دارات التضخيم بالترانزستور - المضخمات السمعية .. الإلكترونيات

Transistor Amplification Circuits - Audio Amplifiers .. Electronics

Transistor Amplification Circuits

Figure (129) shows a circuit for amplifying audio signals using transistors TR1, TR2 • In this circuit, the two resistors MA, M2 are chosen so that a small current passes through the base of TR1, and so that the transistor base voltage is more negative than the emitter voltage.

As for the resistor M3, it works to increase the stability of the transistor operating conditions and the stability of the required negative bias voltage for the emitter. If the current flowing through it increases, this leads to an increase in the voltage drop across the resistor M, and thus to an increase in the negativity of the emitter, so the value of the base voltage decreases. As for the capacitor SA, it works with the resistor M3 to form a resonant circuit at operating frequencies. Any change in the base current, resulting from a change in the original signal to be amplified, leads to a similar change in the collector current. For this reason, the signal voltage appears across the load resistor M4.

Since the reactance of the capacitor S2 is very small at the audio frequency, the amplified signals can easily reach the base of the transistor M through the coupling capacitor S2 to start the next amplification stage through TR02. It is noted in Figure (130) that the potential of both the collector of TR1 and the base of TR2 are equal so that they can be connected directly. This method is called "direct coupling". Figure (131) includes three amplification stages that are also connected directly. The collector voltage in both TR1 and TR2 can be low, but this does not constitute an obstacle to obtaining good results.

129 - Two stages of the transistor amplifier.
130 - Direct connection between the collector and the base.
131 - Direct connection in a hearing aid for the hearing impaired.