2SC1946A 30 Watt VHF Amplifier
Note:
Operating an unlicensed transmitter is illegal in some countries. The circuit
presented here is for educational purposes only, the webmaster can’t be held
responsible for any mis-use regarding this circuit.
Description:
The 30 watt amplifier schematic shown below provides an appropriate power boost
with an input of 4 watt up to 6 watts. The circuit is designed to cover
88-108MHz FM Broadcast Band. However, the circuit is very stable at my place
and provides a clean-output through seven (7) element Butter-worth low-pass
filter.
Notes:
The heart of the circuit is 2SC1946A VHF RF power transistor. The transistor is
specifically designed for operation in frequencies up to 175 MHz, with very
good results.
As you can see, the power line is well decoupled. The amplifier current can be
over 5 amps. All the coils are made from 16gauge laminated wire (or Silver
copper wire can do best) and the RFC can be of HF toroid
core (as shown in the picture) or 6 holes ferrite bead.C3 and R1 forms snubber circuit while R2 and C6 prevent the amplifier from
self-oscillation at VHF, sometimes you need to add 180 ohms in parallel with
L7.That will cause the amplifier to dissipate UNDESIRABLE VHF thereby reducing
spurious level.
The photo below is 60Watts VHF power amplifier using the above circuit. Two of
2SC1946A transistors are arranged at 90 degrees to each other and their outputs
are combined using "Power Combiner Network”. It is quite difficult to
combine powers at VHF and UHF bands.
However, I recommend that hobbies should stick to single power design due to
its complicity and large rate of INTERFERENCE. (in
attempt to go for double transistors which involves power combiner network).
Since the two amplifiers are operating in different phase (out
of phase).
Tuning:
Tuning of the amplifier is not hard at all. You just have to connect the output
to a good antenna with a transmission line (RG214) of 50 ohms. First
match the output network, and then do the same to the input network for a maximum
power output. By way of adjustment, you can increase the output
at its operating frequency.
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