JP2001326417A - Driver circuit for laser diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a driver circuit for a laser diode which can output a stable signal by being impedance-matched to a transmission line. SOLUTION: A node N1 which is installed on the input side of the transmission line 2 is used as an output terminal. The drain terminal of a source grounded MOSFET 7a and the source terminal of a source follower MOSFET 8b1 whose polarity is opposite to that of the MOSFET 7a are connected to the node N1 as the output terminal. The MOSFET 8b1 is turned on and off so as to be delayed by the delay time portion of the transmission line 2 connected to the output terminal (the node N1) more than the MOSFET 7a.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor circuit, and more particularly to a laser diode driver circuit.

[0002]

2. Description of the Related Art In a conventional laser diode driver circuit shown in FIG. 5, a source of a MOSFET 1 is grounded, and only a drain thereof is an output terminal. The transmission line 2 is connected, and a laser diode 3 is connected to the transmission line 2.

In FIG. 5, the characteristic impedance of the transmission line 2 is about 50 Ω, whereas the input impedance of the laser diode 3 is very low, about 5 Ω. Therefore, the signal from the MOSFET 1 is transmitted between the transmission line 2 and the laser diode 3. A large reflection occurs at the connection node N2, and the reflected wave returns to the connection node N1 between the MOSFET 1 and the transmission line 2 again.

Since the impedance of the MOSFET 1 when viewed from the node N1 when the MOSFET 1 transitions from on to off is very high at several GΩ, the laser diode 3
Is reflected again at the node N1, the signal on the transmission line 2 is not stabilized, and the laser diode 3 malfunctions.

Therefore, conventionally, the transmission line 2 was made very short to several mm, thereby forming a lumped constant circuit to avoid the above-mentioned problem. However, the distance between the driver circuit and the laser diode was reduced due to the convenience of mounting. In the case of leaving, it takes a long time for the signal to be transmitted through the transmission line 2, and thus there is a problem that it takes a long time for the signal to stabilize.

As a method for solving the problem of the laser diode driver circuit according to the conventional example shown in FIG. 5, a GTL method (ISSCC Tech) as shown in FIG.
Dig. 1992, p. 58).

In the conventional laser diode driver circuit shown in FIG. 6, when the MOSFET 4 is turned off, the two MOSFETs 5 and 6 are simultaneously turned on for a short time, and the gate and the drain of the MOSFET 4 are connected. It is configured to function like a diode.

In the laser diode driver circuit according to the conventional example shown in FIG. 6, the impedance of the MOSFET 4 as viewed from the node Out is reduced by the above-described configuration, and the reflected wave is absorbed by impedance matching with the transmission line. The signal on the transmission line is stabilized.

[0009]

However, in the conventional laser diode driver circuit shown in FIG. 6, the MOSFET 1 is turned on at the moment when the MOSFETs 5 and 6 are simultaneously turned on for a short time.
The gate voltage of T4 is not exactly the same as the drain voltage, and perfect diode characteristics cannot be obtained.

For this reason, unless the ratio of the on-currents of the MOSFETs 1, 5, and 6 is properly set, the desired characteristics cannot be obtained, and the device is affected by variations in elements, and it is difficult to achieve the intended purpose. There is a problem that is.

Furthermore, since the MOSFET 4 needs to fulfill two roles of a driver and impedance matching, there is a problem that parameter optimization is difficult.

By the way, as shown in FIG. 7, in a conventional laser diode driver circuit, a method of connecting a matching circuit using a resistor and a capacitor to an output terminal has been proposed (Japanese Patent Laid-Open No. 01-214081). ). 7, reference numeral 20 denotes a driving unit, 21 denotes a matching circuit, 22 denotes a transmission line, and 3 denotes a laser diode.

In the system according to the conventional example shown in FIG.
When a high-frequency signal of about GHz is used for a transmission line 22 of about 2 GHz, a capacity of several nF is required to achieve matching. However, a capacity of several nF is difficult to manufacture on a chip. There is a problem that additional parts are required.

Further, in the system according to the conventional example shown in FIG.
Since it takes time to charge and discharge a capacity of several nF, there is also a problem that the switching time is delayed.

