4 Transistor Transmitter
Circuit Notes:
This circuit provides an FM modulated signal
with an output power of around 500mW. The input
Mic preamp is built around
a couple of 2N3904 transistors, audio gain limited by the 5k preset.
The oscillator is a colpitts
stage, frequency of oscillation governed by the tank circuit made from
two 5pF capacitors and the inductor. ( Click here for Colpitt
Oscillator Resonant Frequency Equation.)
Frequency is around 100Mhz
with values shown.
Audio modulation is fed into the tank
circuit via the 5p capacitor, the 10k resistor and 1N4002
controlling the amount of modulation. The
oscillator output is fed into the 3.9uH inductor which
will have a high
impedance at RF frequencies.
The output stage
operates as a class D amplifier , no direct bias is
applied but the RF signal developed
across the 3.9uH inductor is sufficient to drive
this stage. The emitter resistor and 1k base resistor
prevent instability and thermal runaway in this
stage.
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