From Figure 7, it can be seen that when both the driver and the HPA are biased with resistive dividers, the small-signal-gain variation is 4.23 dB for a temperature drift of 40°C (from 0 to 40°C). This gives a variation rate per amplifying stage of 0.0176 dB/°C for all six amplifying stages (the driver has four amplifying stages while the HPA has two). This value is similar to the one obtained and discussed in [7]. When the driver remains biased with a fixed resistive divider and the HPA is biased by means of the proposed active bias network, the small-signal-gain variation is reduced to just 2.8 dB for the same temperature range. This gives a variation rate per amplifying stage of 0.0175 dB/°C (in this case the variation rate is given only by the four amplifying stages of the driver amplifier). The proposed active bias network is thus able to compensate the small-signal-gain variations due to the HPA at different temperatures. Figure 7 demonstrates that the drop in the smallsignal-gain parameter caused by increasing temperatures is compensated by a higher transconductance, as stated in section 2. This increasing transconductance with increasing temperatures is achieved because the active bias scheme applies a more positive gate voltage in order to keep the drain current constant.Of further importance is the fact that, for the same temperature, the 1-dB compression point (P1 dB point) is slightly higher for the active bias configuration than for the resistive divider scheme. For instance, at 30°C the P1 dB points are 23.2 dBm and 22.5 dBm, respectively. CONCLUSIONThis study demonstrates that an active bias network accounts for the drop in MMIC GaAs MESFET high-power amplifier PAE figure caused by the reverse gate currents, which appear under large-signal operation associated with gate-drain breakdown mechanisms. This bias scheme additionally compensates the small-signal-gain variations due to temperature changes. The performance of a Ka-band high-power amplifier biased with the proposed active scheme has been successfully compared to that of the same amplifier biased by means of a conventional fixed resistive network. ACKNOWLEDGMENTSThis work was supported by Project TIC2002-04569-C02-01 and Project TIC2001-3839-C03-01 of the Spanish National Board of Science and Technology (MCYT

The high efficiency operation of GaAs MESFET amplifiers is important for many microwave systems. In this work, three common types of doping profiles, uniform, low-highlow(impu1se) and ion implanted are optimized for maximum power added efficiency using a large signal, physics based MESFET model. X Band efficiencies up to 75% are predicted.Optimum tuning conditions for the input and output side of the MESFET consistent with Class F theory must be met. Optimum profiles exhibit low surface doping density and low I d , , for maximum breakdown voltage.

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