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FM Transmitter Circuit
September 16, 2015 By Administrator(https://www.electronicshub.org/author/elktros/)

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:

FM transmission is done by the process of audio pre amplification, modulation

and then transmission. 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. The
amplification is done by an amplifier, whereas the modulation and carrier signal
generation is done by an variable frequency oscillator circuit. The frequency is
set at anywhere between the FM frequency range from 88MHz to 108MHz. 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.

Related Post: How TV Transmitter Circuit Works (/tv-transmitter-

circuit/)?

Circuit Diagram of 2 km FM Transmitter Circuit:

antenna

(https://www.electronicshub.org/wp-content/uploads/2014/03/Circuit-Diagram-
of-FM-Transmitter.jpg)
FM Transmitter Circuit Diagram – ElectronicsHub.Org
(https://www.electronicshub.org/)

Circuit Components:

COMPONENT
VALUE
NAME

R1 18K
R2 22K
R3 90K
R4 5K
R5 540 Ohms
R6 9K
R7 40K
R8 1K
R9 20K
C1 5uF, Electrolyte
C2 47uF, Electrolyte
C3 0.01uF, Electrolyte
C4 15uF, Electrolyte
C5 0.01uF, Ceramic
C6 20pF, Variable Capacitor
C7 10pF, Ceramic
C8 20pF, Variable Capacitor
L1, L2 0.2uH
30 Inches Long Wire or Telescopic
Antenna
Antenna
V1 9V Battery
:
 Audio Input Microphone

FM Transmitter Circuit Design:

Design of Audio Pre-amplifier:

Here we are designing a simple single stage common emitter amplifier as the
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) Selection of Load Resistor, R4: To calculate the value of load resistor, we

first need to calculate the quiescent collector current. Let us assume this value
to be about 1mA. The collector voltage needs to be about half of Vcc. This gives
the value of load resistor, R4 as : Vc/Iq = 4.5K. We select a 5K resistor for better
operation.

c) 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. The bias current is approximated to be 10 times
the base current. Now base current, Ib is equal to the collector current divided
by the current gain, hfe. This gives the value of Ib to be 0.008mA. The bias
current is thus 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) 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. Let us assume the current through
microphone to be 0.4mA. This gives the value of Rm = (Vcc-Vb)/0.4 = 18.05K.
Here we select a 18K resistor.
:
g) Selection of Bypass Capacitor, C4: Here we select an electrolyte capacitor of
15 uF, which bypasses the DC signal.

[Also Read:How to build Adjustable Timer

(https://www.electronicshub.org/adjustable-timer/)]

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 (https://www.electronicshub.org/150-watt-power-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
(https://www.electronicshub.org/basic-electrical-circuits-componentstypes/) 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:

! signal from the microphone is very low level

Audio " signal, of the order of mill
(https://www.electronicshub.org)
volts. This extremely small voltage needs to be first amplified. A common
emitter configuration of a bipolar transistor, biased to operate in class A region,
produces an amplified inverted signal.

Another important aspect of this circuit is the colpitt oscillator circuit. This is a
LC oscillator where energy moves back and forth between the inductor and
capacitor forming oscillations. It is mainly used for RF application.
:
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?

Audio input from the microphone or any other device is first amplified using the
common emitter configuration of BC109. This amplified signal is then given to
the oscillator circuit through the coupling capacitor. The oscillator circuit
generates a signal with a frequency determined by the value of the variable
capacitor. The output signal from the emitter of the transistor is coupled to the
input of the power amplifier transistor using the coupling capacitor. As this
signal is amplified, the variable capacitor in the power amplifier section tends to
maintain an output matching with that of the oscillator. The amplified RF signal
is then transmitted using 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.

Related Post: FM Bugger Circuit

(https://www.electronicshub.org/fm-bugger-circuit/)

Related Posts:
What is Attenuation? How to Measure it? Attenuation in…
(https://www.electronicshub.org/attenuation/)
What is a Power Amplifier? Types, Classes, Applications
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Pcm Vs Bitstream - Which One is Better?
(https://www.electronicshub.org/pcm-vs-bitstream/)
Pre Out Vs Line Out - Find the Di!erence?
(https://www.electronicshub.org/pre-out-vs-line-out/)
Bluetooth Audio Codecs | Terminology & Types
(https://www.electronicshub.org/bluetooth-audio-codecs/)
Wireless Communication: Introduction, Types and Applications
(https://www.electronicshub.org/wireless-communication-introduction-
types-applications/)
:
 55 Responses

July 8, 2014 at 10:10 am (https://www.electronicshub.org/fm-transmitter-

comsianx circuit/#comment-65353)
says:

how i made L1 and L2 ?

