JPS626376B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a distortion compensation circuit for an optical transmitter used in an optical communication device.

When constructing an optical transmitter for analog signal transmission using a light source device such as a light emitting diode or a semiconductor laser, it is difficult to obtain good analog signal transmission characteristics due to the large distortion generated by the light source device. Therefore, in order to compensate for this distortion characteristic, negative feedback method, feed forward method, etc.
and pre-distortion methods are used, but the negative feedback method has a narrow frequency range that can be handled due to limitations due to the feedback loop propagation time, and the feed forward method has the drawback of complicating the circuit. Recently, the third predistortion method has been relatively commonly used. This is constructed so that the distortion generated by the light source is canceled out by the distortion generated by a nonlinear circuit called a predistortion circuit. However, in this method, as will be explained in detail later, the circuit scale is large for accurate compensation, the adjustment method for nonlinear characteristics is difficult as there are many adjustment points, and the temperature characteristics of the distortion compensation characteristics are difficult to adjust. It was difficult to improve.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a distortion compensation circuit that does not increase in circuit scale, is easy to adjust, and has stable characteristics against changes in ambient temperature.

According to the present invention, in a distortion compensation circuit for compensating for distortion components generated in a light source device such as a light emitting diode or a semiconductor laser when an electric signal is converted into an optical signal, the electric signal is used as an input signal. , a feedback amplifier configured such that the output signal is a signal for driving the light source device, and the amplification degree is determined by the amount of feedback; A distortion compensation circuit is obtained, comprising a field effect transistor arranged such that a change in the source-to-source resistance corresponds to a change in the amount of feedback, and a control voltage generation circuit that provides the control voltage.

Next, a detailed description will be given with reference to the drawings.

FIG. 1 is a diagram showing the configuration of a conventional distortion compensation circuit. The conventional distortion compensation circuit shown in FIG. 1 includes a light emitting diode (LED) 1, a transistor 2 that operates by inputting a signal S, and a nonlinear circuit 3 that compensates for the distortion generated by the light emitting diode 1. , and this nonlinear circuit 3 has three variable resistors 4 in this case.
, three diodes 5, three bias power supplies 6, and a fixed resistor 7. B is a bias power supply for the light emitting diode 1.

In the above configuration, the nonlinearity of the nonlinear circuit 3 can be achieved to some extent by adjusting the values of the variable resistor 4 and the bias power supply 6, and this nonlinear circuit is called a polygonal line approximation. Because we use an approximation method,
For accurate distortion compensation, it is necessary to increase the number of polygonal lines by increasing the number of series circuits consisting of variable resistors, diodes, and bias power supplies. In addition, in order to apply this circuit to light source devices that have different nonlinear characteristics for each type of light source device, or even for each unit of the same type, it is necessary to It is necessary to readjust by changing the resistance value of and the voltage of the bias power supply 6.

Due to the above reasons, the conventional circuit has the following drawbacks, as briefly explained above. In other words, accurate compensation requires a large circuit, there are many adjustment points to adjust nonlinear characteristics, and the adjustment method is difficult.Furthermore, the distortion characteristics of LEDs change with temperature, so compensation circuits are required. It has been difficult to accurately change the nonlinear characteristics of the distortion compensation characteristics with temperature because there are too many adjustment points, and it has been difficult to improve the temperature characteristics of the distortion compensation characteristics.

FIG. 2 is a diagram showing the circuit configuration of the first embodiment of the present invention. Input signal S is amplified by amplifier 11 and applied to LED 13 through resistor 12. A bias current B is applied to the LED 13 through a resistor 14. From the output terminal of the amplifier 11 to the negative input terminal connected to a resistor with a resistance value R ₁ (represented by R ₁ , the same applies hereinafter), one side is connected through a resistor R ₂ and the other is connected to a resistor R ₃ and an electric field. Since the signal is fed back through the series circuit of effect transistors (FETs) 15, the amplification degree A at this stage is calculated as follows: A=1+R ₂ (R ₃ +r)/R ₁ (R ₂ +R ₃
+r). However, r is the resistance value between the drain and source of the FET 15. By the way, the drain of FET
The source-to-source resistance r is the control voltage V applied to the gate.
Although it is mostly determined by _c , it also changes somewhat depending on the source-drain voltage. That is, FET
When 15 is viewed as a resistor with a resistance value r, the resistance value changes depending on the terminal voltage. This is resistor _R3 and FET1
This means that the current and voltage characteristics of the series circuit No. 5 have nonlinearity. Therefore, the second
The amplification degree of the feedback amplifier in the figure also shows nonlinearity according to the amplification degree equation described above.

