Amplifier for telephone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Amplifier for telephone. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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If a person began to hear worse, it is more difficult for him to use the telephone. Of course, you can buy a special device for the hearing impaired, but it costs a lot, and not everywhere is on sale. It is much easier to add a simple amplifier to an existing apparatus - and the problem is solved.

On fig. 1 shows a fragment of the scheme of the device "Tulip" produced by the Polish People's Republic, which turned out to be with the author.

Amplifier for telephone

A telephone line is connected to its terminals X1 and X2 (for example, from a home-made PBX or intercom) in such a way that the voltage of the specified polarity appears on the terminals (this is important for the amplifier connected to the device in the future). It is not at all necessary to measure the voltage at the terminals with a voltmeter, it is enough to measure it at pins 8 and 9, suitable for the handset. As a rule, when the handset is raised (and therefore, the contacts of the SA1 switch are closed) and the BM1 carbon microphone, it can be, according to the author’s measurements, 2,7 ... 4,5 V, but in some other devices it will increase to 8 ... 16 B. This voltage will be the power supply for the amplifier. Depending on it, you should choose the transistor of the amplifying stage.

The amplifier circuit is shown in fig. 2.

Amplifier for telephone

It is easy to see that the phone BF1 of the handset is connected to the output of the amplifying stage, and a variable resistor R10 is connected to pin 1, which was previously connected to the phone, which controls the volume of the sound when talking with the subscriber.

From the variable resistor engine, the signal enters through the decoupling capacitor C1 to the amplifying stage, made on the transistor VT1. The bias on the base of the transistor is set by resistor R2, the DC load of the cascade is resistor R3. Phone BF1 of the handset is connected to the collector of the transistor through a coupling capacitor C2. The supply voltage to the cascade is removed from the capacitor C3, which receives not only a constant voltage falling on the microphone, but also an alternating voltage rectified by the VD2 diode that appears on the microphone during a conversation. In addition, thanks to the VD2C3 chain, self-excitation of the device due to acoustic coupling between the microphone and the phone is eliminated.

Zener diode VD1 in this case acts as a maximum voltage limiter due to a possible surge in the supply voltage, for example, in the event of a microphone disconnection (due to poor contact) or its failure.

Resistor R1 - any type, its resistance should be approximately equal to the resistance of the handset, so as not to disturb the operation of the device. Capacitors - any oxide for a rated voltage not lower than that indicated in the diagram. The zener diode is chosen with a minimum stabilization voltage higher than the maximum possible supply voltage by about 10%.

When setting up an amplifier, resistor R2 is selected so that the voltage between the collector and emitter of the transistor is approximately half the supply voltage. The handset must be lifted.

With the specified refinement of the telephone set, the current consumption from the line practically does not increase.

Author: B.Sadovskov, Chelyabinsk

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