FM Transmitter Circuits: How to Build One Tailored For You

Does your engineering project need an FM transmitter circuit, but you have no idea how to make one? Then, this article is for you.

The FM transmitter circuit is a crucial part of wireless and Bluetooth communication circuit projects and carrier frequency. Plus, It’s efficient and can be tricky—because of its complex diagrams and design.

Image showing a Vector scheme of frequency modulation (FM)

Luckily, this guide will make things easy for you by breaking down the big chunks of information and frustrating diagrams.

Hence, get comfortable, and let’s dive in.

What is an FM Transmitter Circuit?

Image  Showing an FM Transmitter chip

The FM (Frequency modulation) transmitter is an electronic circuit that manipulates a carrier signal wave to transmit useful information or data.

Also, it uses a single transmitter and does not require a massive power supply to transmit audio input signals over long or short distances.

For an FM transmitter circuit to work, you need a portable audio device like an MP3 player or mobile phone.

Therefore, you can connect the transmitter to the headphone jack of your audio device and send sound signals over an FM band frequency.

Any radio station within the range of transmission can pick up any message signal.

FM Transmitters vs. AM Transmitters

AM stands for Amplitude Modulation, which means the overall signal strength, or how far the wave goes up and down, is varied to encode the information (sound) to be transmitted. In this transmission, the frequency remains constant.

But with FM (Frequency Modulation), the frequency is varied to encode the sound information for transmission while the amplitude remains constant.

A signal with frequency modulation

The broadcasting frequency is also a difference between FM transmitters and AM transmitters. AM frequencies are broadcasted in the 550KHz to 1720KHz frequency range, while FM frequencies are in the 88MHz to 108MHz range.

How Does the FM Transmitter Circuit Work?

An FM transmitter circuit handles sound input, pre-amplification, modulation, power amplification, and transmission to get the signal into the air.

An FM radio circuit board

This circuit begins with a microphone to capture the sound input, which directs the sound waves to a high-gain audio amplifier transistor to amplify the signal. You can use the BC239C bipolar NPN transistor for this section.

The signal then goes into a frequency-modulating circuit with another transistor, a variable capacitor, and an inductor. You can use the 2N5179 VHF transistor for this frequency-modulating circuit.

The next section is the oscillator circuit, which generates the carrier frequency (between 88MHz and 108MHz) using an inductor and variable capacitor. This circuit mixes the audio signal with the carrier signal to create the frequency modulation, and you can tune this frequency within the band by adjusting the variable capacitor.

Next, the modulated frequency goes into a power amplifier, which feeds the signal into the antenna for transmission via a capacitor. The power amplifier helps to create low impedance matching with the antenna.

How to Make an FM Transmitter Circuit

Image showing engineer creating an FM transmitter circuit

This section will show you how to make the most straightforward DIY FM transmitter circuit that works perfectly.

FM transmitter Circuit Production Tools

Here are the things you need to make an FM transmitter circuit:

• 9v Battery
• Variable capacitor
• Antenna
• Microphone or any other audio input
• Inductor
• Resistors and capacitors
• Transistors

Circuit Diagram and Description

If you’re new to this, circuit diagrams might look a bit confusing. But, don’t worry, we are here to make things simple. Take a look at this practical diagram of a circuit.

This circuit diagram shows an FM transmitter with a 9v power supply. Plus, the mic is the input device that receives the sound signals.

Hence, you can generate sound radio waves signals when you speak into the mic. Also, the microphone has capacitive plates that create energy from the sounds you make.

Then, it varies the audio wave at the divider junction and turns it into audio signals. After this, the capacitor (C1) cancels out the noise from the audio signal and sends it to the transistor (Q1).

The transistors send the sound signals to the LC tank circuit. Plus, the circuit is necessary because it generates the motion of fixed frequency.

The audio signal coming from the transistor will then modulate the signal of fixed frequency. Then, the modified signal transmits to the antenna, which sends the sound signal to any receiver within 30 meters.

