A Push-Pull 300-Watt Amplifier For 81.36 MHz

By Richard Frey, Senior Applications Engineer
Microsemi Corporation

This design uses low cost power FETs that bridge the gap between typical power devices and specialized RF devices.

Power amplifiers for 80 MHz have typically used expensive ceramicmetal packaged RF devices. The use of an inexpensive TO-247 packaged transistor with an innovative internal device connection provides the basis for the cost effective design presented here. This article describes a 300 watt amplifier for 81.36 MHz using a push-pull pair of plastic packaged devices. The design techniques and construction practices are described in enough detail to permit duplication of the amplifier.

Devices used in the amplifier The devices used in the amplifier are the ARF449A and ARF449B symmetrical pair (see photo). These devices are targeted for high voltage, single frequency, class C operation. The operating voltage for the amplifier will be 125 volts. This was chosen as a compromise between the maximum available gain voltage and ruggedness when operating into high VSWR loads. These are a mirrorimage connected pair of MOSFETs, each with the following characteristics:

Because there are several different applications opportunities around 80 MHz, the second harmonic of the 40.68 MHz ISM (Industrial- Scientific-Medical) frequency allocation was chosen as the operating frequency. The design goals for the amplifier are:

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