Designing Multi-Stage Class C Amplifiers For Pulsed Radar Applications

Abstract
This paper describes a procedure for designing pulsed solid-state amplifiers for radar systems. Solid-state radar systems can be broadly divided into those using phase array antennas, and those using conventional, mechanically steered antennas.

Solid-state phased array radars (PAR's) generally distribute common transmit / receive (T/R) modules across a mechanical assembly that supports individual antennas. In some cases, T/R modules and antennas are integrated into a single assembly that attaches to a mechanical structure. Some systems have a mechanical structure that supports antennas, and signals connect from T/R modules to antennas through transmission lines (i.e. coax or waveguide). In either case, modules are mechanically distributed across the array, and are electronically adjusted in phase to steer the radar beam.

Transmitters in conventional solid-state radar systems combine power from a large number of modules, often using radial combiners. Modules are assembled in cabinets that have a single, high power output connector that feeds a T/R switch and/or other common circuitry (e.g. filter, circulator, etc.). The system connects to a mechanically steered antenna.

RF transmitter power in both types of systems is generated by a large number of common amplifiers. The procedure described in this paper is suitable for designing amplifiers used in either system.

Introduction
An amplifier design starts by reviewing its specification for overall performance. Requirements are compared to performance of available components to develop an architecture, determine performance of individual stages, and loss of passive components. A spreadsheet or similar tool projects performance of the overall assembly. Spreadsheet results lead to a block diagram that shows the amplifier's architecture and lists performance of individual stages and passive components. A successful design results when overall specifications realistically represent performance that can be achieved using available components. Performance, reliability and cost are all important aspects of a design. While all three are discussed, the focus of this paper is electrical performance.

This paper is illustrated by a typical three stage, S-Band, class C transmit amplifier design. The example could be used as the RF Power Amplifier (RF-PA) subsection of a T/R module in a Phased Array Radar system, or as one of many RF-PA modules in a Conventional Radar system.

Reprinted with permission. Copyright © 2005 James R. Curtis.