Reply

July 16, 2014 at 6:49 am (https://www.electronicshub.org/fm-transmitter-

Administrator circuit/#comment-68011)
says:

They are available in market by built.

Reply

August 16, 2014 at 10:32 pm (https://www.electronicshub.org/fm-transmitter-

RF MasterX circuit/#comment-89541)

says:

C4 & R5 it should be parallel, not serial.

Reply
:
 August 23, 2014 at 10:56 am (https://www.electronicshub.org/fm-transmitter-
Mustafa circuit/#comment-91588)
says:

can u make the audio input a 3.5mm jack female

Reply

August 26, 2014 at 12:22 pm (https://www.electronicshub.org/fm-transmitter-

preetha circuit/#comment-92793)
says:

i am doing my mini projct with this circuit,,will it works correctly?if any

corrections required plz infrm me,,its lil urgent fr me,,cn u?

Reply

January 18, 2016 at 7:15 am (https://www.electronicshub.org/fm-transmitter-

Shekar circuit/#comment-199609)
says:

did you do the project on this ckt..? did it work..?

Reply

March 5, 2017 at 11:33 pm (https://www.electronicshub.org/fm-transmitter-

vijayakumar circuit/#comment-259356)
says:

hi preetha how to increase the transmit range

Reply
:
 September 12, 2014 at 4:31 pm (https://www.electronicshub.org/fm-transmitter-
sai mukund circuit/#comment-97420)

says:

which power transistor i have to take

Reply

September 17, 2014 at 12:45 pm (https://www.electronicshub.org/fm-transmitter-

circuit/#comment-98767)

udayakumar.p says:

iam doing Fm transmitter in what frequency it will transmit .only in tat part we r
stuck plz admin say in which freq

Reply

September 19, 2014 at 1:58 pm (https://www.electronicshub.org/fm-transmitter-

Sunny circuit/#comment-99715)

says:

what about the voltage rating of the capacitors and if it did not work or one of
the transistor start getting hot what should i do.

Reply
:
 September 20, 2014 at 4:18 am (https://www.electronicshub.org/fm-transmitter-
Abhi circuit/#comment-99991)

says:

Hi,
Can you provide some more info on the components? Like C1 = 5uF but what
about volts? Same with R1, it is 18K but what about watts? Is it 1/4 watts? When
I go to an online site to buy these components I see additional Volts and Watts,
not sure what to choose.

I am new to making electronic circuits so my questions might sound vague.

Looking forward to some help.

Thanks

Reply

March 13, 2018 at 1:08 pm (https://www.electronicshub.org/fm-transmitter-

ranjan banik circuit/#comment-310797)
says:

all R/k you can use 1/4 watt.

Reply
:
 September 27, 2014 at 9:31 am (https://www.electronicshub.org/fm-transmitter-
Faisal circuit/#comment-101808)
says:

How to make L1&L2 using copper wire.(What my question exactly is that that,
how many turns do i take and what should be the diameter of the coil.)Please
help.

Reply

September 27, 2014 at 9:35 am (https://www.electronicshub.org/fm-

Faisal transmitter-circuit/#comment-101810)

(https://www.yahoo.com)
says:

What should be the diameter of the coil L1&L2 and how many rounds should i
take on the rod of that diameter.Please help.

Reply

October 2, 2014 at 12:51 pm (https://www.electronicshub.org/fm-transmitter-

Junior circuit/#comment-103004)

says:

Please, what is the value of Q1 and Q2

Reply
:
 October 7, 2014 at 3:05 pm (https://www.electronicshub.org/fm-transmitter-
partha circuit/#comment-104338)
says:

i dint find any of the range of L1 and L2…..pls help me….

Reply

November 8, 2014 at 2:37 am

mintu (https://www.electronicshub.org/fm-transmitter-
circuit/#comment-109643)

(https://www.electronicshub.org/fm-
transmitter-circuit/) says:

0.01uF, Electrolyte i think this is ceramic capacitor not electrolyte ?

