FM Transmitter Variable Range

Introduction: FM Transmitter Variable Range

--by Sanjeev Kumar and Maninder Bir Singh Gulshan

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The FM transmitter (Frequency modulation) circuit is made up of a single transistor or a BJT. In wireless communication, the (frequency modulation) FM carries the data or information by changing the frequency of the carrier wave as per the information or a message signal.

The FM transmitter achieves the application of (very high frequency) VHF radio frequencies of 87.5 HZ to 108 MHZ in both transmission as well as in reception of the Signal. The FM transmitter completes the most excellent volume within less power.

The performance and working of the FM transmitter circuit is dependent upon the variable capacitor and inductor. In this article, you are going to learn that how you can prepare a FM transmitter circuit and how does it work with its various applications.

Step 2: Power Consumption

The FM transmitter is a circuit that uses a very low power to operate and does uses (Frequency Modulation) FM Waves to transmit the sound. With the help of such FM transmitters we can easily transmit the audio signals through the carrier waves with different frequencies.

The frequency of the carrier wave would be the same as for the audio signal with an amplitude. The FM Transmitter produces a range of VHF from 88 HZ to 108 MHZ.

Step 3: Block Diagram For FM transmitter circuit

Components required for FM transmitter circuit are modulator, oscillator, RF-Amplifier, Audio pre-amplifier, microphone and antenna. The Diagram shows the Block diagram for FM transmitter circuit. There are two types of frequency in the signal :

  • Carrier signal (with carrier frequency)
  • Audio signal (With audio frequency)

The carrier frequency is obtained by modulating the audio signals. The (Frequency Modulation) FM signal is obtained by differentiating the carrier frequency and by allowing the Audio frequency. The transistor is used for oscillator purpose in order to obtain Radio Frequency signal.


  1. MIC
  2. RESISTOR 4.7 k ohm
  3. RESISTOR 330 ohm
  4. Capacitor 0.001mf
  5. Capacitor 22pf – 100pf
  6. Variable Capacitor 0 – 100pf

Step 5: MIC (Microphone)

A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates is made of very light material and acts as the diaphragm. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance.


  1. 4.7k ohm
  2. 330 ohm


  1. 0.001uf
  2. 22 uf – 100 uf

Variable Capacitor: 0 – 100pf


BC547 is a NPN transistor hence the collector and emitter will be left open (Reverse biased) when the base pin is held at ground and will be closed (Forward biased) when a signal is provided to base pin. BC547 has a gain value of 110 to 800, this value determines the amplification capacity of the transistor. The maximum amount of current that could flow through the Collector pin is 100mA, hence we cannot connect loads that consume more than 100mA using this transistor. To bias a transistor we have to supply current to base pin, this current (IB) should be limited to 5mA.

When this transistor is fully biased then it can allow a maximum of 100mA to flow across the collector and emitter. This stage is called Saturation Region and the typical voltage allowed across the Collector-Emitter (V­CE) or Base-Emitter (VBE) could be 200 and 900 mV respectively. When base current is removed the transistor becomes fully off, this stage is called as the Cut-off Region and the Base Emitter voltage could be around 660 mV.

BC547 as Switch

When a transistor is used as a switch it is operated in the Saturation and Cut-Off Region as explained above. As discussed a transistor will act as an Open switch during Forward Bias and as a Closed switch during Reverse Bias, this biasing can be achieved by supplying the required amount of current to the base pin. As mentioned the biasing current should maximum of 5mA. Anything more than 5mA will kill the Transistor; hence a resistor is always added in series with base pin. The value of this resistor (RB) can be calculated using below formulae.


Where, the value of VBE should be 5V for BC547 and the Base current (IB depends on the Collector current (IC). The value of IB should not exceed mA.

