This is a common emission set amplifier circuit, which adjusts the base bias. When the analog signal is input, the transistor collector current changes with the analog signal strength, so the output optical power of LED also changes with the analog signal. Using the improved form of this circuit, the circuit adopts 5V DC working point, and the design R1 = 30 Ω, R2 = 2K Ω. An isolation capacitor CF = 10uF is connected between the transmitting end and the signal source. Since the gain of LM386 can reach 200, in order to reduce the amplitude of the signal at the transmitting end (otherwise the signal amplitude is too large to be adjusted without distortion at the receiving end), a resistance RB = 100 Ω is connected between the capacitor CF and the ground. Since the ICQ needs to reach about 20mA, it can be seen from the calculation of R1 and R2 bias that the resistance connected to the emitter is about tens of ohms, which is designed as RE1 = 20 Ω and re2 = 52 Ω, and a DC isolation capacitor CEF is 100uF in parallel on re2.

    Transmitting circuit

The receiving tube converts the optical signal into an electrical signal. Infrared receiving and amplifying circuit is realized with operational amplifier LM386. The port connection method of op amp LM386 is shown in the figure.

       Receiving circuit

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