An object of the present invention is to provide a laser diode driver circuit capable of outputting a stable signal by impedance matching with a transmission line.

[0016]

In order to achieve the above object, a driver circuit for a laser diode according to the present invention is a driver circuit for a laser diode having a driver and a reflection / absorption unit, wherein the driver comprises:
The reflection / absorption unit is connected in parallel with the driver, drives and controls the driver by delaying by a delay time of the transmission line, and outputs the drive signal of the laser diode connected to the transmission line. This is to absorb the reflected wave from the transmission line.

Further, the reflection / absorption unit comprises a delay circuit and an impedance matching device.

Further, the driver and the impedance matching device are composed of transistors.

The transistor constituting the driver is of a common source type in order to extract a large current.

The transistor constituting the impedance junction device is of a source follower type in order to reduce the output impedance.

The transistor constituting the driver uses an nMOSFET as a common source type.

The transistor constituting the impedance junction device uses a pMOSFET as a source follower type.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, the laser diode driver circuit according to the present invention has a driver 7 and a reflection / absorption unit 8,

The driver 7 outputs a drive signal for driving the laser diode 3 connected to the transmission line 2.

The reflection / absorption unit 8 is connected in parallel to the driver 7, controls the drive of the driver 7 with a delay corresponding to the delay time of the transmission line 2, and absorbs the reflected wave from the transmission line 2. It has become.

The reflection / absorption unit 8 is specifically composed of a delay circuit 8a and an impedance matching unit 8b.

As described above, according to the present invention, the reflected wave propagating in the transmission line 2 toward the driver 7 can be absorbed by turning on the reflection / absorption unit 8 connected in parallel at the moment when the driver 7 is turned off. , And can output a stable signal.

Next, a specific example of the present invention will be described in detail with reference to FIG.

The reflection / absorption unit 8 includes a delay circuit 8a and an impedance matching unit 8b, and the driver 7 and the impedance matching unit 8b include transistors.

The transistor constituting the driver 7 is of a source-grounded type for extracting a large current, and the transistor 8b1 of the impedance junction 8b is of a source-follower type for lowering output impedance. It was done.

In the embodiment shown in FIG. 2, the transistor forming the driver 7 uses an nMOSFET 7a as a source grounded type, and the transistor forming the impedance junction 8b is a pMOSFET 8b
1 is used as a source follower type.

As shown in FIG. 2, the driver circuit of the laser diode according to the embodiment of the present invention has a node N1 provided on the input side of the transmission line 2 as an output terminal, and the node N1 as an output terminal has a source grounded type. MOSFET7
a, the source terminal of the source follower type MOSFET 8b1 having the opposite polarity to the MOSFET 7a is connected, and the source follower type MOSFET 8b1 is delayed from the grounded source type MOSFET 7a by the delay time of the transmission line 2 connecting to the output terminal (node N1). On and off.

As shown in FIG. 2, an nMOSFET 7a is used as a transistor constituting the driver 7 of FIG.
PMOSFET 8b1
Is used as a source follower type, and a source grounded type n
The ID-VD characteristic of the MOSFET 7a becomes the characteristic shown in FIG.
Are the characteristics shown in FIG. 3B.

As shown in FIG. 3A, a common source type nM
The OSFET 7a shows a saturation characteristic because the source is grounded.
As D increases, the slope of the ID-VD characteristic becomes gentler, and the differential resistance indicated by the dotted line increases.

On the other hand, as shown in FIG. 3B, the source follower type pMOSFET 8b1 has a diode characteristic. As VD increases, the slope of the ID-VD characteristic increases and the differential resistance indicated by the dotted line decreases.

Therefore, the source follower type pMOS
Since the FET 8b1 has a small differential resistance even with a small gate width, a low impedance of about 50Ω can be easily obtained, and matching with the transmission line 2 can be achieved.

In the circuit shown in FIG. 2, the input changes from low to high.
gh, the nMOSFET 7a is turned off first. At this time, the input node N3 of the pMOSFET 8b1
Are three-stage delay circuits I1, I2 and I3 (FIG. 1)
(Corresponding to the delay circuit 8a of FIG. 3), the input signal is delayed by the amount corresponding to the delay of the delay circuit 8a.

