Negative resistance low noise, reflection mode transistor amplifiers for microwave and millimetre wave applications

Negative resistance, reflection mode amplification using GaAs FETs or HEMTs offers the possibility of realising the low noise performance associated with such devices whilst achieving a higher gain per stage than conventional transmission mode amplifiers. This possibility is of particular interest in the MM-wave region, where the gain per stage of conventional FET and HEMT amplifiers, when tuned for optimum noise measure, is low. An additional potential benefit, of particular interest for radar LNA applications, is the probable existence of a low loss bypass path through a reflection amplifier after failure of the active device. In this paper, the circuit conditions for optimum noise measure in negative resistance transistor amplifiers are determined, and several examples are given. Design details and measured results are given for an example in X-band. The suitability of the technique for MM-wave frequencies is assessed as far as possible using published S-parameter and noise data for a HEMT device, and a theoretical circuit design is presented