JPH01243487A - Semiconductor laser driving circuit - Google Patents

Abstract

PURPOSE:To contrive stabilization of signal current by a method wherein the gate potential of an FET is clamped by a clamping circuit consisting of a diode and a resistor, and a resistor to be used for current limitation is inserted between the source of the FET and a negative power terminal. CONSTITUTION:The gate voltage of an FET 13 is applied through the intermediary of a clamping circuit 52 for the stabilization of a signal current, and the clamping circuit is biased by a constant current source 51. Also, a resistor 54 is connected to the source of the FET 13, and the fluctuation of the signal current caused by the variation in environmental condition and the manufacturing deviation of the FET 13 is suppressed utilizing a current negative feed-back effect. Said clamping circuit is composed of a diode 53. Utilizing the temperature dependency of the above- mentioned diode 53, terminal voltage gradually decreases when the temperature of the diode goes up, the gate potential of the FET 13 comes down with the rise of temperature, and the current. As a result, the temperature dependency of the signal current can be erased by setting the current density of the diode 53 in such a manner that the amount of the above-mentioned current attenuation and the amount of current increased by the temperature dependency of the threshold voltage of the FET 13 will be cancelled each other.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a drive circuit for a semiconductor laser light source in optical communications, optical disks, etc., and particularly relates to a laser drive circuit suitable for integration into an IC using FETs.

[Conventional technology]

In recent years, the development of Gb/s optical transmission systems has become active, but in order to put such systems into practical use, monolithic systems are required from the viewpoints of speeding up equipment, improving reliability, reducing power consumption, and making it smaller. The situation is such that the introduction of E=C technology is essential. Gb/s transmission ICs have been put into practical use mainly by applying Si bipolar technology. but,
It is difficult to realize a driving IC for a semiconductor laser (hereinafter abbreviated as LD) that requires high-speed, large-current switching using Si bipolar technology, and therefore GaAs MESFET (Schottky junction field effect transistor) technology has been applied.

FIG. 2 shows an example of a conventional configuration of an LD driving IC realized using the GaAs MESFET technology. Since a semiconductor laser has an oscillation threshold current, it is usually driven with a current obtained by superimposing a signal current on a bias current. In FIG. 2, FET II, 12°13 constitutes a current switch 14, which switches the current supplied from FET 13 according to the first and second signal input levels.
The LD connected to terminal 5 is modulated by the signal current generated by switching by the differential pair I and 12. on the other hand,
The bias current is supplied from the bias current supply circuit 9 to the LD via the terminal 5. Since the oscillation threshold current of the LD has a large temperature dependence, the bias current is controlled by the control signal applied to the terminal 7, and the optical output is stabilized.

Further, the signal current is set by adjusting the voltage level applied to the terminal 6. The bias supply circuit 9 and the level shift circuit 8 are, for example, IEICE Technical Report 5SD.
85-140, respectively, FET and FE
It consists of a T and a diode.

[Problem to be solved by the invention]

Since the signal current has a one-to-one relationship with the amplitude of the optical output signal, it is essential to stabilize it against changes in environmental conditions such as temperature and power supply voltage and manufacturing deviations of the device. The above-mentioned conventional technology does not take these points into consideration, so an external circuit is required to supply voltage to the terminal 6, and this circuit also includes a There are problems in that it requires complicated functions, tends to be large in size, the signal current sensitively reacts to the voltage at the terminal 6, and is not easy to use.

The object of the present invention is to overcome the drawbacks of the prior art mentioned above.

It is an object of the present invention to provide a semiconductor laser drive circuit suitable for IC implementation, which can be incorporated internally and can stabilize a signal current with a simple compensation circuit.

[Means to solve the problem]

The object of the present invention is to clamp the gate potential of FET 13 by a clamp circuit consisting of a diode and a resistor in FIG.
This is achieved by inserting a current limiting resistor between the two.

[Effect]

The basic operation of the present invention will be explained below with reference to FIG. In order to stabilize the signal current, the F
The gate voltage of ET13 is applied. The clap circuit is biased by a constant current source 51. Also, the resistor 54 is FE
It is connected to the source of T13 and uses the current negative feedback effect to suppress fluctuations in the signal current due to changes in environmental conditions or manufacturing deviations of FET13. FIG. 1 shows a basic embodiment of the present invention, in which the clamp circuit is composed of a diode 53. Now, the output impedance of the constant current source 51 is R
1. Set the voltage across the diode 53 to VD and the flowing current, and set the potential at terminal 1 to OV.

