60 and 94 GHz quasi-optical monolithic PHEMT amplifiers

This dissertation describes the design, fabrication and testing of quasi-optical monolithic array amplifiers at 60 and 94 GHz. The 60 GHz amplifier is based on the "grid" amplifier approach integrating small cells 1/8 wavelength on a side. Each cell consists of a differential amplifier built from two pseudomorphic high mobility electron transistors. The cell receives and transmits from stripe antennas which also convey the gate and drain bias voltages across the array. The amplifier also uses polarizers to keep the cross-polarized input and output signals separated. Also described is a focusing test set comprising transmit and receive horns and two ellipsoidal reflectors as well as the amplifier itself. This gives greater sensitivity than the conventional test method, and allows the input and output return losses to be measured in addition to the gain. A further advantage is the greater precision with which the interaction between amplifier and optical signal beams can be investigated. The test results indicated that the gain of the circuit was -1.3 dB, rather than +7 dB. This disparity is investigated along several fronts. Detailed analysis of the optical system/test set is discussed. wHis is based on Gaussian beam analysis, and losses due to reflector spillover, beam truncation and beam mismatch at the amplifier surface and at the transmit and receive horns are considered. The models for the passive circuitry are modified based on results calculated by a moment method computer code written for this work. Losses due to beam spreading and scattering between amplifier active surface and polarizers are analyzed in detail and found to be significant. The work concludes with a report on an initial attempt to construct a large cell array for 94 GHz using monolithically fabricated active circuitry along with patch antennas for input and output.