BC547 as Amplifier

A Transistors acts as an Amplifier when operating in Active Region. It can amplify power, voltage and current at different configurations. Some of the configurations used in amplifier circuits are

  1. Common emitter amplifier
  2. Common collector amplifier
  3. Common base amplifier

Of the above types common emitter type is the popular and mostly used configuration. When uses as an Amplifier the DC current gain of the Transistor can be calculated by using the below formulae

DC Current Gain = Collector Current (IC) / Base Current (IB)



Step 10: Working of Simple FM Transmitter Circuit

The circuit diagram shown for FM transmitter circuit and the electronic components are the resistor, capacitor, trimmer or variable capacitor, inductor (coil), transmitter, mic, 9v of power supply or 7809 Voltage regulator (in case if you are using input voltage more than 9v) and an antenna. The mic or a microphone is assumed to grab the sound signals and there is a presence of sensor with capacitance value inside the mic. The change in pressure of air or AC signal causes in production of such capacitance.

The Oscillation circuit could be made with the help of Transistor 2N3904, Inductor and the variable capacitor. The Transistor 2N3904 used in FM Transmitter Circuit. It is a NPN transistor which is basically used for signal and voltage amplification purposes. If the current is passed through L1 Inductor and the variable capacitor, the FM Transmitter Circuit will start oscillating with the resonant to that of the carrier frequency (I.e frequency of the carrier signal).

The negative current or the negative feedback will result C2 Capacitor to the Transmitter Circuit. The oscillator is required in FM Transmitter Circuit in order to generate the radio frequency carrier waves. The transmitter circuit is capable of storing the energy for the oscillation as it is a derived from the (inductor and capacitor) LC circuits. The audio I.e input signal obtained from the mic is passed through the base of the transistor in order to modulate the output signal of the LC circuit in FM (I.e frequency modulation wave) Form.

Here, the main purpose of the variable capacitor becomes that to vary the resonant frequency in order to obtain the best FM Signal Frequency band. The modulated signal is then transmit or radiated as a radio wave with the frequency of FM frequency range. The antenna is nothing but a piece of good conductor, in our application we’d used a copper wire of 30cm long and 26 gauges thick. You can use copper wire up to 25-27 inches long as an antenna in the circuit, the length of the antenna however must be significant.

Step 11: PCB Layout for the FM Transmitter

Step 12: Application Of FM Transmitter Circuit

  • FM Transmitter Circuits are used in a sound system as a signal transmitter.
  • These are used in wireless components for vehicles and offices.
  • The circuits are used to make FM transmitters used to reduce the noise in certain places.

Step 13: Advantages and Disadvantages Of FM Transmitter Circuit


  • The FM Transmitter Circuit could be made very easily as the very common components are used which are very cheap and easily available.
  • Above circuit gives the very high efficiency in transmitting signal in a shorter range.
  • The circuit is best suited for demonstration and FM transmitter circuit project.
  • The circuit does have a large and complicated components.
  • The transmitter circuit is capable to neglect the noise signal through the amplitude variation.


  • There is a large wider channel needed.
  • With the Circuit, the transmitter as well as the receiver tends to become more difficult.
  • The received signal gets poor quality as there an interference occurs in the medium.
  • The above circuit is not good enough for high power FM transmitter circuit purpose

Step 14: Ordering the PCBs From JLCPCB

Full Process is shown using Screenshots Step-wise.
Now we have got the PCB design and it’s time to order the PCB’s. For that, you just have to go to, and click on “QUOTE NOW” button.

JLCPCB are also sponsor of this project. JLCPCB (ShenzhenJLC Electronics Co., Ltd.), is the largest PCB prototype enterprise in Chinaand a high-tech manufacturer specializing in quick PCB prototype and small-batch PCB production. You can order a minimum of 5 PCBs for just $2.

To get the PCB manufactured, upload the gerber file you downloaded in the last step. Upload file or you can also drag and drop the gerber files.

After uploading the zip file, you’ll see a success message at the bottom if the file is successfully uploaded.

You can review the PCB in the Gerber viewer to make sure everything is good. You can view both top and bottom of the PCB. After making sure our PCB looks good, we can now place the order at a reasonable price. You can order 5 PCBs for just $2 but if it’s your first order then you can get 10 PCBs for $2.

To place the order, click on “SAVE TO CART” button. My PCBs took 2 days to get manufactured and arrived within a week using DHL delivery option. PCBs were well packed and the quality was really good.

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