1. Field of the Invention
The present invention relates to an LED (light emitting diode) driver circuit for high-speed optical communication systems and in particular to an LED driver circuit which is capable of significantly reducing the fall time of a long-wavelength LED.
2. Description of the Prior Art
A long-wavelength light emitting diode (hereafter abbreviated as LED) can operate at high-speed exceeding 100 Mb/s. In recent years, therefore, the LED seems to be promising as an optical source for LANs (Local Area Networks), computer networks, and interoffice communication systems with a repeater spacing of 1 km to 2 km. Unfortunately, unlike short-wavelength LEDs, long-wavelength LEDs have an unfavorable performance characteristic in that the fall time is two to three times as long as the rise time. For example, when long-wavelength LEDs have the radiation layers with impurity concentration of 5.times.10.sup.18 cm.sup.-3, those fall time are generally 4 to 5 ns. Then, those rise time are typically 1.5 ns. For practical use of long-wavelength LEDs, therefore, reduction of the fall time is a significant technical subject.
FIG. 1 shows a circuit configuration of LED driver circuit of the prior art. Such a driver is described in Japanese Utility Model publication No. 7790/84. In general, reduction of the fall time of the LED can be achieved by fast discharging of the electric charge stored in the parasitic capacitance of the LED, during the cutoff state of the LED. Based on this viewpoint, a circuit configuration illustrated in FIG. 1 has been proposed. A series circuit consisting of a resistor 4 and a capacitor 5 is connected to the collector of a transistor 1 and in parallel to an LED 2 and to a resistor 3. The resistors 3 and 4 as well as the capacitor 5 are used to discharge electronic charge stored in the parasitic capacitance of the LED 2. In this case, the time constant required for discharging the electric charge is almost determined by the product of the parasitic capacitance of the LED 2 of the parallel resistance of the resistors 3 and 4. The smaller the values of the resistors 3 and 4 are, the shorter fall time the LED 2 will have. If the resistors 3 and 4 have small values, however, almost all the collector current of the transistor 1 flows through the resistors 3 and 4 when a pulse signal supplied from a terminal 8 rises up. That is to say, no current flows through the LED 2. Therefore, the desired improvement effect of the rise time due to a resistor 6 and a capacitor 7 is not attained. On the contrary, the rise time is significantly degraded. In addition, since a large current flows through the resistor 3 even in the steady state of the pulse signal, the transistor 1 is required to supply not only the current for driving the LED 2 but also the undesirable current for driving the resistor 3. In conclusion, it is difficult to reduce both the rise and fall times of the LED 2 in the circuit configuration of the prior art as illustrated in FIG. 1.