JPH031854B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a driving circuit for a light emitting diode used in optical fiber communication.

Since the relationship between the light emitting output and the light emitting current when directly modulating a light emitting diode is not linear, modulation distortion occurs when the light emitting element is directly modulated with a modulation signal.

Conventionally, the distortion compensation method for light emitting diodes is as follows:
The predistortion method is often used because it can be configured with a simple circuit.

First, the predistortion method will be explained with reference to FIG.

The modulation signal input to the terminal _T1 is amplified by the transistor T, the light emitting diode LED is driven by the current I, and a light emission output P corresponding to the modulation signal is sent to the optical fiber L. The characteristics of the light emitting output P and drive current I of the light emitting diode LED are nonlinear as shown in Figure 2a, but the compensation diode D and resistor R connected to the emitter of the transistor T are also nonlinear. Since the impedance is nonlinear with respect to the emitter current, the light emitting diode
This diode D,
and can be compensated by a resistor R.

However, such conventional compensation methods
Even if it is possible to compensate for the I-P characteristics of the light emitting diode LED, it is not possible to compensate for the phase characteristics of the light emitting diode LED.

In other words, while FIG. 2b shows that the frequency characteristics of the light emitting output P of the light emitting diode LED change depending on the light emitting currents _I1 and _I2 , the circuit shown in FIG. Since it is not possible to compensate for the phase, there was a drawback that when a light emitting diode LED was driven using a TV signal as a modulation signal, the color tone changed depending on the magnitude of the brightness current.

The present invention has been made in view of these points, and provides a drive circuit that compensates for the phase characteristics of a light emitting diode.

The light emitting element drive circuit of the present invention will be explained below.

FIG. 3 shows an embodiment of this invention.
DP indicates a phase compensation circuit, and DG indicates an amplitude compensation circuit.
The phase compensation circuit DP consists of a circuit network having an input terminal T ₁ and an output terminal T ₂ , a resistor R, and a voltage variable capacitance element.
It is formed by VC, variable resistor RV, and capacitor C, and its output is input to transistor _TR1 .
The amplitude compensation circuit DG is formed of a Zener diode DZ, a transistor Tr that sets the operating point of the Zener diode DZ, and a variable resistor RV'.
Its output is connected to transistors TR ₂ and TR ₃ that drive light emitting diodes LED.

This circuit applies predistortion to the analog modulation signal applied to the input terminal _T1 by the phase compensation circuit DP and the amplitude compensation circuit DG, thereby correcting nonlinear distortion of the light emitting diode LED.
Regarding the amplitude compensation circuit DG, as explained above, a Zener diode is used instead of the diode D.
This utilizes the nonlinearity of the DZ, and although a detailed explanation will be omitted, the temperature characteristics of the Zener diode DZ and its nonlinearity are
When compensating for the I-P characteristics of a diode, it is more effective in compensating for amplitude distortion than using a normal diode.

In the phase compensation circuit DP, when the voltage applied to both ends of the voltage variable capacitance element VC is ν _C , and its capacitance value is C(ν _C ), the transfer function F(s) is F(s)= It is expressed as 1/(1+SC(ν _C )R). (However, S=jω) This transfer function F(s) exhibits linear low-pass filter characteristics, and the output is attenuated at high frequencies as in FIG. 2b.

By the way, it is known that when a varactor using the junction capacitance of a diode is used as the voltage variable capacitance element VC, the capacitance value changes non-linearly depending on the applied voltage.

Therefore, if the bias voltage applied by the variable resistor RV is ν _B and the modulation signal at the input terminal T ₁ is ν _X , then the capacitance C of the voltage variable capacitive element VC (ν _B +
By changing the bias voltage _νB , the rate of change of _νX ) can be made into an f-V characteristic that fluctuates in the opposite direction to the f-P characteristic shown in Figure 2b, and the phase characteristics of the light emitting diode LED can be compensated. .

Figure 4 shows a combination of two different types of varactors VC ₁ and VC ₂ as voltage variable capacitance elements, and their total capacitance change rate is used to compensate the phase of a light emitting diode LED.The operation is similar to that shown in Figure 1. It is the same as the phase compensation circuit DP.

