With new architectures, things changed for many reasons: first the allocated bandwidth is increasing in HTS satellites, or is split in sub-bands spread over 2.5GHz or even 2.9GHz in Ka-band. Second, amplifiers are working in multicarrier mode and the instantaneous bandwidth must be large. This constraint is increased further when new architectures such as multiport amplifiers or phased arrays are used. Today, TWTAs are designed to be capable of handling the full bandwidth (2GHz in Ku-band, 2.9GHz in Ka-band).
Efficiency is the most stringent requirement that power amplifiers must comply with. It is easy to understand why, as roughly 80% of the regular payload DC power is allocated to power amplifiers (this percentage is less when a digital processor is embarked, but the consequence is to put more pressure on electrical efficiency). One has to keep in mind that improving efficiency not only reduces the required primary power from the solar cells; it also and maybe more significantly reduces the dissipation constraints of a platform, which has only one way to evacuate excess heat: thermal radiation into space.
The EPC efficiency is typically between 93 and 95%, so the main constraint rests on the shoulders of the amplifier device itself. For many years (and still today) TWT efficiency was specified at saturation level, knowing that many of them were actually used close to this configuration. Great progress has been made during the last decades for TWTs, whose saturation efficiency increased from 50% in the 1980s to the current 70%. Room for improvement is still in front of us (some devices has already being measured at 75%), and 80% should be achievable.
But efficiency at saturation doesn’t tell the full story, as more and more amplifiers are used in multicarrier configuration. It is at this new operating point, i.e. in back-off, that the efficiency must now be optimized (back-off operation is required in multi-carrier operation to guarantee the required level of linearity). Of course, the first requirement is to keep the back off as low as possible, so most of amplifiers used in space are linearized.
For many years, efficiency improvement at output back-off (OBO) followed the improvement of efficiency at saturation, but with new TWT multi-stage collector designs, improving efficiency at 2 to 4dB OBO are now possible, and very significant gains have been demonstrated by Thales in the last few years, with close to 8% more in efficiency (from 42 to 50%) at 3dB OBO. This has a tremendous impact on the global efficiency of HTS payloads.