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Solid State Products
Traveling Wave Tube Amplifiers (TWTAs) & Solid State Power Amplifiers (SSPAs)
For many years, the only source for high power microwave amplifiers was a vacuum device such as a TWT. Now, with the advent of high power solid state devices such as LDMOS ICs for frequencies below 1.5 GHz, GaN PHEMT ICs for frequencies from 1 GHz through 18 GHz and GaAs, as well as InP ICs through 94GHz, new Solid State Power Amplifier (SSPA) designs are possible. For some microwave transmitter applications, SSPAs offer a cost effective alternative to traditional traveling wave based amplifiers. Typically available at lower power levels, SSPAs can offer an alternative to TWTs for narrow band applications. The chart below compares TWT availability to the current frontier state-of–the-art in SSPAs.
Power Amplifier Availability
Solid State Power Amplifiers
While the basic functionality of the amplifier is the same in a high power solid state amplifier, multiple devices are combined into unit building blocks or power modules which, in turn, are combined to develop the needed gain and power. The network is fed by a high current, low voltage supply and, similar to TWTAs, added control and protection circuitry is usually incorporated into the amplifier. An example of an SSPA block diagram and power module is shown below.
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Figure C. Block Diagram
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Figure D. Power Module |
Depending on the cost and performance goals, solid state amplifiers can be built with commercial package parts, mil-spec parts, or bare die. Packaged parts, which incorporate 1 to 3 discrete ICs, are usually partially matched to ease the design and manufacturing process. Alternatively, discrete die allow for maximum flexibility with an increase in design expense. For most customers, discrete die are used when the goal is to maximize performance in the smallest unit volume.
TWTA and SSPA Comparison
Depending on the particular system and application, both SSPAs and TWTAs have advantages and disadvantages. While there are many variables used to determine the most appropriate choice, the matrix below can help to refine the decision
TWTA & SSPA Matrix
Given the relative merits of TWTAs and SSPAs, it is not surprising that each technology lends itself to particular applications. In general, wideband, applications such as EW systems are typically implemented using TWT technology. Conversely, narrow band applications, such as single frequency satellite uplink terminals, are often based on SSPAs. Narrow band high power applications, such as those found in radar systems, represent a middle ground with some systems using TWTs and others, especially phased array systems, using solid state amplifiers.
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