The signal transmitted by the optocoupler can be digital or analog, and the requirements for the device are different. Therefore, when selecting, the corresponding optocoupler should be selected for the input signal. The optocoupler used for analog signals is often referred to as a linear optocoupler. The principle of optocoupler in transmitting signals is the same as that of an isolation transformer, but it is small in size, high in frequency, and easy to use. Generally, optocouplers are packaged in DIP. The light-emitting device of the optocoupler used for transmitting analog signals is a diode, and the optical receiver is a photosensitive transistor. When a current passes through a light-emitting diode, it forms a light source that illuminates the surface of the photosensitive transistor, causing the transistor to experience a collector current. The magnitude of this current is proportional to the intensity of the light, that is, the forward current flowing through the diode. Due to the transmission of optical signals between the input and output ends of the optocoupler, the two parts are completely isolated electrically, without any electrical signal response or disturbance. Therefore, the function is stable and the anti-interference ability is strong The coupling capacitance between the light-emitting tube and the photosensitive tube is small (about 2pf) and the withstand voltage is high (about 2.5KV), so the common mode rejection ratio (CMRR) is very high. The electrical isolation between input and output depends on the insulation resistance between the two power supplies. In addition, due to its small input resistance (about 10 Ω), the noise of the high internal resistance source is equivalent to being shorted. Therefore, the analog signal isolation circuit composed of optocouplers has excellent electrical performance. In fact, an optocoupler is a current transfer device controlled by photocurrent, with output characteristics similar to those of ordinary bipolar transistors. Therefore, it can be directly used as an ordinary amplifier to form an analog amplification circuit, and electrical isolation can be achieved between input and output. However, this type of amplification circuit has poor working stability and no practical value. There are two main reasons for this: firstly, the linear working range of the optocoupler is narrow and varies with temperature; The second is the current transmission coefficient of the common emitter of the optocoupler β The reverse saturation current ICBO (i.e. dark current) of the collector is significantly affected by temperature changes. Therefore, in practical applications, in addition to using optical couplers with a wide linear range and high linearity to achieve analog signal isolation, effective measures must also be taken on the circuit to minimize the impact of temperature changes on the working state of the amplification circuit. From the relationship between the transfer characteristics of optocouplers and temperature, it can be seen that if the analog isolation circuit composed of optocouplers is stable and practical, the influence of dark current (ICBO) should be eliminated as much as possible to improve linearity, and the static working point IFQ should be automatically adjusted with temperature changes to maintain symmetry of the output signal, so that the dynamic range of the input signal can automatically change with temperature changes to offset β The influence of temperature changes on the value ensures the stability of the circuit's working state