Telephone Amplifier Circuit

Section Ul-a is configured as a high-gain inverting voltage amplifier that is inductively coupled to the phone line via LI. Inductor LI is a homemade unit that consists of 250 turns of fine, enamel-coated wire that is wound on an iron core. The op amp receives the few mV produced by LI via. CI and Rl and amplifies the signal. Capacitor CI acts as the negative-feedback component that limits the circuit`s high-frequency gain, while R3 limits the low-frequency gain. Resistor R3 is particularly important because without it, the amplifier would saturate. Op amp Ul-b is configured as a difference amplifier.
Telephone Amplifier Circuit - schematic

It receives a signal from Ul-a via 03 and R4 and amplifies the difference between it and half of the supply voltage. Transistor Ql is configured as a common-collector amplifier ensuring sufficient signal to drive the speaker. Capacitor C5 is used to remove any dc component provided by transistor Ql.

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