Here, we present a 50W FM-linear which is based on the BLW86. BLW86 was produced by Philips until 2006. Nowadays, BLW86 is still in production from ASI. ASI (Advanced Semiconductor, Inc.) manufactures a line of RF power transistors which replace those from Motorola, Philips, and SGS Thomson.
The BLW86 is designed for class-C amplifiers in VHF applications up to 175 MHz. Although BLW86 is still in production there is not much bibliography about practical applications around it. On March 1998, Application Note 98031 was published from Philips Semiconductors. AN98031 described how to build a wideband 300W push-pull FM amplifier using BLV25. For driving that FM amplifier, a specific driver amplifier circuit was proposed on page 21. That driver amplifier uses the BLW86 power transistor and it is able to deliver over 50W of RF power, on a 50 Ohm load, when driven from a 5W FM exciter. The circuit schematic and the PCB layout of the 50W FM Linear Amplifier are presented below.
The BLW85 linear amplifier operates normally when powered from a 28V power supply unit. The current consumption, for 50W output, is about 3A. The power gain is about 10-13db, inside the 88-108MHz Band (FM Band/ Europe) and the efficiency approaches 70%.
The prototype (suggested from Philips) uses an epoxy fibre-glass, er=4.5, printed board. The dielectric thickness is 1/16inch (1.59mm). The bottom side of the PCB is copper-covered and 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.
Printed Circuit Board and Components Placement Guide