JPS598973B2 - Light emitting diode drive circuit - Google Patents

Description

[Detailed description of the invention] The present invention relates to a high-speed driving circuit for light emitting diodes.

Light emitting diodes are easy to handle and highly reliable.

For this reason, it is often used as a light source for optical signal transmission along with semiconductor lasers and the like. However, since the former has a lower response frequency than the latter, when driving at a modulation speed higher than the response frequency, it is necessary to take measures to increase the speed of the drive circuit. Generally, the response frequency of a light emitting diode is
It is determined by the carrier recombination time, diffusion capacitance, and junction capacitance at the junction of the light emitting diode.

Therefore, by rapidly charging and discharging the diffusion capacitance and the junction capacitance, it is possible in principle to increase the modulation speed up to the response frequency determined by the carrier recombination time. A conventional method for increasing speed is to use a parallel circuit of resistors and capacitors to reduce the transient drive impedance at the rise and fall of the drive pulse, and to apply peaking to the light-emitting diode drive current. There is. A method is also known in which the drive voltage pulse is made into a pulse waveform with overshoot and Young shoot to apply peaking to the drive current of the light emitting diode. FIG. 1 shows an example of a conventional drive circuit for modulating a light emitting diode with a pulse code, such as an RZ code, having both peaking.

In FIG. 1, reference numeral 1 denotes a drive pulse generation circuit that generates a voltage pulse having overshoot and antanate in response to an input signal. 2 is a power supply terminal, 3 is a transistor, 4 is a capacitor, 5 is a resistor, and 6 is a light emitting diode.

Figure 2 shows an example of the signal waveform in the main part of Figure 1,
When the input signal voltage VI is applied to the drive voltage pulse generation circuit 1, the drive voltage pulse VB applied to the base of the transistor 3, the current IL flowing through the light emitting diode 6), and the light emitting output P of the light emitting diode 6 are shown. It is.

Due to the peaking effect of both the drive voltage pulse VB having overshoot and anthanate and the capacitor 4 and resistor 5 reducing the transient drive impedance, the LED drive current IL increases as shown in FIG.
can cause large overshoots and anthanumates. As a result, the diffusion capacitance and junction capacitance of the light emitting diode 6 (hereinafter, both capacitances are abbreviated as the capacitance of the light emitting diode 6) can be rapidly charged and discharged, and the rise time and fall time of the light emitting output P can be shortened. can be driven at a modulation speed that exceeds the response frequency of However, in this method, the charging route and the discharging route for the capacitance of the light emitting diode 6 are the same. That is, the route is "transistor 3, capacitor 4 and resistor 5, and light emitting diode 6", and transistor 3 must be in an on state both during charging and discharging. Therefore, in order to make full use of the peaking effect even at the falling edge, it is necessary to keep the bias current 1F corresponding to the anthanate side current constantly flowing even when there is no input pulse as shown in FIG. Therefore, the conventional high-speed driving method of a light emitting diode requires a constant bias current of 1 F, which has an undesirable drawback in terms of reliability. In addition, compared to semiconductor lasers, light-emitting diodes have lower coupling efficiency with optical fibers, and the power incident on the optical fibers is about an order of magnitude lower. Therefore, especially when performing high-speed optical signal transmission, it is important to consider the S/N design of the transmission system. is tough. In the above-described high-speed light-emitting diode driving method, the maximum value of the light emission output that can be effectively extracted as a signal from the light-emitting diode 6 is, assuming that the maximum permissible current of the light-emitting diode 6 is 1max as shown in FIG. and an anthanate current of 1F are required, [Imax-(IR+IF)]
The light emitting output is limited to the current value corresponding to the current value. For this reason, in high-speed optical signal transmission using light-emitting diodes, the conventional high-speed driving method for light-emitting diodes has the drawback of making the S/N design of the transmission system significantly stricter. The present invention aims to improve the above-mentioned drawbacks of the conventional technology, and is based on the basic principle of eliminating the need for a bias current of 1F by separating the route for charging and the route for discharging the capacitance of a light emitting diode. The features include a drive circuit that supplies current to the light emitting diode for a time corresponding to the pulse width of the input pulse, and a circuit that rapidly charges the capacity of the light emitting diode in synchronization with the rise time of the input pulse; The light-emitting diode drive circuit includes a light-emitting diode and a circuit that rapidly discharges the capacitance of the light-emitting diode in synchronization with the fall time of an input pulse. Embodiments will be described in detail below with reference to the drawings. FIG. 3 shows one embodiment of the invention, where 7
8 is the main drive pulse generation circuit, 8 is the charging pulse generation circuit, and 9 is the main drive pulse generation circuit.
is a discharge pulse generation circuit. Further, 11, 12, and 13 are transistors, 14 is a diode, and 15 is a transistor.
16 is a resistor, and 17 is a light emitting diode. The main drive pulse generation circuit 7 generates a drive voltage pulse for a time corresponding to the pulse width of the input signal VI. The charging pulse generating circuit 8 generates a short driving voltage pulse in synchronization with the rising edge of the input signal I, and the discharging pulse generating circuit 9 generates a short driving voltage pulse in synchronization with the rising edge of the input signal VI.
A short drive voltage pulse is generated in synchronization with the falling edge of . As is well known, generation of these drive voltage pulses can be easily realized by a combination of an ordinary logic circuit and a current switching circuit. FIG. 4 shows an example of the signal waveform of the main part of FIG. 3.

