Speaker / microphone Transistor amplifier
Électronique / Electronic

This category of experience dealing amplifiers. As you already know, many of the projects you've worked on were the amplifiers. Then you probably already understand how this type of circuit. This project demonstrates how a speaker can be used as microphone and how a transistor amplifies the voltage hundreds of times.

The amplifier circuit is similar to those used in radios, telephones, hearing aids and sound amplifiers. In short, the speaker changes sound energy into electrical energy, which is then amplified by the transistor stage before moving on to the earphone, which converts electrical energy into sound energy. The speaker can produce electrical energy because it has all the essential parts of an electrodynamics microphone.

That is to say the wire coil which is connected to the cone paper suspended in the magnetic field of a powerful permanent magnet. As the sound waves move the cone and the coil, the movement of the coil in the magnetic field produces a voltage. This voltage has all the required features of the acoustic waves (sound).

The output transformer is used in reverse to obtain a high voltage. A transformer can be used in one direction or the other, and in this case, a voltage increase is desirable because it helps the overall voltage amplification circuit.

The transistor is in a so-called common emitter amplifying circuit (CE). This term comes from the fact that the emitter of the transistor is connected directly to the input and output circuits, that is to say that its connection is common to the input and the output.

The PNP transistor therefore requires a negative base voltage of about 0.15 volts next to the transmitter. You can remember this by looking at the schematic symbol of the transistor. The junction B-E is like a diode and is to receive a bias current in the right direction to turn the transistor on. Remember that a bias current in the right direction should always be in the opposite direction of the arrow symbol of the diode (and transistor).

Resistors 100K and 4.7K form a voltage divider across the battery to supply the polarity and magnitude required for the bias voltage B-E in the right direction. This kind of base bias is called a fixed bias voltage.

The transformer is used to the circuit collector to obtain a high impedance to without losing much power and audio frequency D.C. This and earpiece none of them support output.

To demonstrate the power of fantastic amplification of this circuit, temporarily try to move the earphone through the input B-E. The advantage of this earphone is that it can be connected anywhere in a low voltage circuit without damaging anything and without loading the circuit. So do not worry about connecting it where you like it.