Simple AM Transmitter

Transmitters are devices that are capable of transmitting audio as radio waves from an audio device. There are two types of transmitters; FM & AM. We have many posts on FM transmitters, so we decided to make one on AM transmitter.

AM transmitters create radio waves using Amplitude Modulation (that’s why AM). That means only the amplitude of the waves gets changed but the frequency remains the same. In this case, the frequency of career frequencies falls within a predefined range called Carrier Frequency. The carrier frequency changes from country to country.

Description

Here is the circuit diagram of a simple AM transmitter circuit that can transmit your audio to your backyard. This circuit is designed with limited power output to match the FCC regulations and still produces enough amplitude modulation of voice in the medium wave band to satisfy your personal needs. You will love this!.

The circuit has two parts, an audio amplifier and a radio frequency oscillator. The oscillator is built around Q1 (BC109) and related components. The tank circuit with inductance L1 and capacitance VC1 is tunable in the range of 500kHz to 1600KHz. These components can be easily obtained from your old medium wave radio. Q1 is provided with regenerative feedback by connecting the base and collector of Q1 to opposite ends of the tank circuit.

C2, the 1nF capacitance , couples signals from the base to the top of L1, and C4 the 100pF capacitance ensures that the oscillation is transfered from collector, to the emitter, and through the internal base emitter resistance of the transistor Q2 (BC 109) , back to the base again. The resistor R7 has a vital part in this circuit. It ensures that the oscillation will not be shunted to ground trough the very low value internal emitter resistance, re of Q1(BC 109), and also increases the input impedance such that the modulation signal will not be shunted to ground.

Q2 is wired as a common emitter RF amplifier, C5 decouples the emitter resistance and unleashes full gain of this stage. The microphone can be electret condenser microphone and the amount of AM modulation can be adjusted by the 4.7 K variable resistanceR5.

AM Transmitter Circuit Diagram with Parts List 

Am Transmitter Circuit
Am Transmitter Circuit Diagram

Notes

  • The transmission frequency can be adjusted using the variable capacitance C3.
  • Use a 200uH inductor for the L1 in the tank circuit.
  • Power the circuit using a 9V battery for noise free operation.
  • Use a 30 cm long insulated Copper wire as the antenna.

We have more Radio Transmitter Circuits that you may like to take a visit;

1. Long Range FM Transmitter

2. Single Chip FM Transmitter

3. Simple FM Transmitter

4. FM Tracking Transmitter

5. FM Telephone Bug

Author

89 Comments

  1. CHris james A. Reyes

    I just simulated the circuit using Multism 12.0 and there was no clear Amplitude Modulation . The circuit may not be working at all .

  2. sir please provide a am transceiver circuit plzzzz

  3. Please i need the Receiver circuit for this transmitter

  4. Philippe

    My simulation of this circuit don’t work… Did someone succeed in doing it ?

  5. manasa k

    sir,could u plz tell me what is the necessity of c1…how is it related withot c1 wat will happen

    • C1 is decoupling capacitor. Before feeding any signal to transistor base, the signal is decoupled so as to remove any noise present in it, if any. The circuit may not function effectively without C1, if noise dominates the signal. In that case, it will be a directly coupled circuit. Though, capacitor coupled circuit is always preferred.

  6. sharvari

    this circuit works real nicely.however its range is very small.sir could you explain how the dsb-fc generation actually happens in this circuit??

  7. karrar hussain

    hi,,,i have implemented its circuit but there is something i dont understand about ,,how i will check my output ,,??? what actually the output should be ,,,i m using L1 =220uH can it affects my circuit ???
    if u have the calculations plz share it with me ,,??
    what should be the value of Beta for both of the transistors ..plz answer my questions because i am doing it as my semester project …

  8. seetharaman

    Hi Shavari you are correct. the corrections should be “The oscillator is built around Q2”; “Q2 is provided with the regenerative feedback”; ” base and collector of Q2 to opposite ends”; “re of Q2(BC 109)”,

  9. sharvari

    sir please explain the working of the circuit.is there a mix up int the explanation?it is written that Q1 is the oscillator and Q2 as the amplifier??

