This idea is used in old days. Old engineers could not get the good audio transistors. Therefore , they tried to construct an audio power amplifier with a chap switching transistor.
I tried to use this method with a favorite 555 type IC. See the circuit.
LM308 is an operational amplifier. The gain of this amplifier is 36dB.
NE555 is used as a free-running oscillator. The oscillated frequency is 166kHz. This frequency is too high to be heard by the year of the man. In normal oscillator , the time-controlling capacitor “0.0022uF” is connected to the ground. But in this case, it is connected to the output of the audio amplifier. Therefore the width of the pulse is modulated by the audio signal. And it is heard by the speaker.
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