JPH02215170A - Light emitting element drive circuit - Google Patents

Abstract

PURPOSE:To improve the symmetry of optical pulses generated from a light emitting element by a method wherein a constant current induced by a third transistor Tr is made to increase at the rise of an operating current of the light emitting element and to increase at the fall of the operating current at a rate larger than that at the rise. CONSTITUTION:A current which is made to flow according to binary signals inputted into a light emitting element 13 which is connected in series to a first transistor Tr 12 of a light emitting element drive circuit is switched. A second Tr 14 is connected to the first Tr 12 in parallel and executes a switching operation contrary to the Tr 12. A third Tr 15 is connected to the Trs 12 and 14 in series, and a constant current is made to flow through the Trs 12 and 14 connected in parallel through the Tr 15. Then, binary signals, which enable a current to flow through the element 13, are generated from a differential signal generating circuit and supplied to a first and second pulse generating circuit, 18 and 19. The current of the Tr 14 is made to increase at the rise of operating current of the element 13 through the circuits 18 and 19 and to increase at the fall of the operating current at a rate larger than that at the rise.

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.

C. Prior Art] As a conventional light emitting element drive circuit of this type, there is one disclosed in, for example, Japanese Patent Application Laid-Open No. 63-67791.

This publication discloses a light emitting element driving circuit having a circuit configuration as shown in FIG. 4, with the aim of improving the high frequency characteristics of the light emitting element driving circuit.

That is, the binary signal shown in FIG. 5(A) is input to the differential signal generating circuit 3, and the base of the transistor 1 is supplied with a binary signal having the same phase as this binary signal. Furthermore, a binary signal having an opposite phase to this is applied to the base of the transistor 2. Therefore, the current flowing through the light emitting diode (LED) 4 is changed by 100% by each transistor 1 and 2 performing opposite switching operations, and the LED 4 repeatedly emits and extinguishes light. At this time, each transistor 1
．． The current flowing through 2 is kept constant by transistor 5 and resistor 6. Further, the binary signal applied to the base of transistor 1 is differentiated by capacitor 7 and resistor 8, and this differentiated signal is applied to the base of transistor 5. Therefore, the rising and falling current waveforms of the current applied to the LED 4 are shaped into the waveform shown by the solid line in FIG. 5(B), and compared with the conventional current waveform shown by the dotted line in FIG. 5(B). They have improved. Note that the light pulse emitted from the LED 4 is proportional to the current applied to the LED 4, and the waveform of this light pulse is also shown as the solid line in FIG. 4B.

[Problem to be solved by the invention]

In this way, in the above conventional circuit, when the current flowing through the LED 4 rises and falls, the transistor 5
The configuration is such that the same differential pulses of different polarities are applied to the bases of the two. Therefore, the rise and fall of the waveform of the current applied to the LED 4 is improved to the same extent.

However, the fall time still remains long compared to the rise time, and no matter how symmetrical the input signal waveform is, the waveform of the current applied to the LED 4 has poor symmetry.

Therefore, this poor symmetry hinders the increase in transmission speed in optical communications, and also poses a problem in that it makes it difficult to design an optical receiving circuit.

[Means to solve the problem]

The present invention has been made to solve these problems, and it generates a first pulse when starting to conduct current to a light emitting element, and converts this first pulse into a third transistor that generates a constant current. A first pulse generation circuit applied to the base, and a second pulse having a larger amplitude and opposite polarity than the first pulse when the current flowing through the light emitting element is cut off, and converting this second pulse into a third pulse. and a second pulse generation circuit that supplies the pulse to the base of the transistor.

[Effect]

When the current passing through the light emitting element rises, the constant current generated by the third transistor increases, and when the current passing through the light emitting element falls, the constant current generated by this third transistor increases at the rate of increase at the time of rise. decreases by a large percentage.

〔Example〕

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

The differential signal generation circuit 11 to which the binary signal V1n is input,
A binary signal in phase with this binary signal v1n and this 26f
! A binary signal having a phase opposite to that of the signal vIn is output.

The first transistor 12 is connected in series to the LED 13, and a binary signal in phase with the signal v1□ is applied to the base from the differential signal generation circuit 11. And transistor 12
switches the current supplied to the LED 13 in accordance with this binary signal. The second transistor 14 is connected in parallel to the series circuit composed of the transistor 12 and the LED 13, and has a switching operation opposite to that of the transistor 12. The third transistor 15 is connected in series with each of these parallel-connected transistors 12, 14, and has a resistor 16 connected to its emitter. Further, a predetermined voltage V is applied to the base via a resistor 17. is applied, and a constant current is applied to each transistor 12 and 14.

