# FM Transmitter Circuit

Here we are building a wireless FM transmitter which uses RF communication to transmit the medium or low power FM signal. The maximum range of transmission is around 2 km.

### Outline

• FM Transmitter Circuit Principle:
• Circuit Diagram of 2 km FM Transmitter Circuit:
• Circuit Components:
• FM Transmitter Circuit Design:
• Design of Oscillator Circuit:
• Design of Power Amplifier Circuit:
• Selection of Antenna:
• Theory Behind FM Transmitter Circuit:
• How to Operate FM Transmitter Circuit?
• Limitations:

## FM Transmitter Circuit Principle:

FM transmission involves several essential processes, including audio pre-amplification, modulation, and signal transmission. In our approach, we’ve adapted this procedure by initially amplifying the audio signal. Next, we employ an oscillator to generate a carrier signal, which is subsequently modulated with the amplified audio signal.

The amplification is executed by an amplifier, while the modulation and carrier signal generation are achieved through a variable frequency oscillator circuit. This oscillator allows us to set the frequency anywhere within the FM band, typically ranging from 88MHz to 108MHz. Following modulation, the FM signal is further amplified by a power amplifier to provide a low impedance output that matches the antenna requirements.

## FM Transmitter Circuit Design:

### a) Selection of Vcc:

Here we have selected the NPN Bipolar Junction Transistor, BC109. Since VCEO for this transistor is around 40V, we choose a much lesser Vcc, of about 9V.

### b) Load Resistor Selection, R4:

To determine the load resistor value, we need to start by calculating the quiescent collector current, which we’ll assume is approximately 1mA. To achieve this, the collector voltage should ideally sit at around half of the Vcc voltage. With these parameters, the load resistor, denoted as R4, can be computed using the formula Vc/Iq, resulting in a value of 4.5K. For enhanced performance, we opt for a 5K resistor.

### c) Resistors R2 and R3 of the Voltage Divider:

We must compute the bias current as well as the voltage across the resistors in order to determine the value of the voltage divider resistors. The bias current is around ten times that of the base current. Now the collector current is reduced by the current gain, hfe, to get the base current, Ib. As a result, the value of Ib is 0.008mA. As a result, the bias current is 0.08mA.

The voltage across the base, Vb is assumed to be 0.7V more than the emitter voltage Ve. Now assume the emitter voltage to be 12% of Vcc, i.e. 1.08V. This gives Vb to be 1.78V.

Thus, R2 = Vb/Ibias = 22.25K. Here we select a 22K resistor.

R3= (Vcc-Vb/Ibias = 90.1K. Here we select a 90K resistor.

### d) Selection of Emitter Resistor R5:

The value of resistor R5 can be determined using the equation Ve/Ie, where Ie represents the emitter current and is typically quite close to the collector current. This computation results in a value of R5 equal to 540 Ohms. In practice, we opt for a 500 Ohms resistor, which effectively serves the role of bypassing the emitter current.

### e) Selection of coupling capacitor, C1:

In this context, this capacitor fulfills the role of modulating the current passing through the transistor. A higher capacitance value corresponds to lower frequencies, typically associated with bass, while a lower capacitance value enhances treble, which pertains to higher frequencies. In this instance, we opt for a 5 uF capacitor.

### f) Microphone Resistor R1 Selection:

The intention behind this resistor is to restrict the current flowing through the microphone, ensuring it remains below the microphone’s maximum current capacity. Assuming a current of 0.4mA across the microphone, we can calculate the resistor value using the formula Rm = (Vcc – Vb) / 0.4, yielding a value of approximately 18.05K. In this instance, we’ll employ an 18K resistor for this purpose.

### g) Selection of Bypass Capacitor, C4:

Here we select an electrolyte capacitor of 15 uF, which bypasses the DC signal.

### Design of Oscillator Circuit:

a) Selection of tank circuit components – L1 and C6: We know the frequency of oscillations is given by

f = 1/(2∏√LC)

Here we require a frequency between 88 MHz to 100 MHz. Let us select a 0.2uH inductor. This gives value of C6 to be around 12pF. Here we select a variable capacitor in the range 5 to 20pF.

b) Selection of Tank Capacitor, C9: This capacitor serves the purpose of keeping the tank circuit to vibrate. Since here we are using BJT 2N222, we prefer the value of C9 between 4 to 10 pF. Let us select a 5 pF capacitor.

c) Selection of bias resistors R6 and R7: Using the same method for calculation of bias resistors, as in the preamplifier design, we select the values of bias resistors R6 and R7 to be 9 K and 40 K respectively.

d) Selection of coupling capacitor, C3: Here we select electrolyte capacitors of about 0.01 uF as the coupling capacitor.

e) Selection of emitter resistor, R8: Using the same calculations as for the amplifier circuit, we get the value of emitter resistor to be around 1K.

### Design of Power Amplifier Circuit:

As we aim for a low-power output, our preference lies in employing a class A power amplifier featuring an LC tank circuit at its output. The values of the components in the tank circuit match those used in the oscillator circuit. In this case, we choose a biasing resistor of approximately 20 K and a coupling capacitor of about 10 pF.

### Selection of Antenna:

Since the range is about 2 km, we can prepare an antenna using a stick antenna or a wire of 30 inches approximately which would be about 1/4th of the transmitting wavelength.

## Theory Behind FM Transmitter Circuit:

The audio signal from the microphone starts at an extremely low voltage level, typically in the millivolt range. It’s imperative to amplify this minute signal first. We achieve this by utilizing a bipolar transistor configured as a common emitter amplifier, which is biased to operate within the class A region, yielding an amplified and inverted signal.

In addition, a significant element of this circuit is the Colpitt oscillator circuit, an LC oscillator primarily utilized in radio frequency (RF) applications. This oscillator creates oscillations by transferring energy between the inductor and capacitor. When this oscillator receives a voltage input, the output signal becomes a composite of the input signal and the oscillating output signal, resulting in a modulated signal. In simpler terms, the oscillator’s frequency varies in response to the input signal, thus producing a frequency-modulated signal.

## How to Operate FM Transmitter Circuit?

The BC109 transistor, configured in the common emitter mode, initially amplifies the audio input from sources such as the microphone. Following amplification, the signal is directed to the oscillator circuit via a coupling capacitor. The frequency of the signal produced by the oscillator circuit relies on the variable capacitor’s value.

A coupling capacitor serves to link the output signal from the transistor’s emitter to the input of the power amplifier transistor. Within the power amplifier section, a variable capacitor is utilized to ensure that the output aligns with the oscillator’s frequency as the signal undergoes amplification. Subsequently, the boosted RF signal is transmitted through the antenna.

### Applications of FM Transmitter Circuit:

This circuit can be used at any place to transmit audio signals using FM transmission, especially at institutions and organizations.

### Limitations:

This circuit is for educational purposes and may require more practical approach.

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