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What's going on in this simple FM transmitter, step by step?

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New Member

I'm trying to understand this circuit and so far I read a lot about fm transmitter circuits, but there's no clear description which explaines what's happening in the circuit since we turn it on. Which capacitors get charged first, when does the transistor open and close and why is the C2 feedback hooked up to emitter-collector, instead of the base? So I'm looking for someone who understands and can explain what's happening from powering up the circuit.

Tony Stewart

Well-Known Member
Most Helpful Member
This classic design uses the ratio of collector to emitter impedance for gain and collector to emitter with a small amount tapped to the emitter for positive feedback . the soft limiting when Vce<2V resul;ts in gain reduction to prevent saturation from reduced gain. The inductor to V+ gives a large high impedance swing at the output centered around V+.

If the cap was returned to base it would result in negative feedback and gain reduction. The condition of oscillation is 0 or 360 phase shift and gain >=1 and for sine wave oscillators Gain=1 The soft limiting gives this gain reduction. The Ratio of collector load resistance to L or C impedance magnitude is equal to the Q which determines the noise bandwidth of the oscillator.

The Base resistor provides the minimum DC current for voltage gain and oscillation while the common base capacitor lowers the impedance for the emitter and reducss supply noise.

The impedance and gain of the antenna coupling to "free space impedance" determines the loaded Q and effective radiated power.


Well-Known Member
Most Helpful Member
The simple circuit is missing "pre-emphasis" then it will sound muffled on a normal FM radio. All FM broadcast band transmitters use pre-emphasis to boost high audio frequencies then all FM radios use de-emphasis to cut the boosted highs back down to normal then any added hiss is reduced. The de-emphasis in an FM radio will also cut high audio frequencies from this simple transmitter.

Another problem with this simple circuit is that it is missing a voltage regulator. As the battery voltage drops then the radio frequency slowly changes.

Another problem is that the LC tuned circuit connects directly to the antenna so the frequency changes if something moves towards or away from the antenna.

I improved this circuit by adding pre-emphasis, adding a voltage regulator and adding a transistor between the LC tuned circuit and the antenna.


Dick Cappels

Active Member
Both the circuit you cited and audiogur's improved circuit achieve frequency modulation my varying the base voltage with the audio signal. This causes a slight modulation of the capacitance between the base and the collector (notice that the base is bypassed to ground) and the result is the resonant frequency of the collector circuit is altered as a function of the audio signal.
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