9 - After the Power Amplifier

Abstract

Most transistor failures in solid-state amplifiers occur at load mismatch phase angles presenting a high current mode of operation to the output transistor(s), which results in an increase in the power dissipated by the transistor(s). The most commonly used method to accomplish solid-state RF power amplifier protection against load mismatches is the reflectometer, VSWR sensing. A standard reflectometer principle is used in this chapter to detect the RF power amplifier output mismatch. It is also commonly known as a VSWR bridge, and its use can be extended to microwave frequencies with proper mechanical design. There are mechanical restrictions that clearly place a limit to the bandwidth of the circuit. Specific amounts of load mismatch must be developed for testing a system. Filtering for output harmonic reduction is required virtually with all solid-state RF power amplifiers and especially those intended for radio communications. Paralleling of multiple capacitors provides a means to increase their current carrying capabilities and allows the use of inexpensive disc types. In addition, it is easier to compose values closer to the non-standards required in many instances. The PIN diode switches are about the only way to switch signals in RF power applications at VHF and UHF; however, at higher power levels, this technique is costly due to the number of diodes required per filter.