Challenges in the design of wideband GaN-HEMT based class-G RF-power amplifiers

In GaN-based power amplifiers (PA) the gain under continuous wave (CW) excitation compresses slowly over a wide power range. Also, due to memory-effects in the GaN-HEMT, the gain for modulated excitation with varying envelope differs from the CW value. On the other hand, design approaches for supply voltage modulated systems based on post-processing of CW measurements are in widespread use. This paper scrutinizes these approaches in a case study for a class-G modulated PA at 2.7 GHz, i.e. a PA with discrete supply modulation. The errors related to the CW design approach are quantified by comparison with dynamic measurements. Test-signals with equal power distributions but varying modulation bandwidths in a range of 4 MHz to 32 MHz are used. It is found that the dynamic measurement results deviate from the expected CW performance by up to 5% in PAE and 1.5 dB in gain. Responsible for this are on one hand differences in CW and modulated gain. On the other hand the efficiency degrades with increasing supply voltage switching frequency, which is related to the modulation bandwidth and the supply voltage levels.