The first pulse generation circuit 18 inputs the opposite-phase binary signal outputted from the differential signal generation circuit 11, and generates a first pulse at the time of signal change of the binary signal when current starts to flow to the LED 13, This first pulse is applied to the base of transistor 15. The second pulse generation circuit 19 has an LED 13
A second pulse having an amplitude larger than that of the first pulse and having a polarity opposite to that of the first pulse is generated at the time of signal change of the binary signal when the current applied to the second pulse is interrupted. 2 pulses are applied to the base of transistor 15.

Therefore, the constant current generated by the transistor 15 increases when the current flowing through the LED 13 rises, and the constant current generated by the transistor 15 when the current flowing through the LED 13 falls is greater than the rate of increase at the rising edge. It decreases by a large percentage. Therefore, L
The degree of correction of the falling waveform of the current flowing through the ED 13 is greater than the degree of correction of the rising waveform.

FIG. 2 is a circuit diagram showing the detailed configuration of the embodiment shown in FIG. 1, and the same reference numerals are used for the same or corresponding parts as in FIG. 1, and the explanation thereof will be omitted.

The internal details of each pulse generation circuit 18 and 19 are configured as follows, and the signals output from each pulse generation circuit 18 and 19 are connected to the transistor 1 via capacitors 27 and 34.
5, the circuit configuration is the same as that shown in FIG.

That is, the pulse generating circuit 18 is composed of a differentiating circuit 21 and an inverting amplifier circuit 22. The differentiating circuit 21 includes a capacitor 23 whose one end is connected to the negative phase signal output terminal of the differential signal generating circuit 11, and a resistor whose one end is connected to the other end of the capacitor 23 and whose other end is raised to the power supply voltage. It consists of 24. In addition, the inverting amplifier circuit 22 includes a PNP transistor 25 whose base is connected to the connection point of the capacitor 23 and the resistor 26 and whose emitter is raised to the power supply voltage, and one end is connected to the collector of this transistor 25 and the other end is grounded. It is composed of a resistor 26.

The pulse generating circuit 19 includes a differentiating circuit 28 and an inverting amplifier circuit 29
It is composed of. The differentiating circuit 28 includes a capacitor 30 whose one end is connected to the negative phase signal output terminal of the differential signal generating circuit 11, and a resistor 31 whose one end is connected to the other end of the capacitor 30 and whose other end is grounded. ing. The inverting amplifier circuit 29 also includes a transistor 32 whose base is connected to a connection point between a capacitor 30 and a resistor 31 and whose emitter is grounded, and a resistor 33 whose one end is connected to the collector of the transistor 32 and whose other end is raised to the power supply voltage. It is composed of.

The operation of this embodiment in such a configuration will be explained in the third section.
A detailed description will be given below with reference to waveform diagrams shown in FIGS. (A) to (I).

A digital signal V1n, which is a binary signal shown in FIG. 3(A), is input to the differential signal generating circuit 11, and the differential signal generating circuit 11 receives a digital signal VIn shown in FIG.
It outputs a binary signal that is in phase with , and a binary signal that is opposite in phase to the digital signal 1n shown in FIG. This in-phase 2
A value signal is applied to the base of the transistor 12, and a binary signal of opposite phase is applied to the base of the transistor 14. As each transistor 12.14 performs opposite operations, the current flowing through the LED 13 changes by 100%. be done.

Therefore, the constant current applied to the LED 13 by the transistor 15 is interrupted, and the LED 13 repeatedly emits and extinguishes light.

At the same time, the opposite-phase binary signal output from the differential signal generation circuit 11 is applied to each differentiation circuit 21.28 in each pulse generation circuit 18.19. Pulse generation circuit 18
The differentiating circuit 21 inside differentiates this anti-phase binary signal and outputs a differentiated pulse that swings in both positive and negative polarities as shown in FIG. The inverting amplification circuit 22 inputs this differential pulse, and the transistor 25 performs an inverting amplification operation only during the negative polarity period of this differential pulse, thereby producing a selection pulse having only positive polarity components as shown in FIG. occurs,
Output this pulse.