FM Transmitter Circuit Design

The FM transmitter circuit has various designs ranging from simple to complicated. So, let’s look at two basic techniques that are common and easy to create.

Image showing a vintage device using the old FM transmitter

Image showing a modern device with the latest FM transmitter

Wireless Design

The wireless circuit design sends signals that transmit over a radio tuned to the matching frequency band.

The frequency depends on how the inductor gets placed and the values of C1, C2, and C3. Plus, you can manipulate the coil turn distance or diameter to make the perfect response over FM receivers.

A small wire antenna (about 3 inches) may be attached at the shown point to make the bug highly responsive and create distortion-free signals.

So, here’s the design of the wireless FM transmitter circuit.

One transistor design FM transmitter circuit diagram

 One Transistor Design

This design is the most straightforward transmitter circuit to make. However, its simplicity creates some certain drawbacks like:

Small transmitting range

It operates a 1.5v battery with limited capabilities

The one transistor design doesn’t use a microphone as its sound input device. Instead, its antenna serves a dual function (it detects and transmits sound vibrations). Also, it doesn’t have a frequency-determining stage. Thus, it cannot be called a tuned transmitter circuit.

Here’s what the design looks like:

Wireless FM transmitter circuit diagram

Hence, let’s look at the design of the parts you need to create an FM transmitter Circuit.

Design of Audio Pre-amplifier

This design represents a preamplifier with a simple single-stage common emitter amplifier.

• Selection of Vcc

We picked the NPN Bipolar Junction Transistor, BC109. Plus, it has a voltage of around 40V, so we selected a smaller Vcc (9V).

• Load Resistor, R4

Diagram  showing load resistance

Source - Wikimedia Commons (free to use)

Here, calculating the quiescent collector current will give you the value of the load resistor. Thus, the collected voltage should be ½ of the selected Vcc. Again, it means the value of our fixed load resistor, R4, is 4.5k. So, we chose a 5K load resistor for maximum performance.

• Voltage Divider Resistors R2 and R3

You can get the value of the voltage divider resistors by calculating the voltage across all resistors and the bias current.

Plus, the bias current has an approximate value of 10x the base current. The base current (lb) here is 0.008mA -- Therefore, our bias current is 0,08mA.

Also, the voltage across the resistors (Vb) has an assumed value of 0.7v more than the emitter voltage (Ve). So, for example, if our Ve is 12% of the Vcc (1.08v), our Vb will be 1.78v.

Hence, R2 = Vb/lbias = 22.25k. So, we chose a 22k resistor.

R3 = (Vcc-Vb/lbias = 90.1k. So, we chose a 90k resistor.

• Emitter Resistor R5

To get the value of R5, use the formula Ve/le. Le is the emitter current and has the same value as the collector current. Hence, R5 =(Ve/le) = 540 Ohms. So, we choose a 500 Ohms resistor because it can bypass the emitter current.

• Coupling Capacitor, C1

The capacitor’s purpose is to modulate the current flows through the transistor. Thus, large values show low frequencies (bass) while lesser values show higher frequencies (treble). Here, we choose a value of 5uF for our C1.

• Microphone Resistor R1

This resistor limits the amount of current that passes through the mic so it remains below the maximum of what the mic can handle. Also, if the maximum current value of our mic is 0.4 mA, then the value of Rm = (Vcc-Vb)/0.4 = 18.05k. Since it should be lesser, we choose an 18k resistor.

• Bypass Capacitor, C4

For the C4, we chose an electrolyte capacitor that bypasses the DC signal with a value of 15 uF.

Design of Oscillator Circuit

Here is the design for a simple oscillator circuit:

Tank Circuit Components- L1 and C6: We need oscillation frequency between 88 MHz to 10 MHz for this selection. Thus, we choose a capacitor in the range of 5 to 20pF. Using a 0.2uH inductor will give our C6 an approximate value of 12pF.

