JPH0683047B2 - Light emitting element drive circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a light emitting element drive circuit in which rising and falling characteristics of a light emitting element used in digital optical communication are improved.

[Prior Art] FIG. 3 shows the conventional light emission shown in, for example, "Transmission / reception modules for optical fiber transmission for civilian use from low price to high-performance products" (Nikkei Electronics, July 18, 1983, P143). In the figure, 1 is an emitter follower transistor, 2 is a light emitting device connected between the emitter of the emitter follower transistor 1 and ground, and 3
Is a NAND gate that outputs the inverted signal of the modulation signal, 28 is NAND
A capacitor 29 is connected between the output of the gate 3 and the base of the emitter follower transistor 1, and 29 is connected in parallel with the capacitor 28 between the output of the NAND gate 3 and the base of the emitter follower transistor 1. A resistor, 31 is a diode provided between the output of the NAND gate 3 and the emitter of the emitter-follower transistor 1, and is a diode from the emitter of the emitter-follower transistor 1 to the NAND.
The gate 3 is connected so that the direction toward the output is the forward direction. 30 is a power supply and an emitter follower transistor 1
It is a resistor connected between the collector and.

When the emitter follower transistor 1 is in the conducting state, the resistance 29 is reduced by the current amplification factor.
It is provided to prevent an excessive current from flowing out from the ND gate 3. The value of the capacitor 28 is
The input resistance and the input capacitance when the emitter follower transistor 1 is conducting are Rin and Cin, respectively, and the capacitor 28 and the resistor 2
When 9 is C and R respectively, it is decided that CR = CinRin (1).

The capacitor 28 is provided to prevent the rise of the output of the NAND gate 3 from being slowed by the resistance 29 and the input capacitance of the emitter follower transistor 1 to prevent the rise of the output of the light emitting element.

Further, the diode 31 becomes conductive when the NAND gate 3 falls, so that the charge accumulated in the light emitting element 2 and the emitter follower transistor 1 is rapidly discharged. That is, the diode 31 forcibly lowers the drive voltage of the light emitting element to the anode potential V _D1 when the diode 31 is conducting, and is provided to prevent the fall of the output of the light emitting element.

FIG. 4 is a waveform diagram of each part of the light emitting element driving circuit in FIG. 3, in which 32 is a voltage between terminals of the light emitting element (hereinafter, abbreviated as light emitting element driving voltage), 33 is a switching voltage level of the light emitting element ( That is, a threshold level), 34 is a ground voltage, 35 is a light emitting element drive current, and 36 is a light emitting element optical output.

In FIG. 4, the falling line shown by the dotted line shows the falling characteristic when the diode 31 is not provided, and the solid line shows the falling characteristic when the diode 31 is provided. There is.

Next, the operation will be described.

The modulation signal is inverted by the NAND gate 3, and the emitter follower transistor 1 is driven by this inverted output. The emitter follower transistor 1 performs saturation switching, and NA
Conducts when the output of the ND gate 3 becomes "H", and the output is "L".
When it becomes, it becomes non-conductive.

Thus, the saturation switching of the emitter follower transistor 1 controls the light output of the light emitting element 2 connected between the emitter of the emitter follower transistor 1 and the ground.

[Problems to be Solved by the Invention] As can be seen from the waveform of the optical output 36 of FIG. 4, in the above conventional light emitting element drive circuit, when the diode 31 is present as compared with the case where the diode 31 is not present (dotted line) (Solid line) is
The falling speed of the light emitting device light output 36 is improved.

However, the anode potential V when the diode 31 is conducting
The difference between _D1 and the light emitting element switching voltage V _DF cannot be made large, and the internal resistance of the light emitting element 2 increases as the light emitting element drive voltage 32 decreases. Therefore, the light emitting element drive current that determines the discharge time of the accumulated carriers of the light emitting element 2 There is a problem that the fall amount of 35 cannot be made large enough and the trailing edge of the light output 36 of the light emitting element remains.

Further, there is a problem that even when the light output 36 of the light emitting element rises, no measure is taken to improve the portion exceeding the rise time limited by the response speed of the light emitting element 2.

The present invention has been made to solve the above-mentioned problems, and it is possible to increase the speed of rising and falling of the light output of a light emitting element and to easily control the rising and falling speeds of the light emitting element. Aim to get the circuit.

