JPH0414909A - Light emitting diode driving circuit - Google Patents

Abstract

PURPOSE:To improve the falling time of a light emitting diode driving circuit by applying a bias voltage to the light emitting diode at which it is not stimulated when a noninverting input signal is at a low level and an inverting input signal is at a high level. CONSTITUTION:Output of the emitter of a 2nd NPN transistor(TR) 12 being one of emitter follower outputs receiving an inverting input signal 14 is not only supplied to a diode array 16 but to a capacitor 17 connected in parallel with the array 16. Moreover, a light emitting diode 15, the cathode of the diode array 16 and the capacitor 17 are connected to a common current source 18 and the other terminal of the source 18 is connected to a power terminal 19. When a noninverting input signal 13 changes from a low to a high level, a current proportional to a positive change in the emitter voltage of the 1st NPN TR 11 and to a negative change in the emitter voltage of the 2nd NPN TR 12 flows from the light emitting diode 15 through the capacitor 17. Thus, the rising time of the optical output of the light emitting diode 15 is quickened.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a light emitting diode (Dj) used in an optical transmitter.
od) Regarding drive circuits, in particular, a light-emitting diode drive circuit using a current switching type logic circuit A drive circuit that performs a switching operation, as shown in FIG. 1, is used for light-emitting diodes. That is, the first and second NPN transistors (TransiSte) of the emitter follower output with the collector grounded
r) The emitter of 1.2 is connected to the anode (Amode) of the light emitting diode 3 and the diode array 4, and connected to the power supply terminal 6 via the resistor 5 connected to each cathode (Cathode). Base of transistor 1 (Ba
5e) and an in-phase input 8 is connected to the base of the transistor 2 to form a drive circuit. In this circuit, when the in-phase cuff is at a high level, the light emitting diode 1 is turned off and a forward current determined by the forward voltage and the resistor 5 flows through the diode array 4.

At this time, a low bias (B
ias) will be applied. On the other hand, when the negative phase input 8 is at a high level, the diode 4 turns off and a forward current determined by the forward voltage and the resistor 5 flows to the light emitting diode 1.

(Problems to be Solved by the Invention) The conventional light emitting diode drive circuit shown in FIG. The optical output response has improved rise and fall times, making it possible to perform higher-speed digital transmission, resulting in superior high-speed performance. However, although this circuit is affected by the characteristics of the light emitting diode itself, improving the fall time is more effective than improving the rise time.

In other words, the effect is large when the light-emitting diode itself has characteristics where the rise time is somewhat short and the fall time is slightly long, but conversely, the effect is large when the light-emitting diode itself has characteristics where the fall time is long to a certain extent and the fall time is slightly short. is small. In other words, depending on the characteristics of the light emitting diode, it is necessary to further improve the rise time.

The present invention has been made under these circumstances, and an object of the present invention is to further improve the rise time of a light emitting diode drive circuit and increase its high speed.

[Configuration of the Invention (Means for Solving the Problem) A first transistor with an emitter follower output that receives an in-phase signal as input, a second transistor with an emitter follower output that receives an opposite phase signal as input, and the first transistor a light emitting diode whose anode is connected to the emitter of the light emitting diode, a current source connected to the cathode of the light emitting diode, and a parallel circuit of a diode string and a capacitor whose anode is connected to the emitter of the second transistor and whose cathode is connected to the current source. There are features of the light emitting diode drive circuit according to the present invention.

(Function) In the light-emitting diode drive circuit of the present invention, when the common-mode input is at a low level and the negative-phase input is at a high level, a bias voltage of such a magnitude that the light-emitting diode does not emit light is applied. To reduce the fall time of a light emitting output necessary for switching a light emitting diode from a light emitting state to an erasing state by switching from a high level to a low level. In addition, when switching the common mode input from a low level to a high level to switch the light emitting diode from an erased state to a light emitting state, a capacitor connected in parallel with the diode allows overcurrent to flow through the light emitting diode, thereby reducing the rise time of the light emitting output. do.

In addition to improving the fall time, which was done in conventional light emitting diode drive circuits, this also improves the rise time at the same time, increasing pulse response and achieving higher speed.

(Example) An example according to the present invention will be described with reference to FIGS. 2 and 3, and parts that overlap with the prior art will be described using new numbers to aid understanding.

As is clear from FIGS. 2 and 3, the collector (Col)
The bases of the first and second NPN) transistors II and I2 of the emitter follower output with the
An in-phase input signal I3 and an anti-phase input signal 14 are input, and the anode of a light emitting diode I5 and the anode of a diode array 16 are connected to the emitter. However, the negative phase input signal 1
The output of the emitter of the transistor 12 is connected not only to the diode array I6 but also to the capacitor 17 installed in parallel thereto. Furthermore, the light emitting diode I5 and the diode array 16
0 cathode and capacitor 17 are connected to a common current source I8
and connect this to the power supply terminal 19.

