Introduction
The Need to Amplify Signals
An amplifier is one of the most common electrical elements in any system. The requirements for amplification are as varied as the systems where they are used. Amplifiers are available in a large number of form factors ranging from miniscule ICs to the largest high-power transmitter amplifiers. In the following discussion the focus will be on solid state power amplifiers used at microwave frequencies, particularly in test and measurement applications.
Microwave power amplifiers may be used for applications ranging from testing passive elements, such as antennas, to active devices such as limiter diodes or MMIC based power amplifiers. Furthermore, other applications include testing requirements where a relatively large amount of RF power is necessary for overcoming system losses to a radiating element, such as may be found at a compact range, or where there is a system requirement to radiate a device-under-test (DUT) with an intense electromagnetic field, as may be found in EMI/EMC applications.
As varied as the system requirements may be, the specific requirements of a given amplifier can also vary considerably. Nevertheless, there are common requirements for nearly all amplifiers, including frequency range, gain/gain flatness, power output, linearity, noise figure/noise power, matching, and stability. Often there are design trade-offs required to optimize any one parameter over another, and performance compromises are usually necessary for an amplifier that may be used in a general purpose testing application.
The following discourse includes a description of amplifier topologies introducing the basics of spatially combined distributed amplifiers, a discussion of typical amplifier specifications and a review of performance verification measurements.
Broadband Microwave Power Amplifiers
There are numerous techniques for designing microwave power amplifiers. These may be broadly split between tube and solid state technologies. For high power requirements (> 100 Watts), typically these are satisfied with tube based designs. Tube amplifiers, such as Traveling Wave Tube Amplifiers (TWTAs), require a high voltage power supply, typically require warm-up time, and have significant aging related issues. For solid state amplifiers to achieve similar performance often requires switching between
narrow-band amplifiers, with deleterious effects to the overall linearity and gain/power flatness. The switches themselves embody performance compromises. Mechanical switches, while quite linear and relatively low loss, have switching speed limitations, and are subject to failure after repeated switching cycles. Solid state switches may overcome the speed issue, but are not nearly so linear or low loss. Both the signal fidelity and loss issues limit the usefulness of solid state switches for high power microwave amplifiers. Furthermore, switching between narrowband amplifiers requires external stimulus with the software control complication that entails.
A topology often favored for generating modest amounts of microwave power output is to combine the outputs of several relatively low output power amplifiers. The individual amplifiers usually have a “distributed” or “traveling wave” topology1. The distributed amplifier topology achieves a large frequency range by arraying individual transistors; each representing shunt capacitances between series inductances, to create a semi-lumped representation of a transmission line (see Figure 1). This amplifier topology is often fabricated using MMIC techniques, and has been optimized to the point where single amplifiers can provide up to nearly 1 Watt of saturated power output. Nevertheless, it is no trivial task, using conventional planar circuit techniques, to combine the power output of even a small number of these distributed amplifiers over a full decade frequency range, without incurring unacceptable losses or poor flatness characteristics.