Schematics > RF circuit > AM Transmitter
AM Transmitter
Notes:
It is illegal to operate a radio transmitter without a license in most
countries. This ircuit is deliberately limited in power output but will provide
amplitude modulation (AM) of voice over the medium wave band.
The circuit is in two halfs, an audio amplifier and an RF oscillator. The
oscillator is built around Q1 and associated components. The tank circuit L1 and
VC1 is tunable from about 500kHz to 1600KHz. These components can be used from
an old MW radio, if available. Q1 needs regenerative feedback to oscillate and
this is achieved by connecting the base and collector of Q1 to opposite ends of
the tank circuit. The 1nF capacitor C7, couples signals from the base to the top
of L1, and C2, 100pF ensures that the oscillation is passed from collector, to
the emitter, and via the internal base emitter resistance of the transistor,
back to the base again. Resistor R2 has an important role in this circuit. It
ensures that the oscillation will not be shunted to ground via the very low
internal emitter resistance, re of Q1, and also increases the input impedance so
that the modulation signal will not be shunted. Oscillation frequency is
adjusted with VC1.
Q2 is wired as a common emitter amplifier, C5 decoupling the emitter resistor
and realising full gain of this stage. The microphone is an electret condenser
mic and the amount of AM modulation is adjusted with the 4.7k preset resistor
P1.
An antenna is not needed, but 30cm of wire may be used at the collector to
increase transmitter range.
Title: AM Transmitter
Source: www.electronics-lab.com
Published on: 2005-02-03
Reads: 686
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