If the voltage at terminal 2 is v58, the following formula holds true: 6R1I
+ V□= Vss '・'・”−(1) Also,
I and V. Which relationships are given in. Here, IO and β are physical constants. Therefore, the power supply voltage V. V due to fluctuations in . The change in is expressed as equation (1)
(2) It becomes more. Since 1/β is about 25'mV, I=1mA
Then, r6 becomes 25Ω. Also, Rf is V.

If it is -5.2V and Vd is 0.6V, it becomes 4.6Ω. Therefore, from equation 2 (3), V. The amount of change in is 1 of the power supply voltage.
=2.8 (mV) for a 0% variation, which is extremely low, making it possible to minimize changes in the signal current due to variations in the power supply voltage. Incidentally, since the value of the resistor 54 is about 10Ω, the amount of change in the signal current is about 0.3 mA.

By the way, the threshold voltage ■t□ and the mutual conductance coefficient of the FET 13 have temperature dependence, and as the temperature rises, the former becomes deeper and increases the current, and the latter becomes smaller and acts in the direction of decreasing the current. However, overall, the variation in threshold voltage is larger than the variation in value, so reducing the temperature dependence of threshold voltage is an issue. As a measure against this, the present invention utilizes the temperature dependence of the diode 53. In general, the terminal voltage of a diode gradually decreases as the temperature rises.

Therefore, according to the connection shown in FIG. 1, the gate potential of the FET 13 decreases as the temperature rises, and the current decreases. Therefore,
The temperature dependence of the signal current can be eliminated by setting the current density of the diode 53 so that the decrease in current is offset by the increase due to the temperature dependence of the threshold voltage of the FET 13.

In G a As M E S F E T, the manufacturing deviation of the threshold voltage is the largest compared to other constants. Therefore, this measure is essential for stabilizing the signal current. If the value of the resistor 53 is Rg and the signal current is I8, then Is=K(
Vo Vt - RI s)''. Therefore, the change in signal current ΔI8 is given by equation (4), where ΔVt is the change in threshold voltage. Incidentally, Vt=IV, R
g=10Ω.

If I, = 50mA, Vt) = -0.6V, then = 41
mA/V", so the amount of fluctuation in the signal current is 3.8 m
It becomes A/V. ±0.2 threshold voltage variation range
When it is set to v, the amount of signal current fluctuation can be suppressed to ~0.8 mA.

〔Example〕

FIG. 3 shows a configuration diagram of an embodiment according to the present invention.

In the figure, a clamp circuit 52 is constructed of a diode 6 and voltage dividing resistors 63 and 64 in order to generate a clamp voltage having an arbitrary temperature coefficient.

Further, the constant current source 51 is replaced with a resistor 62.

The bias supply circuit 9 is composed of a FET 66 and a current limiting resistor 65 (which may be omitted).

61 is an input buffer circuit for shaping the waveform of the input signal and driving the current switch 14 at high speed. Since the current switching operation has been explained with reference to FIG. 2, stabilization of the signal current will be described here.

The value and temperature coefficient of threshold voltage of FET 13 are respectively δ6. δ
Vts also clamp voltage V. If the temperature coefficient of (voltage between point A and negative power supply terminal 2 in Figure 3) is δva, FET13
The temperature coefficient δrs of the signal current flowing through is given by the following equation from equation (4).

・・・・・・・・・ (6) Here, KOI vaot vto is the standard value at the time of initial setting.

R, , R,4 indicates the resistance value of the resistor 63.64. G a As M E S F E T with gate length of 1 μm
Then, normally, δ/KO is approximately -0.3%, and δvt is -
1.6 mV/℃, and δvo (clamp diode 5
3) is -1.4 mV/'C. Using these values, Figure 4 shows the relationship between the temperature coefficient of the signal current and the clamp voltage VQ when the signal current is set to 50 mA, the value of the resistor R54 is set to 10 Ω, and the voltage across the diode 53 is set to 0.6 V. It is something. As can be seen from the figure, the temperature coefficient of the signal current can be arbitrarily set by the clamp voltage va, and as vo increases, the negative coefficient increases, that is, the signal current decreases. When the temperature of the LD is controlled using a Peltier element or the like, the lightning output signal is stabilized because the temperature coefficient of the signal current is close to zero. According to FIG. 4, the clamp voltage is 0.
It can be seen that when the voltage is 225V, the temperature coefficient of the signal current can be made zero. Note that the LD is often used without temperature control.

In such a case, the efficiency of optical output with respect to current (referred to as external differential quantum efficiency) decreases as the temperature rises.

