Electronic hearing aid circuit

The picture shows a simple electronic hearing aid circuit, which is a multi-stage low-frequency amplifier. The weak signal from the microphone is amplified by a two-stage amplifier composed of transistors VT1 and VT2, and then output by the emitter of the emitter auxiliary output VT3. VT4 amplifies the power and drives the earphone to make sound.

In the figure, VT1, VT2, and VT3 use direct-coupled feedback amplifier circuits, which have the functions of high gain and stable static working point of transistor VT1. Since the inter-electrode coupling capacitor is omitted, the frequency band can be widened to reduce frequency distortion. VT4 power amplifier adopts voltage divider bias circuit, using diode VD as the lower bias resistor, which can play a good role in temperature compensation and has certain automatic volume control performance. The microphone can be replaced by an internal magnet speaker; the earphone uses low impedance 8Ω; transistors VT1, VT2, and VT3 use 3AX23, requiring VT1, VT2 amplification factor = 110, VT3 amplification factor> 250, VT4 uses 3AX31C, amplification factor = 150; diode VD can use 2AP or 2AK.