  10. This ckt is nt working i hav used 220 microhenry insted of 200microhenry. . .is this right?
    Plz help m

  11. which type antenna that i want to use in this circuit…

  12. Navratna

    This circuit is not working, i hav used 470 pf instead of 500pf plz help.

  13. duedeath

    heY! can i have an AM transmitter design? i’m badly neede it.. thanks 🙂

  14. how do we make a 200microhenry??how many tuns?? and what core??
    is it ok to change the 500pF to 40pF???

  15. Is this the perfect,simple & suitable circuits for am transmitter?

  16. Plz, tell me how the amplified signal from Q1 and carrier signal from tank circuit could be modulated by Q2. I can’t understand the physical principle.

  17. I just simulated this circuit using PSPICE

    then I couldn’t find clean amplitude modulated signal.

    I used Vsin (Offset = 0, amplitude = 1mV, Frequency = 1kHz) rather than cond mic

    and I set the 4.7k potentiometer parameter (set = 0.5).

    I didn’t connect the anntena and just connect voltage marker to that node. other setting is identical to the diagram.

    The simulation result was distorted sine signal (I couldn’t find out any carrier frequency)

    Is there any problem I did?

  18. Thank you sir,But How the potentiometer works with the AM transmitter!!!

  19. Seetharaman

    Hi Azim if the supply voltage is 12volts DC ensure all capacitors are rated for 16 volts or more on the safe side. This is the simple thumb rule.

  20. Thank you Sir, But I Tell How I Understand The capacitors Voltage to The Circuit Diagram Where Not given The Voltage(Ex:C1: 2.2uF).Please Tell Me Any Website or PDF File Which is Help Me.

  21. Seetharaman

    Hi Azim all capacitors are to be rated for 12 volts or higher. Capacitor voltage will be written on the capacitor itself. if not with colour code or any alphabet refer to manufacturers catalog.

  22. Sir,I am a first year new Electrical & Electronic Engineering student. Please tell me what the volt of C1,C,C3,C4,C5,C6. And you tell me how I understand the capacitors Voltage….Please please…

  23. MRelectro

    Hi
    I just simulated this circuit using PSPICE

    then I couldn’t find clean amplitude modulated signal.

    I used Vsin (Offset = 0, amplitude = 1mV, Frequency = 1kHz) rather than cond mic

    and I set the 4.7k potentiometer parameter (set = 0.5).

    I didn’t connect the anntena and just connect voltage marker to that node. other setting is identical to the diagram.

    The simulation result was distorted sine signal (I couldn’t find out any carrier frequency)

    Is there any problem I did?

  24. Seetharaman

    Hi Dadirac7 cooling clip is clip on heat-sink used for T05 T018 packages.First shift positive of the battery to C2 R6 side of R8. This itself will increase the power. for further increase replace Q2 with 2N2218 or 2219, raise the battery voltage to 12 volt.

  25. Hi Seetharaman:
    Thanks to you I’m in tune. My question now is, whats a cooling clip? Like a heat sink atached to the transistor? I want to increase my transmiting range, can we do anything to the antenna?

  26. Seetharaman

    Hi Dadirac7 use a 15mm plastic former (PVC Tube)and close wind 100 turns of 32SWG wire (do not over lap, wind side by side without any gap) use 10 to 50PF variable trimmer to tune to high end of MW band. To tune to lower end of MW you require 500PF.

  27. Hi: I’m building the AM transmitter and I want to use it in 1460khz. Can I use a variable capacitor 10-50 uF, for C3? Is it posible really to use two 100uH RF Chokes in series to get 200uH for L1? Is there a diferent name for variable capacitor? I can’t comunicate with the people at Radio Shack or do I have to get it (variable capacitor) by mail?

  28. chavan priyanka pratap

    Good day!

    Can you please give me instructions on how to make the L1 inductor with the value 200uH?

    This would be a very big help…

  29. Seetharaman

    Hi Bale C2 & C4 are ceramic disc capacitors. C3 is a variable capacitor. C1 is electrolytic, positive terminal is to be connected to potentiometer R5.
    Hi Michael you may have to increase C4 by trial and error to get the circuit into oscillation. no other change is required.

