Crown FM150, User Manual

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4.11 RF Driver (FM150/FM300)
The RF Driver assembly is mounted on a 100 mm x 100 mm plate in the under side of the
chassis.
Illustration 6-14 and accompanying schematic complement this discussion.
The driver amplifies the approximate 20 milliwatts from the frequency synthesizer to about 8
watts to drive the RF power amplifier. An MHW6342T hybrid, high-gain, wideband amplifier,
operating at about 20 volts, provides about one watt of drive to a single BLF245 MOSFET
amplifier. The BLF245 stage operates from a supply voltage of approximately 20 volts.
The circuit board provides for input/output coupling and for power supply filtering.
4.12 RF Amplifier (FM150/FM300)
The RF power amplifier assembly is mounted on back of the chassis with four screws, lo-
cated behind an outer cover plate. Access the connections to the module by removing the
bottom cover of the unit. The RF connections to the amplifier are BNC for the input and out-
put. Power comes into the module through a 5–pin header connection next to the RF input
jack.
Illustration 6-12 and accompanying schematic complement this discussion.
The amplifier is built around a ST MicroElectronics SD2942, a dual power MOSFET rated
for 50 volts DC and a maximum power of about 350 watts. When biased for class B, the
transistor has a power gain of about 20 dB. (It is biased below class B in the transmitter.)
Input transformer, T11, is made up of two printed circuit boards. The four-turn primary board
is separated from the one-turn secondary by a thin dielectric film. R12–R17 are for damp-
ing. Trim pot R11 sets the bias.
Output transformer, T21, has a one-turn primary on top of the circuit board and a two-turn
secondary underneath. Inductors L21 and L22 provide power line filtering.
4-13 Principles of Operation