ficient high-power RF is needed. However, gallium nitride devices exhibit memory effects that have to be understood and managed. Silicon germanium is another option, inherently low noise but with significantly lower leak- age current than GaAs and silicon. However, an additional thin base deposition of germanium (a higher electron mobility material) is required, which adds cost. Breakdown voltage is also generally low in these devices. 5.5.4 Broadband Power Amplifiers Irrespective of the process used it is plausible that a greater degree of PA reuse may be possible. For LTE 800 to use the same power amplifier as LTE900 would imply covering an operational bandwidth of over 100 MHz (810 to 915 MHz) at a centre frequency of 862 MHz. This is 11.6% of the centre frequency. PA operational bandwidths of up to 20% of centre frequency are now possible, though with some performance compromise. 23 As a comparison the 1800 MHz band is 4.3%. Any efficiency loss for a UMTS/LTE 900 handset incurred as a consequence of needing to also support LTE 800 would likely to be unacceptable. This implies incremental R and D spending in order to deliver an acceptable technical solution. The general consensus is that 15% is OK, 30% is stretching things too far, which would mean a power amplifier covering say 698 to 915 MHz would be unlikely to meet efficiency and/or EVM and/or spectral mask requirements. The PA will also need to be characterised differently for GSM (higher power) and for TDD LTE. The power in full duplex has to be delivered through a highly specified (linear) RX/TX duplexer with a TDD switch with low harmonics. The design issues of the PA and switch paths are well understood but if both FDD and TDD paths need to be supported then the design and implementation becomes problematic, particularly when all the other existing switch paths are taken into consideration. Wideband amplifiers are also being promoted where one PA (with suitable linearisation and adaptive matching) can replace two or three existing power amplifiers. Th