The use of optocouplers for audio amplifier circuits not only plays a role in signal isolation and prevents interference between stages, but also can amplify audio signals. The photocoupler can effectively replace the
Audio Transformer in the circuit, and overcome the shortcomings of signal loss and distortion during the primary coupling of the audio transformer, and also has the advantages of reducing electromagnetic interference. In the figure, VT1 and VT2 form a two-stage direct coupling amplifier circuit. The sensor or signal source signal is sent to the preamplifier composed of VT1 and VT2 after C1 and R1. The preamplifier converts the weak electrical signal into a DC component signal and adds the amplified and converted signal to the photocoupler. On the light-emitting diode VD, the intensity of the VD will be affected by the strength of the Vi signal. Adding a certain bias voltage to the phototransistor in the photocoupler can convert the optical signal emitted from the VD into an electrical signal and amplify it. Finally, it is amplified and output by the VT3 to the subsequent stage as a signal source. It can be seen that the circuit not only plays the role of isolation between the signal stages, but also plays the role of amplifying the signal, and when the signal is transmitted and amplified, there is no loss, and it will not cause signal distortion to obtain a more original signal data. The bandwidth of the circuit in the figure can reach 120kHz, and the gain can be increased to 20dB, which can fully meet the isolation between the general audio signal front stage and the primary stage, and amplify the signal in the front stage. This circuit can be typically used in the signal processing of a tape recorder, and will directly replace the isolated amplifier circuit formed by the audio transformer.
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