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simple am transmitter

Basic Low Power AM transmitter

This transmitter is basic but allows transmission of audio to an AM radio. It consists of an RF oscillator operating in the AM broadcast band, together with a modulator stage, which mixes the incoming audio and the RF. A signal appears on the output, which has an AM component that can be picked up on a nearby AM radio receiver.

simple am transmitter

The transmitter consists of oscillator stage Q1 and modulator/buffer stage Q2. Q1 is biased via R1, R2, and R3. L1, C3, and C4 form the tank circuit with feedback network C3-C4 providing feedback to the emitter of Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator stage Q2. Q2 is biased by base current produced by RF rectification in the base emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2 form the tank circuit for the collector of Q2. RF is taken from the junction of C9 and C10 and fed to a short-wire antenna. Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes with the RF signal in the collector circuit of Q2, producing a signal that has sum and difference frequencies if the RF carrier and AF input  along with the carrier signal.

An AM signal appears at the collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collector voltage of Q2 is needed for full modulation of the output. Because of the high level of audio needed, the modulation obtained from this circuit is somewhat limited with conventional audio sources because several volts of audio into a few hundred ohms is needed. The circuit demonstrates the principle of an AM transmitter, however, and with a suitable audio drive level, produces a well modulated AM signal.

9 Comments

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  • Ant

    I would like to build this AM transmitter circuit, could someone please tell me what is L1 450UH and L2 450UH on the schematic and maybe give me a link to a product?

  • Salut mon pote

    Hello ! Thanks for you schematic !
    Pardon pour mes fautes de grammaire ou d’orthographe.

  • yolo

    what is the exact frequency of the AM for this circuit to work? 🙂

  • Mali Harsh

    Beuty n jst awesome site ever

  • Chinmoy Mitra

    You have not answered Jim Keith’s question. What about the two series connected capacitors, which don’t seem to have any purpose? And the base of Q1 should be grounded for proper operation, otherwise the circuit may not oscillate.

    • AlbertH

      It’s a copy from another site WITH DELIBERATE ERRORS to prevent idiots copying it.

      It won’t ever work properly as drawn.

  • Jim Keith

    The oscillator section (Q1) is the basic Colpitts type, but should not the base of the transistor be bypassed to gnd via a capacitor so that it functions in the common base mode? Perhaps a connection is missing as the function of C1 & C2 in series is not apparent.

    It would not be difficult at all to add a third transistor to amplify the modulation signal, but to achieve 100% modulation, it would take an 18V supply.

    • AlbertH

      The circuit will not work as presented.

      If the oscillator worked (and it won’t because of the missing parts) the output stage will only be capable of distorted downward modulation.

      It’s a very bad circuit and cannot ever work properly.

      Once again “P. Marian” screws up!

    • Srihari Rao

      We have not used 3rd transistor because to make it a low power AM transmitter. If we use 3rd transistor we should supply still high volts.. But this is basic low power am transmitter.

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