An RF (Radio Frequency) linear power amplifier is used to amplify an RF signal. Its output is proportional to its input, but capable of delivering more power into a load. Here, we present an FM linear, which is capable to deliver about 50W into a 50 ohm load at 88-108MHz (FM Band/ Europe).
The 50W FM-linear is based on the BLW85, NPN silicon RF-power transistor from ASI. The BLW85 is designed for class-C amplifier Applications in VHF transmitters.
The BLW85 linear amplifier operates at class-C and its efficiency is about 60%. The transistor conducts through less than half the RF cycle and behaves quite nonlinear producing some pulses. The tuned filters transform the pulses into sinusoids and restore linearity.
The BLW85 linear amplifier can be powered from a 12-15V power supply unit and absorbs a maximum current of about 9A. The power gain is about 9db (at 108 MHz) and the amplifier delivers about 50W into its load, when driven from an 8-12W RF-source.
The circuit schematic is quite simple. However, take some extra care for providing good grounding and adequate heat sinking. Experienced RF-hobbyists will face no problem at all, during tuning. The use of an RF power-meter and a dummy load, would be quite helpful during the tuning procedure. Tuning is performed by adjusting the trimmer-capacitors for delivering maximum RF-power into the output.
The amplifier was designed for operation inside the FM band (88-108MHz). By scaling the inductors and the capacitors appropriately, the circuit can be easily modified to operate at higher or lower frequencies. The BLW85 can operate well, up to 200 MHz.
The prototype was built on an epoxy FR4 circuit board (1.5mm thickness, er=4.35 – 4.5). The bottom side of the PCB serves as a ground plane and it is directly connected on the transistor’s body and the heat sink. All the components are soldered on the top side of the PCB. Some copper-patches on the top are grounded (connected to the bottom side) threw rivet-vias.
Posted by J.L.
PCB Artwork by A.G
Printed Circuit Board details
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