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.
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
- 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;