Abstract:
A photocoupler having a first and second light-emitting diodes, a compensation circuit, which compensates input signals to the first light-emitting diode and generates input signals to the second light-emitting diode, and further makes the current waveform at the second light-emitting diode complementary to the current waveform at the first light-emitting diode, and at least one photodiode that detects the light emitted from the first and second light-emitting diodes.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an improved photocoupler alone, or a device that uses a photocoupler, comprising light-emitting diodes and photodiodes, and in particular, a photocoupler having a structure for compensating for non-linearity related to current-voltage properties of light-emitting diodes. 
     DISCUSSION OF THE BACKGROUND ART 
     Photocouplers have been used for insulation and separation of signals among input signals and output signals. In recent years there has been a demand for photocouplers that are useful for high-speed analog signal transmission while improving the insulation and separation characteristic. The nonlinearity of photocouplers, particularly the nonlinearity associated with the voltage-current properties of LEDs, becomes a problem in this case. That is, if the photocoupler has a strong nonlinearity, this will result in the distortion of signals during analog signal transmission. 
     A method of applying negative feedback has been proposed as an example of technology for avoiding this problem (See JP (Kohyo) 11[1999]-509367 and JP (Kokai) 61[1986]-36981). By means of this method, an additional photodiode is positioned close to the light-emitting diode. This photodiode has a structure with which the amount of light emitted from the light-emitting diode is monitored and this is fed back to the operating current of the light-emitting diode. 
     Nevertheless, in addition to the difficulty of accurate monitoring, it is extremely difficult to design a high-speed circuit with this structure because the amplifier for amplifying the signals requires a band that is at least 10 times the transmission signal bandwidth and nonlinear elements are used. Consequently, this structure cannot be used for high-speed communications. 
     A method has also been suggested whereby analog signals are not transmitted and instead, analog signals are converted to digital signals and then transmitted, after which they are converted back to analog signals. However, AD/DA converters or additional circuits for modulation-demodulation are necessary in this case, complicating the circuit structure, and high speed operation is still difficult. 
     Therefore, the present invention seeks to improve the nonlinearity related to current-voltage properties of light-emitting diodes in photocouplers and provide a photocoupler with which relatively high-speed analog signal transmission is possible. 
     SUMMARY OF THE INVENTION 
     The present invention provides a photocoupler comprising two light-emitting diodes with almost common I-V properties, one of which is the principal light-emitting element and the other of which is a light-emitting element for compensation, in order to improve the nonlinearity of photocouplers, particularly the non-linearity associated with the properties of the light-emitting diodes. Compensation signals given to the light-emitting diode for compensation are determined from the operating input signals given to the principal light-emitting diode. These compensation signals are given as input of the light-emitting element for compensation. The light emitted from both of the light-emitting elements converges optically and is detected by a single photoelectric conversion detector (photodiode), or is detected by individual detectors, and this output is electrically combined. Transmission output signals with little distortion are obtained by superimposing these optical or electric signals. 
     A negative feedback means for the circuit is not necessary, and analog/digital conversion is not necessary, with the photocoupler of the present invention. Consequently, high-speed operation is possible with the photocoupler of the present invention. For instance, analog signals can be transmitted with little signal distortion at approximately 30 MHz or faster. 
     That is, the present invention provides a photocoupler which comprises first and second light-emitting diodes; a compensation circuit that compensates the input signals to this first light-emitting diode and produces input signals to this second light-emitting diode and makes the current waveform at this second light-emitting diode complementary to the current waveform at this first light-emitting diode; and at least one photodiode that detects the light emission of said first and second light-emitting diodes. 
     It is preferred that the compensation circuit determines that the input voltage waveform given to this second light-emitting diode becomes similar in shape but with a smaller output amplitude than the input voltage waveform to this first light-emitting diode. 
     It is preferred that the compensation circuit operates in such a way that the alternating-current component of this input voltage waveform is amplified by a pre-determined gain. 
     It is preferred that these first and second light-emitting diodes are grounded on one side. 
     It is preferred that this photodiode comprises separate first and second photodiodes corresponding to the first and second light-emitting diodes for receiving the light emitted from the respective light-emitting diode. 
     It is preferred that the photocoupler is structured such that signals that become the simple sum of detection signals are output by these first and second photodiodes. 
     It is preferred that a first group consisting of this first light-emitting diode and this first photodiode and a second group consisting of this second light-emitting diode and this second photodiode comprise separate integrated circuit (IC) packaged photocouplers. 
     It is preferred that these photodiodes become a single element that simultaneously receives the combined light emitted from these first and second light-emitting diodes. 
     It is preferred that this compensation circuit together with these light-emitting diodes and these photodiodes is included in a single IC package. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical circuit that depicts the photocoupler according to the present invention. 