  30. Hi
    I was wandering if C2,C4,C1 are ceramic caps or electrolytic because they do not hve the + sign on the clr.

    thanks

  31. Michael Bitter

    If I wanted to transmit a signal to 58KHZ and using the formula of Fr= (1/2pi*sqr(lc)) which would give me a C3 of 500PF and a L1 of 15mH. Is there anything else that I would need to do to the circuit to work at this frequency? Thank you.

  32. Seetharaman

    HI BARI you can use vero board. my proto was only on a veroboard. now dotted boards are available you have to link them with copper wire. that is good for even HF application.

  33. Thanks Seetharaman!! Its working! now i am going to sold it on vero board! Any suggestion?

  34. Seetharaman

    Hi Bari Being a MW frequency circuit the stray capacitance should not matter anything.

  35. Please tell me if this circuit is designed on bread board, how much its stay capacitance matters? Is it can change the frequency and through the transmission out from the band?
    Reply..
    Waiting……

  36. Seetharaman

    Hi M-E no change is required except the one suggested on 9th october.

  37. Hi!
    If i wanna make this one for 12 volts -wich resistors, etc. shuold i use?
    Thanks!
    /M-E

  38. Seetharaman

    Hi Linta the range may be between 100 to 200 ft. You can use a roof top diagonal antenna of 100ft in length with (seven strands wire)the transmitter.
    as such you can improve the power by shifting the battery from R8 Mic end to R8 RF transistor end.

  39. sir plz tell me the range for the circuit?plz
    can it be increase by using larger wire?
    plzzzzzzzz
    i am waiting?

  40. Seetharaman

    Hi Mulla kindly refer to ADMIN reply on dec 17th

  41. your formula does not have n number of turns of coil.
    can you explain?

  42. Fisch’e

    just got the circuit i hunted almost year round and thank u so much. So my problem is how would i interface with PIC to send morse code using AM band.
    you can just inbox me…matt.fisch@yahoo.com

  43. seetharaman

    Hi Snehal with 500PF you will tune to 500KHz with 10 to 50 PF you will tune to 1500KHz. Higher the capacitor lower the frequency lower the capacitor higher the frequency.

  44. what value of variable capacitor c3 should i use to obtain transmitting frequency of 500khz?

  45. seetharaman

    Hi the power can be increased by adding further RF power amplifier(complicated). a little increase can be done by raising power supply to 12volt and changing Q2 to 2N2219 with cooling clip and reducing emitter resistance to 470 ohms . By increasing C2 to 10n, you are likely to loose little HF response in the modulated Audio. By Disconnecting C1, only the carrier will be produced (blank carrier without information) sufficient to interfere with the nearby receiver not allowing it to receive the required signal (by jamming it & brick wall is not a problem for HF).

  46. Also, how can I adjust the circuit to transmit an empty signal. In other words, how can I adjust it to transmit a signal with no modulation and constant amplitude, so that any radio would just play an empty constant sound. Is it enough to just remove the condenser mic from the circuit, or are other modifications necessary. One more thing, I could only get 10 nF capacitor for C2, will the circuit still work? If not, is there any way to compensate for it?

    Thanks for the help,
    Alvaro

  47. Hello, this circuit is just perfect for a project I’m working on, but how can I increase the power output so that the signal will be able to penetrate a thick brick wall? Don’t worry about any legal issues,as my country allows the use of equipment like this for purely academic purposes.

    Cheers,
    Alvaro

  48. Seetharaman

    Hi Niki could not make out head or tail. pl give the details of the circuit on which you have commented

  49. nikiforos d.

    Circuit Description :
    ———————
    *From THE NEGATIVE POLE connect via a 10 k /1/2 watts RESISTOR TO THE “C” of the transisor
    * a 2 N 2222 SILICON n-p-n transistor.

    Initially the ” b ” OF THE transistor to be left open .

    THE ” E ” OF THE TRANSISTOR CONNECTED TO THE ( + )OF

    THE BATTERY .

    a stronf a.f. oscillation occurs, traceable throuch 2* 32 ohm via a 0.1mf fix capacitor.

    we may use crystal earphone but output very stong ,*danger for ears *, feels a series volume control with one the earphone leds absolutely nessecary e.g 50-100k.

