Electronic – How does this telephone circuit eliminate the sidetone

amplifieraudiotelephonetransistors

I know that the sidetone in telephones is removed by a hybrid transformer or by differential amplifier using an op-amp.

I don't know how this circuit does the job using just 2 transistors (I took the circuit from the ebook: "200 Transistor circuits,Collin Mitchell" and we practically manufactured the circuit and it worked very good ):

telephone circuit from talking electronics 101-200 transistor circuit

This is another sidetone removal circuit using op amps. Its operation is more understandable (I took it from an answer to another question):

sidetone removal circuit using Opamp

I want simple explanation of the transistorized circuit (the following description from the ebook is not sufficient ).

telephone audio circuits

Best Answer

Just an intuition, I didn't do a complete analysis:

  • The first transistor is a common collector and amplifies the microphone signal

  • The second transistor amplifies the line signal but since it has the collector driven by the first emitter it has the microphone signal subtracted (also, it drives the line with the microphone signal)

I also have a feeling that the 4k7 resistor between the microphone and the first emitter gives some feedback but I'm not sure of that

I'm interested if someone has a better analysis

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