A photocoupler is a semiconductor device that converts an electrical signal into an optical signal and then restores the optical signal into an electrical signal. It belongs to an electrical-optical-electrical conversion device. The basic structure is that the light emitter and the photosensitive receiver are housed in the same sealed casing, and are separated from each other by a transparent insulator. A common light emitter is an infrared light-emitting diode with a pin as an input. The transistor diagram can be observed to have a characteristic curve similar to that of a general diode. The photosensitive receiver is a photodiode or phototransistor with its pin as an output. When an electrical signal is sent to the input end of the photocoupler, the light emitting diode emits light through the current, and the intensity of the light is proportional to the signal current, that is, proportional to the magnitude of the forward current flowing through the diode, and the phototransistor at the output end The CE is turned on after being illuminated. When there is no signal at the input end, the LED does not light up, the phototransistor is turned off, and the CE is not connected. Thereby, photoelectric transmission and conversion are realized. With the increasing complexity of various types of electrical equipment control circuits, interference between functional circuits is inevitable. Since the input end and the output end of the optocoupler are transmitted by the optical signal, the two parts of the circuit are completely electrically isolated, so there is no feedback and interference of the electric signal, so the performance is stable and anti-interference ability. Very strong. In general, the transmission of digital signals between circuits can be completely isolated using optocouplers. However, when the analog signal is transmitted, since the linear working range of the photocoupler is narrow, the nonlinear distortion is large, and the conventional modulation and demodulation circuit and the nonlinear compensation circuit are complicated and large. Therefore, the author designed a feedback-type symmetric temperature-compensated analog signal amplifying circuit with high precision, simple circuit and composed of optocouplers. The circuit can complete the isolated transmission of analog signals. 1 Transmission characteristics analysis This paper introduces the feedback-type symmetric temperature compensation analog signal amplification circuit. The power supply voltage in this circuit is 5V. When the resistance R3 is 1 kΩ, the transmission characteristics of the 4N25 are as follows: (1) When the input current I1 is 0, the output current I2 is 0, indicating that the LED does not emit light, and the phototransistor is turned off without illumination; (2) When the input current I1 is 0.5 mA, the output current I2 is 0.22 mA. At this time, Il "I2" indicates that the LED has started to emit light, and the phototransistor has weak illumination and leaves the cut-off region. (3) When the input current I1 is 1 to 4 mA, the output current I2 is 0.7 to 4.19 mA, I1 (4) When the input current I1 is 4.5 mA, the output current I2 is 4.4 mA, I2 "I1", indicating that I2 cannot continue to change linearly as I1 increases to a certain extent. Thereafter, the current transfer ratio drops and the photocoupler begins to enter a saturated state. According to the above analysis, it can be seen that the size of I1 determines the operating state of the circuit. If I1 is too large or too small, the circuit works in the nonlinear region. Only within a certain range, 4N25 works in the linear region. 2 Working principle The optocoupler Icl and IC2 are both selected as 4N25, and Icl and R4 form an output stage for isolating and transmitting analog signals. IC2 and R2 mimic the output form, which can be used to generate feedback comparison signals, and can automatically adjust the operating current of the LED when different current conversion efficiency, to ensure that the optocoupler works reliably in the linear amplification state, and improve the linearity of the circuit. Since the LEDs of the two optocouplers are connected in series, the working states of Icl and IC2 are completely symmetrical, and the same excitation current I1 is shared. And the emitter potential of the two, that is, the collector currents of the two phototransistors are symmetric about the voltage drop generated on R2 and R4, respectively, and are linearly controlled by the input signal Ui, so that the isolation and transmission of the analog signal can be realized. Since the signal transmission between the input and output of the optocoupler is realized by the optical signal, the two parts of the input and output are completely electrically isolated, and there is no feedback and interference of the electrical signal, so the performance is stable and anti-interference. strong ability. The coupling capacitance between the light emitting diode and the photosensitive tube is small and the withstand voltage is high, so the common mode rejection ratio is high. The electrical isolation between the input and output depends on the insulation resistance between the two parts of the power supply. In addition, because of its small input impedance, the noise of the high internal resistance source is equivalent to short circuit. Therefore, the analog signal amplifying circuit composed of the photocoupler has excellent electrical properties. If the non-inverting terminal potential of the operational amplifier A1 deviates from the virtual ground due to the interference signal, the potential of the output terminal of the operational amplifier A1 will increase, and the luminous intensity of the photocoupler IC2 will increase, thereby reducing the collector voltage of the IC2. Finally, the potential of the inverting terminal of the operational amplifier A1 is lowered and returned to the virtual ground. Conversely, if the potential of the inverting terminal of the operational amplifier Al deviates from the virtual ground due to the interference signal, the potential at the output of the operational amplifier A1 will decrease, and the luminous intensity of the photocoupler IC2 will be weakened, and the collector voltage of IC2 will increase. Large, and finally the potential of the inverting terminal of the operational amplifier Al rises back to the virtual ground. 3 Conclusion The photocoupler is a current-switching device controlled by photocurrent, and its output characteristics are similar to those of ordinary bipolar transistors, because it can be directly used as an ordinary amplifier to form an analog amplifying circuit. However, the linear operating range of the optocoupler is narrow and varies with temperature. At the same time, the common emitter current transfer coefficient of the photocoupler and the collector reverse saturation current Iceo (ie dark current) are more affected by temperature changes. Therefore, for the relationship between the transfer characteristics of the optocoupler and the temperature, in order to make the analog isolation circuit composed of the optocoupler work stably, the influence of dark current (Iceo) should be eliminated as much as possible to improve the linearity and make the static work. The point energy can be automatically adjusted according to the temperature change, so that the output signal maintains symmetry, and the dynamic range of the input signal changes automatically with temperature, thereby offsetting the influence of the B value with temperature, and ensuring the stability of the working state of the circuit. In practical applications, in addition to the choice of optocouplers with wide linear range and high linearity to realize the transmission of analog signals, effective measures should also be taken on the circuit. For example, according to the requirements of dynamic work, a suitable static working point can be set, and a feedback symmetric temperature compensation circuit is adopted to eliminate the influence of temperature change on the working state of the amplifying circuit, thereby obtaining the transmission of the signal without distortion.
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