For this reason, at the moment when the reflected wave from the transmission line 2 returns, the pMOSFET 8b1 is turned on.
It can be absorbed by the MOSFET 8b1.

Thereafter, the three-stage delay circuits I1, I2, I3
The node N3 becomes High with a delay of
Since the OSFET 8b1 is turned off, the voltage of the transmission line 2 is stabilized at High.

The delay circuits I1, I2 and I3 shown in FIG.
The number of stages is the number of transmission lines 2 connected to this circuit.
Is determined by the length of The number of stages of the delay circuit is always an odd number, and the total delay time is set to be longer than the time for the electric signal to make one round trip in the transmission line 2 and shorter than to make two round trips.

FIG. 4A is a circuit diagram of the circuit shown in FIG.
FIG. 4B shows a simulation result of a circuit according to a conventional example from which ET8b1 is removed, and FIG.
7 shows simulation results for the circuit of the present invention shown in FIG.

The input signal is low at N0 in the figure at a time of about 2 ns and becomes high at about 2.4 ns.

As shown in FIG. 4A, in the conventional circuit, the influence of the reflected wave is very large, and the waveform of the output node N1 is greatly disturbed.

On the other hand, as shown in FIG. 4B, in the circuit of the present invention, since the reflected wave is absorbed, the disturbance of the waveform is suppressed and the output signal is stable.

As described above, according to the embodiment of the present invention,
Common source type nMOSFET as output transistor
By turning on the low-impedance pMOSFET 8b1 connected in parallel at the moment when 7a is turned off, the reflected wave can be absorbed and a stable signal can be output.

Further, a common-source nMOSFET 7a as an output transistor and a low-impedance pMOS
By using a separate FET 8b1, the characteristics of individual transistors can be optimized, and stable impedance matching can be obtained. Further, since no external components are required, the cost and the area of the substrate can be reduced.

[0048]

As described above, according to the present invention, it is possible to provide a laser diode driver circuit capable of outputting a stable signal by impedance matching with a transmission line.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a driver circuit of a laser diode according to the present invention.

FIG. 2 is a block diagram showing a driver circuit of the laser diode according to the embodiment of the present invention.

FIG. 3 is an IV characteristic diagram of an output transistor used in the embodiment of the present invention.

FIG. 4 is a diagram showing a SPICE simulation waveform.

FIG. 5 is a configuration diagram showing a driver circuit according to a conventional example.

FIG. 6 is a configuration diagram showing a driver circuit according to a conventional example.

FIG. 7 is a configuration diagram showing a driver circuit according to a conventional example.

[Explanation of symbols]

 2 Transmission Line 3 Laser Diode 7 Driver 7a Grounded Source nMOSFET 8 Reflection Absorber 8a Delay Circuit 8b Impedance Matcher 8b1 Source Follower Type pMOSFET

Claims (7)

[Claims] 1. A driver circuit of a laser diode having a driver and a reflection-absorption unit, wherein the driver outputs a drive signal of a laser diode connected to a transmission line, and the reflection-absorption unit is A driver circuit for a laser diode, wherein the driver circuit is connected in parallel with the driver, controls the driving of the driver by delaying by a delay time of the transmission line, and absorbs a reflected wave from the transmission line. 2. The driver circuit according to claim 1, wherein the reflection / absorption unit comprises a delay circuit and an impedance matching device. 3. The laser diode driver circuit according to claim 1, wherein the driver and the impedance matching device are configured by transistors. 4. The laser diode driver circuit according to claim 3, wherein the transistor constituting the driver is of a common-source type for extracting a large current. 5. The transistor constituting the impedance junction device is of a source follower type for lowering output impedance.
A driver circuit for a laser diode according to claim 1. 6. The laser diode driver circuit according to claim 3, wherein the transistor constituting the driver uses an nMOSFET as a grounded source type. 7. The laser diode driver circuit according to claim 3, wherein the transistor constituting the impedance junction device uses a pMOSFET as a source follower type.