Reply

November 24, 2014 at 4:34 pm (https://www.electronicshub.org/fm-transmitter-

Jose Eduardocircuit/#comment-111062)
says:

Hi, what transistors do I use as Q1 Q2 and Q3??

Reply

April 6, 2018 at 8:18 am (https://www.electronicshub.org/fm-transmitter-

Qais circuit/#comment-311789)

says:

search for high frequency transistors, you can use bf199

Reply
:
 November 23, 2015 at 2:49 am (https://www.electronicshub.org/fm-transmitter-
Vedant circuit/#comment-191572)

says:

What about Q3?

Reply

June 13, 2016 at 6:05 am (https://www.electronicshub.org/fm-transmitter-

sai kumar circuit/#comment-221436)
says:

what to be used as Q3 ??

Reply

November 23, 2015 at 3:47 am (https://www.electronicshub.org/fm-transmitter-

Vishal circuit/#comment-191581)
says:

How to change the frequency….means a desired frequency

and how to increase the range…

Reply

November 25, 2015 at 12:27 am (https://www.electronicshub.org/fm-transmitter-

sakshi circuit/#comment-191860)

dhanmeher
says:

which transistors have been used??

Reply
:
 December 5, 2015 at 2:10 am (https://www.electronicshub.org/fm-transmitter-
fahim shah circuit/#comment-193144)

says:

Can u please tell me what will be the tune (carrier) frequency of this circuit??

Reply

January 21, 2016 at 4:41 am (https://www.electronicshub.org/fm-transmitter-

Anusha circuit/#comment-199940)

says:

You can calculate using formula 1/2π√LC for L1,C6

Reply

January 25, 2016 at 3:36 am (https://www.electronicshub.org/fm-transmitter-

Balraj circuit/#comment-200765)
says:

We use some old radio parts

Reply

February 15, 2016 at 2:21 pm (https://www.electronicshub.org/fm-transmitter-

Jakir circuit/#comment-203265)

says:

What is value of transistot Q3???

Reply
:
 February 18, 2016 at 12:22 pm (https://www.electronicshub.org/fm-transmitter-
Roja circuit/#comment-203674)

(https://Gmail)
says:

Which type of antenna should I use for smaller distances??

Reply

March 20, 2016 at 1:27 pm (https://www.electronicshub.org/fm-transmitter-

Divyanshu circuit/#comment-208479)

Ranjan says:

i am making mini projct with this circuit will it works correctly?if any corrections
required please inform me. As soon as possible….

And what’s types of transistor (Q2 & Q3) are use here ..

Can you make L1&L2 if yes …

How??????

Please reply fast

Reply

April 17, 2016 at 2:38 am (https://www.electronicshub.org/fm-transmitter-

jiggo circuit/#comment-212755)

says:

how can i connect the audio input in this circuit?????

Reply
:
 April 24, 2016 at 11:11 pm (https://www.electronicshub.org/fm-transmitter-
mohammed circuit/#comment-214339)

says:

plz what about Q1,Q2&Q3????

Reply

April 26, 2016 at 7:48 am (https://www.electronicshub.org/fm-transmitter-

imran circuit/#comment-214574)

says:

Hi.. Please tell me q1 q2 q3 and l1 l2

Reply

July 3, 2016 at 2:38 pm

muhammad bashir umar (https://www.electronicshub.org/fm-transmitter-
circuit/#comment-223962)
(https://sunnah441@gmail.com)
says:

I want no how to make a transmitter.

Reply

July 13, 2016 at 3:58 am (https://www.electronicshub.org/fm-transmitter-

Parban circuit/#comment-224878)
says:

???? Hy give me your Q1,2,3,transistor name plz……

Reply
:
 July 22, 2016 at 6:58 am (https://www.electronicshub.org/fm-transmitter-
sairaj circuit/#comment-225630)
says:

Transistors names plzz….

Reply

July 27, 2016 at 5:53 am (https://www.electronicshub.org/fm-transmitter-

uday kiran circuit/#comment-225935)

says:

Q1 bc109 and Q2 2n222 and q3 idk..

