A novel technique for measuring small signal S-parameters of an RF/microwave transistor, power amplifying stage for use in power amplifier stability analysis

A novel measurement is described which enables the full small-signal S-parameters of an RF/microwave transistor to be measured while it is simultaneously driven and optimally tuned at a higher frequency as an efficient, class B or class C power amplifying stage. This capability allows a classic, small-signal stability analysis of power amplifiers to be performed across frequencies below the power amplified carrier where parametrically pumped, subharmonic oscillations are often a problem. Measured S-parameters of a GaAs HBT (heterojunction bipolar transistor) operating as an efficient, harmonically tuned, class B, power amplifier stage at 870 MHz are presented across the frequency range 50-700 MHz. Analysis of these results shows the presence of negative resistance in the base and collector that is induced by the carrier.<>