IAN LANG ELECTRONICS
No3. Audio Amplifier
Let us consider the output of a microphone, which is very small, in the region of microvolts. Alone it cannot achieve a level whereby the carrier can be modulated to any appreciable depth and thus they need to be made much stronger. This is the function of the audio amplifier. In our model all stages will be voltage amplifiers, in which the full voltage of the signal leaving the output of one stage of the amplifier will be passed to the input of the next. Below is the first stage of an audio amplifier. This one uses an NPN bipolar junction transistor. This is another diagram provided by NEETS.
It works due to the fact that the varying input at the gate of the transistor (capacitively coupled) to the signal source via C1 causes a varying current at the collector, thus varying the voltage across R3 and providing an AC output signal. The gain of the transistor stipulates the magnitude of amplification. Capacitively coupling the output to the next stage ensures that only AC passes rather than DC.
The signal cannot overcome the breakover voltage of the transistor alone, and R1 is there to bias the transistor on to smooth an easy passage for the AC signal. In effect we get a varying DC which is interpreted by C3 as AC and is in opposite phase to the input signal. When constructed so, an amplifier is said to be operating under class A conditions. This is nowhere near as efficient as the class C amplifiers we looked at which amplified the RF carrier, but it does give the best gain and fidelity which is what we need for audio. If we linked several together like this, gradually increasing voltage through each stage, we could end up with a high voltage signal which is a highly magnified copy of the original. For high power we would eschew transistors and use valves, the response is not necessarily as good but their power-handling capabilities are far superior.
Lastly the audio amplifier will contain, in the early stages, a filter to eliminate higher frequencies. In AM, the frequency of the modulating signal determines the bandwidth, and in commercial broadcasting 4.5kHz is the maximum permissible frequency that can be transmitted.