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This paper describes cryogenic broad-band amplifiers with very low power consumption and very low noise for the 4-8-GHz frequency range. At room temperature, the two-stage InP-based amplifier has a gain of 27 dB and a noise temperature of 31 K with a power consumption of 14.4 mW per stage, including bias circuitry. When cooled to 15 K, an input noise temperature of 1.4 K is obtained at 5.7 mW per stage. At 0.51 mW per stage, the input noise increases to 2.4 K. The noise measurements have been repeated at different laboratories using different methods and are found consistent. Index Terms--Cryogenic electronics, dc power, GaAs, InP high electron-mobility transistor (HEMT), low-noise amplifier (LNA), noise temperature.

The performance of broad-band low-noise low-dc-power-consumption cryogenic amplifiers have been studied in detail with emphasis on minimizing the power consumption and optimizing the amplifier performance at cryogenic temperature. A general approach is presented for the modeling and amplifier design, which helps in minimizing the power consumption and optimizing the performance of the amplifier. A noise temperature below 9 K and 22-dB gain was experimentally obtained in the frequency range of 4-8 GHz with a total power consumption of 4 mW with commercial GaAs transistors. Index Terms--Empirical large-signal models, FET, FET noise models, modeling, noise model extraction, noise parameter.

Typical paraelectric materials (e.g., SrTi[O.sub.3], Kta[O.sub.3], [Ba.sub.x][Sr.sub.1-x]Ti[O.sub.3], x < 0.5) and electrically tunable microwave devices based on these materials are briefly reviewed. The analysis shows that in spite of the recent year's extensive efforts, no considerable improvement in the microwave losses in thin paraelectric films has been achieved. Thin films, regardless of fabrication method and substrate type, have much lower dielectric permittivity than bulk single crystals, and the loss tangent at microwave frequencies (f > 10 GHz) is of the order of 0.01 (at zero dc-bias field) at room temperature. Nevertheless, quite promising component and subsystem level devices are successfully demonstrated. Use of ceramic (bulk and thick film) ferroelectrics in tunable microwave devices, currently considered for industrial applications, offer cost reduction. In this paper, explicitly for the first time, we consider possibilities and benefits of using ferroelectrics in polar phase in electrically controllable microwave devices. Examples of using ferroelectrics in polar state (e.g., [Na.sub.0.5] [K.sub.0.5] Nb[O.sub.3], SrTi[O.sub.3] in antiferroelectric phase) in electrically tunable devices are reported. Index Terms--Ferroelectrics, paraelectrics, tunable microwave devices.

Conformal mapping-based models are given for interdigital capacitors on substrates with a thin superstrate and/or covering dielectric film. The models are useful for a wide range of dielectric constants and layer thicknesses. Capacitors with finger numbers n [greater than or equal to] 2 are discussed. The finger widths and spacing between them may be different. The results are compared with the available data and some examples are given to demonstrate the potential of the models.