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HBT amplifiers in common emitter configuration, at the present state of the art, anegative voltage supply is necessary for FET amplifiers. Therefore, the HBT amplifier's circuits could be simpler and thus lower-cost and higher-yield.Presently, 5.6 W/mm of emitter periphery and a power GaAh'GaAs Power HBTs have been inves%Predfor handsets in mobile cellular conznzunication systems at 1.8 GHz. Different HBT's operating classes have been conzpared, and the choice of the optinzurn operating class has been discussed. Experinzental ineasureinents on a snzall size HBTof 0.4~0.4 n2m2 have shown 1 Woutputpower added efficiency of 68% have been demonstrated at 10 GHz for cW operation However, several important aspects of the HBT for mobile communications such as the optimum operating class still need to be investigated. In this paper, different HBT's operating classes for high output power and high power added efficiency have been compared, and the choice of the optimum operating class has been discussed. Experimental measurements on a small chip size HBT of 0.4x0.4 mm2 have shown 1 W output power with a power-added efficiency of 45%.with a power-added efficiency of 45%.

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