The above-mentioned fluctuation in the frequency characteristics of the light emitting diode LED is also related to the operating temperature of the light emitting diode.

Figure 5 shows a circuit that compensates for fluctuations in phase characteristics caused by temperature fluctuations in the light emitting diode LED.The time constants of the resistor _R2 and capacitor _C2 are combined with the thermal time constant of the light emitting diode LED. Light emitting diode by integrating the driving current
Compensates for the temperature characteristics of the LED. On the other hand, the ambient temperature of the light emitting diode LED is detected by a temperature detection element Th such as a thermistor and amplified by a DC amplifier OP.

Adder circuit AD adds the voltage of the integrated value indicating the temperature change and the output voltage of amplifier OP related to the ambient temperature, and adds the voltage to the varactor via bias circuit B.
Since we are applying voltage to VC ₃ , this circuit will connect the light emitting diode LED along with resistor R ₃ and voltage variable capacitance element VC ₃ .
This means that the f-P characteristics of , including temperature fluctuations, can be compensated for.

FIGS. 6 and 7 show other embodiments of the present invention in which the phase compensation is set significantly.

In Figure 6, _{a second phase compensation circuit is formed by adding an amplifier G 1 with a gain of 1, a resistor R 5 and a voltage variable capacitor VC 5 to the resistor R 4 and voltage variable capacitor} VC 4. . (Note that the bias circuit is omitted).

The transfer function F(s) in this circuit has the characteristics of a quadratic low-pass filter, F(s) = 1/(1+SC ₄ R ₄ )(1+SC ₅ R ₅ ), so light emission with large phase fluctuations Effective compensation is provided for the diode.

In FIG. 7, a modulated analog signal whose phase is reversed by an inverting amplifier _G2 is added to the voltage variable capacitance element _VC6 , and similar to FIG. 6, a large phase compensation is performed. Figure 8 shows voltage variable capacitance element
The operation of the VC ₇ can be switched according to the modulated analog signal. In this circuit, Ds is a switching diode and _C0 is a decoupling capacitor.

While the modulation signal at the input terminal T ₁ is small, the switching diode Ds does not conduct, and the voltage variable capacitance element VC ₇ is connected to the high value variable resistor RV ₇ , so its junction capacitance is used for phase compensation. It has not contributed much. When the modulation signal becomes a large value, the switching diode Ds becomes conductive, so one end of the voltage variable capacitance element _VC7 becomes the ground potential via the decoupling capacitor _C0 .
As described above, the junction capacitance of the voltage variable capacitance element _VC7 functions as a phase compensation circuit.

As explained above, the light emitting element drive circuit of the present invention includes an amplitude characteristic compensation circuit that compensates for the I-P characteristic of the light emitting element and a phase compensation circuit that compensates for the f-P characteristic, which are arranged via active elements. Since both compensation circuits can be adjusted individually by the first and second voltage adjustment means, there is an advantage that arbitrary compensation can be performed without placing a burden on other circuits.

[Brief explanation of the drawing]

Fig. 1 shows a conventional light emitting diode compensation circuit, and Fig. 2 a and b show the I-P and f of the light emitting diode.
-P characteristic diagram, FIG. 3 is a circuit diagram showing an embodiment of correcting the phase and amplitude distortion of the light emitting diode of the present invention, and FIGS. 4 to 8 are circuit diagrams showing other circuits for performing phase compensation of the present invention. It is a circuit diagram showing an example of. In the figure, R is a resistance, VC is a voltage variable capacitance element,
LED indicates a light emitting diode, and RV indicates a variable resistor.

Claims (1)

[Claims] 1. A low-pass type that includes a variable capacitance element whose capacitance changes depending on the voltage applied between both terminals, and a first voltage adjustment means that adjusts the voltage applied between both terminals of the variable capacitance element. a phase compensation circuit, and an amplitude compensation circuit comprising a diode and a second voltage adjustment means for adjusting the voltage applied between both terminals of the diode, respectively, are arranged via active elements, and the amplitude compensation circuit, 1. A light-emitting element drive circuit configured to supply an input modulation signal to a light-emitting element via a phase compensation circuit.