Glossary
Comparison between solid-state and electron tubes amplifiers

SSPA (Solid State Power Amplifier) :

Transistor-based radiofrequency or microwave amplifier. This term covers not only the amplification stages, but also the ancillary functions (low-level stages, linearization, etc.) and power supply.

TWT / TWTA (Traveling Wave Tube / TWT Amplifier) :

Travelling-wave tube and broadband microwave amplifier based on TWT (TWTA), including ancillary functions and power supply as before.

klystron:

Narrow-band tube amplifier, very high peak power.