Technical White Paper
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.
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.
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
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.
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
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
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.
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
Solid state switches may overcome the speed issue, but are not nearly so linear or low loss.