JPH03257885A - Driving circuit for light emitting element - Google Patents

Abstract

PURPOSE:To sharpen the start-up of the optical output of a light emitting element and approximating the width of the on pulse of the optical output to the width of the pulse from a control signal input means by providing a resistance between the other electrode of a third transistor and the control electrode common junction between first and second transistors. CONSTITUTION:A resistance 10 is provided between the other electrode of a third transistor Q3 and the control electrode common junction between first and second transistors Q1 and Q2. Hereby, the potential of the control electrode of the third transistor Q3 becomes high by the amount of the voltage drop with the resistance, considering it on the basis of the control electrode common junction between the first and second transistors, and in case that the light emitting element shifts from OFF to ON, allowance occurs between the control electrode of the third transistor and the other electrode. As a result, in case that the light emitting element shifts from OFF to ON, the third transistor becomes late in turning to OFF, and the potential of the control electrode common junction between the first and second transistors rises sharply than ever, and the start-up of the driving current of the light emitting element is sharpened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a light emitting element drive circuit, and more particularly to a light emitting element drive circuit that can make the rise of a drive current of a light emitting element steep.

[Conventional technology]

FIG. 3 is a circuit diagram showing a conventional semiconductor laser diode drive circuit. In the figure, 1 is a control signal input terminal;
2 is an input circuit, 3 is a high potential power supply, and 4 is a low potential power supply. Ql, Q2 are bipolar NPN) transistors, Q3
is a Schottky clamp)NPN) transistor.

Transistor Ql.

Q2 constitutes a current mirror circuit using transistor Q1 as a reference transistor, and transistor Q3 functions as a driving transistor for driving the current mirror circuit. The base of the transistor Q1 is connected to the input circuit 2,
Collectors are each connected to a high potential power source 3 via a constant current source 5. The transistor Q2 has a base connected to the base of the transistor Q1, a collector connected to the high potential power supply 3 via the resistor 6, and an emitter connected to the emitter of the transistor Q2. A node X is a common connection point between the bases of transistors Ql and Q2. The input circuit 2 controls current sinking of the node X based on the control signal from the control signal input terminal 1.

The transistor Q3 has a base connected to the collector of the transistor Ql, and an emitter connected to the transistor Ql.

A collector is connected to the high potential power supply 3 via a resistor 7 at the base common connection point of Q2.

A semiconductor laser diode (hereinafter referred to as "LD") 8 has an anode connected to the common emitter connection point of the transistors Ql and Q2, and a cathode connected to the low potential power supply 4. 0N10FF of D8 is controlled by the current from transistors Ql and Q2.The operation will be explained next. When the output state of the input circuit 2 becomes "L" due to the control signal from the control signal input terminal 1, that is, the input circuit 2 enters a state where it absorbs the current from the node X, the transistors Ql and Q2 are turned off.
do. At this time, since no current is supplied to the anode of D8, D8 is also at 0FFt.

On the other hand, by turning off the transistor Q1, the constant current source 5
The current flowing from the transistor Q3 increases the potential across the heath of the transistor Q3, and the transistor Q3 turns on.

At this time, the base potential ■B3L of the transistor QB is
The "L" level output voltage of the input circuit 2 is set to V1. , if the voltage between the base and emitter of transistor Q3 is VBE3, then ■"= VL +vBEE "'(
1) 3L, and the base current of the transistor 3 at this time is the output current of the constant current source 5. is equal to Now, the collector current of transistor Q3 is l 1 crystal type 3 The voltage value of power supply 3 is V The resistance value of resistor 7 is R7, CO2 The collector-emitter saturation voltage of transistor Q3 is V
sat3, the saturation emitter current of transistor Q3 is 'E
3, IE3=Io ” 'cs −(
3). At node X, there is a current IE expressed by equation (8).
3 is given, and the input circuit 2 is sucking this current 'E8.

When the output state of the input circuit 2 becomes a high impedance state due to the control signal from the control signal input terminal 1, the input circuit 2
no longer sinks the current at node X, and transistor Q
1. Q2 turns on. At this time, a transistor Ql is connected to the anode of D8.

Since current is supplied through Q2, D8 is turned on.

At this time, the potential Vx of node X is vX” vLD+
vBEI ”・(4)■ = Operating voltage D of D8 vBEl: This is the forward voltage between the base and emitter of the transistor Q1. Also, the base potential V of the transistor Q3 at this time is B3f (V − V x + V BE3 AH -V 10V+V ... (5) LD BE
It becomes IBF2. If we ignore the base current of transistor Q3,
A current IO flows through the transistor Q1.