         FIG. 2  is a theoretical graph depicting the signal waveforms according to the present invention. 
         FIG. 3  is the compensation circuit according to the present invention. 
         FIG. 4  is an electrical circuit that depicts the photocoupler according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a photocoupler with improved nonlinearity related to the current-voltage properties of light-emitting diodes in a photocoupler and with which relatively high-speed signal transmission is possible. 
     Photocouplers according to the present invention will be described in detail while referring to the attached drawings.  FIG. 1  is a diagram describing a first embodiment of the present invention. 
     As shown in  FIG. 1 , photocoupler  10  of the first embodiment comprises an input terminal  20 , a first light-emitting diode (principal light-emitting element)  30 , a second light-emitting diode (light-emitting element for compensation)  40 , a compensation circuit  50  that gives compensation input signals to second light-emitting diode  40 , a first photodiode  60  corresponding to first light-emitting diode  30 , a second photodiode  70  corresponding to second light-emitting diode  40 , and an output terminal  80 . 
     Each element constituting photocoupler  10  is made by mounting several packaged ICs on a circuit board. First light-emitting diode  30  and first photodiode  60  as well as second light-emitting diode  40  and second photodiode  70  must be structured such that they individually transmit and receive light signals and therefore, these groups are usually made into individual photocoupler ICs (refer to reference numbers  91  and  92 ). In this case, the compensation circuit is outside these ICs, but it can also be contained in one of these ICs. Furthermore, photocoupler  10  is an element providing electrical insulation and separation, and the side of light-emitting diodes  30  and  40  as well as the side of photodiodes diodes  60  and  70 , where light emitted from light-emitting diodes  30  and  40  is received, are separate boards. 
     Furthermore, although each group comprised of first light-emitting diode  30  and first photodiode  60  and comprised of second light-emitting diode  40  and second photodiode  70  can be contained in exactly the same package, independent signal transmission is necessary, and therefore, a barrier structure for optical separation is needed between the two groups, making production difficult. In this case, the compensation circuit can also be on a third board in the same package. 
     Input terminal  20  is a terminal that receives the high-frequency signals used for communications. The light signals received by input terminal  20  are guided through resistor R c1  to first light-emitting diode  30 . Pre-determined light emission is produced at first light-emitting diode  30  by the current that is flowing in accordance with the signal voltage waveform of the input signals. The current-voltage properties at first light-emitting diode  30  are usually nonlinear and therefore, light emission intensity to signal intensity is nonlinear and the output of the photodiodes that directly receive this light produce a signal distortion. The photocoupler  10  advantageously includes some elements for preventing this signal distortion, e.g. second light emitting diode  40 . Their effect is described below. 
     The input signals to second light-emitting diode  40  for signal compensation are given by compensation circuit  50 . Voltage signals for compensation are produced by reference to the waveform of signals input to first light-emitting diode  30  (point P). That is, second light-emitting diode  40  receives the signals generated by the compensation circuit for signal waveform compensation and emits light in accordance with these signals. 
     By means of the present embodiment, compensation circuit  50  multiplies the voltage waveform at point P in  FIG. 1  k times to form signals of a similar voltage waveform and these serve as the compensation signals. The compensation signals are given to second light-emitting diode  40  through resistance R c2 . 
     Each signal waveform is shown in  FIG. 2 . The broken line (W V1 ) in the figure is the voltage waveform at point P in  FIG. 1 . The voltage waveform of the signal has already been distorted at the step before its input to first light-emitting diode  30  and its amplitude is relatively small on the positive side, while its amplitude is larger on the negative side when compared to the distortion-free sine waveform (dashed line: W R ). This voltage waveform is distorted due to the fact that point P is at the input position of the positive terminal of the light-emitting diode that has been grounded at the negative terminal side. 
     On the other hand, the waveform of the current that flows through first light-emitting diode  30  at this time is shown by the solid line (W I1 ). The light emission intensity at a light-emitting diode is generally considered to be almost proportional to the current within a normal range and therefore, this current waveform is apparently almost the same as the detected values at first photodiode  60 . As is clear from this figure, the current waveform, that is, the waveform of the detection signals at first photodiode  60 , has a relatively large amplitude on the positive side and a relatively small amplitude on the negative side. 
     As was previously explained, compensation circuit  50  gives the signal waveform for compensation of a shape similar to the voltage signal waveform at point P. When compensation signals are input to second light-emitting diode  40 , the waveform of the input voltage signals to second light-emitting diode  40  further deforms just as the distortion that is produced in the voltage waveform at point P and the signal amplitude of the compensation signal waveform becomes even smaller on the positive side when compared to the negative side, as shown by the dashed line (W V2 ). The waveform of the current that flows to second light-emitting diode  40  at this time (W I2 ) becomes smaller in amplitude on the positive side and larger in amplitude on the negative side, as shown by the solid line. As with first light-emitting diode  30 , this current waveform is approximately the same as the output waveform of second photodiode  70 . 