Figure 3 shows how the video signal VS is passed through the feedback amplifier mentioned above, and its signal level and distortion compensation (DG) characteristics are changed from 0 (corresponding to black) to the maximum practical value (corresponding to white). The results are shown below. As can be seen from the figure, the upward-sloping DG characteristic is realized due to the nonlinearity of the FET 15. Note that this DG characteristic can be changed by changing the value of the control voltage V _c , from a state with no nonlinear characteristics at V _c = 5V to V _c
It can cover a wide range up to the state where the DG characteristic of =2V is 4%. Therefore, the circuit of this embodiment is effective as a distortion compensation circuit for many types of light source devices having downward-sloping DG characteristics.

FIG. 4 is a diagram showing the configuration of a second embodiment of the present invention, and the same parts as in FIG. 2 are given the same reference numerals. As can be seen from the figure, in Fig. 2, amplifier 1
It was inserted between the output terminal and input terminal of 1.
In this case, a series circuit of FET15 and resistor _R3 is connected to the branch connecting the input terminal and ground _.
are inserted as a series circuit. However, resistance
The resistance values of R ₃ and resistor R ₄ are not necessarily equal.
The reason why the negative feedback amplifier in this configuration exhibits nonlinear characteristics is that the amount of negative feedback has nonlinearity, as in the case of FIG. However, due to the difference in the insertion position of the nonlinear circuit, the nonlinear characteristics of opposite polarities are shown in FIGS. 2 and 3. Comparing this with the DG characteristics shown in Figure 3, the DG characteristics in Figure 2
While the characteristic slopes upward to the right, the DG characteristic in Figure 4 slopes downward to the right. Therefore, the circuit of this embodiment is effective for distortion compensation of a light source device whose DG characteristics are upward-sloping.

FIG. 5 is a diagram showing the configuration of a third embodiment of the present invention. In the first and second embodiments, the resistor R ₁ or the resistor R ₂ is used to provide nonlinearity to the amount of feedback.
A non-linear circuit is connected in parallel to , but in this third embodiment, this is done by a voltage dividing circuit consisting of a series circuit of resistor R ₅ , resistor R ₆ and FET 15 . In addition,
Since the circuit of this embodiment exhibits a downward slope of the DG characteristic as in the second embodiment, it is effective for distortion compensation of a light source device having an upward slope of the DG characteristic.

In the first to third embodiments described above, a FET is used in the feedback circuit of the feedback amplifier to provide nonlinearity, thereby creating distortion characteristics opposite to those of the LED. The configuration is simple, there are only a few adjustment points, and it is easy to apply it to distortion compensation for a wide variety of light source devices because all you need to do is change the gate voltage of the FET.

Furthermore, the distortion characteristics of LEDs generally change depending on temperature, but in the circuit of the present invention, the DG characteristics can be easily changed by changing the control voltage of the FET, so it is possible to change this voltage according to temperature. This allows distortion compensation to be achieved over a wide temperature range.

FIG. 6 is a diagram showing an example of a control voltage generating circuit for this purpose. This divides the voltage B by a resistor 16 and a thermistor 17, and amplifies it by an amplifier 18 to obtain a control voltage _Vc . In other words, by appropriately selecting the temperature characteristics of the thermistor, the control voltage V
_c can be changed so that the value of DG becomes a desired value.

As described above, the present invention provides a distortion compensation circuit with a simple configuration and easy adjustment, and furthermore, provides a distortion compensation circuit with excellent temperature characteristics. For this reason, it is possible to downsize optical transmitters for analog signals such as video signals, configure optical transmitters with distortion compensation circuits that can be used with a wide variety of light source devices, and also have excellent temperature characteristics and low distortion characteristics. An optical transmitter can be provided.

[Brief explanation of the drawing]

FIG. 1 shows the configuration of a conventional distortion compensation circuit, FIG. 2 shows the configuration of the first embodiment of the present invention, and FIG. 3 shows the distortion compensation characteristics of the first embodiment. 4 and 5 are diagrams showing the configurations of the second and third embodiments of the present invention, respectively, and FIG. 6 is the configuration of a circuit that generates a control voltage for an FET used in a distortion compensation circuit. It is a figure showing an example. Symbol explanation: 11 is amplifier, 12 is resistor, 13
is LED, 15 is FET, 17 is thermistor, 18
is an amplifier, R ₁ to _{R 6} are resistances (values), r is a resistance between the drain and source of the FET 15, and V _c is a control voltage.

Claims (1)

[Claims] 1. In a distortion compensation circuit for compensating for distortion components generated in a light source device such as a light emitting diode or a semiconductor laser when an electric signal is converted into an optical signal by the light source device, the electric signal is input. As a signal,
A feedback amplifier is configured such that the output signal is a signal for driving the light source device, and the amplification degree is determined by the amount of feedback, and a circuit for providing the amount of feedback includes a drain-source amplifier configured to change the control voltage by changing the control voltage. A distortion compensation circuit comprising: a field effect transistor arranged so that a change in inter-resistance causes a change in the amount of feedback; and a control voltage generation circuit that provides the control voltage. 2. The distortion compensation circuit according to claim 1, wherein the control voltage is a control voltage that changes with ambient temperature.