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Diagram of 100 fF amplifier chip with three levels of magnification [(a)–(c)], adapted from CAD files used for fabrication. The output transformer consists of a set of two overlap capacitors to ground and two series spiral inductors, designed to match over as much of the target band as possible. Another overlap capacitor couples the input line to the quarter wave resonator, which is terminated in the input coil of the SQUID.
Length of resonator as a function of frequency of best input match for a 60 fF input capacitor amplifier. Each point represents a set of measurements on a separate amplifier with a different length input resonator. The continuous line is a fit to a second-order polynomial. Note that the frequency dependence is smooth, allowing for interpolation as needed to design for any frequency in the band. Data are displayed with frequency as the independent variable since this is intended to be used as a design tool where the frequency is selected based on some application, and the length is chosen to match that frequency.
(a) Gains for several flux bias points, ranging over about 5% of a flux quantum, and fixed current bias of the amplifier with a 60 fF coupling capacitor. Note the trade-off between gain and bandwidth in the different curves. (b) High bandwidth bias point (current bias of approximately ) for 100 fF coupling capacitor amplifier. The data shown on this plot demonstrate over 27 GHz of gain-bandwidth product (the integral of linear power gain over the frequency range). (c) Same bias point and amplifier as (b), showing that there are multiple gain maxima, and that feedback effects cause surprisingly complex frequency dependence in the gain.
(a) Total microwave measurement chain drift as a function of time overnight. Inset (b) shows dependence of maximum gain on the flux bias.
Gain at fixed bias point as a function of amplifier input power.
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