[Means for Solving the Problems] A light emitting element drive circuit according to the present invention includes a first emitter follower transistor, a light emitting element connected to the emitter of the first emitter follower transistor, and a modulation signal at the base. Is supplied to the base of the first emitter follower transistor, and the collector is connected to the power supply via a resistor, and the collector of the first emitter follower transistor is connected to the emitter of the first emitter follower transistor based on the inverted signal. A first grounded-emitter transistor that generates a drive voltage for the light emitting element and supplies a drive current to the light emitting element, a rising point detection circuit that outputs a pulse that falls at the rising point of the modulation signal, and this rising point detection at the base. The output of the circuit is supplied,
The collector is composed of a second grounded-emitter transistor connected to the collector of the first grounded-emitter transistor, and a predetermined pulse voltage generated by the output and resistance of the rising-point detection circuit is applied at the rising point of the modulation signal. A rising voltage supply circuit to be added to the drive voltage, a level shift circuit whose input terminal is connected to the collector of the base of the first emitter follower transistor, and the output of this level shift circuit is supplied to the base, and the emitter is It comprises an emitter of the first emitter follower transistor and a second emitter follower transistor connected to the light emitting element, and based on a voltage obtained by level-shifting the drive voltage by a constant value by the level shift circuit, At the trailing edge of the drive voltage, the emitter current is applied to the second emitter follower transistor. And a falling current supply circuit connected to the output side of the light emitting element and lower than the collector potential of the first emitter follower transistor by a predetermined potential or more and higher than the collector potential of the second emitter follower transistor by a predetermined potential or more. The power supply circuit outputs a high voltage, and a resistor connected between the emitter of each of the grounded-emitter transistors and the ground.

[Operation] The rising voltage supply circuit causes the emitter of the first emitter follower transistor to generate an initial driving voltage which is a driving voltage + a predetermined voltage at the rising point of the modulation signal.

The falling current supply circuit causes an emitter current to flow through the second emitter follower transistor at the falling point of the driving voltage, and sucks out the charge accumulated in the junction of the light emitting element.

The rise time of the light emitting element is adjusted by adjusting the value of the resistor connected between the emitters of the first and second grounded-emitter transistors and the ground.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of a light emitting element drive circuit according to the present invention.

In the figure, 1 is a first emitter follower transistor,
A light emitting element 2 is connected to the emitter of the first emitter follower transistor 1. 3 is a NAND gate for obtaining an inverted signal of the modulation signal, 4 is a first grounded-emitter transistor, the inverted signal of the modulation signal is supplied to the base, and the collector is connected to the base of the first emitter follower transistor 1. And power is supplied via the resistor 7.
It is connected to VCC and a resistor 8 is connected between the emitter and ground. The first grounded-emitter transistor 4 causes the emitter of the first emitter follower transistor 1 to generate a drive voltage for the light emitting element 2.

Reference numeral 16 is a rising point detection circuit that outputs a pulse that falls at the rising point of the modulation signal. This is composed of a NAND gate 17 that takes the NAND of the output of the NAND gate 3, a NAND gate 18 that takes the NAND of the output of this NAND gate 17, and a NAND gate 19 that takes the NAND of the output of the NAND gate 18 and the modulation signal. Composed.
Reference numeral 5 is a second grounded-emitter transistor, the output of the NAND gate 19 is supplied to the base, the collector is connected to the collector of the first grounded-emitter transistor 4, and the resistor 9 is connected between the emitter and ground. Has been done. The rising point detection circuit 16 and the second grounded-emitter transistor 5 constitute a rising voltage supply circuit 1A, whereby the rising point detection circuit 16 is provided at the rising point of the modulation signal.
And a predetermined pulse voltage generated by the resistor 7 and the resistor 7 is added to the drive voltage.

Reference numeral 10 is a level shift circuit, which is a circuit connected between the base of the first emitter follower transistor 1 and the collector of the first grounded-emitter transistor 4 and has a direction from the base to the ground. Forward direction 2
This is a circuit formed by connecting two diodes 11 and 12 and a resistor 13 in series. Reference numeral 6 denotes a second emitter follower transistor, which is supplied with the output of the level shift circuit 10 at its base and whose emitter is connected to the emitter of the first emitter follower transistor and the light emitting element 2. . The second emitter follower transistor 6 and the level shift circuit 10 constitute a falling current supply circuit 1B. The falling current supply circuit 1B level-shifts the driving voltage by a constant value, and based on this shift voltage, causes an emitter current to flow through the second emitter follower transistor 6 at the falling point of the driving voltage. Is.

In the light emitting element 2, the anode has a resistor 15 and a capacitor 14
Through a parallel circuit of the first emitter follower transistor 1 and the second emitter follower transistor 6
Connected to the emitter of. In addition, this light emitting element 2
A power supply circuit 20 is connected to the cathode of the. The power supply circuit 20 outputs a voltage 0.7 V or more lower than the collector potential of the first emitter follower transistor 1 and 0.7 V or more higher than the collector potential of the second emitter follower transistor 6.