Tank Capacitor, C9: This capacitor aims to keep vibrating the tanj=k circuit. So, we will choose a 5pF capacitor if our value is between 4 to 10 pF.

Bias Resistors R6 and R7: Based on the calculations for bias resistors in the preamplifier design, our resistors R6 and R7 will be 9K and 40K.

Coupling Capacitor, C3: We chose 0.01 uF electrolyte capacitors for our coupling capacitor.

Emitter resistor, R8:  The emitter resistor will have an approximate value of 1K, based on the previous calculations for the amplifier circuit.

Design of Power Amplifier Circuit

FM transmitter circuits don’t require high power output—so we selected an A-class power amplifier with an LC tank circuit as our output.

Also, our tank circuit has the same values as that in our oscillator circuit. So, we choose a biasing resistor with a value of 20 K and a coupling factor of 10 pF.

Selection of Antenna

The range of our FM transmitter circuit is about 2 km, so we choose a stick antenna which is 1/4th of the transmitting wavelength. Other antenna options also include a 30 inches wire.

Detailed Steps

Here are four steps to help guide you when creating your FM transmitter circuit.

Get the Required Components

Make sure you have all the components you need before you start creating an FM transmitter circuit.

So, for this FM transmitter circuit, you’ll need the 2N3904-2 transistors, five resistors: 100k Ω-1, 100Ω-1, 1M Ω-1, 1k Ω- 1, and 10k Ω -3, one 0.1uH inductor, four capacitors: 0.1 pF - 2, 40 pF trimmer - 1, 4.7 pF - 1, 10pF -1, an antenna, one 9v battery and clip, and one PCB (Printed Circuit Board).

Make a PCB (if You Don’t Have One)

When creating an FM transmitter circuit, the PCB is a must-have. Plus, it's a layout of your circuits in physical form. So, if you can’t get one, you’ll have to make one.

Fortunately, PCBs are pretty easy to create. So, you’ll need a copper-clad, one permanent marker/printed glossy paper, ferric chloride powder, a small hand drill, and some water.

Hence, here’s what you need to do:

• Remove the dust from your copper clad with a scrubber
• Afterwards, draw the layout of the circuit with the permanent marker or iron the printed glossy paper on the clean copper clad
• Next, add some ferric chloride to a bowl of water and mix well with a small stick
• Once mixed properly, place the PCB in the solution to dissolve all unwanted copper
• After this, place the PCB in another clean bowl of water to remove the solution
• Then, clean your PCB with a dry cloth. You will notice the etched layout after you clean the permanent marker
• Finally, place the PCB on any support and drill holes into the circuits

 Circuiting

First, attach the PCB layout and the fritzing file of the circuit. Once your PCB is ready, place the components in the correct circuit and solder it.

Image Showing the soldering of the circuit

Next, make your inductor with a copper wire of 18 gauge or 22 gauge. If you use the 18 gauge copper wire, create a 4-5 turns inductor with ¼ inches (oR). For the 22 gauge, create an 8-10 inductor with ¼ inches.

After creating your inductor, make sure you solder it to the circuit.

Solder your antenna to the circuit. So, you can select an 8-10 cm hook-up wire as your antenna or use a standard antenna.

Tuning the Transmitter

Tuning the transmitter is tricky, and the process takes a while, so it requires patience and caution.

When you vary the trimmer capacitor, you can tune the transmission frequency.

So, vary the trimmer capacity slowly till you hear some distortions. Then, tune in slowly to the distortion area until your transmitter matches with the radio frequency. Afterwards, you’ll hear a clear output from the radio.

Once the tuning is over, you will have one completed FM transmitter circuit.

For a more detailed and practical construction and test, watch the video below:

https://www.youtube.com/watch?v=wslmnYMsalc

FM Transmitter Circuits with Its Special Functions

Here are some special functions of the FM transmitter circuit:

 Circuit Operation

When you turn on the FM circuit, the capacitor prevents the transistor from changing until it's charged up.

