JP3067500B2 - Laser diode pulse drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode pulse drive circuit.

[0002]

2. Description of the Related Art In a circuit for pulse-driving a laser diode, as the pulse width of a driving pulse applied to the laser diode becomes narrower, the delay in the off-time of the output transistor for driving the laser diode cannot be ignored.

Next, a pulse driving circuit for a laser diode according to the prior art will be described with reference to FIG. In the conventional circuit shown in FIG. 5, the base of the driving transistor 3 is grounded (GND) via the resistor 9. The pulse voltage input to the input terminal 10 is current-amplified by the transistor 4 and logically turns the transistor 3 on and off.

When the transistor 3 is turned on, it functions as a high-speed switch, and grounds one end of the resistor 2 connected to the collector. Pulse current I for driving laser diode 1
f is obtained by equation (1).

If = (Vss−Vf−Vce (sat)) / R2 (1) where Vss is a power supply voltage and Vf is a laser diode 1
Vce (sat) is the collector-emitter saturation voltage of the transistor 3, and R2 is the resistance value of the resistor 2.

In FIG. 5, when the pulse voltage applied to the input terminal 10 becomes "0" volt, the input voltage applied to the transistor 3 is grounded (GND) via the resistor 9 to "0" volt. And the transistor 3 is turned off.

However, when the transistor 3 is on, excessive charge is accumulated inside the transistor 3, so that the on state continues even if the input pulse current supplied to the base is lost. Therefore, the transistor 3 is turned off with a delay. The delay of the off time becomes longer as the input current to the base of the transistor becomes excessive.

In order to reduce the delay time, it is necessary to reduce the resistance value of the resistor 9 and to adjust the peak value of the pulse voltage to be input in order to obtain an appropriate input current according to the switching characteristics of the transistor 3. is there.

Next, the operation of the laser diode pulse driving circuit shown in FIG. 5 will be described with reference to FIG. 6 in FIG.
a, 3a and If are various waveforms shown corresponding to each part of the circuit shown in FIG.

4a is an input voltage waveform having a pulse peak value of E volts, 3a is an output voltage waveform of the transistor 4, V4 is a forward voltage between the base and the emitter of the transistor 4, If is an output current of the transistor 3, and 5 is a driving current waveform of FIG.

[0011]

In the case of the conventional circuit shown in FIG. 5, the fall of the output current waveform of the transistor 3 for driving the laser diode 1 in a pulse is slower than the input pulse voltage.

This is due to the charge accumulation effect, because the electric charge excessively accumulated inside the transistor is slowly discharged by the time constant of the resistor 9. Although it can be neglected in applications of relatively long pulses of μs or more, when switching pulses narrow in the order of ns, the pulse width is widened, and the switching function of the transistor 3 is significantly reduced. A problem that the motor cannot be driven.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a high-speed and stable laser diode can be realized by quickly discharging excess accumulated charges in a laser diode driving transistor with a simple circuit configuration. It is an object to provide a pulse drive circuit.

[0014]

According to a first aspect of the present invention, there is provided a laser diode having one end connected to a power supply, the other end connected to a collector via a resistor, and a grounded emitter. A transistor 4 having a collector connected to a power supply and an emitter connected to the base of the transistor 3, a transistor 7 having a base connected to the base of the transistor 4 and a collector grounded, and a transistor 7 connected between the emitter of the transistor 7 and the power supply. A resistor 6 and a capacitor 5 having one end connected to the emitter of the transistor 7 and the other end connected to the base of the transistor 3 apply a reverse bias between the base and the emitter of the transistor 3 when the input pulse voltage is turned off.

In the second invention, the transistor 3 has one end connected to the power supply, the other end of the laser diode 1 connected to the collector and the emitter grounded via the resistor 8, and the collector connected to the power supply and the emitter connected to the transistor. And a transistor 4 connected to the base of transistor 3
7 connected to the base of the transistor and the collector is grounded
And a resistor 6 connected between the emitter of the transistor 7 and the power supply, and a capacitor 5 having one end connected to the emitter of the transistor 7 and the other end connected to the base of the transistor 3. Outputs a proportional constant current,
A reverse bias is applied between the base and the emitter of the transistor 3 at the timing when the input pulse voltage is turned off.

[0016]

By adopting such a structure, in the first structure, a low-impedance reverse bias is applied to the base of the transistor 3 at the same time as the input pulse is turned off, thereby forcibly neutralizing the charge stored in the transistor 3. This makes the turn-off time of the output transistor very short.

In the second configuration, the turn-off time of the transistor 3 is improved by using a part of the first configuration, and a constant current pulse proportional to the peak value of the input pulse voltage is output at high speed.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a pulse driving circuit for a laser diode according to the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a circuit diagram showing a first embodiment of a pulse drive circuit for a laser diode according to the present invention. In the first embodiment shown in FIG. 1, a capacitor 5, a resistor 6 and a transistor 7 are newly added to the configuration of the prior art shown in FIG.