In such a circuit connection, when the common mode input signal 13 is at a high level, the current I of the current source 18 flows through the light emitting diode 15.
Most of it flows and emits light. On the other hand, the negative phase input signal 1
4 is maintained at a high level, the in-phase input signal I3 is at a low level and the light emitting diode J5 is extinguished. Further, most of the current J of the current source 18 flows through the diode string 1B, and the light emitting diode is biased by a voltage obtained by subtracting the difference between the high level and the low level from the forward voltage of the diode string J6. When the common-mode input signal 13 changes from a low level to a high level, the positive change in the emitter voltage of the first NPN transistor 11 receives the common-mode input signal 13, and the second NPN transistor receives the negative-phase input signal 14. A current proportional to the negative change in the emitter voltage of light emitting diode 1
5 through condenser 17. As a result, the rise time of the light output of the light emitting diode 15 is accelerated.

On the other hand, the circuit connection in FIG. 3 is shown as a second embodiment. The difference from the first embodiment shown in FIG. 2 is that the diode array 20
is composed of two diodes, and the first and second NPN transistors 11 each have an emitter follower output and the in-phase input signal 13 and the opposite-phase input signal 14 are input to the base.
, ] 2 and the diode array 20 and the light emitting diode 15. Furthermore, in place of the current source 18, an NPN transistor 24 and a resistor 2 are used.
A current source circuit having 5 connected in series is connected to a power source 19.

The base of the NPN transistor 24 constituting the current source circuit is grounded, and the resistors 22 and 23 serve to release charges from the light emitting diode 15 and the diode array 20. In this embodiment as well, when the in-phase input signal 13 goes from a low level to a high level, a positive change in the emitter voltage of the first NPN transistor 11 to which the in-phase input signal 13 is input and a positive change in the emitter voltage of the first NPN transistor 11 to which the in-phase input signal 14 is input are combined. A current proportional to the negative change in the emitter voltage of the second NPN transistor 12 flows from the light emitting diode 15 through the capacitor 17. As a result, the rise time of the light output of the light emitting diode 15 is accelerated, as in the first embodiment.

In addition, FIGS. 4 and 5 are presented to explain the operations of the first and second embodiments. That is, although FIG. 4 shows exactly the same circuit connection as FIG. 2 representing the first embodiment, the NPN transistor A changes from L (LoνLevel) to H (Hi
gh Level), NPN) transistor B reveals the potential at the moment it switches from H to L. At this time, the emitter potential of the NPN transistor A increases by +Δ■, and the emitter potential of the NPN transistor B decreases by Δ■.

That is, when switching from L to H (Sv1lch), as shown in FIG. 5, an additional current of Δi flows due to the series resistor R of the light emitting diode C and the capacitor C. The same phenomenon occurs when switching from H to L, but when the light emitting diode C is turned off <
Since the series resistance at the time of Off) is large, the effect is not expected to be high.

[Effects of the Invention] In the light emitting diode drive circuit according to the present invention, when the in-phase input signal is at a low level and the anti-phase input signal is at a high level,
When a bias voltage that does not emit light is applied to the light-emitting diode, and the common-mode input signal switches from low level to high level, that is, the fall time of the light-emitting diode output when the light-emitting diode switches from the light-emitting state to the erased state. has been improved from the conventional 8ns to 5ns. Furthermore, when the common mode input signal switches from a low level to a high level and the light emitting diode switches from a light emitting state to a quenching state, i.e., when the light emitting diode switches from a quenching state to a light emitting state, a capacitor connected in parallel to the diode string is connected. As a result, an overcurrent flows through the light emitting diode, and the rise time of the light emitting output decreases from 6ns to 4n.
Improve to s.

In this way, conventionally, the main focus was on improving the fall time of the light emission output, but by simultaneously improving the rise time, it is possible to bring the pulse shape closer to the original waveform, increasing responsiveness and improving high speed. be able to.

These improvements led to improved transmission rates in the infrared region.

If the characteristics of a light emitting diode are that the rise time is somewhat long and the fall time is somewhat short,
The effects of improvement will be greater.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an example of a conventional light emitting diode drive circuit, FIG. 2 is a circuit diagram of a light emitting diode drive circuit according to the present invention, and FIG. 3 is a circuit diagram showing another embodiment of the present invention. 4 and 5 are drawings submitted for explaining the operation of the present invention. 1.2.11, 12: Emisota follower output NPN transistor, 3.15: Light emitting diode, 4, ]6.20: Diode string, 17, Capacitor 5.22.23.25: Resistor, 24: NPN) transistor, 18 Two current sources.

Claims (1)

[Claims] a first transistor with an emitter follower output that receives an in-phase signal as input; a second transistor with an emitter follower output that receives an opposite phase signal as input; a light emitting diode having an anode connected to the emitter of the first transistor; and a light emitting diode. a current source connected to the cathode of the second transistor; and a parallel circuit of a diode string and a capacitor whose anode is connected to the emitter of the second transistor and whose cathode is connected to the current source.