However, this kind of amplifier circuit has poor working stability and is of no practical value. Therefore, it can be used as an ordinary amplifier to directly form an analog amplifier circuit, and the input and output Can achieve electrical isolation. Its output characteristics are similar to those of ordinary bipolar transistors. In fact, the optoisolator is a current transfer device controlled by photocurrent. Therefore, the analog signal isolation circuit composed of optoisolators has excellent electrical performance. In addition, because of its low input resistance (about 10Ω), the noise to the high internal resistance source is equivalent to being shorted. The electrical isolation between input and output depends on the insulation resistance between the two parts of the power supply. The coupling capacitance between the luminous tube and the photosensitive tube is small (about 2pf), and the withstand voltage is high (about 2.5KV), so the common-mode rejection ratio is very high. Since the optical signal is transmitted between the input end and the output end of the optoisolator, the two parts are electrically isolated completely without feedback and interference of electrical signals, so the performance is stable and the anti-interference ability is strong. The magnitude of the current is related to the intensity of the light, that is, the forward current flowing through the diode. The light source illuminates the surface of the phototransistor, causing the phototransistor to generate collector current. When a current flows through the light-emitting diode, a light source is formed. The light-emitting device of the optoisolator used to transmit analog signals is a diode, and the light receiver is a photosensitive triode. Optoisolators are generally DIP packaged. The principle of signal transmission is the same as the isolation transformer, but it is small in size, high in transmission frequency, and easy to use. The optoisolator used for analog signals is often called a linear optoisolator. The signal transmitted by the optoisolator can be a digital signal or an analog signal, but the requirements for the device are different, so the corresponding optoisolator should be selected for the input signal. The optoisolator with the new pigtail input and output has quite good performance, the lowest insertion loss is about 0.5dB, the isolation is 35-60dB, and the highest can reach 70dB. For reverse light, when the linearly polarized light exiting the analyzer passes through the placement medium, the deflection direction is also rotated 45 degrees to the right, so that the polarization direction of the reverse light is orthogonal to the direction of the polarizer, completely blocking the transmission of reflected light.įaraday's magnetic medium usually uses yttrium iron garnet (YIG) single crystal with low optical loss in the wavelength range of 1 μm to 2 μm. Together with the external magnetic field, the Faraday gyromagnetic medium rotates the polarization direction of the signal light by 45 degrees to the right, and makes the low loss pass and the polarizer 45 degrees. Light is the transmission medium, so the input end and the output end are electrically insulated, which is also called electrical isolation.įor the signal light incident in the forward direction, it becomes linearly polarized light after passing through the polarizer. When the input electrical signal is applied to the input light-emitting device LED, the LED emits light, the light receiving device receives the light signal and converts it into an electrical signal, and then directly outputs the electrical signal, or amplifies the electrical signal to a standard digital level output, so that The conversion and transmission of "electricity-optical-electricity" are realized. For polarized light transmitted in the direction of the magnetic field, the rotation angle θ of the polarization direction and the product of the magnetic field strength B and the material length L are proportional. It is also called the magneto-optical rotation effect. The Faraday effect is the first observation by Faraday in 1845 that a non-optically active material rotates the polarization direction of light passing through the material under the action of a magnetic field.
The optoisolator mainly uses the Faraday effect of the magneto-optical crystal.
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