In the first embodiment, the base of the transistor 4 and the base of the transistor 7 are connected in common, and this is used as the input terminal 10. Since the emitter of the transistor 7 is connected to the power supply Vss via the resistor 6 to form an emitter follower circuit, a voltage equal to the pulse voltage amplitude applied to the input terminal 10 is output to the emitter of the transistor 7.

The emitter of the transistor 4 is connected to ground (GND) via the base-emitter junction of the transistor 3.
Thus, a modified emitter follower circuit using the input of the transistor 3 as an output load is formed. Further, the emitter of the transistor 4 is connected to the emitter of the transistor 7 in an alternating manner via the capacitor 5, and the pulse amplitude is equal to the input voltage, and the voltage which is DC-shifted by the base-emitter forward voltage of the transistor 4 is applied to the base of the transistor 3. Circuit configuration. In the first embodiment, the drive current If of the laser diode 1 is given by the above-mentioned formula (1) shown in the prior art.

In FIG. 1, an input pulse voltage applied to the input terminal 10 is taken out as an emitter follower output of the transistor 7, and its pulse voltage amplitude is equal to the input pulse voltage, and the output impedance at the falling edge of the pulse waveform is extremely small.

Similarly, an input pulse voltage applied to the input terminal 10 is also taken out as an emitter follower output of the transistor 4. The pulse voltage amplitude here is smaller than the input pulse voltage by the forward voltage between the base and the emitter of the transistor 4 without the capacitor 5, and the output impedance at the rising edge of the pulse waveform is extremely small.

Therefore, the transistor 4 and the transistor 7
Is connected via a capacitor 5, a voltage equal to the forward voltage between the base and the emitter of the transistor 7 is generated across the capacitor 5, and the transistor 7 is driven to a low falling impedance to generate the emitter potential of the transistor 4. Is lower than the ground (GND) potential by the forward voltage between the base and the emitter of the transistor 4 when the input pulse voltage is 0 volt.

When the transistor 3 is turned on,
It is driven by the low impedance of the transistor 4 and is reverse-biased by the low impedance of the capacitor 5 and the transistor 7 when the transistor is turned off.
The stored charge inside is discharged instantaneously.

FIG. 3 shows operation waveforms of various parts of the circuit according to the embodiment of FIG. In FIG. 3, reference numerals 4a, 7a, 3a, and If represent various waveforms corresponding to the respective portions of the circuit shown in FIG. In FIG. 3, E is the peak value of the input pulse voltage, V7 is the forward voltage between the base and the emitter of the transistor 7, V4 is the forward voltage between the base and the emitter of the transistor 4, If is the output current of the transistor 3, and 4 is a driving current waveform of the diode 1. The drive current If of the laser diode is given by the above equation (1).

The input pulse voltage of the peak value E is
As a result, the DC voltage is shifted by the voltage value V4, converted into the operation waveform 3a shown in FIG.

For this reason, the transistor 3 is strongly reverse-biased at the falling edge of the input pulse, so that the internal accumulated charge is instantaneously discharged and turned off.

The capacitor 5 has a function of absorbing the accumulated charge with respect to the transistor 3, and the size thereof is sufficiently large as compared with the equivalent input capacitance when the transistor 3 is in a saturated state. For example, in the first embodiment, the value of the capacitor is 1000 pF.

FIG. 2 is a circuit diagram showing a second embodiment of a pulse drive circuit for a laser diode according to the present invention.

The difference from the configuration of the first embodiment is that the resistor 2 is connected between the laser diode 1 and the collector of the transistor 3 in the first embodiment, but the resistor 2 is not provided in the second embodiment. The other point is that the emitter of the transistor 3 is grounded via the resistor 8, and the other circuit configuration is the same as that of the first embodiment shown in FIG.

As in the case of FIG. 1, the pulse voltage applied to the input terminal 10 of FIG. 2 is DC-shifted by the base-emitter forward voltage of the transistor 4 and is input to the base of the transistor 3. And outputs a constant current proportional to the peak value. Here, the pulse current If for driving the laser diode 1 is obtained by equation (2).

If = (E−V4−V3) · hfe / (R8 (1 + hfe)) (2) where E is the peak value of the input pulse voltage, and V4 and V3 are between the base and emitter of transistor 4 and transistor 3. The forward voltage value, hfe is the amplification factor of the common emitter current of the transistor 3, and R8 is the constant of the resistor 8.

Next, the transistor 3 is strongly reverse-biased at the fall of the input pulse, and is turned off instantaneously.

The pulse voltage of the peak value E applied to the input terminal 10 is subtracted by the forward voltage between the base and the emitter of the transistor 4 and the transistor 3 and is taken out as a current output across the resistor 8. The magnitude of the output current here is substantially equal to the drive current for the laser diode 1. While the transistor 3 in FIG. 1 operates as a saturated switch, the transistor 3 in FIG. 2 operates as an unsaturated switch and outputs a constant current determined by the value of the resistor 8.

