CONSTRUCTING FM RF AMPLIFIERS
RF
Amplifiers are somewhat easy to build provided certain guidelines
are followed. Component leads must be kept as short as possible.
The transistor is soldered in last after it has been bolted down to
the heat sink with a thin coating of heat sink thermal compound
between the mounting area of the transistor and the heat
sink.
A number of amplifier designs are presented. These amplifiers do
not necessarily need a pre-made printed circuit board although
either full kits or just the circuit boards can be ordered from
Free Radio Berkeley. Cut a piece of doubled-sided (copper foil on
both sides) fiber glass (G-10 or equivalent) circuit board to the
dimensions given in the construction diagram. Then cut out smaller
pieces of circuit board for the individual mounting pads as show in
the diagram. Try to cut them to about the same dimensions of the
pads on the diagram. The pads are glued down on the larger board,
duplicating the layout in the diagram.
The first amplifier is a 6-8 watt no-tune, broad band design. It is
very easy to build and use. Unfortunately, the transistor, 2SC1971,
in this design is destined to be discontinued sometime in 2003.
Supplies should still be available for some time after that. This
amplifier requires 1⁄2 to 1 watt of drive, operating at 13.8
volts DC.
The last design is a 100 watt amplifier which uses an RF power FET,
MRF173. This amplifier requires 4-5 watts of drive power at 28
volts DC for 100 watts of output power. When used with the 6-8 watt
amplifier, lower the drive power to the 6-8 watt amplifier to bring
the output power down to 4-5 watts. This can be done with a T-pad
at the input of the 6-8 watt amplifier. A modification can be made
to the FRB PLL exciter for variable output power. Contact FRB for
details.