The differentiating circuit 28 in the pulse generating circuit 19 differentiates this anti-phase binary signal and outputs a differentiated pulse having both positive and negative polarities as shown in FIG. The inverting amplification circuit 29 inputs this differential pulse, and the transistor 32 performs an inverting amplification operation only during the positive polarity period of this differential pulse, thereby producing a selection pulse having only negative polarity components as shown in FIG. occurs. The amplitude of this selection pulse is shown in the same figure (
The amplitude is generated larger than the amplitude of the selection signal by the pulse generation circuit 18 shown in E), and the inverting amplifier circuit 29 outputs this pulse.

Therefore, the pulse shown in FIG. 12 (H) is applied to the base of the transistor 15 that supplies constant current to each transistor 15, 16. This pulse is a combination of the small pulse of positive polarity shown in (E) of the same figure and the large pulse of negative polarity shown in (G) of the same figure, and this pulse has amplitudes of both positive and negative polarities. The voltage is based on the voltage VO input to one end of the resistor 17. Therefore, the constant current generated by the transistor 15 changes when current starts to flow through the LED 13 (positive phase binary signal (see figure (B))).
is increased by a small pulse of positive polarity at the rising edge of
Further, when the current flowing through the LED 13 is cut off (at the fall of the positive phase binary signal), the current is greatly reduced by a large pulse of negative polarity. The rate of decrease in current at the time of falling is greater than the rate of increase in current at time of rise, and therefore the current waveform applied to the LED 13 becomes as shown by the solid line in FIG. 1 (1).

In other words, the rising waveform and the falling waveform have good symmetry, and are more improved compared to the conventional current waveform shown by the dotted line. Therefore, the waveform of the light pulse emitted from the LED 13 also has good symmetry in proportion to the current waveform.

In addition, in the description of each of the above embodiments, the case where an LED was used as a light emitting element was explained, but the invention is not limited to this, and a laser diode (LD) or the like may also be used, and in this case, the above embodiments may also be used. It has the same effect as the example.

〔Effect of the invention〕

As explained above, according to the present invention, the constant current generated by the third transistor increases when the a-current of the light emitting element increases, and the constant current generated by the third transistor increases when the current flowing through the light emitting element falls. The constant current generated decreases at a rate greater than the rate of increase during startup.

Therefore, the symmetry of the optical pulse waveform emitted from the light emitting element is improved.

Therefore, by using this circuit in an optical communication transmission circuit, it is possible to fully bring out the performance of the light emitting element.
Further, it has the effect that the transmission speed of the optical communication system can be increased without changing the light emitting element.

Moreover, it also has the effect of further extending the transmission distance of optical communication.

Furthermore, since the symmetry of the optical pulse is improved, it has the effect of facilitating the design of the optical receiving circuit, and also has the effect of improving the optical receiving sensitivity. Furthermore, this circuit can be constructed using all elements that can be integrated into an IC, and the circuit scale can be constructed to be relatively small, so the entire circuit can be easily integrated into a monolithic IC. It also has the effect of making it possible.

3(A) to 3(I) are signal waveform diagrams in each part of the circuit shown in FIG. 2, FIG. 4 is a circuit diagram showing the conventional configuration, and FIG. 5 ( A).

(B) is a signal waveform diagram in a conventional circuit.

DESCRIPTION OF SYMBOLS 11... Differential signal generation circuit, 12... First transistor, 13... LED, 14... Second transistor, 15... Third transistor, 18... First
pulse generating circuit, 19... second pulse generating circuit.

Patent applicant: Sumitomo Electric Industries, Ltd. Representative patent attorney Yoshiki Hase
Salt 1) Tatsuya

[Brief explanation of the drawing]

Figure 1 is a block diagram showing a schematic configuration of an embodiment of the present invention, and Figure 2 is a detailed diagram of the embodiment shown in Figure 1.

Claims (1)

[Claims] A first device connected in series to a light emitting element and switching a current flowing through the light emitting element in accordance with an input binary signal.
a second transistor connected in parallel to the first transistor and having a switching operation opposite to that of the first transistor; a second transistor connected in series to the first and second transistors connected in parallel; a third transistor that causes a constant current to flow through the first and second transistors; and a third transistor that generates a first pulse at a signal change point in the binary signal at which current begins to flow through the light emitting element; a first pulse generating circuit that applies a current to the base of the third transistor;
generating a second pulse having an amplitude greater than the first pulse and a polarity opposite to the first pulse at the time of signal change of the value signal, and applying the second pulse to the base of the third transistor; A light emitting element drive circuit comprising: a second pulse generation circuit;