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The circuit diagram of the deaf hearing aid is shown in the figure below, which is actually a multi-audio amplifier composed of transistors VT1~VT3. Among them, the transistor VT1 and its surrounding capacitance and resistance constitute a typical resistance-capacitance coupling amplifier circuit, which is used for pre-audio voltage amplification; the transistor VT2 and VT3 constitute a two-stage direct coupling power amplifier circuit, and the three transistors are common To achieve audio amplification.
The electret microphone B is used to receive the sound signal and convert it into a weak electrical signal. The signal is amplified by the base of VT1 after being coupled by C1, and then output by the collector of VT1 after being amplified, and then coupled to by C2 At the base of VT2, a second-stage amplification is performed. Finally, the amplified signal is output from the emitter of VT3, and then reaches the earphone through the jack XS and enters the human ear.
In this circuit, C3 is a filter capacitor, mainly used to reduce the AC internal resistance of battery G, to prevent self-oscillation generated by the circuit when the battery is about to be scrapped, so that the headset emits a clearer sound. C4 is a bypass capacitor, which is mainly used to bypass various noises in the output signal and improve the sound quality of the earphones.