# 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:

Audio pre-amplification, modulation, and transmission are all used in FM transmission. We’ve modified the same formula by first amplifying the audio signal, then using an oscillator to generate a carrier signal, and finally modulating the carrier signal with the amplified audio signal. An amplifier performs the amplification, while a variable frequency oscillator circuit performs the modulation and carrier signal production. The frequency can be set anywhere between 88MHz and 108MHz on the FM band. The FM signal from the oscillator is then boosted by a power amplifier to provide a low impedance output that is matched to the antenna.

## FM Transmitter Circuit Design:

### Design of Audio Pre-amplifier:

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: We must first compute the quiescent collector current before we can calculate the value of the load resistor. Let’s say this value is around 1mA. The collector voltage should be around half of the Vcc voltage. The value of the load resistor, R4, is Vc/Iq = 4.5K. For improved performance, we choose 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 R5 is given by Ve/Ie, where Ie is the emitter current and is approximately equal to the collector current. This gives R5 = (Ve/Ie) = 540 Ohms. Here we select a 500Ohms resistor.  It serves the purpose of bypassing the emitter current.

e) Selection of coupling capacitor, C1: Here this capacitor serves the purpose of modulating the current going through the transistor. A large value indicates low frequency (bass), whereas a lesser value increases treble (higher frequency). Here we select a value of 5 uF.

f) Microphone Resistor R1 Selection: This resistor’s purpose is to limit the current that passes through the microphone, which should be less than the maximum current that the microphone can handle. Assume a current of 0.4mA across the microphone. Rm = (Vcc-Vb)/0.4 = 18.05K is the result of this. We’ll use an 18K resistor in this case.

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:

Since we require a low power output, we prefer using a class A power amplifier with LC tank circuit at the output. The values of the tank circuit components are same as that in oscillator circuit. Here we select the biasing resistor to be about 20 K and 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 microphone’s audio signal is very low-level, on the order of millivolts. This incredibly low voltage must be boosted first. An amplified inverted signal is produced by a bipolar transistor with a common emitter configuration that is biassed to function in the class A region.

The colpitt oscillator circuit is also an important part of this circuit. This is an LC oscillator, which generates oscillations by moving energy back and forth between the inductor and capacitor. It is mostly employed in radio frequency (RF) applications.

When this oscillator is given a voltage input, the output signal is a mixture of the input signal and the oscillating output signal, producing a modulated signal.  In other words, the frequency of the oscillator generated circuit varies with the application of an input signal, producing a frequency modulated signal.

## How to Operate FM Transmitter Circuit?

The common emitter configuration of BC109 amplifies audio input from the microphone or any other device first. The coupling capacitor then sends the amplified signal to the oscillator circuit. The frequency of the signal generated by the oscillator circuit is determined by the value of the variable capacitor. The coupling capacitor couples the output signal from the transistor’s emitter to the input of the power amplifier transistor. The variable capacitor in the power amplifier portion tends to maintain an output matching that of the oscillator as the signal is amplified. The antenna subsequently transmits the boosted RF signal.

### 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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