15-10-2012, 10:32 AM
Four-Stage FM Transm itter
Four-Stage.pdf (Size: 241.67 KB / Downloads: 226)
This FM transmitter circuit uses
four radio frequency stages:
a VHF oscillator built around
transistor BF494 (T1), a preamplifier
built around transistor BF200 (T2), a
driver built around transistor 2N2219
(T3) and a power amplifier built
around transistor 2N3866 (T4). A condenser
microphone is connected at the
input of the oscillator.
Working of the circuit is simple.
When you speak near the microphone,
frequency-modulated signals are
obtained at the collector of oscillator
transistor T1. The FM signals are amplified
by the VHF preamplifier and
the pre-driver stage. You can also use
transistor 2N5109 in place of 2N2219.
The preamplifier is a tuned class-A RF
amplifier and the driver is a class-C
amplifier. Signals are finally fed to the
class-C RF power amplifier, which de-
delivers
RF power to a 50-ohm horizontal
dipole or ground plane antenna.
Use a heat-sink with transistor
2N3866 for heat dissipation. Carefully
adjust trimmer VC1 connected across
L1 to generate frequency within 88-
108 MHz. Also adjust trimmers VC2
through VC7 to get maximum output
at maximum range.