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Two-Power Supply Differential Amplifier
Électronique / Electronic

This is the last in the series of microphone amplifiers. Now we use the operational amplifier as a differential amplifier. It is a two-power source type amplifier, and this time we use the speaker as a microphone. Slide the switch to position OFF and assemble the circuit. When you finish the wiring, apply the earphone to your ear, slide the switch to position ON to turn on the power, and tap the speaker lightly with your finger. Can you hear the tapping sound from the earphone?

The dual operational amplifier works as a differential amplifier when two inputs are applied at the same time to its positive (+) and negative (-) terminals. The transformer plays an important role in this amplifier circuit. Its two different outputs from out terminals of the transformer supply the opposite inputs for terminals 2 and 3.

You should remember that the speaker has a coil and a magnet inside. When used as a normal speaker, electricity flows through the coil, and a magnetic field is created around the coil. The magnet attracts or repulses the coil depending on the magnetic field created by the coil. So the coil moves, this movement of is transferred to the cone paper attached to it and this creates the sound you hear.

Here, the speaker is used as microphone, so the opposite takes place. When sound makes the coil move, the distance from the magnet changes, and the strength of the magnetic field changes so that the voltage appears at both end of the coil. This small voltage is applied to the primary of the transformer, which then results in larger voltage at the secondary side of the transformer. So using the microphone simplifies the circuit. In order to use the earphone as in previous projects see OP_AMP section, we would need to make a far more complex circuit.








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