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I found this simple FM transmitter circuit:

enter image description here

I built it an it works rather nicely. There are many simple FM transmitter circuits on Google, most of which look basically like this:

enter image description here

These work by modulating the base-collector capacitance of the BJT using the sound signal, which appears in parallel with the L1-VC1 resonant circuit, whose frequency changes with the capacitance.

The one I built however seems different (the top one):

1) There the Rf OUT is taken from the emitter of the BJT, while in the ones resembling the bottom picture, output is taken from the collector. The circuit on the bottom Picture is operating in common-base mode, so taking the output from the collector makes sense. But what amplifier configuration is used in the first Picture? Only time I know when the output is taken from the emitter is the common collector configuration, but that one has no voltage amplification so it seems strange that it would be used here.

2) In the bottom one, C1 and C3 are huge capacitors for the high frequency RF signal, and therefore in essence short the base of the BJT to supply, and so the base-collector capacitance appears in parallel to L1 and VC1. But what is happening in the top circuit? My quess is that C3 in the top one does the same as C3 in the bottom one, as it sits parallel to the battery (one side grounded, one side to +9V). But in the top one, we have C4 connected to ground from the collector, instead of being connected in parallel to L1. Which components form the resonant circuit in the top circuit?

Thank you!

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Concerning the radio-frequency oscillator, both circuits are the same. Components that affect frequency are the same too:

  • C4 of circuit 1 (10pf) is equivalent to VC1 of circuit 2

  • C5 of circuit 1 (10pf) is equivalent to C2 (4.7pf) of circuit 2

As you have noted, the antenna goes to different places in each circuit. Both places offer a radio-frequency signal. The emitter is a lower-impedance point. Attaching antenna there likely causes frequency to shift less than attaching to the collector.

A half-wavelength antenna wire at the collector would be appropriate. A somewhat shorter-than-half wavelength antenna would be appropriate at the emitter.

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  • \$\begingroup\$ Really good answer, straight to the point. \$\endgroup\$ – Janka Aug 23 at 21:02
  • \$\begingroup\$ Thank you for the answer! However, I fail to understand what you mean when you say the emitter is "low impedance". Why is it low impedance? \$\endgroup\$ – S. Rotos Aug 24 at 7:07
  • \$\begingroup\$ The base is grounded through a large-value capacitor as far as RF is concerned. So it is a common-base circuit for RF. High impedance at collector versus low impedance at emitter is a fundamental property of common-base amplifiers. \$\endgroup\$ – glen_geek Aug 24 at 13:21
  • \$\begingroup\$ But in common base amp output is taken from the collector. Here it is taken from the emitter. And when you say "low impedance", do you mean low impedance to ground? I still don't get why the output is taken from the emitter; the collector is the amplified output of a common base amp, so why don't we take the output from there? The emitter is the input to common base amp, so the voltage should be lower there. \$\endgroup\$ – S. Rotos Aug 24 at 17:44
  • \$\begingroup\$ I find the concept of low and high impedance points confusing. Low and high impedance between what things? \$\endgroup\$ – S. Rotos Aug 24 at 17:48
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what amplifier configuration is used in the first Picture?

Certainly not common-emitter, and C2 shorts the base to ground at RF, so by a process of elimination, common-base.

But what is happening in the top circuit? Which components form the resonant circuit in the top circuit?

Again by a process of elimination, the resonant circuit is formed by C4, C5, and L1 -- L1 is the only coil, and C4 and C5 are the only caps that won't be shorts in the FM band.

Roughly what is happening is that C4 and C5, in parallel with L1, perform an impedance transformation from the high-impedance node at the transistor collector to a lower-impedance node at the emitter. This means that even without voltage amplification there's a chance for power amplification -- which clearly must be happening, because, well, you're seeing oscillation.

I suspect that the FM modulation is coming from the same modulation of collector-base capacitance that the lower circuit shows.

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  • \$\begingroup\$ Both circuits are common-base. \$\endgroup\$ – Kevin White Aug 23 at 20:23
  • \$\begingroup\$ D'oh. Answer corrected. Thanks for bringing this to my attention. \$\endgroup\$ – TimWescott Aug 23 at 20:52
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FM radio was invented to be high fidelity. All FM radio stations use pre-emphasis to boost high audio frequencies and all FM radios use complementary de-emphasis to cut high audio frequencies back to normal and cut hiss. But these extremely simple FM transmitters are missing pre-emphasis then audio heard from them on normal FM radios will sound muffled like your stereo with its treble tone control turned all the way down.

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