If the current mirror ratio of transistors Ql and Q2 is N, then the collector current I of transistor Q2 is N・1o
becomes. The sum of the base currents of transistors Ql and Q22 is the emitter current of transistor Q3.
Equal to E3.

Therefore, Current amplification factor of β-transistor becomes.

When the output of the input circuit 2 shifts from "L" to a high impedance state, that is, in a transient state where D8 shifts from OFF to ON, the output potential of the input circuit 2, that is, the common base potential of the current mirror circuit, The potential VX of the node X increases from vL to V +V.

LD BEI By the way, there is a parasitic capacitance between the constant current source 5 and the base of the transistor Q3 and between the collector of the transistor Q1 and the base of the transistor Q3. Therefore, when the output of the input circuit 2 shifts from "L" to a high impedance state, the base potential of the transistor Q3 is delayed in rising due to the above-mentioned parasitic capacitance. As a result, the potential difference between the base and emitter of transistor Q3 becomes smaller, and transistor Q3 becomes OF.
Along with this, the transistor Ql.

Since the current mirror circuit formed by Q2 also turns OFF, an increase in the drive current of LD8 is prevented, and the waveform of the drive current Ip of LD8 becomes a waveform with a blunted rise as shown in FIG. Furthermore, as shown in FIG. 4, the drive current I
When F is less than the threshold current Ith, the optical output P is almost 0, and the drive current l is the threshold current lth.
When it exceeds 1p, the previous output P increases linearly in proportion to the drive current 1p.

Therefore, when the rising edge of the drive current IF is blunted, the rising edge of the waveform of the optical output P is also blunted as shown in FIG. 5, and the pulse width of the optical output P is
is smaller than the pulse width of

[Problem to be solved by the invention]

Since the conventional semiconductor laser diode drive circuit is configured as described above, the rise of the drive current IF of the LD8 becomes blunt in the transient state when the LD8 shifts from OFF to ON, and as a result, the waveform of the optical output P becomes distorted. The rising edge may be blunt, or the on-pulse width of the optical output P may be lower than the potential V.
There was a problem that the pulse width was significantly narrower than the pulse width of x.

This invention was made to solve the above-mentioned problems, and the light output of the light emitting element has a steep rise, and the width of the on-pulse of the light output is about the same as the width of the pulse from the control signal input means. The purpose is to obtain a certain light emitting element drive circuit.

[Means to solve the problem]

The light emitting element driving circuit according to the present invention includes a control signal input means, a first transistor having a control electrode connected to the control signal input means and one electrode connected to the first power source, and a control electrode connected to the first power source. a second transistor having one electrode connected to the first power supply and the other electrode connected to the other electrode of the first transistor, forming a current mirror circuit together with the first transistor; a control electrode is connected to one electrode of the first transistor;
a third transistor having one electrode connected to the first power source, the other electrode of the third transistor and the first . A resistor connected between the common connection point of the control electrode of the second transistor and one electrode of the first transistor. A light emitting element whose other electrode is connected to the second power source is provided at the common connection point of the other electrode of the second transistor.

[Effect]

In this invention, the other electrode of the third transistor and the first . Since a resistor is provided between the common connection point of the control electrodes of the second transistor, the potential of the control electrode of the third transistor is based on the common connection point of the control electrodes of the second transistor. becomes higher by the voltage drop of
When the light emitting element shifts from OFF to ON, there is a margin in the voltage between the control electrode of the third transistor and the other electrode.

〔Example〕

FIG. 1 is a circuit diagram showing an embodiment of a light emitting element driving circuit according to the present invention. In the figure, the difference from the conventional circuit shown in FIG. 3 is that a resistor 10 is newly provided between the emitter of transistor Q3 and node X. Other configurations are the same as before.

Next, the operation will be explained. When the output of the input circuit 2 becomes a high impedance state, the input circuit 2 no longer absorbs the current of the node X, and the transistors Ql and Q2 are turned on. Transistor Q1. The sum of the currents flowing through Q2 is given to LD8 as drive current IF (-1° (1,000 N)). Here, if the emitter current of the transistor Q3 is I, the emitter 3H vine current I becomes 3H. At this time, the voltage drop V across the resistor 10 is IOR.If the resistance value of the resistor 10 is RIO, then DlOH R10XIE8H FR10X- β (8). Base potential V of transistor Q3 at this time
B 3 nit·v'-v+v+v BAHX DIOHBEi3 (9).

