FM Transmitter Circuit Providing 500mW Output

Figure 1 

The circuit was designed to produce a 500mW output power transmitter whose signal modulated by FM using four transistors.

• Transmitter – an electronic device that can produce or amplify a carrier wave signal, modulates it with a significant signal, and radiates the resulting signal from an antenna which are being utilized in television, telecommunications, and radio
• Frequency Modulation (FM) – transmits its signal or information over a carrier wave by changing its frequency but it can also be taken into account as a special case of phase modulation where the carrier phase modulation is the time integral of the FM modulating signal
• 2N3904 – NPN device designed for general purpose low power amplifier and switch applications; extends to 100 mA as a switch and to 100 MHz as an amplifier
• 7001 (NTE123AP) – a small signal bipolar transistor having an N channel polarity; collector emitter voltage of 40 V; collector base voltage of 60 V; DC current gain minimum of 20; collector emitter breakdown voltage of 40 V, and continuous collector current
• Class D Amplifier – suitable for high power and portable applications because of its high efficiency

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 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 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.

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. 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.

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 is commonly used at VHF radio frequencies for high fidelity broadcasts of music and speech. It is also used at intermediate frequencies by all analog VCR systems, including VHS, to record both the luminance and the chrominance portions of the video signal. It can also be used at audio frequencies to synthesize sound.

Class D amplifiers are widely used in audio amplifiers where a much higher frequency pulse modulated signal is achieved by converting analogue signals. Because of this, they are now being used in several audio appliances where quality is not a factor.

Source:www.zen22142.zen.co.uk/Circuits/rf/4txtr.htm