Next, the operation will be described with reference to FIGS. 2 and 1. Second
In the figure, 37 is an input signal of the NAND gate 3 (hereinafter abbreviated as a modulation signal), 38 is a base input signal of the first grounded-emitter transistor 4, 39 is a collector current of the first grounded-emitter transistor 4, and 40 is Output signal of NAND gate 18,
41 is the output of the rising point detection circuit 16, 42 is the collector current of the second grounded-emitter transistor 5, 21 is the voltage between the terminals of the light emitting element (hereinafter abbreviated as light emitting element drive voltage), and 22 is the switching of the light emitting element 2. The voltage level V _DF (that is, the threshold level), 23 is the cathode voltage level of the light emitting element 2,
24 is the emitter current of the first emitter follower transistor 1, 43 is the base input voltage of the second emitter follower transistor 6, and 44 is 0. from the cathode voltage level of the light emitting element 2.
7V lower potential, 25 is the second emitter follower transistor 6
Is the emitter current, 26 is the drive current of the light emitting element 2, and 27 is the light output of the light emitting element.

Note that, in FIG. 2, the case where the modulation signal 37 for causing the light emitting element to emit light at t1 to t2 and t3 to t4 is input will be described.

First, the rising operation of the light output of the light emitting element and the operation during light emission will be described. The NAND gate 3 inverts the modulation signal 37 and
The base input signal 38 of the grounded-emitter transistor 4 is output. The first grounded-emitter transistor 4 converts this input signal into a current, and outputs the collector signal current 39 of the first grounded-emitter transistor 4 to the resistor 7.

The falling point detection circuit 16 inputs the output signal 40 of the NAND gate 18 delayed by td [seconds] and the modulation signal 37 through the NAND gates 3, 17 and 18 to the NAND gate 19 to generate the modulation signal 37. Falling in synchronization with the rising point of, duration td1 [sec]
Output 41 of rising point detection circuit 16 that keeps low level only
Is output to the second grounded-emitter transistor 5.

The second grounded-emitter transistor 5 is a rising point detection circuit.
The output 41 of 16 is converted into a signal current, and the collector current 42 of the second grounded-emitter transistor 5 is output to the resistor 7. The collector current 42 of the second grounded-emitter transistor 5 and the collector current 39 of the first grounded-emitter transistor 4 are summed in the resistor 7. Therefore, the current flowing through the resistor 7 is minimized from the rising point t1 of the modulation signal 37 to td1 [seconds], and then a constant current flows until time t2 when the modulation signal 37 falls. Correspondingly, the light emitting element drive voltage 21 generated in the emitter of the emitter follower transistor ₁ is a waveform having V _{2 at} the rising edge of the modulation signal and V ₁ at the steady state with reference to the output voltage V _EE of the power supply circuit 20. Becomes

The first and second emitter-floor transistors 1 and 6 and the level shift circuit 10 constitute a push-pull class B operation amplifier, and the load is the parallel impedance of the capacitor 14 and the resistor 15 and the light emitting element 2. That is, the base input voltage 43 of the second emitter follower transistor 6 is a diode input of the base input voltage of the first emitter follower transistor 1.
The signal is obtained by lowering the potential by about 1.4V by 11 and 12, and therefore has a waveform similar to the light emitting element drive voltage 21 and shifted by 0.7V. The cathode potential V _EE 23 of the light emitting element 2 is the power supply circuit 2
0 is set between the low level V ₃ of the light emitting element drive voltage 21 and the switching voltage level V _DF of the light emitting element, and the first emitter follower transistor 1 is turned on at t1 to t2 and t3 to t4. Since the second emitter follower transistor 6 is cut off, the emitter current 24 of the first emitter follower transistor 1 flows to the light emitting element 2. Here, the resistor 15 is a steady-state amplitude V ₁ of the light-emitting element drive voltage 21,
The light output power of the light emitting element 2 is determined together with the switching voltage V _DF of the light emitting element. Further, the capacitor 14 compensates the parasitic capacitance of the light emitting element 2.

Emitter current of the first emitter follower transistor 1 24
The current pulse I ₁ generated at the rising edge of ₁ drives the light emitting element 2 larger than the steady-state current I ₀ , so that the light output 27 of the light emitting element 2 can rise faster.

Next, the extinction operation and the falling operation will be described. Light emitting element drive voltage
When 21 becomes equal to or lower than the cathode voltage level V _EE 23 of the light emitting element 2, the first emitter follower transistor 1 is cut off and the second emitter follower transistor 6 is turned on. The base input voltage 43 of the second emitter follower transistor 6 is the cathode voltage level of the light emitting element 2 during the period from t2 to t3.
Since the voltage is 0.7 V or more lower than V _EE , the emitter current 25 of the second emitter follower transistor 6 flows into the light emitting element 2. The emitter current 25 of the second emitter-follower transistor 6 absorbs the electric charge accumulated in the junction of the light emitting element 2 with the leading pulse current I ₂ and quickly lowers the voltage between the terminals of the light emitting element 2 to output light. The fall of can be accelerated.
When the voltage between the terminals of the light emitting element 2 decreases and the light output falls, the impedance of the light emitting element 2 increases, so
The emitter current 25 of the emitter-follower transistor 6 of is almost no longer flowing.

