I'm an electronics student and I have been trying to understand this FM transmitter diagram (taken from electronicshub.org) for a long time. I had put efforts to learn it by surfing the web but parts of the circuit remain totally puzzling to me. It would be really grateful if anyone could explain the diagram thoroughly and as detailed as possible from top to bottom.

I even don't understand why the capacitors C2, C4 (I thought C4 should be parallel to R5 as in voltage biasing circuits bypass capacitors are used, don't know why it is in series), C5 and C9 are used in the circuit. Some websites say the capacitor C9 is used to stabilize the LC oscillation in the tank circuit but I want to know how it does so?

And what about the C8 and L2 tank, why is another tuned oscillator added at the last? Mathematical explanations are very much welcomed. enter image description here

  • 3
    \$\begingroup\$ Please respect your readers and capitalise your sentences, proper nouns and acronyms and add punctuation as per standard English rules. It will make it much easier to read. For paragraphs hit <Enter> twice. \$\endgroup\$
    – Transistor
    Sep 25, 2016 at 10:32
  • \$\begingroup\$ sorry im a newbie \$\endgroup\$
    – user124726
    Sep 25, 2016 at 10:58
  • 3
    \$\begingroup\$ Newbie to English or the site? ;^) You didn't even manage it in your comment! Anyway, Rat has fixed it for you. \$\endgroup\$
    – Transistor
    Sep 25, 2016 at 11:05
  • 4
    \$\begingroup\$ @Transistor: the perfect way to remove the incentive to do it right the next time :/ \$\endgroup\$
    – PlasmaHH
    Sep 25, 2016 at 13:49

1 Answer 1


Q1 amplifies audio with a gain of approximately R4/R5. This audio feeds Q2 (an oscillator) and modulates its base-collector miller capacitance thus producing a varying output frequency. Q3 is an RF amplifier to provide more RF signal to the antenna.

That's the intention but there is a problem (as mentioned), Q1 won't work and indeed C4 should either be shorted out or possibly bypass R5.

C5 is to remove RF artefacts from Q2's base. At RF, the miller capacitor is in series with C5 and, if the power supply is correctly decoupled that series circuit (as far as RF is concerned) appears in parallel with CF6 (the tank capacitor)

C9 is positive feedback to allow Q2 to self oscillate.

The tuned circuit on Q3 is just filtering to ensure the FM output doesn't produce harmonics that might overlap onto other stations.

  • \$\begingroup\$ Another way of looking at C5 is that it should be a short circuit to RF, (though not to audio), putting Q2 into "grounded grid" cough common base configuration, hence allowing C9 to provide positive feedback in a version of the Colpitts oscillator. C9 will be quite small. \$\endgroup\$
    – user16324
    Sep 25, 2016 at 10:24
  • \$\begingroup\$ can you provide me with more knowledge about the c9 capacitor and its working? That is really confusing me \$\endgroup\$
    – user124726
    Sep 25, 2016 at 10:47
  • \$\begingroup\$ For the miller cap to modulate it has to appear to be across the tank capacitor. C9 helps it there via the power rails. Remember, at ac and rf one power rail (0 volts) has an extremely low impedance to the other power rail. \$\endgroup\$
    – Andy aka
    Sep 25, 2016 at 11:24
  • \$\begingroup\$ C9 cannot be too high in value or it will roll-off the higher audio frequencies though. Probably about 1 nF will be about right. \$\endgroup\$
    – Andy aka
    Sep 25, 2016 at 11:26

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.