I am new to looking at FM transmitter and have a few questions regarding the FM Transmitter circuit below. Please break down your answers as simple as possible as I am not as well versed in circuits at this point. Thank you all in advance for your help and guidance.enter image description here

1.I understand that the base current effects the collector current but I don't understand how the frequency from the Tank circuit changes based on the signal going to the base of the transistor. Please break down in simple terms how the frequency from the tank circuit at the collector is changed to a higher or lower frequency based on the signal going to the base of the transistor. Are the 2 signals combined somehow, sorry I really don't understand how this happens.

2.My understanding is that the capacitor across the transistor C2 is for signal feedback. Is this correct? Why is signal feedback needed? Does this feedback signal need to be in phase with the tank circuit? If so, how do you know that simply placing a capacitor across the transistor will achieve an in phase feedback signal?

3.Does C1 and C3 effect the frequency of the signal going to the antenna? If so, please breakdown how they effect the frequency going to the antenna. Also, does C1 and C3 serve another purpose in this circuit? Please explain what will happen in the circuit if they were not there.

4.Please explain what is the purpose of R2 and R1. Please explain what will happen in the circuit if they were not there.

  • \$\begingroup\$ If you remove R2 and R1, the circuit will pretty much just do nothing. \$\endgroup\$
    – Hearth
    May 7, 2019 at 14:41

1 Answer 1


All 'FM' circuits of this type are horrible for beginners because the FM component is generated by parasitic effects in the transistor (Which are second order effects and not covered in beginner descriptions of transistors). You actually have to go quite a long way down the semiconductor physics rabbit hole to really understand why this sort of works.

These circuits are also frustratingly poor FM transmitters and are picky about layout even by the standards of 100MHz electronics.

C1 and C3 are bypass caps the serve to remove RF voltage difference from the points they connect.

R2 provides both mic bias and biases the transistor to a somewhat reasonable operating point, R1 is the emitter load resistor necessary to limit current and develop a point to inject the feedback.

The transistor is operated in a common base configuration with feedback into the emitter (a low impedance point).


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