That is, the rising voltage supply circuit 1A generates a driving voltage obtained by adding the pulse voltage based on the pulse current I ₁ to the steady-state voltage to the emitter of the first emitter follower transistor 1, and the pulse current of the light emitting element driving current 26. The rise of the light output 27 of the light emitting element can be accelerated by I ₁ .

Further, since the electric charge at the time of light emission is accumulated in the junction portion of the light emitting element, it is necessary to suck out the accumulated electric charge in order to speed up the fall.

Therefore, by forcibly flowing the emitter current 26 of the second emitter follower transistor 6 by the falling current supply circuit 1B, the charge accumulated in the junction of the light emitting element 2 is sucked out, and the light output 27 of the light emitting element is output. Speed up the fall.

That is, an emitter current 24 of the first emitter follower transistor 1 and an emitter current 25 of the second emitter follower transistor 6 are obtained by the transistor pair that performs push-pull class B operation, and when the light emitting element drive current 26 rises. The generated pulse current I ₁ accelerates the rise of the light emitting element optical output 27, and the pulse current I ₂ generated when the driving voltage falls causes the light emitting element optical output 27 to quickly fall.

Further, the time required for the rise of the optical output can be adjusted by changing the value of the resistor 8 or the resistor 9.

As described above, according to the present invention, the rising voltage supply circuit causes the emitter of the first emitter follower transistor to generate the drive voltage + pulse voltage at the rising point of the modulation signal, By flowing an emitter current having a large driving force, the rise of the light output of the light emitting element is accelerated, and at the same time, by the fall current supply circuit,
By passing an emitter current through the emitter follower transistor, the electric charge remaining in the junction of the light emitting element is absorbed, and the fall of the light output of the light emitting element is accelerated.

Further, the time required for the fall can be adjusted by changing the resistance values of the first and second grounded-emitter transistors.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram of each part of a light emitting element drive circuit, FIG. 3 is a configuration diagram of a conventional light emitting element drive circuit, and FIG. 4 is FIG. It is a waveform diagram of each part of the light emitting element drive circuit shown. 1 ... First emitter follower transistor, 2 ... Light emitting element, 3 ... NAND gate, 4 ... First grounded emitter transistor, 5 ... Second grounded emitter transistor,
6 ... Second emitter follower transistor, 7-9 ...
Resistance, 10 ... Level shift circuit, 16 ... Fall point detection circuit, 20 ... Power supply circuit, 1A ... Rising voltage supply circuit, 1B ...
… Falling current supply circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadayoshi Kitayama 5-1-1, Ofuna, Kamakura-shi, Kanagawa Mitsubishi Electric Corporation, Information Electronics Laboratory (56) Reference Japanese Patent Laid-Open No. 54-152955 (JP, A) Kai 56-89130 (JP, A) JP 55-138927 (JP, A)

Claims (1)

[Claims] 1. A first emitter follower transistor, a light emitting element connected to the emitter of the first emitter follower transistor, an inverted signal of a modulation signal is supplied to a base, and a collector is the first emitter. The base of the follower transistor is connected to a power source through a resistor, and the drive voltage of the light emitting element is generated in the emitter of the first emitter follower transistor based on the inverted signal to drive the light emitting element. It is composed of a first grounded-emitter transistor that supplies a current and a plurality of logic circuits. The modulation signal is supplied to one input of the final-stage logic circuit and the inverted signal of the modulation signal is logically supplied to the other input. The processed signal is supplied,
A rising point detection circuit that outputs a pulse that falls at the rising point of the modulation signal, and a second emitter whose output is supplied to the base and whose collector is connected to the collector of the first grounded-emitter transistor A rising voltage supply circuit which is composed of a grounding transistor and which adds a predetermined pulse voltage generated by the output of the rising point detection circuit and a resistance to the driving voltage at the rising point of the modulation signal; and a first emitter follower transistor. A level shift circuit whose input terminal is connected to the base and the collector of the first grounded-emitter transistor, and the output of the level shift circuit is supplied to the base, and the emitter is the emitter of the first emitter follower transistor and the light emission. A second emitter follower transistor connected to the device, Based on a voltage obtained by level-shifting the driving voltage by a constant value in the shift circuit, a falling current supply circuit for flowing an emitter current to the second emitter follower transistor at the falling point of the driving voltage, and A power supply circuit connected to the cathode for outputting a voltage lower than the collector potential of the first emitter follower transistor by a predetermined potential or more and higher than the collector potential of the second emitter follower transistor by a predetermined potential or more; A light emitting element drive circuit comprising a resistor connected between an emitter of a transistor and ground.