    YES THE POLARITY IS REVERSED ,THE TRANSISTOR WORKS IN UNUSUALL mode /WORKS AS A spasistor IN A RELAXATION BEAK-DOWN MODE LIKE A U.J.T.

    I DIDNT HAVE ACCESS WITH OTHER TRANSISTOR TYPES

    LIKE E.G. BC 109 ETC….///

    HAPPY EXPERIMENTATION —NIKIFOROS D. /GRECE./ATHENS.///
    *we may experiment with E CONNECTING IT TO C OR E ALWAYS THROUGH PROTECTING CAPACITOR AND /OR RESISTANCE.

  50. hiiiii a really good circuit diagram for a am transmiter…. but i have a big problem that how to make a 200uh inductor can u help me on this .
    give me the procedure for it . plz
    just send it on my id
    rahul.bhandari71@gmail.com

    its really urgent for me
    i hope u will help me…….

    rahul

  51. in d formula
    L = (d²l²)/(18d+40l)

    where is the n used????/

  52. How does the AM modulation occur in this circuit ?

  53. we have a problem on making l1 inductor.can u help?

  54. HI, I AM GRATEFUL FOR THE CIRCUIT DIAGRAM OF AM TRANSMITTER I GOT FROM YOUR SITE BUT COULD YOU FURTHER HELP ME WITH THEIR ANALYSIS AS WELL? I WILL BE GLAD IF YOU DO. THANKS

  55. Seetharaman

    For L1 I have used 77turns of 28SWG enameled wire wound side by side on a 5/8″ dia 8″ ferrite rod. The audio quality is good. C6 I have increased to 220 mfd which improved the audio quality. Also I used 10uF for C5.

  56. kurimaw100

    hello.. I cant find a bc109c transistor do you know any alternative transistors? Tnx

  57. kurimaw100

    hello.. I cant find a bc109c transistor do of any alternative transistors? Tnx

    • admin

      Other options are BC107, BC548,BC148, 2N2222 etc.

  58. hi men , thaks for the diagram , you saved my ass XD i want to ask ou something , how could i build that inductor ? do you have some manual , i have the problem that here in my city electronic stores are very bad so they dont have any inductor XD

    Saludos desde México , oaxaca jejeje

    sorry for the grammar mistakes lol

    ED…

    • admin

      The approximate inductance of a single-layer air-core coil may be calculated from the simplified formula:
      L = (d²n²)/(18d+40l)
      where:
      L = inductance in uH,
      d = coil diameter in inches,
      l = coil length in inches, and
      n = number of turns.
      This formula is a close approximation for coils having a length equal to or greater than 0.4 d.

  59. Forgot:
    5. how do i do to connect some sound-sorce like a cd-player or my computer to the transmitter?

    • admin

      You must remove R9 and condenser mic. The sound source can be connected across to the base of Q1 and ground. A coupling capacitor of 10uF must be also added to the base of Q1.

  60. Hi!
    Some Qs:
    1. i don’t have a cond.-mic. -how do i do to use an mic. wich is build with a magnet and a coil? (i don’t remember it’s name right now)
    2.how strong is this transmitter?
    (in watts)
    3.if i don’t have the transistors bc109 -witch other can i use?
    4.how do i do to make this transmitter stronger?
    -Thank you!

  61. rhea coros

    hello…

    is it ok to use a phone plug instead of a microphine…

    thanks…

  62. rhea coros

    hello…
    is it ok to use a 5K variable resistor instead of a 4.5K?

    thanks…

  63. if u have 2 100Uh you could add them in parrallel

  64. Good day!

    Can you please give me instructions on how to make the L1 inductor with the value 200uH?

    This would be a very big help…Thanks

    sam

    • sir
      can you pls give me instuction on how to make .

      L1 make copper wire?indutor col with the value 200uh?and this transmitter range pls and on transmitter how mw,sw? transtor us bc109c .2n2219,2n2221.2n2222? Thanks

      • Hello junaid. This am transmitter circuit is work or not?