Reply

July 27, 2016 at 5:51 am (https://www.electronicshub.org/fm-transmitter-

uday kiran circuit/#comment-225934)

says:

please provide us the value of transistor Q3…..the values of Q1 and Q2 are

given as BC109 and 2N222 or 2N3904…

Reply

August 10, 2016 at 12:18 pm (https://www.electronicshub.org/fm-transmitter-

aman circuit/#comment-227761)
says:

what is the value of other two transistors Q1 and Q2

Reply
:
 September 21, 2016 at 10:31 am (https://www.electronicshub.org/fm-transmitter-
teja circuit/#comment-234243)

says:

can u specify the transistors Q1,Q2,Q3? which one u have used in this circuit?

Reply

October 5, 2016 at 7:41 pm (https://www.electronicshub.org/fm-transmitter-

Breakcal circuit/#comment-237019)
says:

how to calculate the output impedance of that circuit?

Reply

January 3, 2017 at 9:15 am (https://www.electronicshub.org/fm-transmitter-

Kelvin L circuit/#comment-251372)
says:

How can we add audio input from mp3 prayer to this circuit?

Reply

April 1, 2017 at 5:08 am (https://www.electronicshub.org/fm-transmitter-

V.VARUN circuit/#comment-263602)
KUMAR says:

R2 and R3 values should be interchanged i.e R2=90K and R3=22K. The formula
used is also exactly reverse.

Reply
:
 April 3, 2017 at 12:01 pm (https://www.electronicshub.org/fm-transmitter-
unknown circuit/#comment-263925)

says:

what is the bandwidth of this transmitter

Reply

April 3, 2017 at 12:03 pm (https://www.electronicshub.org/fm-transmitter-

unknown circuit/#comment-263926)

says:

what frequency it transmits?

Reply

April 21, 2017 at 4:39 am (https://www.electronicshub.org/fm-transmitter-

Sunil Shah circuit/#comment-265919)
says:

I am thinking of doing this project. Please tell me the values of Q1, Q2 and Q3.
Also suggest any other correction requited. Please reply fast..

Reply

April 28, 2017 at 6:54 pm (https://www.electronicshub.org/fm-transmitter-

Suleiman Itakure circuit/#comment-266773)

Asmau says:

WHAT IS THE GAIN AT EACH STAGE?

Reply
:
 June 13, 2017 at 12:46 pm (https://www.electronicshub.org/fm-transmitter-circuit/#comment-
ola 272329)

says:

hi, I am working on a transmitter project in my school and I want to confirm if

this circuit has been tested working before proceeding to get the parts.
pls your urgent reply will be really appreciated.
thanks

Reply

June 20, 2017 at 5:49 am (https://www.electronicshub.org/fm-transmitter-

Anusha circuit/#comment-273466)

says:

We have not tested this circuit.This circuit is designed based on theory

Reply

June 22, 2017 at 9:58 am (https://www.electronicshub.org/fm-transmitter-

mahmud circuit/#comment-273830)
says:

What the value of q3

Reply
:
 August 22, 2017 at 9:33 pm (https://www.electronicshub.org/fm-transmitter-
Rahul pandey circuit/#comment-284257)
says:

Which type of resistor and capacitors is used in 2 km fm range

transmitter.Please send me the picture of resistor and capacitor picture

Reply

January 9, 2018 at 8:28 am (https://www.electronicshub.org/fm-transmitter-

manoz circuit/#comment-306666)

says:

name of transistor

Reply

January 12, 2018 at 5:29 pm (https://www.electronicshub.org/fm-transmitter-

Farhan Sheikhcircuit/#comment-306966)
says:

how much power will be consumed for 2 km transmission ?

Reply

May 1, 2018 at 5:25 pm (https://www.electronicshub.org/fm-transmitter-

MASUM circuit/#comment-312768)
says:

how I receive this RF signal. which receiver circuit I use?

Reply
:
 May 6, 2018 at 1:34 pm (https://www.electronicshub.org/fm-transmitter-
hamza circuit/#comment-312933)
says:

How can i Check that this circuit works? or which software will help me
simulate this?

Reply

March 15, 2022 at 12:45 pm (https://www.electronicshub.org/fm-transmitter-

Kishore circuit/#comment-2456226)
says:

What is the value of C9?

Reply

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