When the input signal VI is applied in FIG. 3, the drive voltage pulse VBN from the main drive pulse generation circuit 7 is generated as shown in FIG.
is connected to the base of the transistor 11, and the charging pulse generating circuit 8 is connected to the base of the transistor 11.
Since the driving voltage pulses VBR from VBR are simultaneously applied to the bases of the transistors 12, a current 1L having an overshoot at the rising edge flows through the light emitting diode.
Therefore, the capacitance of the light emitting diode 17 is rapidly charged, and the rise time of the light emitting output P is accelerated. Next, when the input signal V disappears, the drive voltage pulse VBF from the discharge pulse generation circuit 9 is applied to the base of the transistor 13, so unlike during charging, a discharge consisting of "light emitting diode 17, transistor 13, diode 14," At this time, since the on-resistance of the transistor 13 and the on-resistance of the diode 14 are both small, the capacitance of the light emitting diode is rapidly discharged, and the fall time of the light emitting output P is accelerated. In the invention, since the route for charging and the route for discharging the capacity of the light emitting diode are different, the light emitting diode can be driven at high speed without flowing the bias current 1p to speed up the fall time of the light emitting diode as in the conventional example. .

Therefore, in FIG. 4, the maximum permissible current of the light emitting diode 17 is expressed as IR
R, and if the anthanate current is IFF, then the light emission output P that can be effectively extracted as a signal from the light emitting diode 17
The maximum value of is determined by the current (Imax-1RR), and the anthanate current IPF does not become a factor that limits the light emission output P. As explained above, since it is not necessary to constantly flow a bias current to the light emitting diode, reliability does not decrease even if the light emitting diode is driven at high speed. In addition, in high-speed optical signal transmission using light-emitting diodes, the anthanate current does not become a factor that limits the light-emitting output of the light-emitting diode, so a large light-emitting output can be obtained and the transmission system's S
/N There is an advantage that the design becomes easy. Note that since the circuits that generate the main drive pulse and the charging pulse supply current to the light emitting diode through the same route, they may be configured by the same circuit.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional light emitting diode drive circuit.
Fig. 2 is a signal waveform diagram of the main part of the conventional circuit shown in Fig. 1;
FIG. 3 is a circuit diagram showing an embodiment of the light emitting diode drive circuit according to the present invention, and FIG. 4 is a signal waveform diagram of the main part of the circuit shown in FIG. 3. 1... Drive pulse generation circuit, 2...
Power supply terminal, 3...Transistor, 4...
・Capacitor, 5... Resistor, 6... Light emitting diode, T... Main drive pulse generation circuit,
8...Charging pulse generation circuit, 9...Discharge pulse generation circuit, 10...Power terminal, 11,
12, 13...transistor, 14...
・Diode, 15, 16...Resistance, 1T...
...Light emitting diode.

Claims (1)

[Claims] 1. A drive circuit that supplies current to the light emitting diode for a time corresponding to the pulse width of the input pulse, and a circuit that rapidly charges the capacity of the light emitting diode in synchronization with the rise time of the input pulse; 1. A driving circuit for a light emitting diode, characterized in that a circuit for rapidly discharging the capacitance of the light emitting diode in synchronization with the falling time of the light emitting diode is connected to the light emitting diode.