Audio amplifier circuit based on photoelectric coupler design

The use of photocouplers in audio amplifier circuits not only plays a role in signal isolation and prevents inter-stage interference, but also can amplify audio signals. Photocouplers can be equivalent to replacing audio transformers in circuits, and overcome the shortcomings of signal loss and distortion during primary coupling of audio transformers, and also have 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 passing through C1 and R1. The preamplifier converts the weak electrical signal into a DC component signal, and adds the amplified and transformed signal to the light-emitting diode VD of the photocoupler. The light intensity of VD will be affected by the intensity of the Vi signal. By adding a certain bias voltage to the photosensitive transistor in the photocoupler, the optical signal emitted by VD can be converted into an electrical signal and amplified, and finally amplified and output to the next stage as a signal source through VT3. It can be seen from this that the circuit not only plays a role in signal isolation between levels, but also plays a role in amplifying signals, and the signal is lossless and will not cause signal distortion when it is transmitted and amplified, so as to obtain 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 pre-stage and the primary stage of general audio signals, and amplify the signal in the pre-stage. This circuit can be typically used in the signal processing of radio recorders, and will directly replace the isolation amplifier circuit composed of audio transformers.