ED5 will not emit light when it is extinguished. Further, the current applied to the LED 5 is approximately proportional to the voltage difference between the applied voltage and the light emission threshold voltage vth.

When an applied voltage with such a waveform is applied to the cathode of the LED 5, the signal waveform of the light pulse emitted from the LED 5 is shown in FIG.
Time according to the same scale as B).

The vertical axis represents the light emission power of the LED 5. In other words, compared to the signal waveform of the optical pulse from the conventional light emitting element shown by the dotted line in FIG.
Signal rise waveform and timing T2 at 1
The falling waveform of the signal becomes steep. Therefore, the time required for the signal state to change at each timing TI, T2 is shortened. In addition, the luminous power is L
It is proportional to the current applied to the ED5.

According to this embodiment, it is possible to further increase the transmission speed of optical communication. Furthermore, since this circuit uses active elements such as transistors 14.15 and 21 and diodes 17.18 to determine the circuit characteristics, the circuit characteristics are determined regardless of the capacitance and dynamic resistance value of the LED 5. . Therefore, the conventional trouble of changing each circuit constant each time the LED is changed is eliminated.

Further, the resistor 20, which is a passive component, has a small value, and this circuit can be implemented as an IC in a package, and furthermore, can be implemented as a monolithic IC. Therefore, it becomes possible to downsize the optical digital transmitter and reduce power consumption, and it becomes possible to reduce the cost of the optical digital communication system. Also, L from the pre-bias circuit 7
Since the voltage applied to the cathode of the ED5 is set to be always lower than the light emission threshold voltage vth when the LED 5 is extinguished, the LED 5 is completely extinguished when there is no signal.

In addition, in the description of the above embodiment, L is used as a light emitting element.
Although the case where an ED is used has been described, the present invention is not limited to this, and a laser diode (LD) or the like may be used, and even in this case, the same effects as in the above embodiment can be achieved.

〔Effect of the invention〕

As explained above, the present invention includes a switching circuit that changes the current applied to the light emitting element by 100% according to a binary signal, and a pre-bias that applies a voltage to the light emitting element that does not exceed the light emission threshold voltage. By including the circuit, the voltage applied to the light emitting element quickly reaches a predetermined voltage, and the rise time and fall time of the current flowing through the light emitting element are shortened.

Therefore, the present invention has the effect that optical communication can be performed at a higher transmission speed than before. Furthermore, since the determination of the characteristic factors of the circuit characteristics depends on the active elements, there is an effect that the trouble of changing the circuit constants of the passive components depending on the type of light emitting element used is eliminated.

Further, it has the effect that it is easy to integrate it into an IC, making it possible to make the device smaller and cheaper.

Further, since there is no need to apply peaking to the current flowing through the light emitting element as in the conventional case, there is an effect that overload is not applied to the light emitting element. In addition, there is no need to increase the power supply voltage for peaking, and there is an effect that power consumption can be achieved and the circuit scale can be suppressed, so that the device can be provided at a low cost.

Further, unlike in the prior art, current does not flow through the light emitting element when there is no signal, and there is no need to monitor the presence or absence of a signal on the receiving side.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of the present invention, FIG. 2 is a circuit diagram showing a detailed configuration of the embodiment shown in FIG. 1, and FIG. 3(B) is a waveform diagram of the voltage applied to the LED 5 shown in FIG. 2, and FIG. 3(C) is a waveform diagram of the digital signal Vin input to the base of the transistor 14 shown in FIG. A waveform diagram of the light emission pulse of this LED5, Figure 4 (All1. (B) is a signal waveform diagram in a conventional circuit, Figures 5 (A) and CB) are a circuit diagram showing an example of the conventional circuit, and Figure 6 (A ) and (B) are signal waveform diagrams in another conventional circuit, and FIG. 7 is a signal waveform diagram in another conventional circuit. 5...LED, 6...Switching circuit, 7...
Pre-bias circuit. Figure 1 Figure 2 Recycle! ! One signal area type N3 diagram Conventional valley diagram 4 Conventional Geisha (1)! Figure 5 Sub-5 Technique philosophy (2) Figure 6 Conventional technique #r (3) Figure 7

Claims (1)

[Claims] 100% of the current applied to the light emitting element according to the binary signal
1. A light-emitting element drive circuit comprising: a switching circuit for changing the light-emitting element; and a pre-bias circuit for applying a voltage to the light-emitting element to an extent that the voltage does not exceed the light-emission threshold voltage of the light-emitting element.