Diplexer-like bias circuits for stabilizing GaAs HBTs during low frequency measurements

GaAs based heterojunction bipolar transistors are used in microwave oscillators to provide low phase noise operation. Prediction of phase noise requires transistor models to be accurate at low frequencies as well. Measurements of low frequency noise and network parameters however, are not straight forward because of the instabilities of the transistor in the measurement setup. We designed and built diplexer-like bias circuits to stabilize the transistors between DC and 50 GHz for successful low frequency measurements between 10 Hz and 1 MHz.