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

  • \$\begingroup\$ which of the two circuits are you asking about? \$\endgroup\$ Mar 15, 2021 at 21:01
  • 3
    \$\begingroup\$ (1) You need to credit the authors of those two circuits if they are not your own. This is site policy. (2) Who says the first circuit removes sidetone? Please edit to fix. \$\endgroup\$
    – Transistor
    Mar 15, 2021 at 21:03
  • \$\begingroup\$ @MarcusMüller I want to know the operation of the first circuit , the second one is obvious \$\endgroup\$
    – Zaki
    Mar 16, 2021 at 5:15
  • 1
    \$\begingroup\$ @Hearth I actually soldered the circuit and it worked!!!! \$\endgroup\$
    – Zaki
    Mar 16, 2021 at 5:20
  • 1
    \$\begingroup\$ @Lorenzo Marcantonio, I remember I was interested in this as a student when I made my own answering machine... \$\endgroup\$ Mar 19, 2021 at 16:52

1 Answer 1


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

  • \$\begingroup\$ Lorenzo Marcantonio, Congratulations for the attempt to explain the (second) circuit! Only I think the first emitter is fixed at 5.1 V so the second collector is not driven by it. Regarding the 4k7 resistor, I think its role is to connect galvanically the lower plate of the 100n capacitor to the 5v1 Zener diode. Today I lost a lot of time thinking about the two circuits, but I never found them doing what was announced... \$\endgroup\$ Mar 17, 2021 at 18:03
  • 1
    \$\begingroup\$ It's possible that 5.1V is the baseline on top of which is developed the microphone signal to have enough collector juice for the second bjt. Could be that. \$\endgroup\$ Mar 18, 2021 at 7:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.