Attached to the circuit is a
preamplifier input microphone that is developed within the area of 2N3904 transistors having an audio gain that was preset to the threshold of the 5K ohm trim potentiometer.
This potentiometer will supply a small percentage of modification which is often used with an uneven control. The capacitors have a common connection to the emitter circuit since it is a
transistor version but for electron tube version, common connection to the cathode. This tank circuit controls the frequency of the oscillation. The output of the oscillator is supplied to the 3.9 uH inductor which will obtain high impedance when the circuit is tuned to RF frequencies. The oscillation of the circuit is being defined the parallel tuned tank circuit known as the colpitts oscillator which comprises of two voltage dividing ceramic capacitors (5 pF) in series and an inductor.
The 2N3904 FET transistors were mainly used as switching transistors for use in pulse and square wave applications, as general purpose amplifier, in flyback converter for auxiliary power and charging applications, simple current limiting power supply, current sensing low side MOSFET driver, in high precision comparator using op-amp, protected high or low side MOSFET driver, and in small signal transistors. The 7001 or NTE123AP Silicon NPN transistor is used in audio amplifier and switch.
Fm Transmitter 2N3904 Circuit
There is no direct bias or DC voltage applied to the transistor to determine the desired operating point. On the contrary, the RF frequency acquired by the 3.9 uH inductor is just enough to energize this stage. To prevent instability and drastic thermal changes, the 7001 transistor utilizes its emitter resistance and the 1K ohm resistor. Using the values of the circuit components, the frequency will be around 100 MHz. Through the 5 pF capacitor and 10K ohm resistor, the modulation of audio signal is supplied to the tank circuit. The amount of modulation is being managed by the 1N4002, a general purpose rectifier diode. The output stage is functioning as a class D amplifier where the output transistors act as a switch.