Low Power FM Transmitter Circuit Using Transistor

The circuit of the FM transmitter is shown in the figure is simple design. The first stage is the oscillator, and is tuned with the variable capacitor. This circuit is use two transistor BC549 as op amp the input signal. Select an unused frequency, and carefully adjust C3 until the background noise stops. Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

How does this circuit work? The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF. A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz, this is a benefit for a simple circuit like this, and even commercial FM is usually limited to a 15 kHz bandwidth. Q1 is the oscillator, and is a conventional design. L1 and C3 (in parallel with C2) tune the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio. This provides the frequency modulation (FM), that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver usually ± 75 kHz. With the value shown for C1, this limits the lower frequency response to about 50 Hz (based only on R1, which is somewhat pessimistic), if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15 Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarized. If polarized, the positive terminal must connect to the 10k resistor.