Once the 22n capacitor gets discharged, it switches off the transistors until it recharges--this procedure quickly generates frequency across the coil and sends it to the antenna for transmission.

Using Tuned Circuit

Here, the FM circuit has a frequency-determining stage (tuned circuit) built into its PCB. If you want the best performance from this circuit, use the traditional wound coil type and avoid etched antenna coils.

Incorporating Q Factor

The circuit here utilizes the “Q factor” to generate high voltages. This power comes from the capacitor and coil of the tank network. With the Q factor, the circuit has improved performance and can transmit over longer distances.

 Better Saturation Capability

A circuit with a better saturation capability has a common-emitter design that’s different from common base types. It has inductors at its base that give better saturation capability and a healthier transistor response.

Adjustable Coil Slug

This design uses a slug-based variable inductor that makes it far superior to its other counterparts. You can tune your transmitter by simply adjusting the slug core with a screwdriver. This circuit has the best transmitting range, but it’s not very stable.

Improved Stability

I mentioned the adjustable coil slug circuit was not stable—thankfully, you can improve its stability by adjusting the antenna from one part of the coil. Plus, this improves the overall performance of the circuit.

Transmitting Music

If you want music rather than eavesdropping through frequencies, then this design will intrigue you. With this transmitter, you can combine a stereo input with the source, allowing the electronic audio signal inside both radio channels to be transmitted satisfactorily. Here’s an image of the design.

Analyzing a Two Transistor Spy Circuit

When you add a second transistor to the previously mentioned single transistor FM transmitters, it will increase the design’s sensitivity to the extreme. Also, the added transistor prevents the MIC from overloading.

IC 741 Transmitter Using Wire Connection

If your project requires you to transmit sound through wires to a loudspeaker, this transmitter is for you. Here, the IC 741 serves as the preamplifier stage, and you can change its gain to fit your needs with ease.

Morse Code Transmitter

As the name implies, this design sends morse codes when you tap the switch connected to the R3. The transmitter has a very long range of a thousand miles, plus UHF and VHF band receivers can receive these codes.

Applications of FM Transmitter Circuits

FM has the advantage of having a larger signal-to-noise ratio than AM, meaning it rejects interference better than AM. Therefore, the most common application of FM transmitter circuits is in music transmission over FM radio.

Car FM modulators use this circuit to enable old radios to play audio from USB or modern phones via AUX or Bluetooth without any modification or replacement.

Car MP3 players with USB input

Home sound systems and large wireless sound systems in halls contain these circuits as well and so do correctional centers, which use them to minimize noise in common areas.

Military and police cars, taxis, ambulances, and tour van radios also utilize these circuits for communication (CB radio). Basically, most radios that you install in your vehicle for communication use frequency modulation and, hence, FM transmitter circuits.

A police car fitted with a CB radio

Advantages of FM Transmitters

Frequency modulation transmitters are easy to build, which makes their price cheap. They are also easy to use.

On the performance aspect, these transmitters are highly efficient and give you the flexibility of operating within a wide range to prevent interference. FM is better at rejecting noise than varying the amplitude in AM transmission circuits.

Disadvantages of FM Transmitters

Compared to AM transmitters, FM circuits are more complex, especially when you integrate the transmitter and receiver in the same circuits, such as in a CB radio. The transmission requires wider channels (200KHz) to make the radio wave less vulnerable to noise.

FM circuits also have a shorter range because they transmit at higher frequencies than AM.

Lastly, FM antennas must be closely placed to enable clearer signal transmission and reception.

Final Words

An important factor in deciding what FM transmitter circuit best fits your DIY projects is knowing what designs you’re capable of making. Again, there are simple designs as well as complex ones.

Some of these designs require you to be careful and patient if you want the best results. Also, follow the steps which I listed earlier to get yourself a working FM transmitter circuit.

Let us know what design worked best for you, and feel free to contact us if you need more information.