     What should be noted is that alternating current signals that are virtually distortion free (W out ) can be reproduced from the sum of the waveform of first light-emitting diode  30  and the current waveform of second light-emitting diode  40 , that is, the sum of the output waveform of first photodiode  60  and the output waveform of second photodiode  70 , but optimizing the value of above-mentioned “k.” That is, the distortion of signals can be compensated by using first and second light-emitting diodes  30  and  40  and the corresponding first and second photodiodes  60  and  70 . 
     If phase distortion of the current waveform signals is large, distortion compensation may not be sufficient when compensated by the sum of the current waveforms of first light-emitting diode  30  and second light-emitting diode  40 . A structure may be used here for combining the phase of two signals when the sum of current waveforms is used in order to efficiently reduce the distortion. An example of a specific means is the method whereby a buffer with the same delay as compensation circuit  50  is introduced behind photodiode  60  in order to improve the symmetry of the circuit structure. Furthermore, results that are satisfactory for practical application can be obtained by speeding up compensation circuit  50 . 
       FIG. 3  is a circuit diagram showing an example of the compensation circuit. The compensation circuit is shown together with the photocoupler that is used with the circuit. Compensation circuit  50  includes differential amplifier  51 . The voltage signals that branch at the input side of first light-emitting diode  30  are input to the positive terminal of differential amplifier  51  through capacitor C 1 . As shown in the figure, this positive terminal is grounded via resistor R 1  (10 kΩ) and the negative terminal is grounded via resistor R 2  with a smaller resistance (2.15 kΩ). Furthermore, although the negative terminal is connected to the output side via variable resistance R V , the resistor can also have a resistance that has been set at an optimal value. 
     As shown in the figure, the output of compensation circuit  50  is connected to a bias terminal via capacitor C 2  and is input to the LED terminal of photocoupler  92 . As a result, alternating-current signals that have been amplified to a pre-determined intensity by the compensation circuit are input to the photocoupler. 
     The output waveforms of first photodiode  60  and second photodiode  70  are electrically combined and output as sum signals in the embodiment in  FIG. 1 . As a result, alternating-current signals that have been input to input terminal  20  are transmitted up to output terminal  80  as they are being brought to minimal distortion by being electrically insulated. Furthermore, signal treatment, such as the necessary amplification and so forth, is performed on the detection signals of first and second photodiodes  60  and  70 , or the sum signal of these detection signals, but a conventional amplification method can be used, and therefore, a description for this is not given. 
       FIG. 4  is a diagram showing a photocoupler that is a second preferred embodiment of the present invention. This photocoupler  110  comprises input terminal  120 , first and second light-emitting diodes  130  and  140 , compensation circuit  150 , photodiode  160 , and output terminal  180 . Photocoupler  110  is usually made into an individually packaged IC  190  as in the first embodiment, but it can also be made into an IC package that contains compensation circuit  150 . 
     The difference from the first embodiment is that a single photodiode  160  receives light from first and second light-emitting diodes  130  and  140 . That is, as with the first embodiment, input signals are given from input terminal  120  to first light-emitting diode  130 , and signals that are obtained when the signal voltage at the input side of the first light-emitting diode  130  is compensated by compensating circuit  150  are given to second light-emitting diode  140 . The light that has been emitted by light-emitting diodes  130  and  140  in accordance with these signals is received by a single photodiode  160 . 
     Consequently, in contrast to the fact that by means of the first embodiment, signals from first and second photodiodes  60  and  70  are electrically synthesized and output as a sum signal, by means of the second embodiment, they are synthesized as the sum of the amount of light (the sum of the number of photons) when light is received at photocoupler  160  and electric signals corresponding to this sum are output at photodiode  160 . That is, taking  FIG. 2  into consideration once again, the signal waveform at the input side of first light-emitting diode  130 , that is, at point P, and the signal waveform at the input side of second photodiode  140  are optically synthesized. In other words, the sum waveform shown to the right in the figure is the same as the amount of light received by the single photodiode  160  and is understood to be the output from this photodiode  160 . 
     An advantage of the second embodiment is that the number of elements that are used can be minimized and as a result, the device can have a simpler structure. In particular, in addition to there being only one photodiode, additional circuits for amplification and synthesis of electrical signals in later steps are not necessary and therefore, there is a practical advantage in this case. 
     Preferred embodiments of the present invention were described above, but these are only examples and a variety of alterations and modifications by persons skilled in the art are possible. For instance, the number of light-emitting diodes in the present embodiments was two, but it is possible to add more light-emitting diodes. In this case, it is also possible to add photodiodes in combination with these light-emitting diodes, or it is possible for one photodiode to receive the light of three or more light-emitting diodes.