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Input Impedance and Gain of a Gigahertz Amplifer Using a DC SQUID in a Quarter Wave Resonator



Lafe F. Spietz, Kent D. Irwin, Jose A. Aumentado


Due to their superior noise performance, SQUIDs are an attractive alternative to high electron mobility transistors for constructing ultra low noise microwave amplifiers for cryogenic use. The primary problem to be overcome in designing such an amplifier is coupling power from the 50 transmission line impedance standard in microwave circuits to the low and frequency dependent input impedance of a SQUID. This has been solved in the past by using the input coil as a microstrip resonator either of one quarter or one half of the input wavelength or by shrinking the physical size of the SQUID until it may be treated as a lumped element. Previous work has led to devices with noise performance approaching the quantum limit, but above 1 gigahertz, the gain has diminished significantly, and the devices have proven dificult to model accurately in detail. We describe the use of a lumped element SQUID inductively coupled to a quarter wave resonator. The resonator acts as an impedance transformer and also makes it possible for the first time to accurately measure the input impedance and intrinsic microwave characteristics of the SQUID. We present a model for input impedance and gain, compare it to the measured scattering parameters, and describe how to use the model for the systematic design of low-noise microwave amplifiers with a wide range of performance characteristics.
Applied Physics Letters


amplifier, squid


Spietz, L. , Irwin, K. and Aumentado, J. (2008), Input Impedance and Gain of a Gigahertz Amplifer Using a DC SQUID in a Quarter Wave Resonator, Applied Physics Letters, [online], (Accessed February 25, 2024)
Created August 29, 2008, Updated February 19, 2017