I stumbled across a relatively simple FM radio transmitter online.

credit: makezine.com Credit: makezine.com

I wanted to understand how it works (it is called "super simple" on the website), but several hours I have nothing but a vague understanding.

I believe R2, L1, C3 form an oscillator, and then the transistor Q1 is used to modulate the signal. C1 would just be used to remove any DC offset. However, this leaves R1, C2, C4, C5, and R3 and, for these components, I have no clue what they do.

Could someone please explain how this "super simple" transmitter works?


3 Answers 3


This is a variation on a Colpitt's oscillator.

L1 and C4 make up the main LC resonant circuit.

C5 is used to provide feedback to the emitter of the transistor which amplifies the signal and makes up for losses and the power radiated from the antenna.

The transistor is in a configuration known as "common base" or "grounded base" as the signal to be amplified is fed into the emitter and comes out of the collector, the base is the common terminal.

R2 and R1 provide the bias for the transistor to ensure it has the right operating conditions. R3 provides a path for the transistor current back to the battery but still allows the signal to be injected into the emitter by C5.

This circuit is somewhat unusual in that the antenna is connected to the emitter, often is connected to the collector but they both have the oscillating signal present. C3 is a decoupling capacitor to provide a stable power supply at high frequencies.

Although the frequency of the oscillation is mainly determined by L1 and C4 it is also affected somewhat by the characteristics of the transistor that change with the audio signal injected into the base of the transistor through C1. These changes modulate the frequency of the oscillation in time with the audio.


L1 and C4 are the parallel-resonant tank circuit for the RF oscillator, and Q1 is wired as a common-base amplifier that provides the gain required for oscillation, with the feedback (and output coupling) provided through C5.

In the common-base configuration, the input is applied to the emitter and the output is taken from the collector. The base is held at ground (at radio frequencies) by C2.

C3 bypasses the positive end of L1 to ground at radio frequencies, and it has negligible effect on the frequency.

R1 and R2 provide the base bias for the transistor, and R3 provides the DC path for the emitter current to ground.

R1, C1 and C2 form a bandpass filter for the audio signal. As you suspected, C1 blocks DC.

The audio signals modifies the base bias voltage, causing the operating point of the transistor to shift slightly. One effect of this is that the effective capacitance at the collector node varies slightly, causing the resonant frequency of the tank to shift with it, and this is what generates the FM signal at the output.


Q1 is the oscillator transistor. It's what provides the gain to sustain the oscillation.

The entire oscillator is formed with Q1, L1, C4, C5, R3.

C5 is what provides the feedback that keeps the oscillator running.

R1 & R2 bias the transistor in the linear region. C2 makes the base of the transistor appear to be grounded at the oscillator frequency.

C3 is a bypass capacitor that ensures the DC supply appears to be a very low impedance at the oscillator frequency.

C1 couples the modulation into the base of Q1. The varying signal at the audio input shifts the operating point of Q1 - this causes both amplitude and frequency modulation. The amplitude modulation is actually not desired but is ignored by FM receivers, at least while the transmitter is close to the receiver.

The combination of L1, C4, C5 determines the oscillator frequency.


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