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Distributed Amplifiers Challenge Wide Bandwidths

Distributed Amplifiers Challenge Wide Bandwidths

Trends in solid-state distributed amplifiers continue to grow in gain and bandwidth at higher frequencies.

Distributed amplifiers may be better known in their electron-tube forms as traveling-wave-tube amplifiers (TWTAs), although their goal by any name is to provide reasonable gain and output power over a wide bandwidth. Most engineers may not realize that these are amplifier designs with roots in the 1930s and 1940s, during which times they were based on vacuum-tube active devices rather than solid-state transistors.

But to some researchers, these wide-bandwidth amplifiers are quite useful at high frequencies whether in solid-state or electron-beam form. Gholamreza Nikandish, Robert Bogdan Staszewski, and Anding Zhu with the School of Electrical and Electronic Engineering of University College Dublin, Dublin, Ireland, collected summaries of the state-of-the-art distributed amplifiers in different solid-state device technologies to show what can be done in terms of performance.

As the researchers note, the first monolithic microwave integrated-circuit (MMIC) version of a distributed amplifier, based on gallium-arsenide (GaAs) metal-epitaxial-semiconductor field-effect-transistor (MESFET) technology, was demonstrated by Yalcin Ayasli of Varian Associates in 1982. The amplifier had four 1-μm-gate-length MESFET devices and achieved 9-dB gain from 1 to 13 GHz.

Although the many researchers represented by the work on distributed amplifiers worked in so many different solid-state device technologies, they were facing similar design challenges in terms of reaching for wide bandwidths, high gain, and high power-added efficiencies (PAEs).

Capacitive coupling is one way to reduce the parasitic input capacitance that limits the bandwidth of a distributed amplifier. Placing a capacitor in series with the gate of the transistors reduces the effective input capacitance, and a large resistor is placed in parallel with the capacitor to provide a path for the gate bias. This reduces the voltage gain of the distributed amplifier because of the voltage division (the resistor) at the input of the transistors.

Another way to reduce the capacitive loading effect of the transistors’ input impedance in a distributed amplifier uses a common-source amplifier with RC degeneration as the gain stage in the distributed amplifier. The amplifier’s input impedance is derived as a series resistance and capacitance. When the circuit elements are set properly, the input capacitance of the overall amplifier is reduced, although with degraded transconductance.

The researchers provide a collection of tables listing high-performance distributed amplifiers based on different solid-state technologies, including GaAs, InP, GaN, and SiGe BiCMOS, with bandwidths as wide as 180 GHz (with InP and SiGe semiconductor processes). They note that the trends of increasing bandwidth should continue if anything, with higher-power amplifiers as a result of the availability of GaN semiconductor processes.    

See “The (R)evolution of Distributed Amplifiers,” IEEE Microwave Magazine, June 2018, Vol. 19, No. 4, p. 66.

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