Broadband VHF RF Power Amplifier with 40W Output

Figure 1 

Boosting the output power of low power FM broadcast band exciters is the goal of the project which was based on Motorola MRF171A MOSFET.

The design of the project is based on a MOSFET device with the advantages of high efficiency, ease of tuning, and high gain. The construction was done in a small aluminum die cast box where coaxial sockets are used for the RF input and output connections. A ceramic feedthrough capacitor bolted in the wall of the box routes the power supply. The techniques of construction gave way to the prevention of RF radiation escaping from the amplifier due to excellent shielding. Significant amount of RF radiation as well as harmonic radiation could be radiated without it and could interfere with other sensitive circuits.

A 7-pole low pass filter was implemented in this design which reduces the harmonics to an acceptable level. It includes 7 reactive elements consisting of 3 inductors and 4 capacitors. Better stopband attenuation is obtained from more poles but at the expense of more passband insertion loss and increased complexity. The passband ripple is constraint to a level since this design is wideband.

Rest of the project