![]() This extremely small voltage needs to be first amplified. Theory Behind FM Transmitter Circuit:Īudio signal from the microphone is very low level signal, of the order of mill volts. 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 1/4 th of the transmitting wavelength. Here we select the biasing resistor to be about 20 K and coupling capacitor of about 10 pF. The values of the tank circuit components are same as that in oscillator circuit. Since we require a low power output, we prefer using a class A power amplifier with LC tank circuit at the output. Let us select a 5 pF capacitor.Ĭ) 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.ĭ) Selection of coupling capacitor, C3: Here we select electrolyte capacitors of about 0.01 uF as the coupling capacitor.Į) 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. Since here we are using BJT 2N222, we prefer the value of C9 between 4 to 10 pF. Here we select a variable capacitor in the range 5 to 20pF.ī) Selection of Tank Capacitor, C9: This capacitor serves the purpose of keeping the tank circuit to vibrate. This gives value of C6 to be around 12pF. Here we require a frequency between 88 MHz to 100 MHz. G) Selection of Bypass Capacitor, C4: Here we select an electrolyte capacitor of 15 uF, which bypasses the DC signal.Ī) Selection of tank circuit components – L1 and C6: We know the frequency of oscillations is given by This gives the value of Rm = (Vcc-Vb)/0.4 = 18.05K. Let us assume the current through microphone to be 0.4mA. Here we select a value of 5 uF.į) Selection of Microphone Resistor R1: The purpose of this resistor is to limit the current through the microphone, which should be less than the maximum current a microphone can handle. A large value indicates low frequency (bass), whereas a lesser value increases treble (higher frequency). ![]() It serves the purpose of bypassing the emitter current.Į) Selection of coupling capacitor, C1: Here this capacitor serves the purpose of modulating the current going through the transistor. Here we select a 90K resistor.ĭ) 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. Now assume the emitter voltage to be 12% of Vcc, i.e. ![]() The voltage across the base, Vb is assumed to be 0.7V more than the emitter voltage Ve. ![]() This gives the value of Ib to be 0.008mA. Now base current, Ib is equal to the collector current divided by the current gain, h fe. The bias current is approximated to be 10 times the base current. We select a 5K resistor for better operation.Ĭ) Selection of Voltage Divider Resistors R2 and R3: To calculate the value of the voltage divider resistors, we need to calculate the bias current as well the voltage across the resistors. This gives the value of load resistor, R4 as : Vc/Iq = 4.5K. The collector voltage needs to be about half of Vcc. Let us assume this value to be about 1mA. Since V CEO for this transistor is around 40V, we choose a much lesser Vcc, of about 9V.ī) Selection of Load Resistor, R4: To calculate the value of load resistor, we first need to calculate the quiescent collector current. Here we are designing a simple single stage common emitter amplifier as the pre-amplifier.Ī) Selection of Vcc: Here we have selected the NPN Bipolar Junction Transistor, BC109. Related Post: How TV Transmitter Circuit Works? Circuit Diagram of 2 km FM Transmitter Circuit: FM Transmitter Circuit Diagram – ElectronicsHub.Org Circuit Components:ģ0 Inches Long Wire or Telescopic AntennaįM Transmitter Circuit Design: Design of Audio Pre-amplifier: The power of the FM signal from the oscillator is then amplified using a power amplifier to produce a low impedance output, matching that with the antenna. The frequency is set at anywhere between the FM frequency range from 88MHz to 108MHz. The amplification is done by an amplifier, whereas the modulation and carrier signal generation is done by an variable frequency oscillator circuit. Here we have adapted the same formula by first amplifying the audio signal, generating a carrier signal using an oscillating and then modulating the carrier signal with the amplified audio signal. Applications of FM Transmitter Circuit:įM transmission is done by the process of audio pre amplification, modulation and then transmission.Circuit Diagram of 2 km FM Transmitter Circuit:.
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