On the other hand, when the output of the input circuit 2 is "L", the transistors Ql, Q2 and LD8 are turned off as in the conventional case, so that the output current I of the constant current source 5. are all the base current of transistor Q3, and the emitter current IE of transistor Q3 is
3L becomes l, -IO+Ic3·(10) EL.

By the way, since I - (1 + N) ・IO, it is a constant current! . is, and when formula (11) is substituted into formula (lO), it becomes. Therefore, the output state of the input circuit 2 is "L".

The base potential VB of transistor Q3 in this case is +VB
E3...(13). (13
) and (1), this example has a higher RIO
・It will be larger by (IF/ (1+N)+Ic3). This means that when the output state of input circuit 2 changes from "L" to high impedance, that is, LD8 is OFF.
When transitioning from ON to ON, the base of transistor Q3
Conventionally, the emitter voltage is RIO・(I,/(1+
This shows that there is a margin of N) + IO3). Also, since β<<(1+N) usually holds, equation (1), (
5) Formula, (9) formula, (13) formula, and V
The amount of change from ' to vB3H8L is smaller than the amount of change from V to VB3L
B3H It gets smaller. Therefore, when LD8 transitions from OFF to ON, there is a margin in the voltage between the base and emitter of transistor Q3, and transistor Q3 turns OFF more slowly than before, and the current mirror circuit consisting of transistors Q1 and Q2 turns OFF. As a result, as shown in FIG. 2, the rise of the drive current 1p of the LD 8 becomes steeper. As a result, as shown in FIG. 2, the rise of the optical output P of the LD 8 becomes steep, and the width of the on-pulse of the optical output P does not differ significantly from the pulse width of the input circuit 2 potential Vx.

Note that although the Schottky clamped NPN transistor Q3 is used in the above embodiment, a transistor that is not Schottky clamped may be used. Furthermore, although a laser diode is used as the light emitting element, similar effects can be obtained by using other light emitting elements, such as light emitting diodes.

〔Effect of the invention〕

As described above, according to the present invention, the other electrode of the third transistor and the first electrode of the third transistor. Since a resistor is provided between the control electrode common connection point of the second transistor and the control electrode common connection point of the second transistor, the potential of the control electrode of the third transistor is the same as that of the control electrode common connection point of the second transistor. When the voltage increases by the voltage effect at the resistor and the light emitting element shifts from OFF to ON, there is a margin in the voltage between the control electrode and the other electrode of the third transistor. As a result, when the light emitting element transitions from OFF to ON, the third transistor turns OFF slowly, and the first... The potential at the common connection point of the control electrodes of the second transistor rises more steeply than in the conventional case, and the rise of the drive current of the light emitting element becomes steeper. As a result, the light output of the light emitting element rises steeply, and the on-pulse width of the light emitting element and the pulse width from the control signal input means are approximately equal.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the light emitting element driving circuit according to the present invention, Fig. 2 is a waveform diagram for explaining the operation of the circuit shown in Fig. 1, and Fig. 3 is a conventional semiconductor laser. Circuit diagrams showing the diode drive circuit, Figures 4 and 5 are shown in Figure 3.
FIG. 3 is a waveform diagram for explaining the operation of the circuit shown in the figure. In the figure, 1 is a control signal input terminal, 2 is an input circuit, and 3 is a control signal input terminal.
is a high potential power supply, 4 is a low potential power supply, 8 is a semiconductor laser diode, 10 is a resistor, Ql and Q2 are NPN transistors, and Q3 is a Schottky clamped NPN transistor. Note that the same reference numerals in each figure indicate the same or corresponding parts.

Claims (1)

[Claims] (1) a control signal input means; a first transistor having a control electrode connected to the control signal input means and one electrode connected to a first power supply; a control electrode connected to the control electrode of the first transistor; a second transistor having one electrode connected to the first power supply and the other electrode connected to the other electrode of the first transistor, forming a current mirror circuit together with the first transistor; and a control electrode connected to the first transistor; a third transistor, one electrode of which is connected to the first power source, and a common connection point between the other electrode of the third transistor and the control electrodes of the first and second transistors; A light emitting element driving circuit comprising: a resistor connected between the two transistors; and a light emitting element having one electrode connected to a common connection point of the other electrodes of the first and second transistors, and the other electrode connected to a second power source.