Increasing low-frequency stability margins in microwave amplifiers from experimental data

A key aspect in the robust design of microwave amplifiers is to ensure circuit stability beyond the nominal operating conditions. In this work, a method for measuring and controlling the stability margin of low-frequency resonances in microwave amplifiers is proposed. The approach adds an extra RF port in series with an R-C stabilization network that is connected to the amplifier bias path. The extra RF port is used to experimentally obtain the critical poles of the circuit. Then, from the obtained pole-zero map, pole-placement techniques are applied to get the R-C values that increase the stability margin of the critical resonances. In this way, the risk of running into a low frequency oscillation when amplifier conditions are varied can be significantly reduced. The complete approach is experimentally validated in a demonstrator prototype built in printed circuit board technology.