FIG. 4 shows operation waveforms of various parts of the circuit according to the embodiment of FIG. 4a, 7a, 3a and If in FIG.
3 are various waveforms shown corresponding to each part of the circuit shown in FIG. E is the peak value of the input pulse voltage, V7 is the forward voltage between the base and the emitter of the transistor 7, V4 is the forward voltage between the base and the emitter of the transistor 4, and If is the output current of the transistor 3 for driving the laser diode 1. It is a current waveform.

The input pulse voltage having the peak value E is DC-shifted by the capacitor 5 by the voltage value V4 and converted into the operation waveform 3a shown in FIG.

In the second embodiment, the capacitor 5 has the function of absorbing the accumulated charge in the transistor 3, and the size thereof is sufficiently large as compared with the equivalent input capacitance in the active state of the transistor 3. . For example, in the second embodiment, the value of the capacitor is 100 pF.

Although the first and second embodiments show the case where the polarity of the power supply is positive, the present invention can be applied to the case where the polarity is negative. In this case,
Transistors 3 and 4 are PNP type, and transistor 7 is N
What is necessary is just to change into PN type.

[0040]

The present invention provides a high-speed and stable laser diode pulse drive circuit which makes it possible to maximize the performance of a transistor used with a single power supply and a simple circuit configuration. Can be.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a laser diode pulse drive circuit according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a laser diode pulse drive circuit according to a second embodiment of the present invention.

FIG. 3 is an operation waveform diagram of the embodiment of FIG. 1;

FIG. 4 is an operation waveform diagram of the embodiment of FIG. 2;

FIG. 5 is a configuration diagram of a pulse drive circuit of a laser diode according to the related art.

FIG. 6 is an operation waveform diagram of the circuit of FIG. 5;

[Explanation of symbols]

 Reference Signs List 1 laser diode 2 resistor 3 transistor 4 transistor 5 capacitor 6 resistor 7 transistor 8 resistor 10 input terminal E peak value of input pulse voltage If laser diode driving current V4 forward voltage between base and emitter of transistor 4 V7 base of transistor 7 Emitter forward voltage

Continuation of front page (56) References JP-A-2-33983 (JP, A) JP-A-63-77172 (JP, A) JP-A-58-7794 (JP, A) JP-A-4-167860 (JP) JP-A-63-38276 (JP, A) JP-A-2-256286 (JP, A) JP-A-57-210682 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) H01S 5/00-5/50

Claims (4)

(57) [Claims] A laser diode (1) having one end connected to a first power supply (V _SS ), and the other end of the laser diode (1) connected to a collector via a first resistor (2), and an emitter connected to the emitter. To a second power supply (GND), a first transistor (3), a collector to the first power supply (V _SS ), and an emitter to the first power supply (V _SS ).
A second transistor (4) connected to the base of the transistor (3), a base connected to the input terminal (10), a collector connected to the second power supply (GND), and an emitter connected to the second
A third transistor (7) having a base connected to the input terminal (10), a third transistor (7) connected to the first power supply (V _SS ) through the resistor (6), and a third transistor (4). And a capacitor (5) connected between the emitters of the transistor (7). The pulse voltage input to the input terminal (10) drives the base of the first transistor (3) to drive the laser diode (1). A pulse driving circuit for a laser diode, which is driven by current. 2. The pulse drive circuit for a laser diode according to claim 1, wherein the first power supply (V _SS ) and the second power supply
If the polarity of (GND) is reversed, the first transistor (3)
And a second transistor (4) of a PNP type and a third transistor (7) of an NPN type. 3. A laser diode (1) having one end connected to a first power supply (V _SS ), the other end of the laser diode (1) being connected to a collector, and an emitter being connected via a third resistor (8). A first transistor (3) connected to a second power supply (GND), a collector connected to the first power supply (V _SS ), and an emitter connected to the first power supply (V _SS ).
A second transistor (4) connected to the base of the transistor (3), a base connected to the input terminal (10), a collector connected to the second power supply (GND), and an emitter connected to the second
A third transistor (7) having a base connected to the input terminal (10), a third transistor (7) connected to the first power supply (V _SS ) through the resistor (6), and a third transistor (4). And a capacitor (5) connected between the emitters of the transistor (7). The pulse voltage input to the input terminal (10) drives the base of the first transistor (3) to drive the laser diode (1). A pulse driving circuit for a laser diode, which is driven by current. 4. The laser diode pulse drive circuit according to claim 3, wherein the first power supply (V _SS ) and the second power supply
If the polarity of (GND) is reversed, the first transistor (3)
And a second transistor (4) of a PNP type and a third transistor (7) of an NPN type.