That is, the optical output signal decreases at high temperatures. Therefore, to alleviate this decrease in the optical output signal, it is necessary to make the temperature coefficient of the signal current positive, and the clamp voltage ■. 0.225V (
It is sufficient to set it to 95mA/V2) or less.

Next, let's find the power supply voltage dependence of the signal current.

If Rg3°R64 is set so that the current flowing through the diode 53 is at least one order of magnitude larger than the current flowing through the resistor 63, 64, the voltage fluctuation across the diode is given by equation (3). Therefore, since this fluctuation is divided by the resistance between R63 and Rg4, the fluctuation of the signal current is R! from equation (3)'. Rg R63+ R64 is given. Here, if va is set to 0.225V, R111
4/ (RG3+Rg4) is 0, 375 tonal,
Compared to the configuration shown in FIG. 1, the resulting current fluctuation is 0.
375, the fluctuation of the signal current is also 0.375 times that of the configuration shown in FIG. By the way, the power supply voltage is ±0.5
The amount of variation in the signal current when v varies is at most ±0.1 mA.

The amount of variation in the signal current due to the manufacturing deviation of the threshold voltage of the FET 13 can be obtained from equation (5). In formula (5), V is 0.225V, K is 95mA/V''Rg is 10
If Ω and vL are -1■, the amount of fluctuation of the signal current is 9,6
mA/V. For FETs with a threshold voltage of 1 V, it is generally easy to suppress the manufacturing deviation to below ±0.2 V, so the practical signal current fluctuation amount is 1.9 mA, which is a small value of ±3.8% of the standard value. It can be suppressed.

FIG. 5 shows another embodiment of the signal current stabilizing circuit. In the same figure, the constant current source 51 of FIG. 1 is an FET 101 whose gate and lease are short-circuited, and the clamp circuit 52 is connected with n diodes 201-1 to 201-1 connected in series.
201-n and resistor R301+302. In this case, since FETl0I functions almost as a constant current source, there is an advantage that the signal current hardly changes with respect to the power supply voltage. The clamp circuit can supply an output voltage with an arbitrary temperature coefficient to the FET 13 by appropriately selecting the number of diodes, the current value, and the value of the resistor R301y 302.

Although the above description has focused on an LD as a load, the present invention is basically a current switching circuit, and the load may be an impedance circuit. It is clear from this that the present invention is also applicable to circuits that need to operate at high speed and large amplitude, such as 50Ω line drivers, signal level converters, and amplifiers.

〔Effect of the invention〕

According to the present invention, by using a simple signal current stabilizing circuit consisting of a constant current source and a clamp circuit, fluctuations in signal current due to changes in environmental conditions such as power supply voltage and temperature, which are important for an LD drive circuit, can be suppressed to a few percent. can. Furthermore, due to the current negative feedback effect of the resistor connected to the source, fluctuations in the signal current can be suppressed to a few percent due to manufacturing deviations of the FET.
The above results make it possible to create a monolithic IC, which makes it possible to realize an easy-to-use IC with a single power supply.

[Brief explanation of the drawing]

FIG. 1 is a basic configuration diagram of an LD drive circuit according to the present invention, and FIG.
Figure 3 is a configuration diagram of a conventional LD drive circuit, Figure 3 is a connection diagram showing an embodiment of the LD drive circuit according to the present invention, and Figure 4 is an example of calculation of the signal current temperature coefficient in the LD drive circuit according to the present invention. FIG. 5 is a connection diagram showing an embodiment of the signal current stabilizing circuit according to the present invention. 14... Current switch, 11, 12, 13, 101.
...FET, 60...Signal current stabilization circuit, 53゜2
01-1 to 201-n... Diode, 51... Constant current source, 54, 63, 64, 301, 302... Resistor, 52... Clamp circuit, 9... Bias current supply circuit, 8... Level shift circuit, 61... Input buffer circuit. Figure 1 Sumire z8 3rd 2-run aden, -% (v)

Claims (1)

[Claims] 1. In a semiconductor laser drive circuit comprising a current switching circuit that supplies a signal current to the laser and a bias current supply circuit that supplies a bias current, the current switching circuit is connected to a source-coupled FET differential pair. A voltage clamp is provided between the gate of the constant current supply FET and the other end of the resistor. A semiconductor laser drive circuit characterized by connecting circuits. 2. The voltage clamp circuit is composed of a single or multiple series-connected diodes and a plurality of resistors that divide the voltage across the diodes, and the output voltage is taken out from the connection point of the resistors to supply the constant current. 2. The semiconductor laser drive circuit according to claim 1, wherein the semiconductor laser drive circuit is configured to supply the signal to the gate of the FET.